WO2013044977A1

WO2013044977A1 - A wall element system for an offshore power storage facility

Info

Publication number: WO2013044977A1
Application number: PCT/EP2011/067122
Authority: WO
Inventors: Lars Stig Nielsen; Janus MÜNSTER-SWENDSEN
Original assignee: Seahorn Energy Holding ApS
Priority date: 2011-09-30
Filing date: 2011-09-30
Publication date: 2013-04-04

Abstract

A wall element system (101) of concrete for installation on a seabed as at least a part of a reservoir (1001) to dam up water, comprising: a pile (104) for fixation of the wall element on the seabed; a wall element (102); and a channel (103) integrated in the wall element by a concrete casting process and extending in a substantial vertical direction when the wall element is placed in a normal position on the seabed; wherein the channel is configured to guide the pile through the channel and into the seabed; and wherein the channel is, at least at a lower portion, configured to accommodate a portion of the pile for fixating the pile and the channel in a rigid interconnection. Further, there is provided an offshore reservoir and a method of building an offshore reservoir using the wall element system above.

Description

A wall element system for an offshore power storage facility

This invention relates to the construction of an offshore power storage facility, a wall element system for assembling an offshore wall, a reservoir and a method of installing a wall element system for an offshore power storage facility.

Background

Renewable energy is receiving much focus due to the dependency on fossil fuels of the current power supply and the mitigation of man-made climate changes. Renewable energy output from wind turbines, wave energy converters, solar panels and other renewable energy converters is intermittent or irregular depending on the presence and strength of wind, waves and sunlight. Conventional fossil fuel operated power plants generate power regulated to meet an estimated demand by the consumers. Power is supplied to the consumers via existing distribution systems. However, in frequently occurring situations the demand from the consumers does not match the presence and strength of wind, waves and sunlight. This causes difficulties since it is not possible to store significant amounts of power in conventional distribution systems.

Conventional fossil fuel operated power plants are regulated to constantly balance consumption and production of power. Further, a stable power supply is a necessity for a modern society, and the power consumption is highly inflexible and follows, to a large extent, a daily cycle. Therefore, it is challenging to meet the power demand with energy from intermittent renewable resources. Power storage is a viable solution to this challenge. Power can be stored when renewable energy production is high and demand is low and be supplied to the power system when renewable energy production is low and demand is high. Thus power storage enables a higher share of intermittent renewable energy in a power system and reduces the need for backup capacity of conventional fossil fuel power production.

The storage of power as hydro potential is a well known and proven technology. But the conventional hydro power solutions including storage require an area to be dammed up, causing severe changes to the local environment. In many places the conventional hydropower has limited resources as it is competing for the land with for instance buildings and recreational areas. Further, there is an increasing pressure for renewable energy converters such as wind turbines to be located away from human residential and recreational areas. But offshore constructions are often difficult and costly to construct, which can lead to increased prices of the energy delivered by the offshore facility. An offshore environment is less controllable than an on shore environment, and weather is an important factor in offshore construction work. Often bad weather causes so-called downtime, where construction work has to be discontinued. Costly measures have to be taken to secure accurate handling and placement during construction work. Thus, conventionally, many forces work against cost-efficient production, conversion and storage of renewable energy offshore. Related prior art

WO2009123465 discloses the working principle of an offshore power plant. However, this document fails to disclose how to build such an offshore power plant in an efficient manner. Summary

There is provided a wall element system of concrete for installation on a seabed as at least a part of a reservoir to dam up water. The wall element system comprises a pile for fixation of the wall element on the seabed, a wall element and a channel integrated in the wall element by a casting process. The channel extends in a substantial vertical direction when the wall element is placed in a normal position on the seabed and is configured to guide the pile through the channel and into the seabed. The channel is, at least at a lower portion, configured to accommodate a portion of the pile for fixating the pile and the channel in a rigid interconnection.

This configuration greatly reduces the number and complexity of operations that are needed to build a wall in an offshore environment. The configuration allows the pile, serving at least as part of the foundation for the wall element, to be installed into the seabed after the wall element has been positioned on a designated position on the seabed. Thereby, positioning of the piles at exact and aligned positions is inherently controlled by the channels of the wall element. This is in contrast to conventional offshore building methods which require accurate and complicated machinery to execute precision positioning of huge elements subjected to offshore conditions with significant wave and wind loads, unless construction is restricted to limited time windows with sufficiently calm weather conditions. Not only are the piles precisely positioned and guided into the seabed by the present invention, the wail element is also - due to its configuration - aligned with the piles by that process. This greatly facilitates the task of installing the wall element. A wall element is a wall that comprises at least one channel and that can connect to other wall elements or other structures. A wall element system is a wall element comprising one or more channels and one or more piles. A channel configured to guide a pile prevents the pile from moving significant in a plane transverse to the longitudinal direction of the channel, but allows for movement in a direction along the longitudinal direction of the channel, thereby ensuring that the pile is aligned with the channel. A channel configured to accommodate a pile encloses a portion of the pile after the pile has been installed in the seabed. The channel and the pile are then connected by a rigid interconnection, e.g. by a casting process. By the term 'rigid interconnection' is intended a connection that is rigid considering normal concrete constructions and is capable of transferring the forces arising from damming up water.

Furthermore, the channel provides a shield against the offshore environment which allows for subsequent interconnection, e.g. by grouting, between the pile and the wall element, without complicated and time-consuming construction of formwork constructions exposed to the offshore wave and wind loads. When a space is formed between the pile and the channel, it provides a casting enclosure that is protected from the offshore environment. The space can be sealed off at the bottom by the seabed or by other means. A channel is a hollow space fully enclosed on the sides by the wall element and open at both ends. A channel is straight such that the pile can move down through the channel. The general direction of the straight channel is referred to as the longitudinal direction of the channel. The cross-sectional area of a channel does not have to remain constant throughout the length of the channel; it may vary.

Since the wall element system can be installed as a complete wall structure comprising foundation piles, it has a high level of survivability during installation in an offshore environment. This reduces risk of failure of the wall element system during the period in which the offshore reservoir is erected. As a cost-saving measure, the wall element is manufactured onshore, where weather-protected series production with conventional manufacturing techniques is possible, and is then transported to an offshore erection site for installation. Thus, a significant reduction of cost and complexity of operations that are needed to build a wall in an offshore environment is achieved.

In an embodiment, the wall element system comprises multiple channels and respective piles, e.g. 2 or 4 or 5 channels; thereby increasing foundation capacity and thus enabling the wall element systems to be longer. Longer wall element systems mean fewer wall element systems for a given wall length, which decreases the number of transportation and installation operations, thus reducing installation costs.

In an embodiment, the wall element system comprises multiple channels and respective piles, and the piles have substantially equal diameters; thereby making the construction, handling and installation of wall element piles easier, as the equipment is required to handle piles with that diameter only.

In an embodiment, the channels are placed substantially equidistantly within the wall element. Thereby the nature of the load which is a hydrostatic load is utilized to reduce the constructional complexity and cost of the wall element. In an embodiment, pile and channel diameters and/or mutual distances between piles are adapted to the nature of the seabed surrounding the respective piles. Thereby severe differences in the geological properties of the seabed can be taken into consideration in the construction of the wail element system.

In an embodiment, the cross-section of a pile is substantially circular. Thereby, a uniform reaction towards transverse loads from different directions is provided. This means that the foundation for the wall element is very versatile and can be utilized for foundation for additional constructions as well, such as a wind turbine, thus reducing the overall foundation costs of the combination of wall element system and additional construction. In an embodiment, the pile of a wall element system is hollow. Thereby material consumption for the pile is reduced while a high stiffness of the pile is maintained, thus reducing pile foundation costs.

In an embodiment, the pile diameter is at least 2 m, e.g. about 4 m or 6 m; thereby ensuring sufficient foundation capability to withstand larger hydrostatic forces. In an embodiment, the cross-section of a pile is an I-beam or H-beam shape, oriented with the flanges parallel to the length direction of the wall element. Thereby the nature of the hydrostatic loading on the wall element, which is mainly transverse to wall element length, is utilized to minimize pile costs, as the I-beam and H-beam provide good stiffness towards loads in that direction.

In an embodiment, the pile is made from steel; meaning that it has high strength with regard to tension, shear and compression as well as a high ductility. This means the pile can have a relatively low pile wall thickness and can be driven into the seabed. Pile driving is a relative inexpensive method for installing a pile compared to e.g. drilling.

In an embodiment, the pile is made from reinforced concrete and is assembled from multiple shorter pile pieces. Thereby it is constructed from relatively inexpensive materials.

In an embodiment, the pile is driven into seabed by a hammer or vibrated or drilled into the seabed or any combination of the three, which are known techniques for pile installation within the field of offshore civil engineering. The wall between channels must be of sufficient strength to withstand the forces acting on it, including the hydrostatic pressure, and the pile provides at least some foundation for the wall element. The foundation must be of sufficient strength to ensure that the wall element does not move or fall over. This means that at least some of the forces acting on the wall between channels must be resisted by the piles.

In an embodiment, the largest inner diameter of a channel is greater than the thickness of that portion of the wall element that extends between the channels. Thereby the diameter of a pile in the channel can be greater than the wall-face thickness, and the foundation capability of the pile is then increased, and the distance between piles in the wall element can be increased. The number of piles and the number of pile installation operations are thus reduced. The portion of the wall element that extends between the channels is also denoted the wall-face or wall panel or panel or a like term.

In an embodiment, a channel is located at or in proximity of the respective ends of the wall element, thus placing more construction material and a foundation pile near said ends. Thereby the ends of the wall element are made more robust, which makes a connection to an end of another wall element easier. A channel located in proximity of a wall end is located such that the distance from the end face of the wall element to the channel is less than 5 m or 2 m, e.g. about 1 m or 50 cm. in an embodiment, the wall element or a portion thereof is made of concrete reinforced with bars or fibres. The reinforcement material can be steel, glass fibre, polypropylene fibre or synthetic fibre e.g. carbon fibre. Thereby the wall element becomes stronger and more durable, which entails a reduction in wall element material consumption. In an embodiment, the wall element between two neighbouring channels comprises a lower portion which is substantially vertical and an upper portion which is inclined relative to the vertical, said upper portion being configured to break the waves of the sea. Thereby the peak load magnitude on the wall element from waves is reduced since waves impacting on the inclined portion of the wall will have a reduced peak load magnitude. The upper portion inclined from vertical is smaller than the substantially vertical portion, e.g. with a vertical height of about 5%, 10%, 15% or 20% of the height of the substantially vertical portion. The inclination angle of the inclined portion is less than 45 degrees relative to the vertical, e.g. about 15 degrees, 25 degrees, 35 degrees.

In an embodiment, the wall element comprises one or more hollow compartments within the wall. Thereby the weight of the wall element is decreased. This makes transportation and installation of the wall element easier. A filler material can later be charged to or filled into the hollow compartments, e.g. concrete, stone or sand. By the term 'hollow compartments' is intended a hollow section within the wall. These hollow compartments can have an opening out through the top of the wall when it is in an upright position or they can be fuily closed on all sides.

In an embodiment, the height of the wall element is adapted to the water depth at the designated location on the seabed of the wall element. The height of the wall element above sea level is adapted to local wave conditions, but is typically a minimum of 2 meters above sea level. The overall height of the wall element is at least 5 meters when it is in an upright position. Thereby, the wall element is suited for constructing an offshore reservoir with a possible height difference of more than 10 meters between the inside reservoir and the surrounding body of water, which is necessary for the reservoir to be of commercial interest. In an embodiment, the wall element has such vertical dimension as to make it reach from the seabed to above the sea level; thereby the water is separated into the reservoir side and the sea side. In an embodiment, the wall element has such vertical dimension as to make it reach from the seabed to near the sea level, but still below the sea level. Thereby wave loading on the wail element during at least a part of the installation phase is reduced, as the wail element is not exposed to the wave forces near the sea level where they are largest. Additional wall elements are later added one after another to make the reservoir wall reach above the sea level. In an embodiment, these additional wall elements comprise wave energy converters. This could be e.g. floaters on lever arms that are driven by the wave motion and then drives a generator via a hydraulic system thus converting wave energy to electricity, or other wave energy converters.

In an embodiment, the channel is filled with sand after the pile has been installed into the seabed. Thereby any clearance between the pile and channel is filled with sand, resulting in good load transferring between channel and pile.

In an embodiment, the channel is configured with a bore giving a first clearance between the inside of the channel and the pile at an upper portion of the channel and a constriction of the channel at a lower portion of the channel with a second clearance that is smaller than the first clearance. This configuration increases the installation speed of the wall element system. The larger first clearance of the upper portion of the channel provides a grouting enclosure, while the constriction of the lower portion of the channel, together with the pile, forms a seal capable of holding back grout. Thereby a rigid interconnection between the pile and the channel can quickly be established, by grout being poured into the grouting enclosure, where the seal keeps the grout in place in the grouting enclosure during the curing of the grout. Further, the smaller second clearance improves the guidance and centring of the pile with respect to the channel. This reduces the need for further equipment and installations to secure proper guidance of the pile when moved through the channel. By the term 'constriction' is intended that the channel changes from one cross-section to a smaller cross-section, smoothly or stepwise. The cross-sectional shape is not necessarily the same along the longitudinal direction of the channel.

In an embodiment, the first clearance provides a grouting enclosure between the channel and the pile which is sufficiently wide to accommodate grout throughout the extent of the grouting enclosure, whereas the second clearance is significantly smaller so as to withhold grout in the grouting enclosure. !n an embodiment, the first clearance is at least two times greater than the second clearance. Thereby the second clearance can hold back grout, while the first clearance can provide a grouting enclosure.

In an embodiment, the second clearance between the pile and the constriction at a lower portion of the channel is between 0-35 mm or 0-50 mm or 0-100 mm. A larger clearance makes it easy to fit the pile inside the lower portion of the channel, where a smaller clearance increases the risk of the pile damaging the channel or getting stuck within the channel. A large clearance does not provide the same guidance of the pile as a smaller clearance, but the ability to guide the pile can be increased by increasing the length of the constricted portion at the lower portion of the channel. If the constricted portion is long, the clearance can be larger and if the constricted portion is short, then the clearance needs to be smaller. In an embodiment, the channel comprises a flexible lip forming a seal capable of withholding grout and placed near the constriction of the channel. Thereby grout poured into the casting enclosure created by the first clearance between the pile and the upper portion of the channel is held in place during curing. This ensures a good connection capable of transferring large loads between the pile and the channel. in an embodiment, a substance with very high viscosity, e.g. silicone, is applied in the second clearance between the pile and the constriction at a lower portion of the channel. Thereby a seal between the lower portion of the channel and the pile is formed, which holds grouting material, poured into the casting enclosure created by the first clearance between the pile and the upper portion of the channel, in place during curing. This ensures a good connection capable of transferring large loads between the pile and the channel. This substance could for instance be applied through pipes within the wall element.

In an embodiment, a grouted interconnection between the pile and the channel is established in both the first and the second clearance. Thereby the area of the interconnection is made larger and thus the interconnection becomes stronger and can transfer larger loads.

In an embodiment, the transition between the upper portion of the channel and the lower portion of the channel is conical. Thereby the pile is guided from the upper portion to the lower portion of the channel. In an embodiment, wedges are driven into the first clearance between the upper portion of channel and the pile subsequent to pile insertion into the seabed, thus fixating the pile and the wall element. Thereby a preliminary interconnection between the channel and the pile is formed by the wedges and by the lower portion of the channel, which will hold the wall element in position e.g. while grout in the grouting enclosure is curing. In an embodiment, the interconnection formed by grouting of the grouting enclosure provided by the larger first clearance of the upper portion of the cannel comprises reinforcing bars or fibres. The reinforcement can be made of steel, glass fibre, polypropylene fibre or synthetic fibre, e.g. carbon fibre. Thereby a strong and durable connection is ensured, which means that the lifetime of the connection and thus the offshore wall is prolonged.

In an embodiment, the channel comprises guide pieces protruding from the inside surface of the channel towards the centre of the channel, where the guide pieces are configured to guide the pile through the channel. Thereby guidance of the pile is provided regardless of the size of the channel, thus making the channel design less dependent on pile size and shape. Further, friction between pile and channel can be reduced e.g. by suitable choice of materials for the guide pieces. Further, the guiding pieces can hold the pile and the channel fixed relative to each other while a grouted connection is curing. The guide pieces are for instance an integral part of the channel constructed in the same casting process as the channel or constructed of e.g. steel or plastic material and casted into the concrete channel or mounted after channel manufacturing e.g. by bolting. A lubricant can be added to reduce friction between guide pieces and the pile.

In an embodiment, the wall element system comprises a transition piece fixed on top of the pile, where the transition piece is configured for installation of a wind turbine. Thereby cost reductions are achieved since the pile serves the dual purpose of being a wall foundation and a wind turbine foundation. Further, a power cable for the wind turbine can be installed on the wall element, thus achieving savings compared to laying the power cable within the seabed as it is the normal procedure for offshore wind farms. Since the piles are inserted substantially vertically into the seabed, and wind turbines such as horizontal axis wind turbines require to be substantially vertical, the wall element system with a transition piece fixed on top of the pile forms a cost effective foundation for a wind turbine.

In an embodiment, the transition piece is secured to a single pile. In other embodiments, the transition piece comprises multiple legs to be secured to respective multiple piles e.g. three piles, where at least one of these piles is a part of a wall element system. Thereby a solid wind turbine foundation is created, as this wind turbine foundation is better suited for resisting the overturning moment coming from the wind turbine, compared to a one-pile foundation. Also different design constraints given by the wall design and the wind turbine design can be met with the same pile diameter, which is economically attractive. The transition piece can be secured to the pile by adapting means and methods known from the offshore wind turbine industry e.g. a grouting process. Thereby installation techniques and equipment known from pile foundation for wind turbines can be implemented when installing the transition piece and the wind turbine. The use of well proven technology and existing equipment lowers the installation costs.

In an embodiment, the transition piece is connected to a channel of the wall element. Thereby the wall element is utilized as a connection piece between the pile foundation and the transition piece, thus reducing pile material consumption and costs.

By the term On top of the pile' is intended that the bottom of the wind turbine is above the top of the pile. This could be directly above the pile or anywhere above a horizontal plane at the top of the pile.

By the term 'wind turbine' is intended a wind turbine in the megawatt range, e.g. about 2 MW, 3.6 MW, 5 MW, 7 MW or 10 MW in rated power. By 'transition piece' is intended a piece suitable for connection with the wall element system and a wind turbine. The transition piece can compensate if the pile or the wall element is not as close to the vertical as is required by the wind turbine. Examples of such transition pieces are found in the offshore wind turbine industry. The transition piece is typically made of steel.

In an embodiment, the wall element comprises a buttress structure configured to counteract overturning moments acting on the wall element, where the buttress structure is integrated in the wall element or connected to the wall element by mechanical fastening means e.g. bolts. This configuration results in a reduction of the material and installation costs of a wall element system capable of resisting large overturning moments. The overturning moment acting on an offshore wall element system must be counteracted in order for the wall element system to stay upright in all weather conditions. The buttress structure utilizes the bearing capacity of the seabed at a distance from the bottom of the wall element to counteract the overturning moment acting on the wall element. The buttress structure converts at least a portion of the overturning moment on the wall element to a substantially downwards force on the seabed or a pile or another buttress foundation. Thereby the amount of the overturning moment to be counteracted by bending of the piles of the wall element system is reduced, which means that the pile penetration depth of the wall element system can be reduced, thus reducing pile material consumption and installation costs. A buttress structure is a structure projecting from the wall element to support and strengthen it. The buttress structure supports the wall element by bracing it. The buttress structure can have a constant height or the buttress structure can have a downwards slope towards the seabed, at least for some length of the buttress structure. The buttress structure extends in a transverse or inclined direction from the wall element. In an embodiment, the buttress structure extends from the upper half of the wall element, where the loading from sea waves mainly occurs. Thereby the stiffness of the upper half of the wall element and its ability to withstand sea wave loading is increased.

In an embodiment, the buttress structure extends to a point substantially horizontally from the bottom of the wall element and with a transversal distance to the wail element of more than 15% of the wall element height. In an embodiment, the transversal distance from the end of the buttress structure to the bottom of the wall element is within the interval 25-65% of the wall element height. This is a good trade-off between ability to counteract overturning moments and material consumption of the buttress structure. In an embodiment, the buttress structure comprises a foot. Thereby the load from the buttress structure is distributed to a larger area of the seabed, reducing the pressure on the seabed and thus the needed bearing capacity per unit area of the seabed. This makes sure the buttress structure will not sink into the seabed and thus loose capability in counteracting overturning moment acting on the wall element.

In an embodiment, the channel, which is integrated in the wall element by a casting process and extends in a substantial vertical direction when the wall element is placed in a normal position on the seabed, is located at a transverse distance from the wall-face in a channel structure. The channel structure is connected to the wall-face by one or more buttress structures. The buttress structures connect to the outside of the channel structure. Thereby the buttress structure can carry larger loads as the pile in the channel transfers these loads to a larger portion of the seabed, and thus the ability of the buttress structure to counteract loads is increased. Further, a buttress structure with a channel and a pile can be placed in a wider variety of seabed characteristics as it is less dependent on the bearing capacity of the top of the seabed because the pile utilizes the bearing capacity of lower layers as well. By 'channel structure' is intended a structure that encompasses the channel that is extending in a substantial vertical direction when the wall element is placed in a normal position on the seabed, The channel structure is constructed as housing for the channel of a wall element system and is capable of transferring forces from the wall element to the pile positioned in the channel. The channel structure does not extend beyond 1 ,5 channel diameters from the channel centre.

In an embodiment, the buttress structure extends from the wall element on the side facing away from the highest water level when the wall element is damming up water. Thereby the forces are transferred to the seabed in compression which obviates the need for anchoring the buttress structure and thus reduces the installation costs. Further the buttress structure will be in compression which is advantageous if the buttress structure is made of concrete.

In an embodiment, the buttress structure is a solid wall.

In an embodiment, the buttress structure is a truss structure or a solid wall with a hole in it; thereby reducing material consumption compared to the solid wall buttress structure. In an embodiment, the wall element comprises multiple buttress structures; thereby reducing the load on each buttress structure.

In an embodiment, the buttress structure is extending from the wall element in a substantially perpendicular direction. Thereby providing the best support for the wall element against hydrostatic pressure acting on the other side of the wall element than where the buttress structure is placed. Thereby the cost of the buttress support is reduced.

In an embodiment, the buttress structure is placed at a channel structure of the wall element, thereby transferring the loads from the buttress structure to a portion of the wall element where the pile foundation of the wall element system is acting. Thereby a typical robust construction of a channel structure surrounding a channel is utilized to also transfer the loads from the buttress structure. This reduces the materia! consumption and costs of the wall element.

In an embodiment, multiple buttress structures of a wall element are connected with a tensioned cable that is substantially horizontal, thereby firstly adding stiffness to the buttress structures in the direction of the cable, but also, secondly, enabling a slimmer structure of the buttresses. This reduces the material consumption and costs of the wall element.

In an embodiment, the lowest portion of the buttress structure is raised to a level, corresponding to a seabed level, above the bottom of the wall element, which is configured to sit in a trench in the seabed. Thereby the volume of trenched material, and thus the trench costs, is reduced.

In an embodiment, the wall element comprises a foot with a width of at least 10% of the height of the wall element. Consequently the weight of the wall element is distributed to a larger area on the seabed, which reduces load concentrations within the wall element around the connection to the pile foundation, as the pile foundation will not have to carry the entire weight of the wall element. Further, a foot will increase the distance water will have to travel through the seabed to come from outside the reservoir to inside the reservoir, thus impeding the flow of water through the seabed underneath the wall element. Flow of water underneath the wall element reduces the total impermeability of the reservoir and can lead to scouring around and underneath the wall element.

In an embodiment, the width of the foot is in the interval 15-45% of the wall element height. Thereby the carrying capacity of the foot is ensured while still keeping material use for the foot to a minimum. The exact width of the foot will depend on the bearing capacity of the seabed at the reservoir site and the wall element weight. In an embodiment, the foot is not extending fully between the channels of the wall element. For instance the foot is divided into portions or multiple foots. Thereby material is saved by a foot being added underneath part of the wall- face only. The channels are connected to the pile foundation which has weight carrying capacity and therefore there is less need for a foot around the channels.

In an embodiment, the foot is capable of supporting the wall element by itself at the manufacturing site. Thereby the wall element can stand by itself while the concrete is curing, thus reducing manufacturing costs.

In an embodiment, the foot is placed asymmetrically on the wall element, with the foot placed more to the reservoir side of the wall element. Thereby the ability of the foot to resist any overturning moment acting on the wall element from outside the reservoir is increased.

In an embodiment, the foot comprises bracing supports, thereby increasing stiffness of the foot, which possibly reduces the material consumption and costs of the foot. In an embodiment, the foot comprises a stop rising upwards and substantially following the outer edges of the foot on at least the reservoir side of the wall element. Thereby filler material placed on top of the foot, for instance stones, seabed material or ballast concrete, will be held in place by the stop. Thereby weight can easily be added to the structure for instance if friction force with seabed needs to be increased in order to keep the wall element in place. Further, the ballast materia! can add stiffness to the wall element towards hydrostatic pressure acting on the outside of the wall element. Further, the hollow box created by the foot and the stop on the foot can help add buoyancy to the wall element to make transportation at sea easier and thus less costly.

In an embodiment, the wall element comprises two neighbouring channels; where the wall element between the two neighbouring channels is arched about an axis substantially parallel to the longitudinal direction of the channels. The curvature of the arched portion of the wall element is substantially homogeneous and with a ratio of distance between channel centres to arc radius of between 1.67 and 0.25 or between 0.25 and 0.01. These configurations result in reduced material consumption in the wall element system, as the wall can be made thinner and the distance between piles can be increased. An arched concrete wall is stronger towards pressure forces on the outside of the arc compared to a straight wall, as the wall experiences a larger part of the wall in compression and concrete is stronger in compression than in tension.

When the ratio of distance between channel centres to arc radius is between 1.67 and 0.25, the arched portion of the wall locally transfers the forces acting on it to the seabed via the piles at the end of the arched portion. The curvature of the wall enables the piles at the end of the arched portions to be distanced further apart and the arched wall portion to be thinner compared to a straight concrete wall. Thus a reduction in material consumption is achieved. When the ratio of distance between channel centres to arc radius is between 0.25 and 0.01 , the entire wall element will form one arc that transfers the forces acting on the wall element to other wall elements or other structures at the ends of the wall element. Thus, the needed foundation capacity and the material consumption of the wall element system are reduced.

In an embodiment, the wall element describes an arc with a radius substantially equal to the reservoir radius. Thereby the finished reservoir wall has substantially the shape of a circle when seen from above which enables equal loads on opposite sides of the reservoir to at least partially cancel out each other. Thus, the needed foundation capacity and the material consumption of the reservoir are reduced.

In an embodiment, the arc axis is located on the side of the wall element that will have the lowest water level when the wall element is damming up water. Thereby the wall-face arches out towards the higher water level, making the wall-face stronger towards the hydrostatic forces from this higher water level towards the lower water level. In an embodiment, the lower water level when the wall element is damming up water is on the reservoir side of the wall element.

In an embodiment, the wall element comprises multiple arched portions that are substantially equal. Thereby the nature of the load, which is mainly a hydrostatic load and thus substantially similar on each arched portion, is utilized to reduce the cost of the wall element.

By the term 'homogenous curvature' is intended that the curvature is constant in the horizontal direction. By the term 'neighbouring channels' is intended that there are no similar channels between the neighbouring channels. In an embodiment, the wall element comprises two neighbouring channels; where the wall element between the two neighbouring channels is arched about an axis substantially parallel to the longitudinal direction of the channels. The curvature of the ached portion of the wall element is substantially homogeneous and the arc radius at an upper portion of said arched portion of the wail element is different from the arc radius at a lower portion. Thereby the different loading levels experienced in different heights of the wall element are utilized to reduce material consumption, as wall curvature and thickness are adapted to the corresponding load of that height. Wave loading is most significant near the sea level and the hydrostatic pressure increases with depth.

In an embodiment, the arc radius is gradually increasing from the bottom of the wall element towards the top, with the arc axis on the reservoir side of the wall element. Thereby the wall element curvature corresponds to the variations in hydrostatic pressure, as the arc radius gradually decreases with increasing hydrostatic pressure, forming a more curved and thus stronger wall with increasing hydrostatic pressure.

In an embodiment, the portions of the wall element with different arc radiuses have a constant arc radius themselves, and the transition between the portions is either gradual or a substantially horizontal step. In an embodiment, the wall element comprises a skirt extending into seabed below the bottom of said wall element, where the skirt is configured to impede the flow of water through the seabed underneath the wall element. Thereby the nature of the seabed, which can be penetrated by a skirt, is utilized to impede flow of water underneath the wall element, which would reduce the height difference between the reservoir water level and the surrounding water level and thus the amount of energy stored within the reservoir. Further, a reduced flow of water underneath the wall element will limit scouring around and underneath the wall element. This is advantageous as scouring can lead to foundation failure. In an embodiment, the skirt is connected to the wall element with a connection impermeable to water. Thereby flow of water between the skirt and the wall element is prevented.

In an embodiment, the skirt reaches into a layer with a hydraulic conductivity of 10^("6) m/s or a layer with a hydraulic conductivity of 10^("7) m/s or lower. Thereby the deeper layers of the seabed are utilized to keep the fiow of water underneath the skirt to a minimum.

In an embodiment, the skirt is installed on the wall element in its final position, before the wall element is lowered into place on the seabed. Thereby the skirt is inserted into the seabed when the wall element is lowered into place and no further installation work is required for the skirt. Further an impermeable connection between the wall element and the skirt can easily be established when the skirt is being installed on the wall element before the wall element is lowered onto the seabed.

In an embodiment, the skirt is inserted into the seabed after the wall element has been installed on the seabed. In an embodiment, the skirt is made from sheet piling.

In an embodiment, the skirt is made of steel or vinyl or a combination thereof.

In an embodiment, the wall element comprises a foot, and the skirt is embedded within the foot of the wall element, thereby using the foot to guide the skirt and hold it place during the installation of the skirt. [n an embodiment, the skirt is constructed at the reservoir erection site with deep-mixing techniques. Deep-mixing techniques are in situ soil treatment technologies whereby onsite material is mixed with cementitious and/or other materials. Thereby onsite material is used to construct the skirt, whereby costs for materials and material transportation are reduced. Further, a skirt constructed with deep-mixing techniques increases the soil bearing capacity and thus enables the skirt to add to the foundation of the wali element, thus reducing material use and costs for other foundation for the wall element. The skirt constructed from deep-mixing techniques can be constructed e.g. as disclosed in connection with the co-pending application with the title "Method of building an offshore power storage facility and an offshore reservoir", filed on the same day, by the applicant of this present application, e.g. in connection with figure 1 and the description thereof on page 3-5 and page 55-56, where the 'coherent foundation' is described and serves as a skirt.

By the term 'skirt' is intended a barrier with the purpose of hindering flow of water through the seabed. It is inserted into the seabed e.g. to a depth of about 1 m or 3 m or 5 m.

In an embodiment, the wall element is longer than 20 meters or 40 meters or 60 meters or 80 meters, thereby reducing the total reservoir costs, as the longer the wall elements are, the fewer wall element transportations and installations are necessary for a given total reservoir wall length. Transportation constraints on size and weight are less important at sea compared to land transportation. However the longer the wall elements are, the more difficult they are to handle during manufacturing, transport and installation. Thus the length of a wall element depends on the manufacturing facilities and transport and installation equipment. In an embodiment, the wall element comprises multiple smaller segments assembled to a single wall element by post-tensioning at the manufacturing site. Thereby, the manufacturing process is easier and thus less expensive, as the smaller segments can be handled better during the manufacturing process and can be cast in one continuous process. By 'post-tensioning' is intended applying tension to cables or rods running through channels in the structure being after the structure is constructed. Thereby, the structure is held together and reinforced by the cable or rods. The channels can subsequently be filled with a filler material, e.g. grout.

In an embodiment, the wall element system comprises multiple wall elements each comprising at least two channels spaced apart by the same distance where the multiple wall elements are configured for a water impermeable interconnection with each other along a horizontal division when stacked on top of each other. This configuration allows for easier handling of the wall element system as it is constructed by stacking wall elements that are lower than a single wall element of full height. The reduced size of the wall elements enable easier handling during manufacturing, transportation and installation, which all contribute to a lower cost. The two channels of the wall elements ensure that the stacked wall elements are aligned, which makes the installation process easier. Further, the water impermeable connection enables the wall element system to be used as a part of a reservoir to dam up water. In an embodiment, the bottom wall element guides the piles when these are inserted into the seabed. The following wall element is then stacked on top of the bottom wall element using the same piles as guidance. Thereby the positioning and the alignment of the piles are controlled by the bottom wall element and the alignment of following stacked wall elements is ensured by the piles. Thus is the installation process simplified. In an embodiment, the wall element stacked on top of a first wall element is moved in the longitudinal direction of the wall element and thus it encompasses a different set of piles. Thereby the wall element spans the connection between the end of the first wall element and the end of a wall element adjacent to the first wall element and thus adds stiffness to this connection. This contributes to a more solid wall, and thus material and cost savings can be achieved.

In an embodiment, a gasket is installed between the horizontal divisions of the wall elements. Thereby the weight of the wall element is utilized to establish a water impermeable interconnection between the stacked wall elements.

In an embodiment, the connection between the horizontal divisions of two wall elements seals off a confined enclosure between the wall elements which is isolated from the surrounding sea. Thereby, the weight of the wall element is utilized to create a grouting enclosure that, when grouted, forms a rigid and water impermeable interconnection between the stacked wall elements.

In an embodiment, the stacked wall elements are interconnected using post- tensioning methods. Thereby, the stacked wall elements are firmly interconnected, using a well known and proven technology. This ensures that the wall element system is strong and durable, while enabling the easy manufacturing and handling of the individual wall elements due to their reduced height compared to a full-height wail element.

There is provided an offshore reservoir comprising a wall element system, wherein multiple of said wall element systems are located next to each other and interconnected with a sealed interconnection to form at least a part of a wall of a reservoir enclosure. By 'next to each other' is intended that two wall elements are arranged as neighbouring elements. They may be adjacently arranged. An intermediate piece may be arranged between such two neighbouring elements. The intermediate piece may be arranged adjacent to the neighbouring wall elements. This configuration allows for a fast and safe erection of an offshore reservoir. Each wall element system is positioned at the designated location on the seabed at the offshore reservoir erection site and subsequently the foundation for the wall element system is installed, thus ensuring alignment between wall and foundation and a fast installation process. As the wall element system is installed complete with foundation, it has good survivability in the offshore environment, thus making the erection of the offshore reservoir fast and safe. The sealed interconnection allows the reservoir to dam up water. The sealed interconnection may comprise an intermediate piece arranged between the wall elements. Sealed interconnection is also referred to by the term joining within this technical field. The respective ends of neighbouring wall elements may be joined together by means of a wet or dry interconnection to establish a joint. In case an intermediate piece is arranged between the neighbouring wall elements, it may be joined to the neighbouring elements by means of a wet or dry interconnection to establish joints. Generally, joints can be classified as wet and dry. Wet joints are constructed with cast-in-place concrete poured between the precast wall elements. To ensure structural continuity, protruding reinforcing bars from the wall elements (dowels) are welded, looped, or otherwise connected in the joint region before the concrete is placed. Dry joints are constructed e.g. by bolting or welding together steel plates or other steel inserts cast into the ends of the precast wall elements for this purpose. Wet joints more closely approximate cast-in-place construction, whereas the force transfer in structures with dry joints is accomplished at discrete points. Combinations of wet and dry joints are also viable for interconnecting wall elements. As a cost saving measure, the wall element systems are manufactured onshore, where weather protected series production with conventional means is possible, and then transported to an offshore erection site for installation.

In an embodiment, the overall shape of the reservoir comprised by the wall elements is substantially circular when seen from above. Consequently the reservoir provides the highest possible inner area for a given reservoir wall length, and thus the highest possible reservoir volume, but requires the wall elements to be arched with an arc radius substantially equal to the reservoir radius. A higher reservoir volume means more energy can be stored within the reservoir.

In an embodiment, lines drawn through the centres of the channels of each wall element in a reservoir will form a substantially regular polygon. Consequently the reservoir provides the highest possible inner area for a given reservoir wall length, and thus the highest possible reservoir volume, when considering wall elements where the channels are placed on a straight line. Wail elements with the channels on a straight line are easier and thus less costly to manufacture, compared to wall elements describing an arc. In an embodiment, the substantially regular polygon formed by the reservoir has more than 12 sides, e.g. 20 sides.

In an embodiment, the interconnection between two wall elements is configured for an internal angle between the wall elements within the interval 180 and 150 degrees. Thereby a construction of a reservoir with the shape of a regular polygon with more than 12 sides is facilitated.

In an embodiment, neighbouring wall element systems are installed at the seabed with a gap between them and subsequently interconnected using a connection wall element which has a length of less than 10% of wall element length and is configured to span the gap. The connection wall element is also referred to as an intermediate piece. Thereby the large and heavy wall element systems are installed on the seabed standing free, which reduces the need for accuracy in the handling of the wall element systems. This means that the handling can be performed by smaller offshore vessels e.g. towboats compared to large and expensive jack-up vessels normally used for offshore installations such as bridges and wind turbines. This reduces the installation costs of the wall element system. The reduced size of the connection wall elements, compared to the wall element systems, makes them easier to handle and thus enables them to be handled by the smaller offshore vessels as well. This reduces the handling requirements and the costs of the installation vessels. Further, when wall elements systems are installed on the seabed standing free, it reduces the risk of collision between wall elements during installation.

In an embodiment, cable connections are established between the connecting wall element and two adjacent wall element systems installed at the seabed. Thereby the connecting wall element is positioned, by utilizing the cable connections to pulf the connecting wall element into position between the two adjacent wall element systems. By 'cable connection' is intended a connection that can mainly transfer tension forces and less compression or shear forces, such as a cable. The mentioned cable connection could also be a connection with a rope, a wire, a chain or a wire rope.

In an embodiment, the wall element systems are placed with contact between the ends of neighbouring wall element systems, thereby establishing the contact between wall element systems upon their installation, which reduces the number of offshore operations required to construct the offshore reservoir. In an embodiment, the wall element systems are interconnected by an in situ grouting process of a confined volume between the wall elements. Thereby a rigid and impermeable interconnection is created which is able to adapt to the size of the confined volume. In an embodiment said confined volume is constructed from plates connecting to both wall element systems, for instance as disclosed in connection with co-pending application with the title "Method of building an offshore power storage facility and an offshore reservoir", filed on the same day, by the applicant of this present application, in connection with figure 6a-6c and the description thereof on page 43-45 and page 60-61.

By the term 'adjacent' is intended that the wall elements are located next to each other.

By the term 'sealed' is intended impermeable to water under a pressure corresponding to that at a water depth equal to the height of the offshore reservoir.

By the term 'standing free' is intended that the wail element system stands alone on the seabed with no connection to other wall element systems and water on all sides.

In an embodiment, the offshore reservoir comprises a pump/turbine system, where, in a first mode, the pump/turbine system is configured to drain the reservoir using electricity and, in a second mode, the pump/turbine system is configured to fill the reservoir with water from the sea while producing electricity. Consequently the reservoir is able to store energy as gravitational potential energy of water, where the height is determined by the difference between the surrounding water level and the water level inside the reservoir. The energy is stored by pumping water from inside the reservoir to the surrounding sea, thus emptying the reservoir. The energy is reproduced by letting water from the surrounding sea into the reservoir through a turbine driving a generator. The height difference between the sea and the reservoir determines how much energy can be extracted from a given amount of water. The offshore reservoir can e.g. be operated with a period where the reservoir is drained by the pump, where the pump consumes energy; and a period where the reservoir remains in an at least partially drained state; and a period where the reservoir is filled, where the turbine generates energy. These periods can e.g. be controlled by energy prices and/or energy demand.

In an embodiment, the pump/turbine system is configured to handle more than 50 m³ of water per second.

In an embodiment, the pump/turbine system comprises a pumping unit and a turbine unit integrated in a pump turbine unit. Thereby the space and material needed for the pump/turbine system are reduced as fewer units are required. This reduces the costs of the pump/turbine system.

In an embodiment, the pump/turbine system is installed at or below seabed level. Thereby the pressure provided by the water inside the reservoir is utilized to reduce problems with cavitation within the system, which can lower performance and damage the system. Further, the possible height difference between the surrounding sea and the water inside the offshore reservoir, and thus the energy storage capacity of the offshore reservoir, is maximized.

In an embodiment, the pump/turbine system is installed in a separate housing facility located inside the reservoir and communicates with the sea through a pipe system. Thereby the pump/turbine system is easily installed below seabed level, as the separate housing facility can be located and installed separately. Further a separate housing facility enables the pump/turbine system to be pre-installed onshore within the separate housing facility. In an embodiment, the pump/turbine system is installed in a separate housing facility located underneath the reservoir wall. Thereby the pump/turbine system has ready access to both sides of the reservoir wall. In an embodiment, the pump/turbine system is installed within the reservoir wall. Thereby the pump/turbine system can be pre-installed within the reservoir wall onshore, thereby reducing the number of offshore installation operations, In an embodiment, the opening from the pipe system to the sea is located close to the seabed. Thereby pipe losses in the pipe system are reduced as the pump/turbine system is also located close to the seabed, and the length of the pipe system can be minimized. In an embodiment, a method of installing a wall element system comprises; manufacturing the wail element system at a manufacturing facility onshore, where the manufacturing facility has launching means for launching the wall element system into sea. Then the wail element system is ferried to an erection site for a reservoir. A first wall element is lowered to a designated location at the seabed, wherein the first wall element is lowered to secure a substantial upright position as it reaches its designated location. Then, the pile is rooted into the seabed via the channel of the wall element.

Thereby manufacturing and installation costs are lowered. An onshore manufacturing environment is more controlled than an offshore manufacturing environment, which results in a standardized manufacturing process that lowers manufacturing costs and ensures a homogenous quality of the wall elements. Further, the installation process of rooting the pile, serving as foundation for the wall element, into the seabed via the channel of the wall element reduces the costs of installing the wall element system at an offshore location, as alignment between the wall element and its foundation is ensured.

In an embodiment, the pile is positioned within the channel before the ferrying process. Thereby, the pile is inserted into the channel in a controlled environment, rather than the offshore conditions at the erection site. With the pile already positioned in the channel, the offshore installation process of the wall element system is faster as the wall element and the pile are already positioned relative to each other. Further, the requirements of the offshore installation equipment are reduced, as it does not need to insert the long pile into the channel of the wall element in the offshore environment. Thereby the installation costs of a wall element system are reduced. A wall element system comprising multiple piles has them all inserted into their respective channels before the ferrying process.

In an embodiment, the first wall element is positioned above its designated location on the seabed and the pile is lowered down through the channel such that it is at least partly rooted into the seabed before the first wall element is lowered to a designated location on the seabed. Thereby the channel of the wall element is utilized to guide the pile down to the seabed. Then the pile that is at least partly rooted into the seabed is utilized to guide the wall element down to its designated location on the seabed. This reduces the requirements of the offshore installation equipment, as the wall element system can guide itself down to the designated location on the seabed, thus reducing the installation costs. The pile can for instance be partly inserted into the seabed by its weight alone.

In an embodiment, the wall element is kept floating by the use of external floaters. Consequently the wall element is not required to be self-buoyant to be transported afloat, thus reducing material consumption by enabling a less constrained design of the wall element system. In an embodiment, external floaters are attached to a wall element with cables and are , keeping the wall element afloat in an upright position. Thereby the need for heavy duty offshore handling vessels is reduced, as the wall element can be lowered to the designated position on the seabed at the reservoir site in an upright position, by slackening of the cables.

In an embodiment, a wall element is towed on a barge, thereby using a known and well proven method for transportation at sea.

In an embodiment, the wall element is self-buoyant, thereby enabling it to be transported with the use of tow boats only.

By the .term 'launching into sea' is intended 'transferring the wall element system from land to sea, where the wail element is either floating by internal or external means or positioned on a boat or a barge'.

By the term 'ferrying' is intended 'transported at sea'. This can be e.g. on a boat, on a barge towed by a boat or the wall element system itself is towed by a boat, where the wall element system is either self-buoyant or kept afloat by external floaters attached to it, or the wail element system can be ferried by external floaters attached to it, where the external floaters have propulsion means. By the term 'external floaters' is intended objects with large buoyancy designed for making a wall element or a wall element system float, e.g. a buoy or a rigid shell structure.

In an embodiment, a method of installing a wall element system further comprises; lowering a second wall element to a location at the seabed designated adjacently to the first wall element. The second wall element is lowered to secure a substantial upright position as it reaches its designated location. The pile is rooted via the channel of the second wail element into the seabed. The first and second wall elements are interconnected with a sealed interconnection. This configuration allows for an offshore reservoir to be constructed element by e!ement. An element by element construction allows for the individual elements to be manufactured onshore in a more controlled environment than the offshore reservoir site. Onshore manufacturing lowers manufacturing costs. In an embodiment, the second wall element is connected with a cable connection to the first wall element system already installed at the seabed. Thereby a fixed position for easier positioning of the second wall element system is provided, and the cable connection can help position the second wall element system.

In an embodiment, a trench is dredged in the seabed at the designated location of a wall element, where the trench is at least the length of the wall element and wide enough for the wall element to be installed on the bottom of the trench. The wall element is installed at the designated location in the trench. Thereby any irregularities of the seabed are removed and a plane surface for the wall element to rest on is ensured. Thus any load concentrations within the wall element due to an uneven seabed are avoided and the risk of unforeseen settling of the wall element is reduced. In an embodiment, the trench is substantially level. Consequently the wall element system is substantially vertical, which makes connection to other wall element systems easier and enables installation of wind turbines that have to be substantially vertical. In an embodiment, the trench is constructed in substantially level sections, where each section has a length corresponding to one or more wall elements. Thereby minimum dredging is required to provide a substantially level seabed for all wall elements, as each dredged section needs only to compensate for water depth differences along the length of a single wall element. Further it is ensured that the steps between substantially level sections of the trench with different height are located between wall element systems, which mean that each wall element system is easier to manufacture as it will have a substantially level bottom and a constant height corresponding to the water depth at its substantially level section of the trench. This reduces the manufacturing costs. This configuration reduces costs of both dredging the trench and manufacturing the wall element systems.

In an embodiment, a gravel bed is laid in the trench. Thereby a level and stable surface for the wall element is ensured.

In an embodiment, the buttress structures of the wall element rest at the seabed outside the trench, thereby reducing the width of the trench and thus the amount of dredging needed to construct the trench. In an embodiment, adjacent wall element systems with the pile rooted into the seabed are interconnected with a sealed interconnection, such that the wall element systems, at their respective designated locations at the erection site, collectively form an enclosure. This configuration provides a cost- efficient erection of an offshore reservoir, as wall element systems manufactured onshore can be interconnected to form an offshore reservoir. Onshore manufacturing lowers the manufacturing costs of the wall element systems. Brief description of the fi ures

fig. 1 shows a wall element system;

fig. 2 shows a cross section of a channel of a wall element system;

fig, 3 shows a wall element with hollow compartments;

fig. 4 shows a wall element comprising different buttress structures;

fig. 5 shows a wall element system with channel structures connected to the wall element with buttress structures;

fig. 6 shows different wall elements with an arched wall-face;

fig. 7 shows a wall element system comprising multiple wall elements, a foot and a skirt;

fig. 8 shows a wail element system being ferried from the onshore manufacturing facility to the offshore reservoir site;

fig. 9 shows an interconnection of two neighbouring wall element systems; and

fig. 10 shows an offshore reservoir for storing power. Detailed description

Figure 1 shows an embodiment of a wall element system 101. The wall element system comprises a wail element 102 with channels 103 integrated in the wall element 102 by a casting process and extending in a substantial vertical direction, and piles 104 for fixation of the wall element to the seabed. The channels 103 are configured to guide the piles 104 through the channels 103 and into the seabed. The largest inner diameter of the channels 103 is greater than the thickness of the wall-face 105. The wall element between neighbouring channels 105 is arched about an axis substantially parallel to the longitudinal direction of the channels 103. The wall element 102 comprises multiple arched portions 105 that are all substantially equal. The wall-face element comprises a foot 106; 107 with a width of at least 10% of the height of the wall element. The foot 106; 107 is not extending fully between the channels 103 of the wall element. One portion of the foot 106 is straight and placed asymmetrically on the wall element. The other portion of the foot 107 is arched with the same curvature as the wall-face 105. Different portions of a foot on a wall element will typically have the same shape. The wall element system 101 has three channels 03, each with a pile 104 positioned within it. The piles 104 are shown in a position they would be in during the pile installation, where they extend deeper than the bottom of the wall element 102 as they will do when they are being inserted into the seabed. The seabed is not shown, and pile installation equipment is not shown. The channels 103 are placed substantially equidistantly along the wall element 102. Channels 103 are located at the respective ends of the wall element 102, thus placing more construction material and a foundation pile 104 near said ends. The pile 104 length is not necessarily scaled correctly to the height of the wall element 102. The pile length is primarily determined by water depth and distance between channels and can vary from site to site. The cross sections of the piles 104 are substantially circular. The piles 104 are hollow. The piles 104 have substantially equal diameters.

Figure 2 shows a cross section of a channel 103 of a wall element system 101 installed in a trench 201 in the seabed. The wall element system 101 comprises a transition piece 202 fixed on top of the pile 104. The transition piece 202 is configured for installation of a wind turbine. The channel 103 is configured with a bore giving a first clearance 203 at an upper portion of the channel 103 and a constriction 204 at a lower portion of the channel with a second clearance that is smaller than the first clearance. The constriction 204 at a lower portion of the channel is configured to guide the pile 104. The first clearance can be grouted to form a rigid connection between pile 104 and channel 103. The wall element 102 comprises a foot 205 placed asymmetrically on the wall element 102. The transition piece 202 can be installed on the pile 104 with techniques known from the offshore wind turbine industry. The wind turbine to be installed on the transition piece 202 is not shown. The wall thickness of the pile 104 and the transition piece 202 are not shown in the right scale, but are exaggerated to make the figure clearer. The wall element system 101 is installed at the designated position on the seabed in a trench 201. The trench 201 is wide enough for the wall element system 101 to be installed on the bottom of the trench 201. The angle of the sides of the trench 201 is determined from the internal friction of the seabed material. The trench 201 is shown with a gravel bed 206 laid in the trench.

Figure 3a-b shows a top view of different wail element systems 101. The channels 103 comprises guide pieces 301 ; 303 protruding from the inside surface of the channel towards the centre of the channel, where the guide pieces 301 ; 303 are configured to guide the pile 104 through the channel 103.

Figure 3a shows a wall element system 101 comprising one or more hollow compartments 302 within the wall. The shown holiow compartments 302 are substantially rectangular in shape. The guide pieces 301 are cast into the concrete channel 103 or mounted after channel manufacturing e.g. by bolting.

Figure 3b shows a wall element system 101 comprising one or more hollow compartments 302 within the wall. The shown hollow compartments 302 are substantially rectangular in shape. The guide pieces 303 shown are an integral part of the channel constructed in the same casting process as the channel. Figure 4 shows a wall element 102 comprising different buttress structures 403; 404; 405. The wall element 102 comprises three channels 103 and a foot 402. The channels 103 are located on the side of a straight wall-face 401. The wall element 102 is shown without a pile needed to make it a wall element system. The channels 103 of a wall element 102 can be placed anywhere from the one side to the other side of the wall-face. The foot 402 is straight and placed symmetrically on the wall element 02 and it extends in the full length of the wall element 102. The foot 402 has a width of at least 0% of the height of the wall element 02. The wall element 102 comprises buttress structures 403; 404; 405 configured to counteract overturning moments acting on the wall element 102. Three different embodiments of buttress structures 403; 404; 405 are shown. All of them are extending from the wall element 102 in a substantially perpendicular direction. The buttress structures 403; 404; 405 have a downwards slope towards the seabed. The first embodiment of a buttress structure is a solid transverse wall 403 integrated in the wall element 102. It is shown placed at the wall-face 401 and extends from the upper half of the wall element 102. The second embodiment of a buttress structure is a solid wall with a hole in it 404 integrated in the wall element 102. The hole serves to reduce material consumption for the buttress structure. The second embodiment of a buttress structure 404 is shown placed at a channel structure 406 of the wall element. The buttress structure 404 extends from the top of the wall element and comprises a foot 407. The third embodiment of a buttress structure is a truss structure 405, shown connected to a channel structure 406 of the wall element 102 with mechanical fastening means 408. It comprises a foot 407 and a cross beam 409. The cross beam 409 is to help prevent buckling in the truss structure buttress 405.

All three buttress structures 403; 404; 405 are connected with a tensioned cable that is substantially horizontal 410. The tensioned cable 410 increases the stiffness of the buttress structures 403; 404; 405 in the direction of the cable and thus reduces the structural requirements of the buttress structures. When a buttress structure comprises a foot 407, it is to distribute the load from the buttress structure to a larger area of seabed and prevent the buttress structure from sinking into the seabed when the wall element 102 is subjected to an overturning moment. The seabed is not shown.

The shown wall element 102 is designed for having the highest water level, when the wall element 102 is damming up water, on the opposite side of the buttress structures 403; 404; 405 and the channel structures 406. Thereby the buttress structures 403; 404; 405 extend from the wall element 102 on the side facing away from the highest water level when the wall element is damming up water. This makes the wall element 02 better at resisting large overturning moments. When the channels 103 are placed to the side of the wall element 102 with the lowest water level, when the wall element 102 is damming up water, then the length of the lever arm of the pile (for resisting the overturning moment from the hydrostatic pressure of the differences in water level) increases. A wall element can comprise buttress structures regardless of the positioning of the channels of the wall element. A wall element can also comprise buttress structures if the wall-face is arched.

Figure 5 shows a wall element system 101 with channel structures 406 connected to wall element 102 by buttress structures 501 ; 502. The channels 103, which are integrated in the wall element by a casting process and extend in a substantial vertical direction when the wall element is placed in a normal position on the seabed, are located at a transverse distance from the wall-face 401 in channel structures 406. The channel structures 406 are connected to the wall-face 401 by one or more buttress structures 501 ; 502. The buttress structures 501 ; 502 connect to the outside of the channel structure 406. The channels 103 in the channel structures 406 are configured to guide the piles 104 of the wall element system 101. The wall element system 101 comprises a straight wall-face 401 and a foot 205 with a width of at least 10% of the height of the wall element 102. The foot 205 is placed asymmetrically on the wall element 102, with the foot placed more to the reservoir side of the wall element. The foot 205 extends such that is makes contact with the channel structures 406. One buttress 501 has a downwards slope towards the seabed for some length of the buttress structure. Two buttress structures 502 have a constant height and extend from the wall-face 401 in an inclined direction. Two buttress structures 502 form a cavity together with the wall-face 401 , the channel structure 406 and the foot 205 of the wall element. This cavity increases the buoyancy of the wall element 102 and thus makes floating transportation of the wall element 102 easier and less costly.

Figure 6a-d shows a top view of different wall elements 102 with an arched wall-face 105. The wall elements 102 comprises two neighbouring channels 103; where the wall element between the two neighbouring channels 105 (denoted the wall-face) is arched about an axis substantially parallel to the longitudinal direction of the channels 103; and where the curvature of the arched portion of the wall element 105 is substantially homogenous.

Figure 6a shows an embodiment with a ratio of distance between channels centres 601 to wall-face arc radius 602 between 1.67 and 0.25. The channels 103 are placed on a straight line. The channels 103 need not be placed on a straight line. They can be placed e.g. along an arc with lower curvature than that of the wall-face 105. The curvature of the wall-face 105 is higher than the curvature of the line on which the channels 103 are placed. Therefore there will be load concentrations at the channels 103 of the wall element when considering a hydrostatic load acting on the outside of the arched wa!i-face 105. Due to the high curvature of the wall-face 105, they can cover a long distance between the channels 103 compared to a straight wall-face of similar thickness. Figure 6b shows an embodiment with a ratio of distance between channels centres 601 to wall-face arc radius 602 between 1.67 and 0.25. The channels 103 are placed on a straight line. The channels 103 need not be placed on a straight line. They can be placed e.g. along an arc, with lower curvature than that of the wall-face 105. The curvature of the wall-face 105 is higher than the curvature of the line on which the channels 103 are placed. Therefore there will be load concentrations at the channels 103 of the wail element when considering a hydrostatic load acting on the outside of the arched wall-face 105. Due to the high curvature of the wall-face 105, they can cover a long distance between the channels 103 compared to a straight wall-face of similar thickness.

Figure 6c shows an embodiment with a ratio of distance between channels centres 601 to wall-face arc radius 602 between 0,25 and 0.01. The channels 103 are placed on an arc with the same radius 602 as the arched wall-face 105. This means there will be little or no load concentrations at the channels 103 of the wall element 102 when considering a hydrostatic load on the outside of the arched wall-face 105. The loads are primarily transferred to neighbouring wall elements or other neighbouring structures. Due to the low curvature of the wail-face 105, they can cover a medium distance between the channels 103.

Figure 6d shows an embodiment with a ratio of distance between channels centres 601 to wall-face arc radius 602 between 0.25 and 0.01. The channels 103 are placed on an arc with the same radius 602 as the arched wall-face 105. This means there will be little or no load concentrations at the channels 103 of the wall element 102 when considering a hydrostatic load on the outside of the arched wall-face 105. The loads are primarily transferred to neighbouring wall elements or other neighbouring structures. Due to the low curvature of the wall-face 105, they can cover a medium distance between the channels 103.

In an embodiment the reservoir is circular and comprised of wall elements with a wall arc radius equal to the reservoir radius, thereby enabling forces on opposite sides of the reservoir to at least partially cancel each other out. Figure 7 shows a wall element system 101 comprising multiple wall elements 701 ; 702, each comprising at least two channels 103 spaced apart by the same distance and being configured for a water impermeable interconnection with each other along a horizontal division. The first wall element 701 comprises a foot 106 with a width of at least 10% of the height of the wall element and a skirt 703 installed along the foot 106. The skirt 703 is extending into the seabed below the bottom of the first wall element 701 and is configured to impede the flow of water through the seabed underneath the wall element 701. The seabed is not shown. The skirt shown 703 is a sheet piling skirt and is shown only along a part of the wall element 701. The channels 103 of the first wall element 701 are guiding the piles 104 as they are being inserted into the seabed. The second wall element 702 is guided by the piles 104 to be stacked on top of the first wall element 701 where a water impermeable interconnection along the horizontal division between the first and second wall element is formed. In an embodiment, the skirt of an offshore reservoir is installed continuously along the perimeter of the entire offshore reservoir. The skirt can be installed on either the seaside or the reservoir side of a wall element, or anywhere on the foot of a wall element. Figure 8 shows a wall element system being ferried 801 by a boat 804 from the onshore manufacturing facility 805 to the offshore reservoir site. A similar wall element system is installed 802 on the seabed at the offshore reservoir site with the piles 104 rooted into the seabed. Another similar wall element system 803 is assembled with the piles 104 positioned within the channels and waiting at the onshore manufacturing facility 805 ready to be launched into sea and ferried to the offshore reservoir site. The wail element system being towed 801 is kept afloat by external floaters 806 located at the sides of the wall element. The piles 104 of the towed wall element system 801 are positioned within the channels of the wall element and are ready to be inserted into the seabed once the wall element reaches the designated location on the seabed at the offshore reservoir site, after being lowered to secure a substantial upright position as it reaches the designated location on the seabed.

Figure 9 shows an interconnection of two neighbouring wall element systems 901 ; 902. The neighbouring wall element systems 901 ; 902 are installed at the seabed with a gap 907 between them and subsequently interconnected using a connection wall element 908 which has a length of less than 10% of wall element length and is configured to span the gap 907. The first wall element 903 comprises two neighbouring channels 103; where the wall element 903 between the two neighbouring channels is arched about an axis substantially parallel to the longitudinal direction of the channels 103; and where the curvature of the ached portion of the wall element is substantially homogeneous and the arc radius at an upper portion 905 of said arched portion of the wall element 903 is different from the arc radius at a lower portion 906. The arc radius of the wall element 903 is gradually increasing from the bottom of the wall element towards the top, with the arc axis on the reservoir side of the wall element 903. The second wall element 904 has been lowered onto a location at the seabed designated adjacently to the first wall element 903. Both wall element systems 901 ; 902 comprise piles 104 rooted into the seabed and fixed to the channels 103 in a rigid interconnection. The seabed is not shown. The connection wall element 908 is shown connecting to channels structures 406 of the wall elements 903; 904. A connection wall element can also connect to a wall-face part of a wall element.

Figure 10 shows an offshore reservoir 001 for storing power comprising a pump/turbine system in a separate housing facility 1002. The offshore reservoir comprises multiple wall element systems 101 located adjacent to each other and interconnected with a sealed interconnection 1003 to form a reservoir enclosure. The shown reservoir 1001 comprises six wail element systems 101 installed at the seabed and interconnected with a sealed interconnection 1003. The wall element systems 101 are placed with contact between the ends of neighbouring wall element systems. The wall elements systems 101 comprise wall-faces 105 arched about an axis substantially parallel to the longitudinal direction of the channels 103. The axis is located on the reservoir side of the wall element 101. The pump/turbine system is located in a separate housing facility 1002 within the reservoir and communicates with the surrounding sea through a pipe system 004. The wail elements 101 comprise channels 103 located at or in proximity of the respective ends of the wall element, thus placing more construction material and a foundation pile near the ends of a wall element 101.

In an embodiment, a reservoir comprises more than 12 wall elements.

In an embodiment, most of the wall elements in a reservoir are the same embodiment of a wall element. Thereby manufacturing, transportation and installation processes are simplified.

Generally, other terms for a 'wall element' could include terms like 'panel' or 'wall panel' or similar terms.

Claims

1. A wall element system (101 ) for installation on a seabed as at least a part of a reservoir (1001 ) to dam up water, comprising:

- a pile (104) for fixation of the wall element (102; 701 ) on the seabed;

- a wall element;

- a channel (103) integrated in the wall element by a casting process and extending in a substantial vertical direction when the wall element is placed in a normal position on the seabed; wherein the channel is configured to guide the pile through the channel and into the seabed; and wherein the channel, at least at a lower portion, is configured to accommodate a portion of the pile for fixating the pile and the channel in a rigid interconnection.

2. A wall element system according to claim 1 , wherein the channel is configured with a bore giving a first clearance between the inside of the channel and the pile at an upper portion of the channel and a constriction (204) of the channel at a lower portion of the channel with a second clearance that is smaller than the first clearance,

3. A wall element system according to any of claims 1 -2, wherein the channel comprises guide pieces (301 ; 303) protruding from the inside surface of the channel towards the centre of the channel, wherein the guide pieces are configured to guide the pile through the channel.

4. A wall element system according to any of claims 1-3, comprising a transition piece (202) fixed on top of the pile, wherein the transition piece is configured for installation of a wind turbine.

5. A wall element system according to any of claims 1-4, comprising a buttress structure (404; 501) configured to counteract overturning moments acting on the wail element, wherein the buttress structure is integrated in the wall element or connected to the wall element by mechanical fastening means.

6. A wall element system according to any of claims 1-5, comprising a foot (106; 205) with a width of at least 10% of the height of the wall element.

7. A wall element system according to any of claims 1-6, wherein the wall element comprises two neighbouring channels; where the wall element between the two neighbouring channels is arched about an axis substantially parallel to the longitudinal direction of the channels; and wherein the curvature of the arched portion of the wall element is substantially homogeneous and with a ratio of distance between channel centres to arc radius between 1.67 and 0.25 or between 0.25 and 0.01.

8. A wall element system according to any of claims 1-6, wherein the wall element comprises two neighbouring channels; wherein the wall element between the two neighbouring channels is arched about an axis substantially parallel to the longitudinal direction of the channels; and wherein the curvature of the ached portion of the wall element is substantially homogeneous, and the arc radius at an upper portion of said arched portion of the wall element is different from the arc radius at a lower portion.

9. A wall element system according to any of claims 1-8, comprising a skirt (703) extending into the seabed below the bottom of said wall element, wherein the skirt is configured to impede the flow of water through the seabed underneath the wall element.

10. A wall element system according to any of claims 1-9, wherein the wall element is longer than 20 meters or 40 meters or 60 meters or 80 meters.

11. A wall element system according to any of claims 1-10, comprising multiple wall elements each comprising at least two channels spaced apart by the same distance, wherein the multiple wall elements are configured for a water impermeable interconnection with each other along a horizontal division when stacked on top of each other.

12. An offshore reservoir comprising a wall element system according to any of claims 1- 1 , wherein a plurality of said wall elements are located next to each other and interconnected with a sealed interconnection to form at least a part of a wall of a reservoir enclosure.

13. An offshore reservoir according to claim 2, comprising a pump/turbine system, wherein, in a first mode, the pump/turbine system is configured to drain the reservoir using electricity and, in a second mode, the pump/turbine system is configured to fill the reservoir with water from the sea while producing electricity.

14. A method of installing a wall element system according to any of claims 1-11 , comprising:

- manufacturing a wall element system at a manufacturing facility on shore, where the manufacturing facility has launching means for launching the wall element system into sea;

- ferrying the wall element system to an erection site for a reservoir;

- lowering a first wall element to a designated location at the seabed, wherein the first wail element is lowered to secure a substantia] upright position as it reaches its designated location;

- rooting the pile via the channel of the wall element into the seabed.

15. A method of installing a wall element system according to claim 14, further comprising: - lowering a second wall element to a location at the seabed designated adjacently to the first wall element, wherein the second wall element is lowered to secure a substantial upright position as it reaches its designated location;

- rooting the pile via the channel of the second wall element into the seabed; and

- interconnecting the first and second wall element with a sealed interconnection.

16. A method of installing a wall element according to claim 4, comprising:

- dredging a trench (201 ) in the seabed at the designated location of a wall element, where the trench is at least the length of the wall element and wide enough for the wall element to be installed on the bottom of the trench; and

- installing the wall element at the designated location in the trench.

17. A method of building an offshore reservoir according to claim 15, comprising:

- repeating the steps of claim 15, such that the wall elements, at their respective designated locations at the erection site, collectively form an enclosure.