Section 1 General anchor requirements

1.1.1 This chapter relates specifically to the following anchor types for floating offshore installations at a fixed location: high holding power (HHP) drag embedment anchors, driven and drilled and grouted pile anchors, suction installed pile/caisson anchors and gravity base anchors. Other anchor types will be specially considered. It also relates to the use of catenary and taut mooring line configurations.

1.1.2 An anchor point is considered to consist of the anchor itself and the chain or mooring line embedded in the seabed.

1.1.3 The anchor design shall be based upon the expected site and seabed conditions at the proposed anchor point locations. The anchor type, dimensions, weight and other characteristics are to be determined by their ability to develop sufficient vertical, lateral and torsional capacity to resist the design loads with appropriate factors of safety based on a working stress design approach; except where stated.

1.1.4 The following information is to be submitted:

• Data, calculations and analysis supporting the selection of anchor.
• Anchor details.
• Proposed test loading or line pull in loads at installation.

and in addition for floating offshore installations at a fixed location:

• Soils data for the anchor locations.

1.1.5 Anchor design using a load and resistance factor design approach will be specially considered.

1.1.6 Consideration could be given to performing special tests, such as centrifuge model tests, to provide a better understanding of anchor behaviour

1.1.7 Installation tolerances on anchor pile orientation and verticality shall be defined during the design process and accounted for in capacity calculations and anchor acceptance.

1.1.8 Consideration is to be given to the anchor installation tolerances on verticality and orientation when designing the connection between the anchor line and the anchor.

1.1.9 The connection between the anchor line and anchor is to be designed so as to minimise disturbance to the seabed soils during pile installation, as this could reduce the axial and lateral resistance provided by the anchor. Any reduction in anchor capacity is to be taken into account.

1.2.1 Geotechnical design shall take account of the nature of the loading placed upon the anchors as defined by Pt 3, Ch 10 Positional Mooring Systems.

1.2.2 It should be noted that the static load case, as defined within these rules, contains an element of dynamic loading.

1.2.3 The possible variation in inclination of the applied loading to the foundation is to be taken into account. The envelope of maximum and minimum axial, lateral and torsional loads should be determined based upon the possible range of chain angles, verticality and orientation of the anchor upon installation.

1.2.4 Except for drag anchors, the effective weight of the anchor should be accounted for in the analyses. In general, this should be added to the anchor loads.

1.3.1 Sufficient and professional oversight shall be applied to the setup, configuration, verification and acceptance of a vessel’s surface and sub-surface positioning systems where used to compute the real-time absolute positions of anchors and locations on the seabed.

1.3.2 Full consideration shall be made to ensure that current operations and procedures, together with all previous survey and site data information, references a single and consistent geodetic datum in terms of spheroid, datum and projection.

1.3.3 Where surface positioning is undertaken using GNSS systems, operations should be undertaken in alignment with IMCA / OGP 015. Alongside tests and checks should be undertaken in port to verify system setup and achievable accuracy at the computational point (e.g. stern roller or crane location.)

1.3.4 Where sub-surface positioning is undertaken using USBL systems, operations should be undertaken in alignment with IMCA / OGP 017. Verification and offshore calibration operations may be required dependent upon installation tolerance.

1.4.1 Estimated values of anchor point movements in service are to be submitted together with the basis and details of the calculations made. These estimates are to account for the difference between any installation test load or mooring line pull-in load and the loading the anchor may be subjected to in-service.

1.4.2 The anchor points are to be so designed that their movements remain within tolerable limits. Where applicable other factors such as the effect of reservoir subsidence and post-seismic induced settlement should be considered.

1.4.3 The potential for the introduction of mooring line slack to the system (from chain cut in during storm loading for example) should be considered. Where applicable, estimates should be made of the amount of slack generated and checks performed to ensure the estimated slack can be accommodated by the mooring system.

1.4.4 The definition of tolerable limits on anchor point movement is to take into account factors such as allowable line slack, mooring line angle at fairlead, ability to remove line slack in-service, proximity to other subsea infrastructure, effect on other connected infrastructure such as risers. Other factors may also affect the definition of tolerable limits on anchor point movement.

1.5.1 Structural strength of the anchors is to be checked for intact, damaged and installation cases in accordance with Pt 3, Ch 10 Positional Mooring Systems or a recognised structural design code. Where necessary a detailed finite element stress analysis is to be carried out.

1.5.2 A fatigue damage assessment shall be performed for both the anchor and connection between the anchor line and anchor taking into account stress ranges due to environmental loading. Particular attention should be given to any stiffening arrangement of the connection between the anchor line and pile.

1.5.3 For driven anchor piles the effects of driving shall be taken into account in the fatigue damage assessment.

1.6.1 The geotechnical design of anchors should follow the requirements within these rules and can be performed in accordance with industry recognised methods such as those contained within the latest revision of API RP 2SK, ISO 19901-7 or a similar internationally recognised standard.

1.6.2 Analysis of the anchor/soil interaction under design loading is to take account of the non-linear stress/strain behaviour of the foundation soils, stress history and cyclic loading effects on soil resistance.

1.7.1 The influence of erosion of soils from around and beneath the anchor is to be taken into account in its design. Erosion due to the following causes is to be investigated:

• The effect of waves and currents passing over the seabed at velocities sufficient to dislodge and transport particles of bed materials (scour and sand waves)
• The relief of hydraulic pressures and pore water pressures built up under the foundation due to environmental loading, which may cause the removal of soil from beneath the foundation (sub-surface erosion or piping).
• The effect of interaction between the mooring line and the seabed, for example trenching caused by buried chain around suction anchors.

1.7.2 As per ISO 19901-4, scour is generally considered to constitute global and local components and seabed level change.

Where appropriate these parameters should be defined. Once these parameters are defined they should be considered within the definition of scour inspection frequency periods, acceptance criteria and trigger points for remedial action.

1.7.3 The methods proposed, for the prevention of and/or protection against erosion, are to be submitted for approval. Options for scour protection include skirts, rock dump, grout bags and frond mats.

1.7.4 Any erosion protection system laid on the seabed is to be designed that it will permit free dissipation of pore water pressures that may be generated in the surface soil under cyclic loading conditions.

1.8.1 Where the anchor point is located on or near a slope, the influence of the slope on the anchor is to be considered.

1.8.2 The possibility failure of the slope due to wave or earthquake loading is to be investigated.

1.8.3 The results of any calculations or tests are to be submitted.

1.9.1 Where appropriate, the influence of earthquake loading on the anchor point stability is to be fully accounted for in the design in relation to the particular site conditions. This assessment is to consider the site response, potential for seismic liquefaction and any other aspects that may influence anchor points such as slope stability.

1.9.2 Where appropriate, the possibility of post-seismic induced settlement and its magnitude should be accounted for during anchor design/selection.

1.9.3 Seismic design and seismic criteria should be considered in accordance with the latest revision of ISO19901-2.

1.10.1 The site investigation and subsequent design should take into account the presence of unconventional soils, such as those listed in ISO19901-8. It should be recognised that design methods that have been developed and used for design of offshore anchors in conventional soils may not be applicable to unconventional soils and that further investigation and testing may be required. Anchor design for unconventional soils will require special consideration.