Practical Approach for the Evaluation of Cyclically Induced Excess Pore Pressure around Offshore Foundations in Sand-Paper

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The paper presents a practical numerical framework for estimating how cyclic storm loading builds excess pore pressure around offshore foundations in sand and reduces their post-storm bearing capacity. Such effects are important for design, but existing verification methods are either costly, limited to calibrated conditions, or computationally demanding.<br /><br />The approach combines a simplified finite-element analysis of foundation loading with empirical relationships derived from cyclic direct simple shear (DSS) tests. At each soil integration point, pore-pressure accumulation is estimated from the mean effective stress, cyclic shear-strain amplitude, and number of load cycles. The procedure also models pore-pressure dissipation during the storm through consolidation. The resulting effective-stress reduction is represented by a lower equivalent friction angle, which is then used to calculate post-cyclic foundation capacity.<br /><br />Laboratory tests on dense, poorly graded sand established the strain-based relationships used in the model. In an example analysis of an 8 m diameter monopile subjected to 60 equivalent cycles, the highest pore-pressure accumulation occurred near the pile and within the passive-side shear zone. Because the sand was highly permeable, substantial pressure dissipated during the storm. The calculated ultimate lateral capacity fell from 39.7 MN to 37.6 MN, a reduction of about 5%.<br /><br />The framework offers a relatively simple, modular alternative to detailed cyclic constitutive modelling and can be calibrated using a manageable number of laboratory tests. However, it is still conceptual and relies on simplifying assumptions, including constant cyclic strain amplitudes and no consideration of cyclic deformation accumulation, stress redistribution, or changes in void ratio. The authors conclude that further refinement and validation against laboratory and field measurements are needed before general practical application.

Keywords

cyclic storm loading

excess pore pressure

offshore foundations

sand bearing capacity

finite-element analysis

cyclic direct simple shear

effective stress reduction

pore-pressure dissipation

monopile lateral capacity

post-cyclic capacity