Section 6 Anchor lines

6.1 General

6.1.1 Anchor line length is to be sufficient to avoid uplift forces occurring at the anchor point for damaged condition loads, unless the anchor point is specially designed to accept a vertical component of loading.

6.1.2 An anchor line integrity monitoring system or device is to be provided for floating unit mooring systems, to detect line breakage and significant tension and offset irregularities under ambient environmental conditions as well as more severe storms within the envelope of design environmental conditions.

The mooring line integrity monitoring system shall be able to detect failure of any part along the line (between attachment point to Offshore Unit to at least the seabed touch down or embedment point.

Ability to detect line failure beyond seabed touch down or embedment should be assessed and documented. The results should be taken into consideration when setting the scope of Offshore In Water Survey.

The precision and accuracy of the system is to be documented for a load range up to at least 90% of the breaking strength of the mooring lines and 100% of the offset range.

Detection of tension anomalies or line breakage is to raise an alarm (at least visual). The system should be able to be interrogated on demand and present sufficient redundancy so that the system remains operational after failure of any one component and to enable inspection or testing, maintenance and repair without loss of operability.

Calibration checks are to be carried out at least once a year.

Calibration and maintenance procedures and schedule are to be documented in the Operation Manual of the unit.

This is generally not a requirement for offloading buoy systems.

6.1.3 Specific steel wire rope, chain and fibre rope design requirements can be found in Pt 3, Ch 10, 7 Wire ropes, Pt 3, Ch 10, 8 Chains and Pt 3, Ch 10, 9 Fibre ropes respectively.

6.1.4 In general, the break strength of the anchor line is not to be greater than the load bearing capacity of the connecting structure.

For chain or rope loose fittings, sockets, shackles, connectors etc. the design shall be based on mooring line pull at least equal to the as new nominal minimum break strength of the mooring line main component (steel wire rope, chain or fibre rope) applying a minimum contingency factor of 1.1.

For fairleads and stoppers see Pt 3, Ch 10, 10 Fairleads and cable stoppers. For support structure see Pt 4, Ch 6, 1 General requirements

6.1.5 In general the mooring analyses should provide all of the loading parameters required for the detailed design of the mooring lines components and the associated supporting structures they interact with (pad-eyes, fairleads, bend shoes etc). The detailed structural or mechanical design of complex or non-standard (e.g. special D-shackles with dimensions not conforming with ISO 1704, or special connectors) component is generally substantiated by finite element calculations. Suitable elastic plastic models need to be used to model elastic plastic behaviour (e.g. Ramberg-Osgood law) at the contact points. Convergence should be demonstrated for the large displacement nonlinearities, contact related nonlinearities as well as nonlinear material properties. Alternatively elastic analysis is also acceptable.

The detailed design calculations of components should address both strength and fatigue aspects. For fatigue calculations principal stresses at the model mesh are to be refined at hot spots locations and at surface of the modelled component to ensure characteristic mean principal stresses in the surface plane are captured.

Special non-standard mooring components shall be designed so that local yielding only occur for a few load cycles imparting a shake-down effect after which no further yielding occurs. The analysis shall be based on cyclic material properties and cyclic loading shall demonstrate an effective shakedown after few cycles.

Deformation under design loads from (intact and one line damage case) shall not adversely affect the performance of the component.

Conservative plastic strain and stress curve and characteristics plastic strain limit shall be reported for the selected material with reference to recognised code or standard and substantiated by material test records.

6.1.6 Kenter links are not permitted on long term permanent mooring systems. Connectors purposely designed (for project specific strength and fatigue loading) (e.g. H-Links) and manufactured under LR Survey shall be preferred.

6.1.7 Locking mechanisms of pin parts of mooring line component connections on long term positional mooring systems should be redundant and not be located within the main load path.

6.2 Factors of safety – Strength

6.2.1 Minimum factors of safety applicable to the steel wire rope, chain and polyester anchor lines of moored floating units are given in Pt 3, Ch 10, 6.2 Factors of safety – Strength 6.2.1. For fibre ropes, see Pt 3, Ch 10, 9.2 Design aspects 9.2.2.

Table 10.6.1 Minimum factors of safety for anchor lines for floating offshore installations at a fixed location

Design case	Factor of safety, see Note 2
	Intact	Damaged
Extreme storm, or maximum environment, with floating unit attached	1,67 see Note 1	1,25 see Note 1
NOTES
1. The factors of safety given in this Table are associated with the following conditions:
(a) Arrangements being available to shut down production and/or transfer of oil or gas through risers in event of anchoring system failure.
(b) The floating unit being located in an open sea area. Special consideration will be given to factors of safety when the unit is in close proximity to another installation, or is located near the shore.
2. Factor of safety = Anchor line minimum breaking strength basis to be documented. A reduction factor may require to be applied to the standard assigned minimum breaking strength of anchor line components in some cases (e.g., where component test database is small: for non-standard components), or where anchor line components are not new.
3. Maximum tension to be based on assessment by dynamic analyses. See also Pt 3, Ch 10, 5.5 Combination of low and high frequency components – Design values 5.5.3 on maximum tension.

6.2.2 Factors of safety applicable to the steel wire rope, chain and polyester anchor lines of offshore loading buoys (CALMs, turret mooring buoys which may remain temporarily disconnected without mooring line integrity monitoring etc.) are given in Pt 3, Ch 10, 6.2 Factors of safety – Strength 6.2.4. For generic fibre ropes, Pt 3, Ch 10, 9.2 Design aspects 9.2.2.

6.2.3 PM notation (including PM TA(1), PM TA(2) and PM TA(3)). Minimum factors of safety applicable to steel wire rope and chain anchor lines for mobile offshore units are given in Pt 3, Ch 10, 6.2 Factors of safety – Strength 6.2.4.

6.2.4 PMC notation (including PMC TA(1), PMC TA(2) and PMC TA(3)). Minimum factors of safety applicable to steel wire rope and chain anchor lines for mooring system for mobile offshore units analysed quasi-statically and dynamically are given in Pt 3, Ch 10, 6.2 Factors of safety – Strength 6.2.4and Pt 3, Ch 10, 6.2 Factors of safety – Strength 6.2.4 respectively.

Table 10.6.2 Minimum factors of safety for anchor lines of offshore loading buoys

Design case	Factor of safety
	Intact	Damaged
Extreme storm, or maximum storm condition with ship attached	1,85	1,35
NOTES
1. For special cases where allowable offset criteria for risers cannot be met in a Damaged Case (single line break) (e.g. in offshore loading buoy systems for shallow water), the Damaged Case can be omitted in design and an increased intact factor of safety applied. A minimum factor of safety of 2,3 is to be applied in this case. Failure of any one mooring line should still be shown not lead to progressive collapse or incidents of substantial consequences such as loss of life, uncontrolled outflow of hazardous or polluting products, collision, sinking. 2. Maximum tension to be based on assessment by dynamic analyses. See also Pt 3, Ch 10, 5.5 Combination of low and high frequency components – Design values 5.5.3 on maximum tension.

Table 10.6.3 Factors of safety for PM notation

Design case	Description	Factors of safety for PM notation, see Note 1
Design case	Description	Quasi-static analysis	Dynamic analysis
1	Operating (Intact)	2,7	2,3
2	Survival (Intact)	1,8	1,5
3	Operating (Single line failure)	1,8	1,5
4	Survival (Single line failure)	1,25	1,1
NOTES
1. The factors of safety given in this Table apply to units positioned at least 300 m from another unit.
2. The unit is to be positioned to avoid contact with another unit in any of the design cases.
3. See also Pt 3, Ch 10, 5.5 Combination of low and high frequency components – Design values 5.5.3 on maximum tension.

Table 10.6.4 Factors of safety for PMC notation - Quasi-static analysis

Design case	Description	Factors of safety for PMC notation Quasi-static analysis, see Notes
		Unit moored 50 m or less from other structures		Unit moored within 50 to 300 m from other structures
		Critical line	Non-critical line	Critical line	Non-critical line
1	Operating (Intact)	3,0	2,7	3,0	2,7
2	Survival (Intact)	—	—	2,0	1,8
3	Operating (Single line failure)	2,0	1,8	2,0	1,8
4	Survival (Single line failure)	—	—	1,5	1,33
NOTES
1. See also Pt 3, Ch 10, 5.4 Analysis
2. The unit is to be positioned to avoid contact with another unit in any of the design cases.
3. See also Pt 3, Ch 10, 5.5 Combination of low and high frequency components – Design values 5.5.3 on maximum tension.

Table 10.6.5 Factors of safety for PMC notation - Dynamic analysis

Design case	Description	Factors of safety for PMC notation Dynamic analysis, see Notes
		Unit moored 50 m or less from other structures		Unit moored within 50 to 300 m from other structures
		Critical line	Non-critical line	Critical line	Non-critical line
1	Operating (Intact)	2,5	2,3	2,5	2,3
2	Survival (Intact)	—	—	1,65	1,5
3	Operating (Single line failure)	1,65	1,5	1,65	1,5
4	Survival (Single line failure)	—	—	1,35	1,2
NOTES
1. See also Pt 3, Ch 10, 5.4 Analysis.
2. The unit is to be positioned to avoid contact with another unit in any of the design cases.

6.3 Fatigue life

6.3.1 The fatigue life of the main components in the positional mooring system are to be verified. Calculations are to be submitted.

6.3.2 Where applicable tension bending effects are to be considered in the fatigue calculations of the mooring line at the fairleads and stoppers (or at any point within the line where it is subject to a constraint resulting in local bending). The detailed methodology shall be reported and agreed with LR in the early stages of the design. Contingencies should be included to address any uncertainties.

Torsion in the mooring line shall be avoided by design. In cases where this is not possible the performance of the component under such loading regime should be substantiated by a qualification programme agreed with LR.

Note: For top chain connections to stoppers, guidance can be drawn from publications from recent joint industry research projects on Fatigue of Top Chain of Mooring Lines due to In-Plane and Out-of-Plane Bending). Details of the methodology shall be reported and agreed with LR at early stage of design. The associated scope of manufacturing and testing shall be agreed with LR. The bushing performance shall be well documented and substantiated by adequate prototype testing and confirmed by factory acceptance tests. The design shall include contingencies to address any uncertainties (e.g. long term performance of bushing, bush and interlink friction coefficients etc.).

Applicable factors of safety shall be agreed with LR, after review of the detailed design methodology (else the default is 10).

6.3.3 Fatigue life calculations for anchor lines can be carried out in accordance with a recognised Code, e.g., API RP 2SK: Recommended Practice for Design and Analysis of Station keeping Systems for Floating Structures.

Note Where various wind driven wave and swell (potentially multiple) regimes prevail concurrently, the fatigue assessment shall be shown to account for these environmental characteristics and conservatively capture the various peak frequencies and relative directionalities.

6.3.4 Consideration will be given to the use of alternative methods, detailed proposals are to be submitted and agreed with LR.

6.3.5 The minimum factors of safety on the calculated fatigue lives for components of the mooring system are to comply with Pt 4, Ch 5, 5.4 Joint classifications and S-N curves 5.4.2 in Pt 4, Ch 5 Primary Hull Strength .