Clasification Society Rulefinder 2016 - Version 9.25
Clasification Society Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, January 2016 - Part 3 FUNCTIONAL UNIT TYPES AND SPECIAL FEATURES - Chapter 10 Positional Mooring Systems - Section 7 Wire ropes

Section 7 Wire ropes

Parent topic: Chapter 10 Positional Mooring Systems

7.1 General

7.1.1 This Section applies to steel wire ropes for positional mooring systems of offshore units.

7.1.2 Wire ropes, associated fittings, are to be of an approved design.

7.2 Rope construction

7.2.1 When selecting a rope construction the following considerations apply:
  • Required service life.
  • Position in catenary.
  • Axial stiffness properties of rope.
  • Bending over sheaves, etc.
  • Characteristic torsional properties of rope construction.
7.2.2 Various rope constructions can be accepted for long-term mooring applications. These include:
  • spiral strand.
  • locked coil.
  • six-strand.

Other constructions can be considered.

7.3 Design verification

7.3.1 The design of wire rope and associated fittings is to be verified. The following information will be required for appraisal and information:
  • Plans of rope assembly, components and fittings such as terminations/sockets, bearings, pins and locking mechanism, bend stiffeners, electrical insulation and other fittings.
  • Materials specification covering all components and fittings, steel wires, steel fittings, pins, socketing resins, sheathing and blocking or lubricating compound).
  • Corrosion control specification (anodes, steel wire galvanisation, coating, electrical insulation, arrangement, and supporting calculations.
  • Design specification.
  • Purchaser’s specification.
  • Codes and Standards applied.
  • Calculations for the strength and fatigue of rope, socket, fittings, and their corrosion protection.
  • Dimensions of rope assembly and components and fittings as well as associated tolerances.
  • Weight properties and tolerances (to include weight per metre of main rope section inclusive of sheathing).
  • Axial and torsional stiffness data.
  • Wire rope datasheet (inclusive of all rope main characteristics, tolerances, handling and service limiting criteria).

7.3.2 Data from prototype rope tests is to be submitted (inclusive of material, construction and test procedures and data)..

7.3.3 Fatigue life calculations for steel wire ropes can be carried out in accordance with a recommended Code, e.g., API RP 2SK: Recommended Practice for Design and Analysis of Stationkeeping Systems for Floating Structures. Rope bending fatigue effects are to be included where relevant.

7.3.4 The minimum factors of safety on the calculated fatigue lives of wire rope and fittings are to comply with Pt 4, Ch 5, 5.4 Joint classifications and S-N curves 5.4.2 in Pt 4, Ch 5, 5.6 Factors of safety on fatigue life.

7.3.5 The rope termination including the socket, socketing arrangement and pin is to be designed to withstand a load of not less than the minimum breaking strength of the attached wire rope and be shown to withstand no significant plastic deformation under loading equal to 80 per cent of this load.

The rope is to be designed for the maximum design, storage and installation conditions specified for the project.

7.3.6 Pin locking mechanism, anodes and bend stiffeners attachment to the sockets should not be in the main load carrying path of the socket.

7.3.7 Sheathing should be able to match and accommodate the rope flexure and elongation under rope design and service loads without loss of integrity. The sheathing should not rotate in relation to the sockets and be designed for the maximum grip pressure under various tensions specified for the project.

7.3.8 Bend stiffeners and their connection to sockets shall be designed to protect the wire rope end termination from over bending under design loads (including storage, installation and service) for the range of off line pull angles or curvatures specified for the project.

7.3.9 Bend stiffener connections to sockets should be adequately protected against corrosion.

7.4 Materials

7.4.1 Steel wire used for rope manufacture is to be manufactured in accordance with a recognised National Standard:
  1. The steel is to be of homogeneous quality, consistent strength, and free from visual defects likely to impair the performance of the rope.
  2. The minimum tensile strength of the wire is to be the tensile strength ordered. The maximum tensile strength is not to exceed the specified minimum strength by more than 230 N/mm2. The tensile strength should normally be within the range 1420 to 1770 N/mm2.
7.4.2 The material used in the manufacture of sockets is to comply with the following requirements:
  1. Cast sockets:
    • Castings are to be manufactured and tested generally in accordance with Ch 4 Steel Castings of the Rules for the Manufacture, Testing and Certification of Materials (hereinafter referred to as the Rules for Materials).
    • As a supplement to Ch 4 Steel Castings of the Rules for Materials, impact tests are to be carried out at a test temperature of minus 20°C, to satisfy a minimum average energy requirement of 40J, with no more than one individual result from each three test specimens being less than 70 per cent of the required minimum average. Increased material toughness may be required in specific cases.
    • Alternative casting standards to Ch 4 Steel Castings of the Rules for Materials complying with recognised national or proprietary specifications may be accepted, see also Ch 4, 1.1 Scope of the Rules for Materials.
  2. Fabricated sockets:
  3. Socket pins:
    • Socket pins may be cast or forged. Where cast, material requirements are to comply with (a) above. Forged socket pins are to be manufactured in accordance with Ch 5 Steel Forgings of the Rules for Materials.
    • As a supplement to Ch 5 Steel Forgings of the Rules for Materials, impact tests are to be carried out at a test temperature of minus 20°C, to satisfy a minimum average energy requirement of 40J, with no more than one individual result from each three test specimens being less than 70 per cent of the required minimum average. Increased material toughness may be required in specific cases.
    • Alternative standards to Ch 5 Steel Forgings of the Rules for Materials, complying with recognised national or proprietary specifications may be accepted, see also Ch 5, 1.1 Scope of the Rules for Materials.

7.5 Corrosion protection

7.5.1 Wire ropes are to be protected against corrosion. The corrosion protection will normally consist of galvanising or other sacrificial coating of individual wires. Filler wires of zinc or other suitable sacrificial material can be incorporated in the outer layers of the rope, as an addition to, but not in place of, galvanising of individual wires.

7.5.2 Galvanising is to meet the following minimum standards:
  1. Zinc:
    • BS EN 1179.
  2. Zinc weight:
    • ASTM A 603 Table 5, Class A (spiral strand and locked coil).
    • ISO 2232, Quality B (six-strand ropes).
  3. Alternative recognised Codes or Standards providing acceptable equivalence will be considered.

7.5.3 Sockets are to be protected against corrosion by sacrificial anodes or acceptable equivalent.

7.5.4 Suitable arrangements are to be made to insulate the corrosion protected rope/socket assembly from adjacent non-protected elements in the system.

7.5.5 Polyethylene sheathing can also be used on appropriate rope constructions, as an addition to, but not normally as an alternative to, galvanising:
  1. Where sheathing is intended to be fitted, the specification is to be submitted. ASTM D 1248 is an acceptable specification for medium or high density polyethylene sheathing.
  2. A continuous strip of contrasting colour is to be incorporated into the sheathing to aid monitoring for twist. The position of the strip around the circumference sheathing in relation to a reference point on each end socket should be reported on plan.

7.5.6 Compound used as blocking or lubricating material shall as a minimum meet the requirements ISO 4346 Steel Wire Ropes for General Purposes - Lubricants - Basic Requirements or equivalent.

7.5.7 Compound must not adversely affect the long term integrity of the wires and sheathing.

7.6 Manufacture and testing

7.6.1 Steel wire ropes are to be manufactured in accordance with the design standards and procedures and at a works approved by LR. Ropes and fittings will be subject to LR survey during manufacture and testing.

A prototype testing programme is to be agreed with LR and carried out under LR survey. Prototypes are to be of the same materials, construction and termination unless specifically agreed with LR.

7.6.2 A certified ISO 9001/9002 Quality System is to be in place and a quality plan is to be produced and agreed with LR’s Surveyors.

7.6.3 Where sheathing is specified, it is to be carried out in accordance with the Quality Plan.

7.6.4 Cast sockets are to be manufactured and tested in accordance with the requirements of Pt 3, Ch 10, 7.4 Materials 7.4.2.

7.6.5 The following minimum requirements for the non-destructive testing of cast sockets are applicable:
  1. Ultrasonics: All areas of all sockets and pins.
  2. Radiography: Critical areas of first, last, and one intermediate socket selected by the LR Surveyor to be examined. Critical areas to be identified on design drawings, and these to be included in the design submission for verification.
  3. Magnetic Particle Inspection (MPI): 100 per cent of all sockets and pins.
  4. Visual: 100 per cent of all sockets and pins.

7.6.6 The material of plate fabricated sockets is to comply with Pt 3, Ch 10, 7.4 Materials 7.4.2 and welding and NDE to be in accordance with Pt 4, Ch 8 Welding and Structural Details. Post-weld heat treatment to be carried out for thicknesses exceeding 65 mm.

7.6.7 Tests are to be carried out on individual wires for the following:
  • Tensile strength and elongation.
  • Torsion.
  • Reverse bend.
  • Zinc coating; mass, uniformity and adhesion.

Tests are to be carried out in accordance with recognised National Standards such as ISO 2232, and ASTM A603, as appropriate.

7.6.8 Rope production samples are to be tested for the following:

  • Modulus.
  • Minimum breaking strength.
7.6.9 The tests required by Pt 3, Ch 10, 7.6 Manufacture and testing 7.6.8 are to be as follows:
  1. The modulus test is to be carried out on one finished rope sample taken from the first production length. Production sockets need not be fitted for this particular test. Load/extension characteristics and permanent stretch are to be determined and documented. Acceptance criteria for permanent stretch are to be as follows:
    • Maximum of 0,4 per cent for spiral strand and locked coil ropes.
    • Maximum of 0,8 per cent for six-strand ropes.

    The modulus of elasticity is to be calculated and documented. The basis for the calculated value (cross-sectional metallic area, or area of circle enclosing the rope) is to be clearly stated.

  2. Breaking load test is to be carried out on one sample taken from each manufactured length.
    • Where the rope design, the machine, and the machine settings are identical, consideration can be given to a reduction in the number of tests. As a minimum, breaking load tests are to be carried out on a sample taken from each of the first manufactured length, and one other length, selected by LR Surveyors.
    • Tests are to be carried out in accordance with a recognised National Standard such as EN 12385-1 Steel wire ropes – Safety – Part 1: General requirements (method 1).
    • One of the rope samples is to be fitted at one end with a socket taken from the project production batch, and socketed in accordance with approved procedures. Where more than one socket design type is involved, a further assembly is to be tested for each different type of socket.
    • The rope sample and the production socket is to withstand the specified minimum breaking load.

      The socket pin is to be able to be removed after the test, and replaced, without the application of undue force.

    • NDE to be carried out on the socket following testing (100 per cent visual and 100 per cent MPI).

7.6.10 Socketing is to be performed according to the quality plan and is to follow a tested and repeatable procedure drawing on ISO 17558 ‘Steel wire ropes - Socketing procedures - Molten metal and resin socketing’ and be carried out by experienced personnel.

Due attention should be paid to the following parameters:
  • rope termination brush configuration,
  • cleanliness of socket and rope brush,
  • resin mix and mixing technique,
  • brush and socket positioning and alignment,
  • resin pouring technique,
  • control of temperature and duration of curing,
  • scale effect.
7.6.11 The characteristic mechanical properties (e.g. modulus of elasticity, shrinkage, compressive strength) of the socketing resin shall be established from a number of test samples. Guidance on tests methods can be drawn from BS 6319 Testing of Resin Compositions for Use in Construction (especially Part 1, 2, 5 and 12).
  • resin compressive strength and modulus of elasticity (also ISO 604 Plastics – Determination of compressive properties).
  • shrinkage
  • density and hardness (also EN 59 Glass reinforced plastics – Measurement of hardness by means of a Barcol impressor).

The number of test samples is to be such as to establish the properties with sufficient confidence

7.6.12 Bend stiffeners are to be manufactured according to a quality plan and follow a qualified and repeatable procedure. Material properties (e.g. tear strength, elasticity, water absorption, aging) are to be reported.

7.6.13 The maximum allowable curvature of the wire rope under storage, service and installation conditions are to be substantiated and documented by the manufacturers.

7.6.14 Material properties (e.g. elasticity and water absorption) of the sheathing are to be documented. The sheathing should be manufactured such that it does not rotate in relation to the wire rope and sockets. The manufacturer is to substantiate and report curves of maximum allowable grip pressure under various tensions for the sheathed rope provided.

7.6.15 Complete rope assembly characteristics and associated tolerances are to be documented.

7.7 Identification

7.7.1 Each wire rope assembly is to be marked at each end with the letters LR and the Certificate Number.

7.8 Certification

7.8.1 A certificate is to be issued for each rope assembly by LR. The following is to be included in the Certificate:
  • Purchaser’s name and order number.
  • Description of order, including wire rope diameter and construction.
  • Tested minimum breaking load.
  • Design Appraisal Document Number.
  • Socket inspection certificate references.
  • Individual wire certificate references.
  • Sheathing report references.
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