GB2337964A

GB2337964A - Trim correcting mooring system

Info

Publication number: GB2337964A
Application number: GB9912983A
Authority: GB
Inventors: Poul-Eric Christiansen
Original assignee: Kvaerner Oil and Gas Ltd
Current assignee: Kvaerner Oil and Gas Ltd
Priority date: 1998-06-05
Filing date: 1999-06-04
Publication date: 1999-12-08
Anticipated expiration: 2019-06-04
Also published as: GB9912983D0; GB2337964B; GB9812090D0

Abstract

A mooring system for a spar-type substructure 200, comprising means to create a restoring couple, by adjusting the vertical position a, b and c, at which a mooring force takes effect on the substructure. A mooring line 204 may be connected to the substructure by a bridle 203, having upper 205 and lower 206 elements, attached to the substructure, with means to adjust the relative lengths of the elements, thereby varying the effective mooring position. The elements of the bridle may pass through fairleaders 207 and 208, which may alter the relative lengths of the elements, through vertical movement or rotation (Fig 4). The mooring line may be connected to the bridle, by means of a ratchet type chain stopper (216, Fig 5), arranged to adjust the pretension in the mooring line, with respect to the substructure.

Description

TRIM CORRECTING MOORING SYSTEM 2337964 The invention relates to a mooring

system for deep draft floating substructures.

Deep draft floating substructures are currently being used to develop deepwater oil and gas reserves. The SPAR platform concept is a typical example of this type of substructure.

Several SPAR platforms have been installed in the Gulf of Mexico (e.g. Oryx Neptune).

Another concept with similar characteristics is the Kvaerner Multileg Deep Draft Floater (which is described and cJaimed in our UK Patent Specification No 2328408).

These types of substructures have relatively small water plane areas which do not contribute significantly to their stability. Instead, they rely for stability on their centre of buoyancy being above their centre of gravity. The drafts of these substructures are typically in the order of 100-300m.

Deep draft substructures are normally moored with a spread mooring system of the catenary or taut line type. A typical mooring system is shown in W095126294 to SIPM. Any steady state environmental loading imposed on the substructure (e.g. due to wind or current) will be reacted by the mooring system. However, due to the large vertical dimension of the substructure, the vertical distance between the applied load and the mooring reaction can be large. (The vertical distance can be adjusted, as shown in US4702321 to Horton.) This distance can induce a large overturning moment, resulting in a significant trim (out of vertical displacement) of the substructure. The trim may upset the performance of process systems, may be detrimental to human comfort, and may effect other performance parameters.

Shifts in the topsides loading can also produce an overturning moment, which may be difficult to compensate for and hence give rise to a trim on the substructure.

On normal vessels such as ship shaped hulls and semisubmersible platforms, trim is corrected by moving ballast water from one location in the vessel to another, so to create a moment equal and opposite to the imposed moment. However, deep draft substructures normally have only small dimensions in a horizontal plane, and hence short distances between any opposite ballast compartments. Due to the short lever arm available, very large amounts of ballast would.have to be shifted to compensate for the imposed moment. In most instances it is not practical to use ballast for trim correction. Instead, the substructures are fitted with additional solid ballast in their keel to increase the pendulum righting moment effect, and thereby restrict trim to within an acceptable limit.

This method of restricting trim results in large substructures which are more expensive to build, outfit and install, as compared to substructures which are allowed to take up a trim.

S102 Clearly, it is desirable to reduce trim in deep draft substructures, without increasing their size, weight and cost.

The present invention is concerned with the reduction of trim by the application of a restoring couple derived from mooring forces.

The invention provides a mooring system for influencing the trim of a spar-type substructure for use in the development of deepwater oil and gas reserves, the system comprising means to create a restoring couple by adjusting the vertical position at which a mooring force takes effect on the substructure.

It is preferred that the mooring forces are influenced in opposite senses on opposed sides of the substructure to exert a restoring couple.

It is also preferred that a mooring line is connected to the substructure by a bridle having upper and lower elements attached to the substructure, and in which there is means to adjust the relative lengths of the upper and lower elements of the bridle, whereby the force of a mooring line can take effect at different heights on the substructure.

In one preferred form, routing for one of the upper and lower elements of the bridle includes a fairleader arranged for vertical movement with respect to the substructure, such that the length of the element of the bridle passing round that faideader can be adjusted.

In another preferred form, the lengths of upper and lower elements of the bridle are individually adjustable.

In yet another preferred form, the bridle is arranged in a continuous loop between two faideaders, and there is means to move that part of the loop between the fairleaders, so to adjust the relative lengths of the upper and lower elements of the bridle.

Preferably, connection of a mooring line to the means to adjust the vertical position at which the mooring force takes effect is a ratchet type chain stopper arranged to adjust the pretension in the mooring line with respect to the substructure.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:

Figures 1 to 5 are diagrammatic side views of a spar platform with moorings according to the invention; and Figure 6 is a diagram of a ratchet type chain stopper.

The arrangements shown in Figures 1 to 5 control the lines of action of the mooring forces, and hence the effective attachment points on the substructure. In this way, the vertical reaction levels of individual mooring lines can be changed.

Figure 1 shows the consequence of moving the effective attachment point of a mooring line in a vertical direction. An elongate buoyant substructure 200 has a deck 201, and floats in a stable configuration with respect to a sea surface 202. The substructure has a mooring line S102 204, connected to the substructure by a bridle 203 having upper and lower elements 205 and 206. Vertically movable faideaders 207 and 208 attach the elements 205 and 206 of the bridle 203 to the substructure. (Similar reference numerals will be used in Figures 2 to 5) Vertical movement of the faideaders 207 and 208 can move the effective attachment point up (to a) or 5 down (to c).

Figure 2 shows details of one particular arrangement to move the effective attachment point of the mooring line. The mooring line 204 is connected to a bridle 203. Each element of the bridle is routed through a fixed faideader and up to a winch (211, 212) located above the sea surface. Each element of the bridle can therefore be controlled individually, and the connection of the mooring line to the bridle can be shifted in a vertical direction, thereby changing the vertical reaction level of the restoring force produced by the mooring line.

The effect of moving the effective attachment points on opposite mooring lines is illustrated in Figure 3. The mooring lines can be adjusted to create a couple which is equal and opposite to the wind overturning moment, although both mooring lines are attached below the level at which the wind load is applied. Moments introduced due to weight changes/shifts in deck loads can also be corrected for by adjusting the effective attachment points of opposite mooring lines until an equal and opposite moment is produced.

An alternative to routing both bridle elements up to the deck is to use a continuous loop 214 between two faideaders as illustrated in Figure 4. One of the faideaders is powered (for instance with electrical or hydraulic power 215) such that part of the closed loop is moved up or down thereby shifting the connection of the mooring line 204 in the vertical direction.

One area of interest for the bridle and continuous loop arrangement (in Figure 4) is the connection of the mooring line 204 to the bridlelcontinuous loop 203. Part of the connection is the chain stopper 216 arrangement illustrated in Figure 5 (and detailed in Figure 6). The mooring line has a chain at its top end. This chain is pulled through a ratchet type chain stopper attached to the bridle or continuous loop to for instance a work boat on the surface. Once the correct pretension is set, the excess chain can be cut by diver or remote operated vehicle.

S102

Claims

6.

1. A mooring system for influencing the trim of a spar-type substructure for use in the development of deepwater oil and gas reserves, the system comprising means to create a restoring couple by adjusting the vertical position at which a mooring force takes effect on the substructure.

A system as claimed in claim 1 in which the mooring forces are influenced in opposite senses on opposed sides of the substructure to exert a restoring couple.

3. A system as claimed in claim 1 or claim 2 in which a mooring line is connected to the substructure by a bridle having upper and lower elements attached to the substructure, and in which there is means to adjust the relative lengths of the upper and lower elements of the bridle, whereby the force of a mooring line can take effect at different heights on the substructure.

4. A system as claimed in claim 3 in which routing for one of the upper and lower elements of the bridle includes a faideader arranged for vertical movement with respect to the substructure, such that the length of the element of the bridle passing round that fairleader can be adjusted.

5. A system as cJaimed in cJaim 3 in which the lengths of upper and lower elements of the bridle are individually adjustable.

A system as claimed in cJaim 3 in which the bridle is arranged in a continuous loop between two faideaders, and there is means to move that part of the loop between the fairleaders, so to adjust the relative lengths of the upper and lower elements of the bridle.

7. A system as cJaimed in any one of the preceding claims in which the connection of a mooring line to the means to adjust the vertical position at which the mooring force takes effect is a ratchet type chain stopper arranged to adjust the pretension in the mooring line with respect to the substructure.

A mooring system substantially as hereinbefore described with reference to and as shown in any one of Figures 1 to 5.

S102 - A mooring system for influencing the trim of a spar-type substructure for use in the development of deepwater oil and gas reserves, the system comprising. m-cans to create a restoring couple by adjusting the relative veffical positions,at which mooring forces take effect on opposed sides of the substructure to exert the re storing couple.

2. A system as claimed in claim 1 in which a mooring line 'is connected to the substructure by a bridle having upper and lower elements attached to the substructure, and in which there is means to adjust the relative lengths of the upper and lower elements of the bridle, whereby the force of a mooring line can take effect at different heights on the substructure.

A system as claimed in claim 2 in which routing for one of the upper and lower elements of the bridle includes a fairleader arranged for veffical movement with respect to the substructure, such that the length of the element of the bridle passing round that fairleader can be adjusted.

4.

A system as claimed in claim 2 in which the lengths of upper and lower elements of the bridle are individually adjustable.

5. A system as claimed in claim 2 in which the bridle is arranged in a continuous loop between two fairleaders, and there is means to move that part of the loop between the fairleaders, so to adjust the relative lengths of the upper and lower elements of the bridle.

6. A system as claimed in any one of the preceding claims in which the connection of a mooring line to the means to adjust the vertical position at which the mooring force takes effect is a ratchet type chain stopper arranged to adjust the pretension in the mooring line with respect to the substructure.

7, A mooring system substantially as hereinbefore described with reference to and as shown in any one of Figures 1 to 5.