WO1993019280A1 - Load sharing riser tensioning apparatus - Google Patents

Abstract

Tensioner for a riser pipe extending between a seabed and a floating structure, such as a tension leg platform, comprising at least three hydraulic cylinders being connected with the deck of the floating structure, the cylinder rod of hydraulic cylinder being connected with the riser, where the upper end of each hydraulic cylinder being connected with a toroidal ring which ring being supported by struts connected with the deck, the connections between the cylinders and the toroidal ring being positioned between two strut connections, thereby distributing forces from the cylinders into the toroidal ring where the forces are distributed as torsion forces, and further through the struts to the deck substantially without stress peaks in case the forces through some of the cylinders should increase substantially in relation to the forces through the rest of the cylinders due to failure, fire or such.

Description

Load Sharing Riser Tensioning Apparatus

The present invention is related to a load shari riser tensioning apparatus for a riser pipe tensioner accordi to the preamble of the claims.

A recent need for tensioning riser pipes extendi upwards from the seabed and being supported by a floating vess at the water surface has led to the use of riser tensioners. Su tensioners traditionally have consisted of a framework from whi are suspended cylinder assemblies with cylinder rods connecti a central bracket on the riser.

The need for tensioning such riser pipes comes abo due to the requirement to maintain such riser pipes in tensi from the top of the pipe to the sea bottom, and to restrain t pipe from excessive lateral motions. This tension has be determined to be critical to insure the structural survival the pipe, and to protect the environment from any conteϊi spilling from the pipe. It is also needed to maintain the sa operating conditions desired for the platform and its occupant

To control this tension, and to allow for differenti motion between the riser and the platform, hydro-pneumatic ris tensioners are generally employed. These traditionally ha consisted of a frame, with multiple (usually four) hydraul cylinders connected to the frame of the tensioner and the cylinder rods connected to the riser pipe at the tensioner rin The frame of the tensioner is supported at four points in t main deck structure of the platform. Wave and current. action on the riser cause it to mo about, swinging through a small vertical angle or cone. T pressured cylinder rods compensate for this by stroking in a out, maintaining the required tension and sharing the load, mo or less equally. During normal operation the four cylinder rods stro in and out in response to the movements between the riser pi and the platform. Tension is maintained by pressure in ea cylinder, and fluid from the cylinder can move back and for into a connected chamber, the accumulator. Each pair of accum lators and cylinders becomes a gas spring, maintaining tensio 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 bracket of the platform deck.

It is also possible that one or more cylinders ar inoperative, in which case the support of the riser is placed o the remaining cylinders of the riser tensioner, again causin uneven load reactions on its supports. Designers of the platform must consider that both events, both lost cylinders and extrem riser movements can occur together, and design in sufficien support strength at the reaction points in order to maintai sufficient margin of safety.

Added to these operating factors can be emergency load caused by explosions in the deck area. At such times, the cros 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 wit it, may be extensive fire. This can cause intense heat, raisin the temperature of the steel supports to the point they becom weakened, causing them to twist and to fail. Attempts have been made to minimize the load con centrations that can occur when a deck supported tensioner devic connected to an offshore riser pipe is in operation. Such loa concentrations can occur whenever horizontal forces are actin on the riser, or when hydraulic or mechanical failures happen t one or more of the hydro-pneumatic riser tensioner cylinders Such events can cause an asymmetric load condition at the suppor points of the riser tensioner frame. Sudden and potentiall catastrophic load increases can also be experienced when th riser tensioner is fully stroked out due to pressure losses i 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 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 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 critica stresses. This can be accomplished by insulating or otherwis keeping the high temperatures from the fire from 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 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 increase the safe operating margin for th platform under extreme conditions and are commonly used to gai one objective, namely to extend the time that the structure ca withstand exposure to a major, potentially catastrophic fire.

Consideration of the above conditions leads th designer of the riser tensioner to the conclusion that it woul 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 standpoin of economics and weight to do this in a manner simple t fabricate and easily handled. Finally, due to the possibl exposure of the frame to an explosion, with resultant dynami load increases on the supports as the force of the blast i reacted from the frame, the frame cross-sectional profile to th vessel deck region should be minimized.

In the specific embodiment described in the following a vertical riser extends from a floating vessel to the seabe below. Tension is required to maintain the riser in a vertica attitude, even as the vessel heaves and moves about. This is don with a riser tensioner using four hydraulically pressure cylinders, the rods of which being attached to a connectin collar on the riser, known as the riser tensioner ring.

The present invention secures the riser by go distribution of the forces and also in case of a total cylind failure, with the apparatus described by the features stated i the claims.

This disclosure points out the important principle allowing the connecting structure between the load carryi points and the point where the main load is supported, to rota with respect to their normal axes as stationary equilibrium order to maintain significant load sharing between more than o load supports.

The same principle can be extended to more than t supports by a support structure allowing rotation between a pair of supports, in any azimuth. It was found that a practical solution to the abo listed desirable features could be attained by replacing t normal box frame supporting the cylinders with four legs th extend upwards to support a toroidal ring. The base of each l terminates in a mounting pad that seats on the reaction bracke of the vessel, and are bolted thereto.

Cylinder loads are carried into the toroidal ring by "hook" or eye extending inward from the ring. (This appendage m or may not be located where a leg support is attached. ) The ri then transfers this load into the support legs. The tendency the upper end of each leg to rotate downward is resisted by t toroidal section modules of the centre ring member. As i resists, the stresses are distributed to the other legs, so th each leg is involved in sharing loads generated by the oth legs, no matter which legs are actually being pulled on by cyli ders.

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

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

The advantages of the present invention will be mo obvious from the following description in connection with t o drawing of which Figure 1 schematically discloses the tension according to the present invention in a side view, Figure discloses more detailed the support of the toroidal ring as section along II-II in Figure 3 and Figure 3 is a view of t tensioner seen from above in Figure 1. s At the lower end the cylinder rod 7 of each cylinder is secured to a ring bracket 4 such as by an upset profile thereby making the connection adjustable. Hydraulically ea cylinder 2 is a separate unit and connected with a pressuriz accumulator 10 which comprises an upper gas portion and a low 0 fluid portion from respectively to which the fluid in t cylinder is flowing, depending on the movements of the cylind rod 7 and thereby the piston.

The upper end of each cylinder 2 is connected with arm 5 protruding radially inwardly from a toroidal ring 3. T 5 ring 3 is mounted on preferably four brackets or struts 8 whi are firmly connected with the deck. Each arm 5 is secured to t ring in the middle between two struts 8. The forces from t cylinders then will seek to twist the ring and simultaneous press the ring towards or away from the struts 8, depending 0 the forces in the cylinder 2. Hereby the forces from ea cylinder is distributed mainly as torsion forces and shar between the neighbour struts 8 to both sides. Furthermore t torsion stresses are distributed in the ring also to the t opposite struts. 5 When the platform is displaced the riser will assume acute angle to the vertical. The cylinder rod on one side wi be extended, on the other side the rod will be pressed into t cylinder. The load transferred to the toroidal ring thereby al will be a combination of torsion and pressure upwards a downwards on the arms or levers 5 on the respective sides. Fro here the forces are transmitted to the toroidal ring by torsio of the ring and loading the ring in the direction of the cylinde axis. The load hereby is transferred to the deck structure vi at least the two neighbour struts 8. The upper portion of th cylinder 2 may simply be hooked up on the lever 5, thereby bein adapted to transfer forces from the riser to the deck structur in which ever angle the riser is to the vertical.

As mentioned above, the upper portion of the cylinder 2 are connected with the toroidal ring 3. At least three, mos suitably four cylinders 2 are secured at equal angular distance along the toroidal ring 3, each of which being arranged in th middle between two struts 8. Hereby the distribution of force from the cylinder 2 to the deck structure (not shown) is the mos desirable transfer and distribution of forces to the dec structure.

As disclosed in Figure 1, a protection cylinder covers the cylinder rod 7 when extended out of the cylinder, which protection cylinder being arranged around an insulatio surrounding the cylinder 2, thereby reciprocating along with th cylinder rod 7.

By a displacement of the platform, the angle of th upper portion of the riser will be such that the forces betwee the platform structure and the riser are still transferred b means of the cylinders 2 and their cylinder rods 7.

In case of a fire the force transmitting effect of on or two cylinders may be seriously decreased resulting in a increased transfer of forces through the other cylinders. Th distribution of the forces from the cylinder to the platfor deck, however, is very good, resulting in a high degree of safet in case of fire and avoid reduced effectivity of the cylinder due to the heat.

Claims

Patent Claims

1. Tensioner for a riser pipe extending between a s bed and a floating structure, such as a tension leg platfor comprising at least three hydraulic cylinders being connect with the deck of the floating structure, the cylinder rod of ea cylinder being connected with the riser, CHARACTERIZED IN t upper end of each hydraulic cylinder being connected with toroidal ring which ring being supported by struts connected wi the deck, the connections between the cylinders and the toroid ring being positioned between two strut connections, there distributing forces from the cylinders into the toroidal ri where the forces are distributed as torsion forces, and furth through the struts to the deck substantially without stress pea in cases the forces through some of the cylinders should increa substantially in relation to the forces through the rest of t cylinders due to failure, fire or such.

2. Tensioner according to claim 1, CHARACTERIZED IN t upper end of each hydraulic cylinder being connected to a lev protruding inwardly from the toroidal ring which ring bei supported by struts connected with the deck, the connectio between the hydraulic cylinders and the toroidal ring bei positioned between two strut connections, thereby distributi torsion forces from the levers into the toroidal ring, furth through the struts to the deck substantially without stress pea in case the forces through some of the cylinder should increa substantially in relation to the forces through the rest of t cylinders.