GB2162557A - Mooring lines - Google Patents

Abstract

A mooring line comprises rope fibres (9) impervious sheaths (7, 8, 12, 13, 14) and a hollow (10, 11) extending along the length of the rope to be filled with a dense slurry to thereby increase the mass per unit length and stiffness of the rope. Thus a lightweight mooring line can be constructed for use as a catenary mooring line and the weight per unit length/stiffness of the mooring line can be increased at the location of use by filling the tubular element of the line with a slurry. <IMAGE>

Description

SPECIFICATION Mooring lines This invention relates to mooring lines and is particularly although not exclusively applicable to catenary mooring system for marine vehicles with particular applications to floating drilling and oil or gas production vesselsforthe offshore industry.

These vessels need to maintain position within an allowable tolerance above a subsea wellhead during drilling, oil and gas production or work over operations. In many locations, the design of a platform and its operability in heavy weather is severely limited by the capacity of the mooring system being used.

A typical catenary mooring system is shown in the side elevation and plan views of Figures 1 and 2 respectively. The vessel, 1, floating on sea surface, 2, is held on station by an eight leg configuration of mooring lines, 4. These lines could be made up of wire, rope, chain, synthetic fibre rope or a combination of these materials. The mooring lines take up a catenary shape, as shown, due to the combined effects of their own self weight and buoyancy.

The surface platform, 1, can be subject to forces from wind, ocean currents and the steady component of wave drift forces - all of which are acting to move the platform awayfrom its calm weather equilibrium position. These perturbing forces are reacted by restoring forces arising from the catenary mooring lines. These forces are induced by an increase in tension in the lines facing the weather as sections of the lines lift off from the sea bed due to the platform motion. Conversely, mooring lines on the leeward side of the platform are subject to a reduction in tension due to line sections settling on to the sea bed.The mooring line behaviour described above gives rise to a typical restoring force against vessel offset curves as shown in Figure 3 - the offset being defined as the horizontal displacement of the platform from its calm weather equilibrium position. The shape of the curve in Figure 3 generates a 'hardening spring' characteristic which generates low restoring force at low offsets (region 5 in Figure 3) with the restoring force increasing sharply at higher offsets (region 6).

Thus, oscillating platform motions within region 5 due to wave action are resisted by the mooring system to a negligible extent whereas the action of steady forces will drive the platform into region 6 of the curve where a substantial restoring force will develop.

The performance of conventional mooring systems for ships and floating platforms used in the offshore industry is limited by two conflicting factors. These are associated with the fact that mooring line performance is directly due to its self weight per unit length since it is the lifting off or settling on of line lengths at the sea bed which incudes the required restoring forces. On the other hand, lines of high self weight impose a large component of constant vertical force on the vessel, as well as, requiring that the vessel be capable of carrying heavy mooring lines when in transit between work locations.

Another general limit on mooring line design is due to the fact that only certain discrete materials are available for use. Despite the fact that different diameters of wire rope, chain links or synthetic fibre rope are available, the usable range of material properties and weights per unit length are very restricted from the point of view to try two optimise the design of mooring systems.

All catenary moorings suffer from performance limits for shallow and deep water applications. In shallow waters, the lines develop insufficient restoring force since line lengths lifting off or laying on the sea bed are small. However, a more significant limitation arises for deep water applications where the weight of the required lines can exceed the breaking strengths of the lines themselves or load carrying capability of the surface platform both during operation and when in transit.

Thus, the ideal performance goal for a catenary mooring system would be to generate a required restoring force against offset characteristic, like that shown in figure 3, while at the same time minimising the penalties due to platform weight carrying capability and line material choice.

In recent years, the availability of synthetic fibre ropes or aramid fibres has offered an alternative to wire rope or chain for mooring lines. This man made fibre rope has the advantage of very high breaking strengths but for catenary line applications, it suffers from the major disadvantage of having very low submerged weight per unit length. This latter characteristic gives man made fibre rope a very poor performance when used as the only mooring line component in a catenary system.

This invention provides a mooring line comprising a rope having a tubular element coupled thereto and extending along the length of the rope for receiving a flowable material to increase the mass per unit length/stiffness of the rope.

More specifically the submerged weight per unit length of such ropes can be substantially boosted in a controlled manner by constructing the rope with two components -- one being the synthetic fibre bundle for providing the necessary breaking strength whereas the second is a hollow section, either internal or external to the rope, which can be filled with heavy mud slurries of controllable density to generate a desired weight per unit length.

Such features avoid the material selection problem in the design of catenary moorings and allow designs to be very closely tailored to generate required restoring force/offset characteristics. The aspects of adjustable weight per unit length and bending stiffness are developed separately below: Figure 4 shows these typical mooring line crosssections. Figure 4 (a) shows a synthetic fibre rope with an internal fluid carrying hollow, 10, surrounded by an impervious sheath, 8. The synthetic fibres 9 are themselves shrouded by an external protective sheath, 7. Figure 4 (b) shows an external fluid carrying section 11, which can be fitted with spacers at intervals or filled with a very porous spongy material to prevent collapse of the external sheath.Figure 4 (c) illustrates the use of parallel, synthetic fibre bundles, 9, and the fluid carrying section, 11-these being surrounded by impervious sheaths 13 and 14 respectively-the whole being bound together by an external covering, 12.

In order to achieve required weights per unit length for the line elements, these can be filled with mud slurries which can be formulated with high but known densities to give the required line weight per unit length. Such muds have been commonly used for many years as drilling lubricants within the offshore industry. Use of mud slurries has the substantial advantage that the weight of these need not be carried on the vessel during transit. The dry slurry material can be carried on the vessel, mixed with sea water and pumped into the mooring lines during installation. Thus a large proportion of the effective line weight per unit length used need not be carried on the vessel deck during transit. The slurry material can be stowed below deck in semisubmersible vessels which are not sensitive to carrying weight in such areas.Either of the three configurations shown in Figure 4 can have the mud slurry pumped into section 10 or 11 during or just after line installation.

The problems caused by slurry settlement can be eased by incorporating flushing or agitator lines (15, in Figure 4 (c)) within the cross-section.

It is known that pressurisation of a fluid filled pipe can lead to changes in the effective tension within it which manifests itself as a change in the pipe's apparent bending stiffness. Thus, pressurisation of the mud slurry in cavities 10 or 11 of the pipe configuration shown in Figure 4 will lead to changes in the behaviour of the lines which can be used to enhance mooring performance.

This pipe pressurisation can be used in two ways.

In the first scenario, a mooring system design can be used with a specified mud slurry density and mud pressurisation to give the desired restoring force against offset characteristic.

Alternatively, the pressurisation can be rendered active such that sensor indication of vessel offset can be used to vary the pressurisation of the lines around the vessel to yield an improved restoring force/offset curve.

There are a number of surface vessel equipment requirements that have to be satisfied if the light weight mooring system described here is to be used.

The first of these is concerned with rope handling equipment during mooring line installation and retrieval. Such equipment needs to have associated facilities for mixing and pumping the mud slurry as well as for pressurising the mixture, if required.

Mooring line cross-sections of the type shown in Figure 4 can be designed to ensure that the fluid filled sections 10 and 11 are collapsible for ease of handling during installation and retrieval.

In specialised mooring applications, the linking of measured vessel offset to a control of the pressure levels in mooring lines will require purpose built electronic controliers and mechanical pressure regulators.

A part of the above requirements can be met by modified conventional mooring equipment.

It should be pointed out here that the mooring line elements described above can be used in the same line with more conventional wire rope or chain segments. In this case, appropriate connectors will be required to terminate the light weight line and continue to wire rope or chain.

; It is felt that use of light weight mooring lines as described above lead to the following advantages: (a) Mooring effectiveness can be achieved with low vessel transit payloads.

(b) Control of line weight per unit length and internal pressurisation allows an efficient mooring design with smaller vessel offsets than conventional systems.

(c) The flexibility available in mooring line property definition allows operation in a wider water depth and environmental conditions range while at the same time extending the water depths at which acceptable mooring can be achieved.

Claims (14)

1.A A mooring line comprising a rope having a tubular element coupled thereto and extending along the length of the rope for receiving a flowable material to increase the mass per unit length/ stiffness of the rope.

2. A mooring line as claimed in claim 1 wherein the rope and tubular elements are arranged coaxially.

3. A mooring line as claimed in claim 2 wherein the tubular element is arranged coaxially within the rope.

4. A mooring line as claimed in claim 3 wherein the rope has an encircling outer protective sheath.

5. A mooring line as claimed in claim 3 or claim 4 wherein the tubular element has an impervious wall to contain the flowable medium coaxially within the rope.

6. A mooring line as claimed in claim 1 or claim 2 wherein the rope is located coaxially within the tubular element.

7. A mooring line as claimed in claim 6wherein the rope has an encircling sheath and the tubular element is defined between that encircling sheath and an outer sheath defining a capacity between said sheaths to receive flowable medium.

8. A mooring line as claimed in claim 7 wherein means are provided to space the inner and outer sheaths apart to maintain the tubular cavity between the sheaths.

9. A mooring line as claimed in claim 8 wherein the spaced means comprise spacer elements between the inner and outer sheaths or porous spongy material disposed between the inner and outer sheaths.

10. A mooring line as claimed in claim 1 or claim 2 wherein the rope and tubular elements are secured together side-by-side one another.

11. A mooring line as claimed in claim 10 wherein the rope and tubular elements are secured together by an encircling covering.

12. A mooring line as claimed in claim 10 or claim 11 wherein the rope is encircled by an impervious sheath and the tubular element is defined by a further impervious sheath.

13. A mooring line as claimed in any of the preceding claims wherein the rope is a synthetic fibre rope.

14. A mooring line as claimed in claim 13 wherein the synthetic fibre is an aramid fibre rope.

14. A mooring line as claimed in claim 13 wherein the synthetic fibre is an aramid fibre rope.

15. A mooring line as claimed in any of the preceding claims wherein the tubular element is filled with said flowable medium under pressure to stiffen the tubular element and thereby the rope.

16. A mooring line as claimed in any of the preceding claims wherein the flowable medium with which the tubular element is filled is a dense slurry.

17. A mooring line as claimed in any of the preceding claims wherein additional pipelines can be provided for delivering a liquid for flushing or agitating the flowable medium in the tubular element.

18. A mooring line as claimed in any of the preceding claims wherein means are provided for varying the pressure applied to the flowable medium in the tubular element to vary the stiffness of the mooring rope as required.

19. A mooring line substantially as described with reference to and as illustrated in Figure 4a, Figure 4b or Figure 4c of the accompanying drawings.

New claims or amendments to claims filed on 31.7.85.

Superseded claims 1-14 New or amended claims CLAIMS

1. A mooring line comprising a rope having a tubular element coupled thereto and extending along the length of the rope and filled with a liquid based slurry to increase the mass per unit length/ stiffness of the rope.