GB2177995A - Marine umbilical connection arrangement - Google Patents

Abstract

An umbilical connection arrangement between a sub-sea control station 15 and floating vessel stationed above it permits connection to be maintained for high amplitude vessel heave, i.e. greater than +/-3 metres, by suspending the umbilical 16 from a rotationally fixed winch reel of high amplitude support means 17 in a free-deployment mode, with excess umbilical streaming of hanging in the water and accommodating large heave motion by alternately slackening or tautening, but defining a large deployment envelope 20. Such an envelope inhibits work-over operation in low amplitude conditions, i.e. heave less than +/- metres, in which divers or equipment, lowered to the control station risk entanglement with the umbilical within the deployment envelope. To permit work-over, the excess length of connected umbilical is winched onto the support means 17 and low amplitude support means 21, including a subsidiary winch, attaches a clamp to the deployed umbilical and raises it to be supported by, and suspended from, a constant tension device 22. This device reciprocates the support position to compensate for low amplitude vessel motion and the tensioned umbilical deployment 23 is much reduced. Transfer between the two modes of deployment is effected without breaking the umbilical connection. <IMAGE>

Description

SPECIFICATION Marine umbilical connection arrangement This invention relates to marine umbilical connection arrangements and in particular to an umbilical connection arrangement between a sub-sea structure and a floating vessel stationed substantially above the structure.

It is known in off-shore oil and gas installations to connect sub-sea structures such as well-heads or well manifolds to a floating vessel, such as a production platform, (or a mooring buoy), by means of a flexible riser through which fluids recovered from the wells are raised to the vessel. It is also known to connect control umbilicals, comprising electrical and/or hydraulic power and/or signal communication channels, between the vessel and sub-sea structures in order to control the functioning thereof.

This invention is concerned particularly with such control umbilicals.

Floating vessels are of course subject to positional displacement with respect to the structure, most notably the substantially vertical oscillatory, heave motion due to wave action. To compensate for such displacement it has been the practice to connect the control umbilicals to the vessel by way of heave motion compensation means, usually in the form of a constant tension device which varies the length of umbilical deployed to maintain in it a constant tension. Such constant tension devices, and other forms of heave compensation means, are limited by well known practical constraints of available space and operating power in the amplitude of vessel displacement they can accommodate.

In practice such devices may maintain a predetermined tension of the order of two tonnes in an umbilical whilst compensating for a heave (or other displacement) amplitude of + 3 metres and this has hitherto represented the practical limiting operational heave amplitude before taking steps to shut down the umbilical communication channels and disconnecting the umbilical.

It has recently been proposed in GB Patent Specification No. 2,133,111 to deploy a flexible oil production riser in such a manner that it is possible to maintain flow for increased amplitudes of vessel displacement.

As disclosed therein, the floating vessel is stationed above and laterally of the sub-sea stucture, a well manifold, and a flexibie extensible riser thereto is deployed from the vessel in so-called free deployment mode, that is, the length of deployed riser is in excess of the maximum separation between the vessel and structure. The excess length accommodates the vessel displacement and the riser alternately tautens and slackens as the vessel motion varies the tension therein.

When the vessel is closest to the structure the riser, above its termination with the structure, tends to fall back towards the sea-bed in a gentle curve which places little stress on the riser, and/or streams in sea current to one side of the structure, the only precaution being the necessity for a bend restrictor adjacent its termination with the structure to limit the radius of curvature of any bend in the riser caused by said fall back or by the component of tension in the riser (acting laterally of the station) occasioned by the sea currents or laterally displaced vessel.

Such a free deployment of the riser enables operational connection to be maintained for much higher heave displacement amplitudes than the constant tension devices, typically some 100 200% more.

In this specification the term "low amplitude" is used in respect of heave and other displacements which fall within the compensatory abilities of known constant tension devices, e.g. heave of + 3 metres amplitude, and the term "high amplitude" is used in respect of such displacements of the order of magnitude available to such free deployment, e.g. heave of greater than t 3 metres amplitude.

However, there are normally operating constraints which make correspondingly simple free deployment of a control umbilical less satisfactory.

Whereas it is readily possible, and indeed desirable, to station a floating vessel laterally of one or more connection points for production risers it is desirable to station the vessel directly above a subsea central control structure for these well heads/ connection points in order to permit so-called work-over operations involving the lowering of divers or machinery from the vessel to the station whilst it is still in operation and therefore connected with the vessel by way of a control umbilical. It will be appreciated that the free deployment of a control umbilical despite the inherent ability to cope with high amplitude displacement between vessel and structure can by virtue of the excess length deployed interfere with and inhibit-work over operations in conditions of low amplitude displacement.

It is an object of the present invention to provide a method of, and an arrangement for, an umbilical connection between a sub-sea structure and a floating vessel stationed substantially above the structure and a floating vessel stationed substantially above the structure which permits both concurrent work-over operations on the structure from the vessel at low amplitudes of displacement between the vessel and station and the maintenance of operational connection at high amplitudes of displacement.

According to a first aspect of the present invention an umbilical connection arrangement between a sub-sea structure and a floating vessel stationed substantially above the structure, comprises deployment means carried by the vessel from which is suspended a flexible umbilical connected both to the vessel and structure, said deployment means including high amplitude support means operable to assume a fixed position with respect to the vessel and suspend freely deployed therefrom a predetermined length of umbilical, longer than the maximum separation between the deployment means and the structure, the excess length, per mitting high amplitude relative displacement between the vessel and the structure, being accommodated by lateral flexing of the umbilical within a free deployment envelope defining a region surrounding a direct line between the deployment means and vessel, the lateral dimensions of which envelope are a function of the deployed length and forces acting laterally thereon, winching means, operable to shorten the deployed length of the connected umbilical to reduce said excess length and store the withdrawn and connected umbilical, and low amplitude support means operable to suspend the shortened length of umbilical therefrom and including constant tension means responsive to the tension in the suspended umbilical to vary the deployed length in accordance with low amplitude displacements of the vessel to maintain the tension at a predetermined level and define thereby a tensioned-umbilical deployment envelope of smaller extent than that of the freely deployed umbilical.

According to a second aspect of the present invention a method of deploying a control umbilical between a sub-sea structure and a floating vessel stationed substantially above the structure while maintaining connection therebetween, both during relative displacements of the structure and vessel of high amplitude and during the suspension of other bodies from the vessel at relative displacements of low amplitude, comprises (i) during high amplitude displacements, suspending from deployment means on the vessel a predetermined first deployed length of flexible umbilical connected at its terminations to respective by the structure and vessel systems, said first deployed length being in excess of the direct distance between the structure and the storage device, whereby the excess length, permitting said high amplitude relative displacement, is accommodated by lateral flexing of the umbilical between the vessel and structure within an umbilical free deployment envelope surround a direct line between them, the lateral extent of the envelope being a function of the deployed length and forces acting laterally thereon, and (ii) during low amplitude displacements, shortening the deployed length to reduce said excess length and suspending the umbilical from a constant tension device, which device accommodates low amplitude displacement by varying the length of umbilical deployed to maintain a predetermined tension therein, the tensioned umbilical defining a tensioned-umbilical deployment envelope, of smaller extent than the free deployment envelope and permitting direct access between the vessel and the structure outside of the envelope.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 illustrates in elevation an umbilical connection arrangement according to the present invention between deployment means on a floating vessel and a sub-sea structure, Figure 2(a) is an elevation view of the deployment means of Figure 1 in greater detail illustrating the differences between high amplitude support means and low amplitude support means for the umbilical and showing the umbilical suspended from the high amplitude support means, Figure 2b'a) is an elevation view of deployment means similar to Figure 2(a) but showing the umbilical suspended from low amplitude support means, and Figure 3 is a plan view of the high amplitude support means of Figure 2(a) showing a means of connection between the umbilical and vessel.

Referring to Figure 1 an off-shore oil production installation includes one or more oil well heads 11 having riser connection points 11' located on the sea bed 12 or in fixed relationship therewith from which oil is pumped along a production riser 13 to a floating production vessel 14. The, or each, well head or riser connection point contains valve equipment which is monitored and controlled by equipment at a sub-sea control station 15 comprising a structure deployed on the sea bed. The control station contains much equipment which requires attention trom divers or other equipment lowered by the vessel and the vessel is stationed over the control station for the purpose of such socalled work-over operation.For normal production operation, which must continue uninterrupted during work-over operation, the control station receives electrical and hydraulic power and control signals by way of separate communication channels in a flexible inextensible umbilical 16 deployed by suspending it from the vessel 14.

To enable umbilical connection to be maintained up to high amplitude displacements between the vessel 14 and a termination 15' at control station 15 the umbilical is suspended from high amplitude support means 17 in the deployment means to hang under its own weight in a free deployment mode, the length deployed being chosen as a function of the amplitude of any oscillatory heave or lateral motions expected of the maximum separation along a straight line separation axis 19 between the deployment means (suspension point) and control system (termination) and being in excess of this straight line separation.

The vessel 14 is subjected principally to vertically oscillatory motion, that is, heave, due to wave motion and it will be appreciated that with the vessel at the crest of a wave, that is, maximum separation from the structure 15 the excess umbilical is taken up and it assumes a profile substantially as shown by the chain dotted lines 16'.

With the vessel at the trough of a wave, that is, minimum separation between the deployment means and structure 15, the umbilical profile is substantially as shown by the full lines 16 with the excess length flexing the umbilical laterally of a separation axis 19 between deployment means and termination the lateral excursion of the umbilical being a function of the deployed length and laterally acting sea currents, tending to stream laterally in strong currents but in weak currents tending to fold back on the lower portion in the vicinity of the termination 15' due to its suspended weight.

It may be desirable to relieve the station of forces, due to the weight of the umbilical or streaming forces on it, and/or to prevent damage to the umbilical structure due to steep curvature between the termination and a folded back portion by providing buoyancy to a lower part of the umbilical although the effect of such buoyancy, particularly in limiting the sharpness of bends, may be to increase the free deployment envelope.

The maximum lateral excursion of the umbilical from the separation axis 19 defiqes a free deployment envelope, indicated by boundary lines 20, and encloses a region surrounding the separation axis 19 anywhere in which the umbilical may appear in accordance with prevailing operating conditions.

The precise dimensions of the free deployment envelope will vary from situation to situation but Figure 1 serves to illustrate that the free deployment envelope may well extend beyond the structure 15 and/or vessel 14 such that the suspension of equipment or divers in work-over operation is inhibited by the risk of entanglement between the lines of such and the umbilical. It may be pointed out that a tendency for the umbilical to stream with sea currents and be constrained to one part of the region in the envelope is unhelpful as the same currents will tend to stream other descending lines in the same direction.

In general, equipment designs and safety considerations limit over-work operations to low amplitude heave conditions and in accordance with the present invention the deployed length may be shortened in such conditions to reduce the excess length and the umbilical suspended from low am plitude support means 21 including a constant tension device 22. The constant tension device (described hereinafter with reference to Figure 2) functions in accordance with known principles to maintain a constant tension in the umbilical, which umbilical is then held substantially, taut as shown at 16', and respond to the forces acting on the umbilical to constrain its lateral displacement to a smaller region surrounding the separation axis 19 which defines a tensioned-umbilical envelope designated by boundary lines 23.

It will be appreciated that the lateral dimension of such a tensioned-umbilical envelope are suffi ciently less than of the free deployment envelope and permit direct access between the vessel 14 and the structure 15 by items described from the vessel without risk of entanglement with the control umbilical.

The manner in which the umbilical deployment mode is changed between the two ranges of dis placement amplitude whilst maintaining umbilical connection will now be considered with reference to Figures 2(a) and 2(b) and Figure 3.

Within the deployment means 18 the high amplitude support means 17 comprises winching means including a reel 24 rotatable about a substantially horizontal axis 25 by drive motor 26, reel braking means 27, normally biased on, and brake releasing means 28. A length of the umbilical 16 between the connection to the vessel system and structure is wound on the reel which serves as a storage means for excess umbilical withdrawn into the deployment means.

When the vessel is subject to high amplitude displacement the umbilical is freely deployed by unwinding a stored length from, the reel from which it is then suspended by causing the braking means 27 to fix the reel position with respect to the vessel.

As stated the umbilical 16, which contains a plurality of communication channels, is connected to the vessel and this connection is required to be maintained despite changes in its deployed length through winching in and out excess length by reel 24. Any suitable form of connection may be employed between the rotatable reel and the vessel, such as slip rings for electrical connections or rotary couplings for fluid connections, but conveniently the connection is made by way of a flexible jumper umbilical contra-wound on the reel as shown in Figure 3.

Referring to Figure 3 which shows a plan view of high amplitude support means 17, in particular the winching means 22, the deployed umbilical 16 terminates in coupling means 30 which is linked by a more flexible jumper umbilical 31 to a fixed connection 32 on the vessel. A length of the umbilical 16 and jumper 31 including the coupling means 30 is wound around the reel 17, which effectively acts as a capstan winch, rotation in one sense winching in excess deployed umbilical and depositing a corresponding length of the flexible jumper in a storage region, whereas rotation in the opposite sense deploys excess umbilical, replacing it by winding onto the reel a corresponding length of the jumper from the storage region.

The principal characteristic of the jumper umbilical 31 is its flexibility for ease of storage between the reel and connection 32 when displaced from the reel and this is achieved at the expense of the structural weakness (particularly to tension) in relation to the umbilical. To relieve the transmission of tension from the deployable umbilical to the jumper the coupling means 30 may be secured to the reel.

As will be readily appreciated from the Figures 1 and 2(a) in conditions of high amplitude displacement the variations in separation are accommodated by straightening or slackening the suspended umbilical, the tension therein increasing or decreasing continuously and being transmitted to the reel 24 forming the support means as a rotational torque resisted by braking means 27.

Under conditions of low amplitude displacement, either whenever such conditions occur or when it is desired to perform work-over operations, the deployed length of umbilical is shortened by operation of the winching means 22 the excess length winched in being stored on the reel 24 whilst umbilical channel connection is maintained with the vessel.

Referring to Figure 2(a) low amplitude support means 21 includes constant tension device 22 and subsidiary winching means 32, which winching comprises in this embodiment the mechanical components of the constant tension device.

The subsidiary winching means comprises a support pulley 33 over which passes, and from which hangs, a winch cable 34, the winch cable being connected to a reciprocably rotatable subsidiary winch drum 35 by way of a pulley system 36.

The winch cable 34 carries attachment means 27 in the form of a releasable clamp which is adapted to be attached to the umbilical by clamping around a foundation member 38 carried by the umbilical and by which the umbilical is lifted.

The clamp 37 comprises a pair of hinged jaws 39 which may be secured to each other or opened by hydraulically actuated fastening means 40 supplied by separately suspended hydraulic line 41. The jaws have an internal profile conforming to the external profile of a rigid body portion 42 of the foundation member 38 onto which they may be clamped.

The foundation member is itself clamped permanently to the umbilical and the rigid body portion makes frictional contact with the umbilical over a length of its surface so as to spread any local surface loading due to action of the clamp 37 or lifting thereby. The foundation member also has flexible bend restriction means 43 adjacent the rigid body portion and extending at least in the direction of the support means.

The foundation member is located at a position on the umbilical to define an attachment point which is separated from the high amplitude support means, that is contact point of reel 24, by a predetermined length L of umbilical when the deployed length has been shortened ready for transfer to the low amplitude suspension means.

The subsidiary winching means 32 lowers the clamp 37 by means of cable 34 and the jaws of the clamp are closed remotely about the rigid portion 42 of the foundation member. The clamp 37 is then raised by the subsidiary winching means to a position, as typified by Figure 2(b), at which the umbilical is suspended from the attachment means (clamp 37), the attachment point forming a low amplitude umbilical support point from which the umbilical is suspended in a substantially vertical direction.

The subsidiary winching means then is changed to operate as a constant tension device by introducing constant tension control means 44 and a sensor 45 of the forces transmitted to the pulley 33. The control means determines the tension in the umbilical by way of sensor 45 and causes reciprocation of the subsidiary winching means to raise and lower the support point with respect to the deployment means to maintain the umbilical tension constant, effectively countering low amplitude displacement of the vessel with respect to the sub-sea control station.

The profile of the umbilical between the attachment point and the high amplitude support point, that is, reel 24 depends on the predetermined length L and this length is chosen as a function of the maximum operational separation between the attachment point and the reel in the low amplitude mode of operation. Figure 2(b) shows by broken lines 46 and 47 typical upper and lower positions of the support means in the constant tension device and typical extremes of profile of the predetermined length of umbilical.

It will be appreciated that whilst the portion of the umbilical descending from the foundation member at the attachment means remains substantially straight due to the tension maintained therein the portion between the foundations member and the reel 24 is subject to continuous flexing and the flexing characteristics of bend restriction means 43 and the predetermined length L of umbilical are chosen to ensure that the flexed umbilical is not constrained to bend with a radius of curvature which may damage its structure.

It will be appreciated that once the umbilical is deployed from the constant tension device in low amplitude displacement conditions it may remain so whether or not work-over operation is currently in progress and be changed to the free deployment mode of umbilical suspension only when high amplitude displacement conditions are forecast.

To change from suspension by the low amplitude support means to suspension by the high amplitude support means, the constant tension control means 44 is de-actuated and subsidiary winching means 32 merely lowers the attachment means (clamp 37) until the weight of the deployed umbilical is taken by the high amplitude means as effected by the braked reel 24, at which time the winch cable 34 becomes slack. The clamp 37 is opened and withdrawn from foundation member 38 and raised to a convenient stand-by position.

The braking means 27 of the reel is released and the umbilical lengthened by deployment of the excess length required for free deployment, after which the reel is braked.

It will be seen that the present invention provides a method of maintaining umbilical connection between a floating vessel and sub-sea structure over a wide range of displacement amplitudes and tailoring the deployment envelope of the connected umbilical to permit work-over operation.

It will be appreciated that the particular apparatus described for the constant device, subsidiary winching means and indeed the winching and high amplitude support means is exemplary only and may be substituted by other arrangements providing the constraints of the present invention are met.

The particular form of attachment means described, namely the foundation member 38 and cooperating clamp 37, is also exemplary and may be replaced by alternative and simpler attachment means, for instance, a clamp which engages with an eye mounting carried like the foundation member on the umbilical.

Claims (14)

1. An umbilical connection arrangement between a sub-sea structure and a floating vessel stationed substantially above the structure, said arrangement comprising deployment means carried by the vessel from which is suspended a flexi ble umbilical connected both to the vessel and structure, said deployment means including high amplitude support means operable to assume a fixed position with respect to the vessel and suspend freely deployed therefrom a predetermined length of umbilical, longer than the maximum separation between the deployment means and the structure, the excess length, permitting high amplitude relative displacement between the vessel and the structure, being accommodated by lateral flexing of the umbilical within a free deployment envelope defining a region surrounding a direct line between the deployment means and vessel, the lateral dimensions of which envelope are a function of the deployed length and forces acting laterally thereon, winching means, operable to shorten the deployed length of the connected umbilical to reduce said excess length and store the withdrawn and connected umbilical, and low amplitude support means operable to suspend the shortened length of umbilical therefrom and including constant tension means responsive to the tension in the suspended umbilical to vary the deployed length in accordance with low amplitude displacements of the vessel to maintain the tension at a predetermined level and define thereby a tensioned-umbilical deployment envelope of smaller extent than that of the freely deployed umbilical.

2. An arrangement as claimed in claim 1 in which the winching means is operable also to lengthen the deployed umbilical by deploying stored connected umbilical.

3. An arrangement as claimed in claim 1 or claim 2 in which the high amplitude support means comprises a reel, rotatable about a substantially horizontal axis, upon which the umbilical is wound between connection with the vessel system and the structure, and braking means operable to prevent reel rotation, the reel comprising a high amplitude support from which the deployed umbilical is suspended.

4. An arrangement as claimed in claim 3 in which the winching means comprises said reel, a drive motor operable to rotate the reel and means for releasing the braking means.

5. An arrangement as claimed in claim 4 in which the reel is operable to store the withdrawn and connected umbilical.

6. An arrangement as claimed in any one of the preceding claims in which the low amplitude support means comprises (i) subsidiary winching means, including a suspended winch cable, (ii) attachment means carried by the winch cable, said subsidiary winching means being operable to lower the attachment means to supportively attach to the deployed umbilical at an attachment point thereof, separated from the high amplitude support means by a predetermined length of the suspended umbilical, and to raise the attachment means and attached umbilical to an operating position, and (iii) a reciprocable constant tension device including means to support the umbilical at said attachment point, the attachment point forming a low amplitude umbilical support point from which the deployed umbilical is suspended, and operable to reciprocate the position of the umbilical support point with respect to the deployment means in accordance with motion of the vessel and the maintenance of a predetermined tension in the suspended umbilical, said predetermined length of umbilical being in excess of the maximum operational separation between the high amplitude support means and the low amplitude umbilical support point.

7. An arrangement as claimed in claim 6 in which the attachment means comprises a part of the constant tension device and supports the umbilical at the attachment point.

8. An arrangement as claimed in claim 7 in which the constant tension device comprises the subsidiary winching means and includes control means operable to cause the subsidiary winching means to wind the winch cable to lower and raise the attachment means to maintain said constant tension in the deployed umbilical suspended therefrom.

9. An arrangement as claimed in claim 7 or claim 8 in which the attachment means comprises a clamp arrangement adapted to releasably engage the umbilical over a preset length thereof.

10. An arrangement as claimed in claim 9 in which the attachment means includes a foundation member carried by the umbilical and adapted to be engaged by the clamp arrangement.

11. An arrangement as claimed in claim 10 in which the foundation member is arranged to cause the umbilical to be suspended in a substantially vertical direction when carried by the attachment means and includes bend restriction means extending from the foundation member along a portion of said predetermined length of the umbilical between the attachment point and the high amplitude support means.

12. An umbilical connection arrangement between a sub-sea structure and a floating vessel stationed substantially above the structure the arrangement being substantially as herein described with reference to and as shown in the accompanying drawings.

13. A method of deploying a control umbilical between a sub-sea structure and a floating vessel stationed substantially above the structure while maintaining connection therebetween, both during relative displacements of the structure and vessel of high amplitude and during the suspension of other bodies from the vessel at relative displacements of low amplitude, the method comprising (i) during high amplitude displacements, suspending from deployment means on the vessel a predetermined first deployed length of flexible umbilical connected at its terminations to respectively the structure and vessel, said first deployed length being in excess of the direct distance between the structure and the storage device, whereby the excess length, permitting said high amplitude relative displacement, is accommodated by lateral flexing of the umbilical between the vessel and structure within an umbilical free deployment envelope sur rounding a direct line between them, the lateral extent of the envelope being a function of the deployed length and forces acting laterally thereon, and (ii) during low amplitude displacements, shortening the deployed length to reduce said excess length and suspending the umbilical from a constant tension device, which device accommodates low amplitude displacement by varying the length of umbilical deployed to maintain a predetermined tension therein, the tensioned umbilical defining a tensioned-umbilical deployment envelope, of smaller extent than the free deployment envelope and permitting direct access between the vessel and the structure outside of the envelope.

14. A method of deploying a control umbilical between a sub-sea structure and a floating vessel stationed substantially above the structure, the method being substantially as herein described with reference to, and as shown in, the accompanying drawings.