Earthquake Loads on Suction Anchors and Piles for Subsea Facilities-Paper

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The paper presents a simplified method for estimating earthquake loads on piles supporting subsea facilities and on mooring anchor piles in layered soils. For structures such as manifolds, the structure is modeled as a rigid mass on pile-head soil springs, forming a single-degree-of-freedom system.<br /><br />The method follows the three-step soil–structure interaction framework: determine foundation motion from seabed response spectra, calculate frequency-dependent pile–soil stiffness and damping, then evaluate the structure’s inertial response. Pile impedances are calculated using the PILES program and strain-compatible soil properties derived from site-response analysis. The system’s natural frequency is found iteratively, accounting for structural, pile, hydrodynamic, and added-soil mass. Seismic forces are then estimated from the seabed acceleration response spectrum.<br /><br />An example demonstrates the procedure for Extreme Level and Abnormal Level Earthquake conditions. The calculated natural periods are approximately 0.8–0.95 seconds, with maximum horizontal loads of 667–1,038 kN and vertical loads of 786–1,310 kN. These seismic loads are combined with static loads to assess pile capacity. The authors also describe a simplified method for estimating torsional stiffness.<br /><br />For most soil profiles, pile design can focus on inertial-interaction loads. Where strong contrasts in soil stiffness occur between layers, kinematic-interaction forces should also be evaluated. Design should account for loading-rate effects in clay, potential liquefaction in sand, and appropriate damping adjustments to response spectra. Accurate estimation of hydrodynamic mass is particularly important for top-loaded subsea structures.<br /><br />Because a mooring anchor pile has little mass relative to its stiffness, it is assumed to move with the ground; its earthquake load is therefore approximated as peak ground acceleration multiplied by pile mass. This load is combined with environmental loads, accounting for possible soil-strength reductions.

Keywords

earthquake loads

pile foundations

subsea facilities

mooring anchor piles

soil-structure interaction

seismic response spectrum

pile impedance

layered soils

hydrodynamic mass

liquefaction