Section 10 Fairleads and cable stoppers

10.1.1 Fairleads and stoppers are to be designed to permit free movement of the anchor line in all mooring configurations and designed to prevent excessive bending and wear of the anchor lines. The hardness of fairleads and chain stoppers where in contact with the anchor line should be softer than the anchor line. In general, the anchor line should not be in contact with any welds but where this is not possible the welds are to be ground flush and are to be softer than the anchor line.

10.1.2 The minimum operating range of the fairlead to be considered in conjunction with the design load is shown in Fig. 10.11.1.

10.1.3 Fairleads and stoppers and their supporting structures are to be designed for a mooring line pull load equivalent to:

• the mooring line maximum design load as defined from intact mooring load case (as defined in Pt 3, Ch 10, 4 Design aspects, Pt 3, Ch 10, 5 Design analysis and Pt 3, Ch 10, 6 Anchor lines) for a range of mooring line pull angles as substantiated by analyses (including a 5 degrees contingency)

and

• the maximum break strength of the main component (steel wire rope, chain or fibre rope), in “as new” condition, directly acting on or closest in the load path to the structure under consideration. The range of mooring line pull direction shall match that reported in Section Pt 3, Ch 10, 10.1 General requirements. The maximum break load is generally to be based on expected maximum break strength plus two standard deviations (when sufficiently document from manufactureŕs test data), otherwise not less than 110% of the nominal minimum break strength of the mooring line component.
  For this case, special consideration will be given to acceptance of local yielding and deformation of fairleads and stoppers when it can be shown:

  • The support structure to the fairlead or stopper satisfies the requirement ofPt 4, Ch 6, 1.1 General .
  • The deformation does prevent repair or replacement, does not otherwise affect the integrity of the overall hull, does not lead to progressive collapse, has no substantial consequences, such as, loss of life, uncontrolled outflow of hazardous or polluting products, collision, sinking.
  • The fairlead or stopper can be readily inspected and can be repaired or replaced offshore. Specific inspections, repair or replacement procedures are documented in IMR Manual and sparing policy ensures spares are readily and locally available.

The maximum permissible stresses for the design cases given in this sub-Section are to be in accordance with Pt 4, Ch 5, Pt 4, Ch 5, 2.1 General for the intact design load case and Pt 4, Ch 5, Pt 4, Ch 5, 2.1 General for the maximum break strength case. See also Pt 3, Ch 10, 10.1 General requirements and Pt 4, Ch. 6,Pt 4, Ch 6, 1.1 General.

10.1.4 Fairleads, stoppers and support structures shall also be assessed for the mooring line load from damaged mooring load case (as defined in Section Pt 3, Ch 10, 4 Design aspects,Pt 3, Ch 10, 5 Design analysis and Pt 3, Ch 10, 6 Anchor lines) for a range of mooring line pull angles as substantiated by analyses (including a 5 degrees contingency) The maximum permissible stresses for the design cases given in this sub-Section are to be in accordance with Pt 4, Ch 5, 2.1 General .

10.1.5 Materials and steel grades are generally to comply with the requirements given in Pt 4, Ch 2 Materials for primary structures.

10.1.6 Chain cable fairleads are to have a minimum of five pockets.

10.1.7 Wire rope fairleads are generally to have a minimum diameter of 16 times the wire rope diameter.

10.1.8 Special consideration will be given to permissible stresses where the chain is of downgraded quality. There have been cases of closing plates on the fairlead shaft coming loose due to corrosion of the threads of the securing bolts, resulting in serious damage to the fairlead arrangements and the complete jamming of the fairlead and chain. Consequently, the securing bolts should also be checked to ensure that the bolt material does not corrode preferentially should the sacrificial anode system fail to function in way of the fairlead.

10.1.9 For permanent mooring systems it is recommended that those lengths of the mooring lines which lie over a fairlead or other similar curved surface are not maintained for any extended period of time at the operating tension that would normally apply to the main part of the lines, but rather only for temporary line tension adjustments that might be necessary for inspection, maintenance or repair. It is generally preferable to have a suitably designed stopper holding the mooring line load outboard of the fairlead. Where applicable the long term detrimental effect of the wheel type fairlead action on the mooring line should be assessed and documented. Note: API RP 2SK etc.

10.1.10 Hawse pipes, guide pipes, or bend shoes and fairleads etc. used in the mooring system are to have adequate strength for the imposed loads. Detailed assessment of the interaction between these devices and mooring line chain or cable shall be documented. The design should take into account the inter-link locking mechanism and the loads required to align the device with the cable through swivel or articulation as well as the intermittent contact interaction in the area where the mooring line separates from the support or bearing surfaces. Hawse pipes or guide pipes when located inside tanks are to also be designed for sloshing forces). Close fit between mooring line and mooring line bearing arrangement shall be designed to minimise detrimental bending and stress concentrations in both mooring line and the mooring line bearing arrangement. The design shall ensure the bearing arrangement and mooring line bearing on it can be inspected and replaced in service.

10.1.11 Sensitivity of the design to the actual long term performance of the bearings are to be considered.

10.1.12 The fairlead, stoppers and bending shoes shall be protected against corrosion and designed such that their performances are not affected by corrosion.

Figure 10.10.1 Minimum operating range of fairlead