Section 6 Anchor pile

6.1.1 Anchor piles are characterised by being relatively long and slender and having a length to diameter ratio or width ratio generally greater than 10.

6.1.2 Table 14.6.1 Minimum factors of safety for anchor piles for a catenary mooring system defines the design cases and factors of safety to be used for anchor piles for a catenary mooring system. Table 14.6.2 Minimum factors of safety for anchor piles for a taut-leg mooring system defines the design cases and factors of safety for anchor piles for taut-leg mooring system. For anchor pile clusters the group as a whole is to have a factor of safety as required by Table 14.6.1 Minimum factors of safety for anchor piles for a catenary mooring system and Table 14.6.2 Minimum factors of safety for anchor piles for a taut-leg mooring system. Individual piles in a group may have lower factors of safety; for taut-leg anchor piles, the minimum factor of safety for individual piles within a group is to be 1.5.

6.1.3 Table 14.6.1 Minimum factors of safety for anchor piles for a catenary mooring system and Table 14.6.2 Minimum factors of safety for anchor piles for a taut-leg mooring system do not apply to axial capacity of piles installed by vibrating hammers.

6.1.4 The efficiency of the group, that is its capacity compared to the sum of the capacities of individual anchor piles within the group, is to be checked.

6.1.5 Consideration is to be given to the possible formation of a post-hole at the pile head and its effect on capacity.

6.1.6 The influence of pile shoes, internal stiffeners, padeye and any other protrusions should be accounted for within the pile capacity and installation assessments.

6.1.7 For piles subjected to permanent tension loads, consideration is to be given to long term changes in soil stresses around the anchor piles and upward creep.

6.2.1 This sub-Section applies to anchor piles that are either driven or drilled and grouted into the seabed. Piles installed by vibrating hammers are not recommended where axial loading is significant.

6.2.2 Other methods, than those contained within API RP 2SK, ISO 19901-7 and associated standard, of determining axial capacity are acceptable, provided they are supported by sufficient evidence of their validity together with appropriate laboratory testing.

6.2.3 For unconventional soils, such as carbonate soils, particular attention should be given to ensure that appropriate design methodology is used. This applies to cohesive and non-cohesive soils.

6.2.4 The pile design must satisfy the required factors of safety in Table 14.6.1 Minimum factors of safety for anchor piles for a catenary mooring system and Table 14.6.2 Minimum factors of safety for anchor piles for a taut-leg mooring system or pull-out capacity and bearing capacity.

6.2.5 No end bearing should be taken for drilled and grouted piles unless it can be demonstrated that there is no infill at the bottom of the drilled hole, or the calculations account for the compressibility of such infill.

6.2.6 A reduction in axial capacity should be considered where large lateral soil displacements are predicted.

6.2.7 For tension loads, no end bearing (or suction) component at the pile tip is to be considered unless this can be justified based on pile configuration, rate of loading and soil permeability.

6.2.8 Pile capacity in rock is to be specially considered.

6.2.9 Consideration should be given to the effect of close spacing of piles, since the ultimate axial capacity of a group can be less than the sum of the individual capacities. This may be determined by consideration of the group as an 'equivalent pier'.

6.2.10 Appropriate account should be taken of the driving shoe and any other protrusions or add-ons that may affect the internal or external skin friction.

6.2.11 Drilled and grouted pile design will be specially considered.

6.3.1 For anchor piles, the lateral capacity and pile response are normally to be determined using a beam-column non-linear soil/structure interaction finite element analysis. The non-linear axial and lateral soil resistance/pile deflection is to be modelled using t-z and p-y curves, respectively.

6.3.2 It should be demonstrated that the selected p-y curve methods are valid for the soil conditions at the site. For unconventional soil conditions consideration should be given to other p-y curve methods specifically developed for those soil types (see guidance note).

6.4.1 Driving stresses and static stresses due to the weight of the hammer are to be considered in the selection of pile driving hammers and pile wall thickness. Driving stresses are also to be included in the assessment of pile fatigue lives. The stresses induced in the pile during driving can be estimated using a wave equation analyses.

6.4.2 Any effects resulting from the use of a pile guide frame should be considered within the pile design. This may include consideration of disturbance caused during frame installation and removal.

6.5.1 The methods for drilling and grouting and details of the plant and materials are to be submitted to LR for approval.

6.5.2 The construction programme is to avoid leaving holes open for long periods in soils or rock sensitive to exposure to water or drilling fluids.

6.5.3 A specimen record is to be submitted for approval prior to the installation of the first pile.