GB2085939A

GB2085939A - Marine mooring cables

Info

Publication number: GB2085939A
Application number: GB8028212A
Authority: GB
Original assignee: Sir McAlpine Robert and Sons Ltd
Current assignee: Sir McAlpine Robert and Sons Ltd
Priority date: 1980-09-01
Filing date: 1980-09-01
Publication date: 1982-05-06
Also published as: GB2085939B

Abstract

A marine mooring cable comprising an elongate tendon (2) maintained in tension by a continuous sleeve (6) extending over a length thereof, the sleeve (6) being formed by collar members (8) the endmost (10) of which are secured to and fixed relative to the respective adjacent portions of the tendon (2). The collar members are preferably of concrete and grouted. <IMAGE>

Description

SPECIFICATION Marine mooring cables This invention relates to marine mooring cables. A conventional marine mooring cable is subject in use to wide cyclic variation of applied tension under the action of e.g. one or more of wind, tide and waves, this variation being large compared to the average tension experienced. Accordingly a simple conventional mooring cable, if it is not to be subject to early fatigue failure, needs to be considerably stronger (i.e. for a given material and structure, of considerably greater cross-sectional area) than is necessary simply to withstand the maximum working tension under static conditions, so that the cable operates in practice at stress levels which are small compared to its static strength.

The present invention provides a marine mooring cable comprising a tensioned elongate tendon and extending over a length of the tendon a sleeve therefor formed by collar members (e.g. annular members such as rings or elongate cylinders) the end most of which are secured to and fixed relative to respective longitudinally spaced portions of the tendon, the tendon being normally maintained in tension by the sleeve which is normally maintained under compression by the tendon.

With the tendon maintained in tension even under no externally applied load, the minimum operating stress that it experiences is greater than that for a conventional cable (which may be substantially zero) so that the operating stress range in the tendon is reduced and the mean operating stress in the tendon is increased compared to the values for a conventional mooring cable, thus reducing susceptibulity to fatigue failure without use of a tendon much stonger than needed to take the maximum operating stress; within limits, the fatigue problem is transferred from the tension component to the compression component.

The collar members are best of a material having high tolerance to compression load variation, e.g. concrete. The concrete collar members would preferably be of length(s) suitable for factory precast production. One or both endmost collar members suitably comprises a steel or prestressed concrete section.

The composite cable is preferably protected against corrosion, e.g. by grouting with cement. Whatever the detailed construction, the sleeve preferably defines a watertight space around the tendon.

A cable according to the invention is illustrated in the accompanying drawing, in which: Figure 1 is schematic longitudinal sectional view of a cable according to the invention, and Figure 2 is a graph illustrating the distribution of forces between the tendon and sleeve of the Fig. 1 cable, assuming perfectly elastic behaviour.

The Fig. 1 cable has a tendon 2, e.g. cable of steel wires or strands, fixed at each end to the body of a respective shackle 4 by which it can be made fast respectively to an anchoring point (e.g. at the sea bed) and to a floating vessel. Surrounding the tendon 2 over substantially the whole of its length is a continuous sleeve 6 of abutting, grouted, precast concrete cylinders or rings 8. The end most members 10 of the sleeve (which may be of prestressed concrete or instead of steel) are fixedly secured to the adjacent tendon portions. The sleeve 6 is maintained under compression by prestressing of tendon 2 before end members 10 of sleeve 6 are secured thereto before the cable is put into service.

The distribution of forces between the tendon 2 and the sleeve 6, simplified by assuming completely elastic behaviour, is illustrated in Fig. 2. After prestressing, before applying an external force, the tension and compression forces are equal (Point A). When an increasing external force is applied to the cable, the tension in the tension increases and the compression in the annulus decreases until eventually, at Point B, the annulus is fully decompressed. Further increases in external force would then be carried entirely by the tendon (CD). This portion of the curve, implying joint opening, is outside the practical range and relates to forces exceeding the maximum design force.

The extent to which load variation is absorbed by the sleeve in preference to the tendon is determined by the relative stiffnesses of the components nd is indicated by the relative slopes of lines AC and AB in Fig.

2. The sleeve is preferably less compressible than the tendon is extendible, and furthermore is preferably of a material of lower elastic modules than the tendon material, concrete is especially preferred for construction of the sleeve.

1. A marine mooring cable comprising a tensioned elongate tendon and extending over a length of the tendon a sleeve therefor formed by collar members the end most of which are secured to and fixed relative to respective longitudinally spaced portions of the tendon, the tendon being normally maintained in tension by the sleeve which is normally maintained under compression by the tendon.

2. A cable according to claim 1 wherein the sleeve comprises concrete collar members.

3. A cable according to claim 2 wherein the collar members are grounted.

4. A cable according to claim 1 or 2 or 3 wherein at least one endmost collar member is of steel or prestressed concrete.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Marine mooring cables This invention relates to marine mooring cables. A conventional marine mooring cable is subject in use to wide cyclic variation of applied tension under the action of e.g. one or more of wind, tide and waves, this variation being large compared to the average tension experienced. Accordingly a simple conventional mooring cable, if it is not to be subject to early fatigue failure, needs to be considerably stronger (i.e. for a given material and structure, of considerably greater cross-sectional area) than is necessary simply to withstand the maximum working tension under static conditions, so that the cable operates in practice at stress levels which are small compared to its static strength. The present invention provides a marine mooring cable comprising a tensioned elongate tendon and extending over a length of the tendon a sleeve therefor formed by collar members (e.g. annular members such as rings or elongate cylinders) the end most of which are secured to and fixed relative to respective longitudinally spaced portions of the tendon, the tendon being normally maintained in tension by the sleeve which is normally maintained under compression by the tendon. With the tendon maintained in tension even under no externally applied load, the minimum operating stress that it experiences is greater than that for a conventional cable (which may be substantially zero) so that the operating stress range in the tendon is reduced and the mean operating stress in the tendon is increased compared to the values for a conventional mooring cable, thus reducing susceptibulity to fatigue failure without use of a tendon much stonger than needed to take the maximum operating stress; within limits, the fatigue problem is transferred from the tension component to the compression component. The collar members are best of a material having high tolerance to compression load variation, e.g. concrete. The concrete collar members would preferably be of length(s) suitable for factory precast production. One or both endmost collar members suitably comprises a steel or prestressed concrete section. The composite cable is preferably protected against corrosion, e.g. by grouting with cement. Whatever the detailed construction, the sleeve preferably defines a watertight space around the tendon. A cable according to the invention is illustrated in the accompanying drawing, in which: Figure 1 is schematic longitudinal sectional view of a cable according to the invention, and Figure 2 is a graph illustrating the distribution of forces between the tendon and sleeve of the Fig. 1 cable, assuming perfectly elastic behaviour. The Fig. 1 cable has a tendon 2, e.g. cable of steel wires or strands, fixed at each end to the body of a respective shackle 4 by which it can be made fast respectively to an anchoring point (e.g. at the sea bed) and to a floating vessel. Surrounding the tendon 2 over substantially the whole of its length is a continuous sleeve 6 of abutting, grouted, precast concrete cylinders or rings 8. The end most members 10 of the sleeve (which may be of prestressed concrete or instead of steel) are fixedly secured to the adjacent tendon portions. The sleeve 6 is maintained under compression by prestressing of tendon 2 before end members 10 of sleeve 6 are secured thereto before the cable is put into service. The distribution of forces between the tendon 2 and the sleeve 6, simplified by assuming completely elastic behaviour, is illustrated in Fig. 2. After prestressing, before applying an external force, the tension and compression forces are equal (Point A). When an increasing external force is applied to the cable, the tension in the tension increases and the compression in the annulus decreases until eventually, at Point B, the annulus is fully decompressed. Further increases in external force would then be carried entirely by the tendon (CD). This portion of the curve, implying joint opening, is outside the practical range and relates to forces exceeding the maximum design force. The extent to which load variation is absorbed by the sleeve in preference to the tendon is determined by the relative stiffnesses of the components nd is indicated by the relative slopes of lines AC and AB in Fig. 2. The sleeve is preferably less compressible than the tendon is extendible, and furthermore is preferably of a material of lower elastic modules than the tendon material, concrete is especially preferred for construction of the sleeve. CLAIMS

3. A cable according to claim 2 wherein the collar members are grounted.

5. A cable according to any preceding claim wherein the sleeve defines a watertight space around the tendon.

6. A marine mooring cable substantially as hereinbefore described with reference to Fig.

1 of the accompanying drawings.

7. A marine mooring cable substantially as hereinbefore described with reference to Figs.

1 and 2 of the accompanying drawings.