GB2149371A

GB2149371A - Production buoy

Info

Publication number: GB2149371A
Application number: GB08330828A
Authority: GB
Inventors: John Ash; Hugh Malcolm Ian Bell; Raymond Scott Park
Original assignee: MARIS
Current assignee: MARIS
Priority date: 1983-11-18
Filing date: 1983-11-18
Publication date: 1985-06-12
Also published as: GB8330828D0

Abstract

An offshore hydrocarbon production buoy (5) is anchored to the seabed by tensioned resiliently extendable tethers (2, 3) which pass uninterruptedly upwards to the buoy (5), freely over engagement means such as pulleys (4), and downwardly into connection with a riser (1), thereby tethering the buoy and maintaining the riser (1) in tension whilst allowing limited vertical movement of the buoy (5). The tethers (2, 3) preferably pass up through guide passages (10) and down through a central passage (9) which also accommodates the riser (1). <IMAGE>

Description

SPECIFICATION Production buoy This invention relates to an offshore hydrocarbon production buoy with a tether tensioned riser.

An offshore hydrocarbon production system comprises a riser and a surface production support facility. The typical riser is a bundle of rigid pipes connecting the sea bottom to the surface and through which pass the product hydrocarbons, the service fluids and the control fluids.

If the field is large and the water depth is less than approximately 300 metres, the hydrocarbon riser and production support facilities can be incorporated in a rigid platform of steel or concrete. However such platforms can be expensive and the time taken to complete a facility can be excessive.

If the field size and/or water depth is such that a rigid platform would be uneconomic then an alternative is to have a floating, tethered production system. This is also favoured when early production is required.

The riser on a floating production system needs either to be supplied with sub-surface flotation or it needs to be held in tension if it is not to buckle under its own weight. Some floating production systems adopt the former approach, using subsea flotation and accepting the associated penalties of increased current drag forces and increased sub-surface mass.

The alternative approach is to hold the riser in tension. A motion compensating system is used at the surface to ensure that wave and tide induced vertical movements of the floating facility relative to the top end of the riser do not result in compression forces going onto the riser. The tensioning equipment is complex and typically consists of a combined pneumatic and mechanical dynamic arrangement. Such floating production systems have the operational disadvantage that the vertical motion of the floating facility can exceed the compensation capacity of the riser tensioning equipment. When this possibility is imminent the riser has to be disconnected at the seabed and held suspended from the floating facility until the sea state conditions improve. This is a difficult operation which could not be done in very deep water.

According to the present invention there is provided a floating buoy which is free to move independently of the riser in a vertical direction. The buoy is tethered with resiliently extendable tethers, being largely constructed of synthetic fibre or filament material. The tethers pass up into tethering engagement with the buoy and downwardly therefrom into connection with the riser, e.g., they may pass through the buoy and be connected to the top of the riser. The tethers can be re-tensioned by initial ballasting adjustments of the buoy to an extent that the vertical movements of the buoy relative to the riser are partly compensated for by the elasticity of the tethers. The riser is at all times kept under tension.

A specific embodiment of the invention wili now be described with reference to the Figs.

1 and 2 of the accompanying drawings.

Referring to Fig. 1, the group of rigid pipes which comprise the riser terminate at the top in a termination block (not shown) which can include compressible elastomeric elements to equalise tension between the pipes. The tethers are connected to the termination block by chains which pass over pulleys as shown. The buoy has a ballasting system including ballast tanks as the principal means of presetting the tension on the tethers and the riser. Individual tension equalising adjustments to a tether can be made by vertically altering the height of the associated pulley using a screw or other adjusting mechanism (not shown).

Individual tethers may have respective chain stop mechanism (not shown) to occasionally lock a chain to the buoy so that it can be released from or reconnected to the riser.

The installation and recovery of the tether involves the use of an external floating crane or derrick which picks up the free top end of the tether.

The vertical motions of the buoy are partially suppressed by the sympathetic extensions and contractions of the tethers. The corresponding tension changes in the tethers are experienced equally in the riser.

The rigid pipes may be connected at the top terminal block by flexible hoses to an adjacent production support vessel as shown in Fig. 2.

There may be an intermediate section of rigid pipes between the riser top termination block and the flexible hoses. In extreme weather conditions the production support vessel might shut down the well and disconnect from the buoy. In such a case the riser remains in place supported in tension from the buoy. The buoy has a basic emergency shut down equipment (not shown).

The chain pulleys of the illustrated buoy could of course be replaced by other forms of wheel or sheave or any other means for tethering engagement allowing for movement of the tether thereover when it extends and contracts in use; in all cases the tethering engagement means are preferably individually vertically adjustable.

The end of the tether connected to the riser need not be chain, and could for example be of steel cable, or similar to the main part of the tether.

Whilst it is preferred, as shown in Fig. 1, for the riser to extend up into a passage through the buoy, this is not absolutely essential. The buoy according to the riser preferably has guide means to limit non-vertical movement of the buoy relative to the riser.

The invention resides not only in the operative combination of buoy, tethers and riser, but also in the buoy per se adapted for use with the tethers as described and in the buoy and resilient tethers adapted for use with a riser as described.

LEGEND FOR FIGURES 1 AND 2 I : Riser 2 : Tether 3 : Chain 4 Pulley 5 Buoy 6 : Ballast Tanks 7 : Flexible Hose 8 : Production Support Vessel 9 : Central Passage 10 : Guide Passage

Claims

1. A hydrocarbon production buoy which can move vertically relative to an associated riser and is anchored to the sea bed by pretensioned resiliently extendable tethers which pass up into tethering engagement with the buoy and downwardly therefrom into connection with the riser to maintain the riser in tension.

2. A buoy according to claim 1, having guide means to limit non-vertical movement of the buoy relative to the riser.

3. A buoy according to claim 1 or 2, having means to selectively lock individual tethers to the buoy to facilitate the installation and removal of a tether at the riser.

4. A buoy according to any preceding claim, having means for adjusting independently the tension of individual tethers.

5. A buoy according to any preceding claim, wherein a riser top termination block comprises means to equalise the tension on individual rigid pipelines making up the riser.

6. A buoy according to any preceding claim, wherein the end of a tether connected to a riser is a chain or steel cable or of synthetic material similar to that of the remainder of the tether.

7. A buoy according to any preceding claim having an intermediate section of rigid pipes connecting a riser termination block to flexible discharge hoses.

8. A hydrocarbon production buoy substantially as hereinbefore described with reference to the accompanying drawings.

9. A hydrocarbon production buoy for maintaining an offshore riser under tension by means of resiliently extendable sea bed tethers, the buoy being substantially as hereinbefore described.

10. A production buoy and resiliently sea bed tethers therefore, the buoy and tethers being adapted to maintain an offshore riser under tension and being substantially as hereinbefore described.