WO1993019279A1 - Emergency catcher for a riser tensioning apparatus - Google Patents

Abstract

Emergency catcher for a tensioner for riser pipes between a sea bed and a floating structure, such as a tension leg platform, comprising at least three hydraulic cylinders connecting the riser pipe with the deck of a floating structure, in which the catcher comprises two in relation to the riser diametrically arranged supports which with interconnected shock absorbers are connected with a catcher ring arranged around the riser a short distance below the tensioner ring, the catcher ring thereby being adapted to receive the tension ring in case the tensioners fail, thereby to maintain the riser substantially in position.

Description

Emergency Catcher for a Riser Tensioninq Apparatus

5 The present invention is related to an emergenc catcher for a riser pipe tensioner apparatus according to th preamble of the claims.

A recent need for tensioning riser pipes extendin upwards from the seabed and being supported by a floating vesse 0 at the water surface has led to the use of riser tensioners. Suc tensioners traditionally have consisted of a framework from whic are suspended cylinder assemblies with cylinder rods connectin a central bracket on the riser.

A need for tensioning such riser pipes comes about du s to the requirement to maintain such riser pipes in tension fro the top of the pipe.to the sea bottom, and to restrain the pip from excessive lateral motions. This tension has been determine to be critical to insure the structural survival of the pipe, an to protect the environment from any contents spilling from th o pipe. It is also needed to maintain the safe operating condition desired for the platform and its occupants.

To control this tension, and to allow for differentia motion between the riser and the platform, hydro-pneumatic rise tensioners are generally employed. These traditionally hav 5 consisted of a frame, with multiple (usually four) hydrauli cylinders connected to the frame of the tensioner and thei cylinder rods connected to the riser pipe at the tensioner ring. The frame of the tensioner is supported at four points in th main deck structure of the platform. o During normal operation the four cylinder rods strok in and out in response to the movements between the riser pip and the platform. Tension is maintained by pressure in eac cylinder, and fluid from the cylinder can move back and fort into a connected chamber, called an accumulator. Each pair o 5 accumulators and cylinders becomes a gas spring, maintainin tension on the riser while the cylinders stroke.

During extreme motions of the riser, the riser may b pulling at an appreciable angle from vertical, and be located of centre with respect to the four cylinders. This causes uneve reaction loads in the tensioner frame and in the support brackets of the platform deck.

It is also possible for one or more cylinders to be inoperative, in which case the support of the riser is placed on the remaining cylinders of the riser tensioner, again causing uneven load reactions on its supports. The designer of the platform must consider that both events - lost cylinders and extreme riser movements - can occur together, and design in sufficient support strength at the reaction points in order to maintain sufficient margin of safety.

Added to these operating factors can be emergency loads caused by explosions in the deck area. At such times, the cross sectional area of the riser tensioner frame needs to be "open", to allow the majority of the blast force to pass through, minimizing the reaction loads from the blast force.

Finally, following the blast force, or concurrent with it, may be extensive fire. This can cause intense heat, raising the temperature of the steel supports to the point they become weakened, causing them to twist and to fail. During such events, the blast and heat may cause pressure to be lost from the cylinders of the riser tensioner, causing the rods to fully stroke out. The riser is now supported from four "struts" that cannot provide equal forces except in one, unique position. In any other position, as the riser moves about, all of the tension can transfer from four cylinders to any one of the cylinders, bringing the full force to suddenly bear at one reaction point into the frame and into the supports. If this occurs as the structure is hot and weakened, failure is likely. Attempts have been made to minimize the load con¬ centrations that can occur when a deck supported tensioner device connected to an offshore riser pipe is in operation. Such load concentrations can occur whenever horizontal forces are acting on the riser, or when hydraulic or mechanical failures happen to one or more of the hydro-pneumatic riser tensioner cylinders. Such events can cause an asymmetric load condition at the support points of the riser tensioner frame. Sudden and potentially catastrophic load increases can also be experienced when the riser tensioner is fully stroked out due to pressure losses in the cylinders as a result of fire and heat damage.

At least four cylinders are used in each tensioner t provide redundancy in the event one is out of service tempora¬ rily. This condition results in the reaction loads increasing o 5 the support pads of the tensioner frame. Should two cylinder become unserviceable, the loading is increased even further. Finally, in the event three cylinders are lost, all the load i carried by one cylinder. This latter case is not expected t occur under any of the normal operating conditions but could b o the case during an emergency such as a fire or explosion.

In the design of fire safe structures it is importan for a reasonable time to maintain the temperature of th structural components to levels that will remain below critical stresses. This can be accomplished by insulating or otherwis s keeping the high temperatures from the fire in heating th structural member, or by reducing the load on the member, so tha the stresses are kept below dangerous levels, even at elevate temperatures. One method is to provide more and more structur in order to reduce the stresses under all of the foresee 0 conditions to below critical levels. A further method is t provide protective systems, such as water sprays, insulation, o both, to minimize the effect of the heat from the fire.

Both methods are increasing the safe operating margi for the platform under extreme conditions and are commonly used 5 -to gain one objective, namely to extend the time that th structure can withstand exposure to a major, potential catastrop¬ hic fire.

Consideration of the above conditions leads th designer of the riser tensioner to the conclusion that it woul o be desirable to support the cylinders from the tensioner fram in a manner that would create the best sharing of the load by th tensioner supports, no matter which cylinders are operating o how many. Furthermore, it would be desirable from the standpoint of economics and weight to do this in a manner simple to 5 fabricate and easily handled. Finally, due to the possible exposure of the frame to an explosion, with resultant dynamic load increases on the supports as the force of the blast is reacted from the frame, the frame cross-sectional profile to the vessel deck region should be minimized. In the specific embodiment described in the following, a vertical riser extends from a floating vessel to the seabed below. Tension is required to maintain the riser in a vertical attitude, even as the vessel heaves and moves about. This is done 5 with a riser tensioner using four hydraulically pressured cylinders, the rods of which being attached to a connecting collar on the riser, known as the riser tensioner ring.

Wave and current action on the riser cause it to move about, swinging through a small vertical angle or cone. The o pressured cylinder rods compensate for this by stroking in and out, maintaining the required tension and sharing the load, more or less equally..

In case of a fire, there may exist a condition where the cylinders are exposed to high temperatures, causing the seals s to fail and the pressure to escape. The cylinders will then stroke out, and act as steel struts. However, any movement of the riser will immediately put all of the riser weight onto a single cylinder, with no sharing of load. Moreover, this is likely to occur when the cylinder and the piston rod, and its surrounding o supports are hot from the fire, and therefor are weakened. Thus the opportunity to yield and to fail is increased, and the riser may be dropped into the sea, creating additional hazards.

To avoid this hazards designers have proposed the concept of adding additional chains or cables to "catch" the 5 riser. However, they all have the problem of carrying the total load on one structure point, because of the geometry imposed by the requirements of the vessel.

To help gain time to control the fire, fire jackets and other insulative measures, along with deluge or other active fire o suppressant systems are employed. The main objective is to protect the structures against excessive heat, which will cause them to bend and break apart as their load carrying capacity reduces.

All of the above help. With the principle according to 5 the present invention,. however, the load carried by any member during an emergency immediately is reduced by 50 %. This can have the effect of greatly extending the time that the structure can maintain its load during a fire, or conversely, reduce the amount of insulation required, or add to the effectiveness of other protective measures that are employed by the vessel. In addition, structural requirements can be simplified, safety improved, and savings realized.

The present invention secures the riser in case of a s total cylinder failure, with the apparatus described by the features stated in the claims.

The principle relies on the idea of allowing the support structure connected to two articulating support struts, such as two cylinders or cables or chains, to rotate about a o centre point. The centre point is used to carry the riser tensioning load. As the riser moves about, the support structure rotates so that both struts, cables, or other structural supports continue to share the load, more or less equally. It is similar in action to that of the well known porch seat swing, where four s parallel chains are suspending the swing. As it moves, the swing rotates to maintain loads on all four chains. The rotation action also helps keep the occupants seated in the swing.

This disclosure points out the important principle of allowing the connecting structure between the load carrying 0 points and the point where the main load is supported to rotate with respect to their normal axes when at stationary equilibrium in order to maintain significant load sharing between more than one load supports.

The same principle can be extended to more than two 5 supports by a support structure allowing rotation between any pair of supports, in any azimuth.

It was found that a practical solution to the above listed desirable features could be attained by replacing the normal box frame with four legs that are supported at their upper o ends by a toroidal ring. The base of each leg terminates in a mounting pad that seats on the reaction brackets of the vessel, and are bolted thereto.

Cylinder loads are carried into each of the legs of the frame by a pin or a shackle located on the leg. The tendency of 5 the upper end of each leg to rotate downward is resisted by the toroidal section modules of the centre ring member. As it resists, the stresses are distributed to the other legs, so that each leg is involved in sharing loads generated by the other legs, no matter which legs are actually being pulled on by cylin- ders .

This results in a frame where all of the parts partici¬ pate in distribution of the loads, and so a structure which is economic in weight and relative cost is made possible. The cross- section of this frame to the deck region is also minimal, so that impulse forces can be minimized.

The frame also lends itself to enhancing the use of passive fire protection, as the surfaces to be protected are easy to envelope, and the load stresses are distributed rather than concentrated. Lower stresses mean that the frame is more resistant to deformation when subjected to excessive heat as would be the case in a fire.

The present invention is directed to a secondary support, operational in cases where the tensioner cylinders of which ever reason are inoperative, thereby maintaining the riser pipe tension necessary. This is achieved with the support according to the present invention as defined by the features disclosed in the claims.

During a fire, once the cylinders lose pressure and stroke out fully, the worst condition for structural failure is guarded against by providing a secondary support, called herein the "Catcher Ring". It is located around the riser, below the riser tensioner ring at an elevation that will support the riser before the cylinders stroke completely out. This ring is attached at two points to links that are connected back into the deck frame.

The catcher ring is specially shaped to allow it to support the riser tensioner ring at two points 180 degrees apart, but can rotate vertically about an imaginary axis through these two support points. The two support links are attached 90 degrees out of phase with the two tension ring support points.

This configuration allows the riser to swing about, through a vertical angle (as acted on by winds, waves and currents). Within this range of motion, the catcher ring can rotate on its two support points, continuously sharing its tension load between the two support links.

This combination of efficient structural geometry and open shaped support linkage results in a riser tensioner system that minimizes sudden, high, intense loads into the deck frame. resulting in economies of weight and cost. Moreover, the structural members used in the design concentrate their essential steel masses in a manner that promotes the use of effective insulation, and maximizes the ability of the structure to absorb s heat from a fire without overheating at highly stressed points. The end result is increased safety of personnel, the rig, and the environment at less cost, with all its parts open to periodic examination and accessible for maintenance.

Furthermore, during emergencies, when pressure is lost o in all four cylinders so that they stroke fully out (as might occur during a fire or explosion), a secondary linkage is employed. It is an elliptically shaped, lower support ring that "catches" or cradles the riser tensioner ring in a manner that allows two supporting linkages to share the load of the riser, s even as the riser angles about its normal, vertical axis. This load sharing during the heat of the fire is particularly important to survival of the riser since the supports at the deck are potentially weakened due to increased internal temperatures.

The above mentioned objects and advantages are achieved 0 with the catcher ring for a riser pipe tensioner according to the present invention, as described by the features stated in the claims.

The present invention is more clearly understood from the following disclosure in connection with the drawings, where 5 Figure 1 schematically discloses an over all side view of a riser tensioner, Figure 2 discloses the catcher device according to the present invention in a position corresponding with Figure 1 with the riser in a vertical position, Figure 3 corresponds with Figure 2, however with the riser in an angle in relation to the o vertical, Figure 4 shows a ground view of a tensioner with a catcher ring according to the present invention, and Figures 5 and 6 disclose a side view and a top view respectively of the catcher ring.

As disclosed in the drawing a riser pipe 1 is tensioned 5 by cylinders 2 and cylinder rods 7 connected respectively to a platform deck via a toroidal, ring 3 and the riser pipe 1 via a ring bracket 4. The platform or floating structure may be displaced due to current or weather conditions, resulting in an angle or the riser pipe in relation to the vertical, which is disclosed in Fig. 3. The tensioner maintains the tension of the riser even in this situation as hydraulic fluid is transferred between the cylinder 2 and the corresponding accumulator which is maintaining a desired pressure on the fluid by means of a s pressurized gas portion within the accumulator.

The tensioner is covered by insulation as is obvious from Fig. 1, an insulation covering the cylinder rod 7 and telescoping the insulation covering the cylinder itself. Said insulation is protecting the cylinders and the cylinder rods in o cases of fire, thereby delaying the time, during a fire, that the cylinder and/or the cylinder rod has a temperature which means a substantial decrease of the strength. It is, however, obvious that there may occur situations where this point will be reached and the tensioners no longer will be able to hold the riser. s In such cases it is of essential importance that the riser is captured before it falls down as the strength of the tensioners^" decreases.

Figures 2 and 3 disclose one embodiment of the present invention, a catcher ring which holds the riser pipe in cases 0 where the strength of the tensioners has been lost due to fire or failure. Figures 5 and 6 disclose another embodiment of the catcher according to the present invention.

The inventive catcher comprises a bracket connection to the platform structure arranged diametrically on each side of the 5 riser and being connected with a shock absorber 6 of which one part is connected with the bracket and another part is spring biased within the shock absorber and being at the other end connected with a catcher ring 8, such as disclosed in Figures 2 and 3. The securement of the tensioner cylinder rods 7 to the o riser 1 is made by ^"means of a tensioner ring (4) which is threaded to the riser 1. The underside of the tensioner ring 4 may have a conical shape as disclosed in Figures 2 and 3, whereby the catcher ring 8 has a corresponding shape in such a way that the tensioner ring 4 may rest on the catcher ring 8, the both s parts abutting each other on a circle.

Figures 5 and 6 disclose another embodiment where the tensioner ring 4 rests on a catcher ring 8 by a cam 9 resting in a recess 10 on the catcher ring 8. In this case the load is transferred through diametrically arranged points on the catcher ring. The catcher ring 8 comprises an elliptical opening, thereby enabling the catcher ring 8 to assume different angles in relation to the riser 1. As disclosed in Fig. 4, the brackets for the catcher ring are arranged diametrically.

Claims

Patent Claims

1. Emergency catcher for a tensioner for riser pipes s between a sea bed and a floating structure, such as a tension leg platform, comprising at least three hydraulic cylinders connec¬ ting the riser pipe with the deck of a floating structure, CHARACTERIZED IN the catcher comprising two in relation to the riser diametrically arranged supports which with interconnected o shock absorbers are connected with a catcher ring arranged around the riser a short distance below the tensioner ring, the catcher ring thereby being adapted to receive the tension ring in case the tensioners fail, thereby to maintain the riser substantially in position. s

2. Catcher according to claim 1, CHARACTERIZED IN the upper side of the catcher ring (4) having the shape of a circular edge abutting a cone on the underside of the tensioning ring (4).

3. Catcher according to claim 1, CHARACTERIZED IN the catcher ring (8) having an elliptical aperture surrounding the 0 riser (1) thereby allowing tilting of the catcher ring in relation to the riser, the upper side of the ring being adapted to receive a cam (9) on the underside of the tensioning ring in a recess (10).

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