Section 10 Fairleads, bending shoes and stoppers

10.1.1 This Section presents the general requirements applicable to the design of fairleads, bending shoes and stoppers. Within the context of this sub-Section:

• Mooring lines are an assembly of loose mooring components (generally of standard design) connected together. One end is hooked up to and held onto the Offshore Unit via a stopper and may pass through a number of connecting components (e.g. stopper, bending shoe or fairlead etc), the other end is connected to an anchor point (generally on the seabed), mainly transferring tension between one end and the other. Mooring lines and their components can generally be readily connected to, disconnected from, and paid into or out of the offshore unit.
• Fairleads, bending shoes and stopper components consist of all components (generally articulated in relation to their supporting structures) in the load path between mooring line and support structure (generally articulated, or in contact with it but not welded to it) and ensuring the load transfer between mooring line components and their supporting structures as well as ensuring and accommodating the free rotation of the top end of the mooring line (in relation to their supporting structures) at its top end connection point.
• Fairleads, bending shoes and stopper supporting structures generally refer to welded plate and stiffened structures, fixed (generally welded) to the main hull, deck or turret hull structure of the offshore unit as well as part of the hull, deck or turret structure, the design of which is affected locally by loads imparted, by the mooring lines, and transferred through the fairleads, bending shoes and stoppers main components. Parts of the primary hull structure of the offshore unit (e.g. main hull, pontoons, columns, turret body, decks bulkheads or other similarly essential structures) may contribute to structural capacity of the supporting structures to Connecting components and their design shall comply with both the requirements of Pt 4, Ch 5 Primary Hull Strength and Pt 4, Ch 6 Local Strength and be consistent with design requirements of this Section.

10.1.2 Connecting components 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 connecting components in direct contact with mooring line components should be softer than the mooring line components. In general, the anchor line should not be in contact with any welds. Where this cannot be avoided, the welds are to be ground flush and softer than the mooring line (limited to low stress areas of the mooring line components) and an increase in wear allowance shall be considered and substantiated for the specific service conditions e.g. (potential chaffing).

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

1. Actions from the mooring line under maximum mooring line design load as defined from intact and one line failed mooring load cases (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 this load case:
   • The maximum mooring line design load shall be taken as the most probable maximum mooring line load of the most loaded line as defined from intact and one line failed mooring load cases (as defined in Pt 3, Ch 10, 4 Design aspects, Pt 3, Ch 10, 5 Design analysisand Pt 3, Ch 10, 6 Anchor lines).
   • The range of mooring line pull angles shall be as substantiated by analyses augmented by a five degrees contingency.
   • When the supporting structure is affected by loading from more than one mooring line (transmitted through fairlead, bending shoe or stopper), the design shall encompass the most onerous combination of loads from the other lines (when the most loaded line is subject to a load equal or greater than the most probable maximum load) as substantiated by the mooring analyses of corresponding intact and one line failed cases.
   • Support structure to fairlead, bending shoe or stopper shall fully satisfy the requirements on maximum permissible stresses for the design cases given in this sub-Section are to be in accordance with Load case (b) in Table 5.2.1 Factors of safety for the combined load cases in Pt 4, Ch 5, 2.1 General.
   • Fairlead, bending shoe and stopper components shall satisfy the requirements on allowable stresses based on factors of safety from Table 10.11.1 Design load cases, Load case 3 (with the exception of contact areas).
   • Fairlead, bending shoe and stopper components may be subject to very localised yielding limited to contact areas.
   • The design shall demonstrate that such yielding will impair neither their long term performance nor that of the mooring line component they act upon and that further to an initial shake down, further loading (as substantiated from mooring analyses) will not lead to further yielding. As such the design shall demonstrate that after yielding under the worst design load considered in this load case, an effective shake down is achieved ruling out any further detrimental yielding and ratcheting.
2. Actions from the mooring line under maximum break strength of the main line 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. For this load case:
   • The maximum break load is generally to be based on either the expected maximum break strength plus two standard deviations when substantiated by statistically significant test data from the manufacturer of the main line component or a load not less than 110 per cent of the nominal minimum break strength of the mooring line component it interacts with. For cases where the minimum break strength of such a mooring line component, is governed by cyclic fatigue damage rather than extreme or accidental (two lines failed) loads, and the component can be shown to be most unlikely (i.e. so unlikely it can be assumed it will not occur over the installation and service life) to be pulled to its minimum break strength. Special consideration will be given to limiting line pull to the minimum break strength of the mooring component next in line. This will be on the basis of a documented risk assessment taking into account the specific design arrangement, as well as service and mooring line hook-up and pretension phases.
   • The range of mooring line pull angles shall match that reported in Pt 3, Ch 10, 10.1 General requirements 10.1.2. For stoppers, special consideration will be given to limiting the range of mooring line pull angles to only encompass largest angles of pull from the two line failed load case and angles up to the most unlikely (i.e. unlikely but possible to occur once over the installation and service life) on the basis of a documented risk assessment, considering the specific design arrangement, as well as service and mooring line hook-up and pretension phases.
   • When the supporting structure is affected by loading from more than one mooring line (transmitted through fairlead, bending shoe or stopper), the design shall encompass the most onerous combination of loads from the other lines (as substantiated by the mooring analyses of corresponding intact and two line failed cases).
   • Support structure to fairlead, bending shoe or stopper shall fully satisfy the requirements on maximum permissible stresses for the design cases given in this sub-Section in accordance with Load case (d) in Table 5.2.1 Factors of safety for the combined load cases in Pt 4, Ch 5, 2.1 General.
   • The hull structure (offshore unit hull and deck or turret buoy hull etc.) is to be shown in compliance with all other Rules requirements as applicable. See also Pt 4, Ch 6, 1.1 General 1.1.6
   • Fairlead, bending shoe and stopper components shall satisfy the requirements on allowable stresses based on factors of safety from Table 10.11.1 Design load cases, Load cases 1 and 2 (exception made for local yielding areas).
   • Special consideration will be given to acceptance of local yielding and deformation of components of the fairleads, bending shoes and stoppers when it can be shown:
     • Local yielding and associated deformation prevent neither repair nor replacement of the components; affect neither the integrity of the support structure nor the main structure of the offshore unit (e.g. main hull, pontoons, columns, turret, decks, bulkheads or other structures otherwise essential to the integrity of the offshore unit), which are to be shown in compliance with all other Rules requirements as applicable. Local yielding and deformation shall not impair the long term performance of fairleads, bending shoes or stoppers or their supporting structures, and leads to neither progressive collapse, nor substantial consequences (such as, loss of life, uncontrolled outflow of hazardous or polluting products, collision, sinking).
     • Fairleads, bending shoes, and stoppers support structures are generally attached locally to the primary hull or deck structures. Primary hull or deck structures are generally subject to global loads and associated deflections. The design of fairleads, bending shoes, stoppers support structures shall account for the stiffness, loading and associated deflections of the primary hull or deck structures. As such it is recommended that the Finite Element model includes part of the primary hull or deck structures to which fairleads, bending shoes, stoppers support structures are integrated e.g. from a significant primary structural member, bulkhead, or other suitable boundary.
     • Fairleads, bending shoes, and stopper support structures should be designed to be readily inspected following documented specific inspection procedures.
     • Fairleads, bending shoes, and stopper components subject to yielding which could impair the long term performance of their arrangement should be designed to be readily inspected and repaired or replaced offshore (including when and where applicable after expected yielding and deformation). Inspection, repair or replacement procedures shall be documented in an Inspection, Maintenance and Repair Manual including sparing policy, ensuring spares are readily and locally available. Alternatively, prototype tests or pertinent empirical evidence shall further substantiate the capacity of the design to withstand the loading and associated yielding without the long term performance being significantly affected. The scope, procedure and criteria of such prototype tests shall be submitted to LR for review and acceptance. Actual prototype testing is to be witnessed by an LR Surveyor and the results reported to LR for review as part of the design appraisal process.
   • Fairleads, bending shoes, stopper components or associated support structures designed on the sole basis of finite element analyses shall have all main structural welds and areas anticipated by design to be subject to plasticity, subject to non-destructive examination as follows at manufacture:
     • 100 per cent visual.
     • 100 per cent MPI.
     • 100 per cent UT/radiographic, for full penetration welds.

Such examinations shall be performed under LR surveillance during manufacturing survey.

10.1.4 Non-linear finite element methods may be used to assess the structural capacity of components subject to plasticity and the circumstances under which they are able to withstand such yielding. Such a design methodology shall generally follow recognised recommended practice (with respect to such aspects as element types, mesh sizes, application of loads and constraints, boundary conditions, convergence, calibration material curves, imperfections, residual stresses etc.) and be submitted to LR for review and acceptance as part of the design appraisal. The non-linear finite element method shall take into account the following design requirements:

• Pertinent material true stress strain curves from recognised codes or standards, representative of the materials nominal properties and based on conservative idealisation, shall be referred to. Material testing shall confirm the validity of such design curves. When such a curve is developed for the purpose of a specific design, this should be based on sufficient tests to provide a reliable nominal characteristic value. Ramberg-Osgood material model is commonly referred to in such analyses.
• Monotonic material properties should be complemented with cyclic material properties both substantiated by test data demonstrating the conservatism of the model.
• Effective stress-strain shake down shall be substantiated by tests demonstrating that, after application of a load representative of the maximum design load (resulting in a greatest level of plasticity), cyclic plasticity rapidly reduces and the material re-exhibits an elastic behaviour in subsequent load cycles (taking due account of the loading expected in service).
• Such tests shall also be performed to confirm that any incremental plasticity resulting from representative in-service cyclic loading will not accumulate a level of plastic strains exceeding the strain criteria set for the design.

When performing tests to substantiate the properties of the models for a specific design, this should be based on sufficient tests data to provide reliable nominal values (generally based on mean offset by at least two standard deviations) and ensure actual values have less than five percent probability of being unconservative.

The methodology shall propose limits on plastic strain pertinent to the specific design. While a basic limit on plastic strain of five percent is often referred to in the industry and can generally be found appropriate, the proposed strain criteria shall be substantiated (either by reference to recognised and pertinent codes and standards or empirical evidence or more generally by test data) for the specific load case/failure mechanism highlighted above. The material and location of the yielding area in relation to welds, should take due account of cyclic loading effects, service loads and service conditions e.g. temperature and corrosion.

10.1.5 Materials and steel grades of the support structures are generally to comply with the requirements given in Pt 4, Ch 2 Materials for primary structures, except where highly stressed, where grades appropriate to ‘Special‘ structural category shall be considered. Similarly highly stressed fairleads, bending shoes and stoppers components shall be made of material and steel grades assigned to the structural category ‘Special‘ in compliance with Pt 4, Ch 2 Materials.

10.1.6 Chain cable fairleads are to have a minimum of seven pockets. Special consideration will be given to designs with five pockets only. Strength and fatigue design of chain links passing through and bearing on such fairleads shall take into account frictional and bending effects, as well as the additional damage due to their interaction with the fairlead. Increases in wear allowances shall be substantiated for such service conditions.

10.1.7 Wire rope fairleads are generally to have a minimum diameter of 16 times the wire rope diameter. Strength and fatigue design of the steel wire ropes passing through and bearing against such fairleads shall take into account bending and crushing effects and the additional damage due to the interaction with the fairlead. Increases in wear allowances shall be substantiated for such service conditions.

10.1.8 Special consideration will be given to permissible stresses where the chain is of downgraded quality.

10.1.10 Fairleads hawse or guide pipes and bending shoes interacting with the mooring line shall have adequate strength for the imposed loads. Detailed assessment of the interaction between these devices and mooring line chain links or wire rope shall be documented. Their design shall take into account the friction, inter-link locking mechanism and the side loads required to align the device (fairleads, hawse or guide pipes, bending shoes, fairleads) with the mooring line through swivel or articulation, as well as the intermittent contact and interaction in areas where the mooring line separates from their support or bearing surfaces. Close fit between mooring line components and bearing surfaces as well as their geometrical arrangement shall be designed to minimise detrimental wear, bending and associated stress concentrations in both mooring line and the bearing surface arrangement.

Chain or wire rope hawse or guide pipes when located inside tanks shall also be designed for sloshing forces. The design of fairleads hawse or guide pipes and bending shoes shall ensure the mooring line component passing through them and bearing surfaces (points of interaction) can be inspected.

10.1.11 Sensitivity of the design to the actual long term performance of the bearings is to be considered and the loads it may be subject to during its design life taken into consideration.

10.1.12 Fairleads, bending shoes and stoppers shall be protected against corrosion and designed such that their long term performances (throughout their design life) is not affected by corrosion.

Figure 10.10.1 Minimum operating range of fairlead

10.1.13 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.

10.1.14 Fairleads, bending shoes, stopper components and their supporting structures are to be designed for the cyclic loading they will be subject to over their design service life. Design service life of supporting structure shall be assessed using a method consistent with that applied to the hull structure, and that of the components shall be assessed using a method consistent with mooring line components.