Implementation of Advanced Monopile Design Approaches at the Seastar Offshore Wind Farm-Paper

Paper

Pdf Summary

The paper presents an exploratory case study applying advanced PISA-type design methods to the monopile foundation of the Seastar offshore substation in Belgium’s North Sea. Unlike conventional API methods, which represent soil resistance using lateral force–displacement curves alone, the PISA approach also captures distributed moments and forces and moments at the pile base, better reflecting the rigid-body rotation of large-diameter, low-aspect-ratio monopiles.<br /><br />Site-specific soil reaction curves were derived using three-dimensional finite element analysis in PLAXIS 3D, with calibrated constitutive models for clay and sand. These curves were incorporated into an integrated geotechnical and structural workflow. Cyclic loading effects in clay were modelled, while dynamic structural analyses accounted for wave directionality, high-order wave effects, ringing, and second-order structural response.<br /><br />Compared with the project’s conventional API-based design, the PISA-type design reduced the required pile embedment from 39.8 m to 29.8 m and pile weight by approximately 10%. It also produced greater foundation stiffness, lower predicted fatigue damage, and increased predicted design life. The results demonstrate that advanced, site-specific modelling can improve foundation efficiency and reduce project risk.<br /><br />The study also identifies implementation challenges. Finite element design requires comprehensive geotechnical data for constitutive-model calibration, but the available site investigation was not designed for this purpose. Cyclic laboratory data were limited, so cyclic parameters were based on experience from a nearby site rather than site-specific calibration. Structural analysis also required linearising nonlinear soil reactions, and iterative coordination between geotechnical and structural analyses was essential.<br /><br />The authors recommend planning early for suitable soil data, representative cyclic-response methods, and effective integration of monopile soil models into structural design. The findings are illustrative and should not be directly generalised to other sites.

Keywords

PISA design

monopile foundation

offshore substation

finite element analysis

PLAXIS 3D

soil reaction curves

cyclic loading

fatigue damage

Belgium North Sea

geotechnical-structural workflow