GB2188899A - A tensioner for tensioning a traction cable - Google Patents

Abstract

A tensioner for a cable connected between an oil well riser and a sea vessel includes an actuator (12) comprising a cylinder (13), a piston (14) dividing the cylinder into first and second chambers (15, 16), and a piston rod (17) passing through the first chamber (15) and the corresponding end (18) of the cylinder, the free end of the piston rod and the end (21) of the cylinder opposite to the end through which the piston rod passes each being provided with at least one sheave (19, 20). The second chamber (16) of the cylinder is fed with hydraulic liquid from a pressure accumulator system (23, 24) via a duct (28, 29) including a first controlled valve (30) having a first non-return valve (31) connected in parallel therewith to prevent flow from said second chamber towards said pressure accumulator system. The riser includes a weak link (4), Fig. 2 (not shown), and the other end of said cable is fixed to a fixed point (22) of the vessel (7) after being reeved around said sheaves provided on said actuator. The tensioner further includes a very high pressure accumulator device (32) which is connected to said second chamber (16) of the actuator via a second controlled valve (35), with the pressure in said accumulator device being greater than the pressure required to break said weak link. In an emergency the valve (35) is actuated to pressurize the second chamber (16) to cause breakage of the weak link. <IMAGE>

Description

SPECIFICATION Atensionerfor tensioning a traction cable connected at one of its ends to the top end of a riser whose bottom end is fixed to the sea bed The present invention provides a tensioner for tensioning a traction cable connected at one of its ends to the top end of a riserwhose bottom end is fixed to the sea bed, said tensioner being connected to a surface vessel subjected to swell and including an actuator comprising a cylinder, a piston dividing the cylinder into first and second chambers, and a piston rod passing through the first chamberandthe corresponding end ofthe cylinder, the free end ofthe piston rod and the end of the cylinder opposite to the end through which the piston rod passes each being provided with at least one sheave, the second chamber ofthe cylinder being fed with hydraulic iiquidfroma pressureaccumulatorsystemviaaduct including afirstcontrolled valve having afirst non-return valve connected in parallel therewith to prevent flow from said second chambertowards said pressure accumulator system, said riser including aweak linkandthe otherend of said cable being fixed to a fixed point of said vessel after being reeved around said sheaves provided on said actuator.

Suchtensionersarewell known,for example in offshore oil drilling, in orderto keep the tubing vertical which provides well continuity from the sea bed uptothesurface.Suchtubing is commonly called a "riser".

In such a tensioner, in the event of an emergency requiring the system to be disconnected rapidly, the weak link can be broken by closing said controlled valve connecting the second chamber ofthe actuator to the pressure accumulator system. I n this way, the non-return valve allows the actuator to be freely filled with hydraulic liquid while the vessel moves down in a wave trough,thereby moving the rod out from the actuator, and the link is broken by the large increase in cable tension as the vessel rises on the crestofawavewhilethe hydraulic liquid is prevented from passing into the accumulator by the non-return valve.

The motion of the vessel is thus the sole driving forcefor enabling thetensionerto be disconnected rapidly. In fact, such disconnection is made possible only bythe amplitude ofthe swell. However, it can happen that thins amplitude is not great enough to give rise to sufficient increase in cable tension to cause the weak linkto break.

According to the invention, the above tensioner further includes a very high pressure accumulator device which is connected to said second chamber of the actuator via a second controlled valve, with the pressure in said accumulator device being greater than the pressure required to break said weak link.

Preferably, said very high pressure accumulator device is constituted by a membrane accumulator.

A second non-return valve preventing fluid return towards the very high pressure accumulator device may be disposed in series with said second controlled valve.

An embodiment of the invention is described by way of example with reference to the accompanying drawings, in which: Figure lisa diagram ofthe hydraulic circuit ofa tensionerfortensioning a traction cable in accordance with the invention; and Figure2 is a diagram showing a riser held in the vertical position by the Figure 1 tensioner.

Figure 2 shows a riser 1 which is fixed at its bottom endtoan underseaconnector2atthe head of a well (not shown). The sea bed is marked 3. The riser includes a weak link4 of less strength than the rest of the riser. Close to its top end, the riser is fixed to a carriage5which is guided in averticalwell6 provided in a floating vessel 7. Two cables 8 and 9 are fixed to the carriage 5, and each of them is put under tension by a tensioner as shown in Figure 1.

The entire tensioner shown in Figure 1 is connected to the vessel 7, as are pulleys 10 and 11 for exerting vertical traction on the riser 1. In contrast, the carriage 5 is fixed relative to the riser 1, and consequently the carriage Sand the vessel 7 move relative to the each other because of the waves, with the carriage sliding in the well 6.

The tensioner for tensioning the cable 8 is now described with reference to Figure 1.

The tensioner includes an actuator 12 comprising a cylinder 13 fixed to the vessel 7, a piston 14which divides the cylinder into a first chamber 15 and a second chamber 16, and a piston rod 17 which passesthrough thefirst chamber 15 and the end 18 of the cylinder.

One or more sheaves 19 placed side-by-side are fixed to the free end of the piston rod 17, and similarly, shaves 20 are fixed to the opposite end 21 ofthecylinder 13.

After passing over the pulley 10 (Figure 2), the cableSisreeved aroundthesheaves 19 and 20 and is then fixed to a fixed point 22 on the vessel 7.

The second chamber 16 ofthe actuator 12 isfed with hydraulicfluid,e.g. awaterand glycol mixture, via a pressure accumulator system comprising a bank 23 of compressed gas, e.g. air, and a gas-liquid transfer separator 24 constituted by a cylinder having a free piston 25 therein dividing its internal volume into a first chamber 26 connected to the bank of compressed air and a second chamber 27 filled with hydraulic liquid and connected to the second chamber 16 ofthe actuator 12 via a duct 28,29 fitted with a first controlled valve 30 connected in parallel with a first non-return valve 31 for preventing hydraulic liquid from flowing from the actuator towards the transfer separator 24 when the valve 30 is closed.

Avery high pressure accumulator device 32 is also connected to the second chamber 16 of the actuator 12 via a duct 28,33, 34, with duct portion 33 including a controlled valve 35 connected in series with a non-return valve 36 preventing hydraulic liquid from flowing from the actuator 12 towards the very high pressure accumulator 32.

The valve 35 is a three outlet electrically-controlled distributor valve. In the position shown, duct portion 33 is closed and communication is provided between ductportion 33 a and adrainduct37fordraining possible leaks from the non-return valve 36 to a receptacle 38 or to the tank 40.

When the electromagnet 39 of the valve 35 is excited, communication is established between the right-hand and the left-hand parts of duct portion 33.

Theveryhigh pressure accumulator32 is a membrane accumulator inflated with nitrogen,the liquid portion of the accumulator is constituted by the same liquid as that used in the actuator 12 and the transfer separator 24, and it is fed continuously by a hydro-pneumatic pump 39 drawing liquid from a tank 40. The outlet pressurefrom the pump is adjusted by a control 41 which adjusts the pressure of the air supplied to the motor 42 ofthe hydro-pneumatic unit 39.

This outlet pressure ensures thatthere is sufficient pressure in the very high pressure accumulator 32 to breaktheweak link4.

A non-return valve 43 is disposed in a duct44, 45, 34 connecting the pump 46 of the unit 39 to the accumulator32. Aduct47 is connected in parallel with duct portion 44 and includes a pressure gauge 48 which is isolated by a valve 49. A safety valve 50 constituting a pressure limiter is connected in parallel with duct 34, and the outlet therefrom leads to the receptacle 38.

Finally, means are provided to make good any possible leaks from the circuit comprising the actuator chamber 16, the ducts 28 and 29, and the chamber 27 of the transfer separator 24 by connecting a duct 51 to the duct 45. The duct 51 is connected to the duct 28 and includes an electrically controlled distributor valve 53.

In the position shown, duct 51 is closed and the valve 53 allows any leaks from the non-return valve 52 to be drained via a duct 54to the receptacle 38.

The tensioner operates as follows.

In normal operation, the valve 30 is open, allowing liquid to flowfreelyfrom the chamber 27 ofthe transfer separator 24to the chamber 16 of the actuator 12, and vice versa. The distributor valve 35 is in the position shown, i.e. the accumulator 32 is isolated from the actuator and the transfer separator 24.

The distributor valve 53 is also in the position shown, isolating the actuator 12 and thetransfer separator 24 from the hydro-pneumatic unit 39.

Underthe influence ofthe swell, the vessel 7 rises and falls, with the actuator rod 17 moving into the cylinderwhen the vessel rises and moving outwhen itfalls in the trough of a wave, and with the hydraulic liquid moving in alternation from the actuator towards the chamber 26 of the transfer separator 24 under the traction force from the cable or vice versa when this force is relaxed as the vessel moves downwards. The cable is kept under an average tension by the compressed air bank 23.

In an emergency, when it is desired to disconnect the system by breaking the weak link4, thevalve 30 is closed and the distributor valve 35 is electrically excited so as to establish liquid communication from the very high pressure accumulator 32 towards the actuator 12. As a result, a sufficient quantity of hydraulicfluid is expelled from the membrane accumulator 32 towards the actuator chamber 16 to put the cable 8 under enough tension to reach the breaking force ofthe weak link4. The non-return valves 31 and 36 prevent the liquid from leaving the chamber 16 and going towards the transfer separator 24 and the accumulator 32.

Thus, even the absence of sufficient swell,the invention makes it possible to be sure of breaking the weak link4 by influx of high pressure hydraulic liquid from the membrane accumulator32.

Byway of example, the pressure in the membrane accumulator 32 is 250 bars (25.106) Pascals) and the pressure in the transfer separator 24 is 70 bars (7.106 Pascals).

When possible leakage from the non-return valve 52 isto be made good,the distributorvalve 53 is electrically excited, thereby putting the hydro-pneumatic unit 39 into communication with the chambers 16 and 27.

Claims (5)

1. Atensionerfortensioning atractioncable connected at one of its ends to the top end of a riser whose bottom end is fixed to the sea bed, said tensioner being connected to a surface vessel subjected to swell and including an actuator comprising a cylinder, a piston dividing the cylinder into first and second chambers, and a piston rod passing through the first chamber and the corresponding end ofthe cylinder, the free end ofthe piston rod and the end of the cylinder opposite to the end through which the piston rod passes each being provided with at least one sheave, the second chamber of the cylinder being fed with hydraulic liquid from a pressure accumulator system via a duct including a first controlled valve having a first non-return valve connected in parallel therewith to prevent flow from said second chambertowards said pressure accumulator system, said riser including a weak link and the other end of said cable being fixed to a fixed point of said vessel after being reeved around said sheaves provided on said actuator, the tensioner further including a very high pressure accumulator device which is connected to said second chamber of the actuator via a second controlled valve, with the pressure in said accumulator device being greaterthan the pressure required to break said weak link.

2. Atensioner according to claim 1,wherein said very high pressure accumulator device is constituted buy a membrane accumulator.

3. Atensioneraccordingtoclaiml or2,wherein a second non-return valve preventing return towards the very high pressure accumulator device is disposed in series with said second controlled valve.

4. Atensioner according to any one of claims 1 to 3, wherein said pressure accumulator system comprises a bank of compressed gas followed bya gas-liquid transfer separator.

5. Atensioner substantially as herein described, with reference to the accompanying drawings.