Section 8 Gravity anchors

8.1.1 Gravity bases should be designed in accordance with industry recognised practice such as ISO 19903 and ISO 19901-4.

8.1.2 A material factor for soil of 1,25 is to be applied to the design undrained shear strength, tangent of the angle of internal friction of the soil and tangent of the angle of interface friction between the soil and the gravity anchor.

8.1.3 Appropriate load coefficients will be specially considered for particular applications for catenary and taut-leg mooring systems.

8.1.4 Appropriate failure modes for the soil are to be considered when evaluating the ultimate capacity of gravity anchors. The installation tolerances are to be taken into account when assessing failure modes for the soil.

8.1.5 The stability and characteristics of any ballast will require special consideration. In particular the ballast should be capable of withstanding cyclic loading.

8.1.6 The gravity anchor is to have sufficient strength to account for the stresses due to the applied loading conditions.

8.1.7 Wave loads on the seabed are to be considered when such loads are unfavourable to stability.

8.2.1 Calculations of foundation movements are to include the effects of short-term and long-term loading.

8.2.2 The following possible causes of vertical movements are to be investigated:

• Immediate settlement
• Secondary settlement due to steady, or permanent, loading
• Long-term consolidation and settlement due to dynamic, static and steady loading.
• Recoverable displacements due to transient loading.

8.2.3 Particular account is to be taken of the possibility of differential settlement due to variations in soil conditions over the foundation base area.

8.2.4 The extent of horizontal movements and tilt are to be assessed. The relative magnitudes of recoverable and non-recoverable displacement are to be determined.

8.3.1 Complete contact with the seabed is to be maintained at all times and the stresses imposed by the foundations on the seabed are to be compressive under all loading conditions.

8.3.2 Calculations of the local contact stresses between the foundation and the seabed are to take into account the results of the seabed survey.

8.3.3 Local soil reactions on gravity foundations are to be based on the highest expected values of soil strength in the upper soil layers.

8.3.4 Unless specifically considered in the design, any voids remaining beneath the foundation after installations are to be filled with cementitious grout (for example).

8.4.1 Where foundations have skirts, dowels or other seabed penetrating elements which transfer load to the seabed, the effect of these components is to be taken into account when determining the efficiency of, and loads in, the foundations for bearing capacity and sliding resistance. These items are to be designed as structural members.

8.4.2 The resistance of skirts, dowels and other seabed penetrating elements to penetration of the seabed during installation of the foundation and their effect, if any, on water flow beneath the foundation during installation is to be taken into account in the design calculations.

8.4.3 The penetration resistance of elements such as skirts and dowels is to be based on conservative (upper bound) estimates of soil strength. Also, by considering more typical penetration resistances, account may be taken of the foundation in formulating possible eccentric ballasting requirements.

8.4.4 The gravity base anchor installation should ensure that any skirts or other seabed penetrating elements penetrate as required by the design.

8.4.5 Provision is to be made for the relief of water pressure generated within the skirts during installation of the structure.

8.5.1 The positioning of the foundation is to be properly related to the location of the site investigation.

8.5.2 Any significant obstructions identified by the seabed survey carried out prior to the installation are to be removed before emplacement.

8.5.3 Differential ballasting may be required to accommodate non-uniform soil properties or a sloping seabed. In general, reduction of pressure beneath the foundation is not to be used to aid installation, unless it can be demonstrated that washout or flow of soil will not occur.

8.5.4 Records of settlement and tilt of the structure are to be made during installation and properly correlated to those required to be kept while the structure is in service.