The increasing scale of offshore energy infrastructure has created a demand for foundation concepts that remain technically and economically viable for larger topsides and greater water depths. While monopile foundations dominate shallow-water applications due to their simplicity and cost-effectiveness, their applicability becomes increasingly constrained by manufacturing, transportation, installation, and structural limitations. Larger offshore topsides are therefore commonly supported by jacket foundations, which provide efficient load transfer and high structural robustness. However, jackets require more complex design, fabrication, and installation processes, resulting in longer project schedules and increased costs. This research investigates the technical feasibility of an alternative support concept consisting of four large-diameter piles, referred to as a four-megapile foundation, for offshore topside structures.

A simplified numerical modelling framework was developed in Python to evaluate the global behaviour of offshore structures supported by large-diameter piles. Based on the three-dimensional matrix method, the model incorporates soil-structure interaction, hydrodynamic loading, structural dynamics, and limit state assessment procedures. Soil behaviour was represented using linearized stiffness formulations derived from the PISA framework. The model was verified against commercial software and benchmark calculations before being applied to a representative North Sea case study.

The verified model was used to assess the structural performance of a four-megapile-supported topside under Ultimate, Fatigue, and Serviceability Limit State conditions. Sensitivity analyses were performed to evaluate the influence of uncertainties in soil properties, environmental loading, and modelling assumptions. In addition, a parametric study compared monopile and four-megapile concepts across a range of water depths and topside dimensions.

The results demonstrate that the four-megapile concept is technically feasible within the investigated range of conditions. ULS, FLS, and SLS requirements can be satisfied through an appropriate combination of pile diameter, penetration depth, wall thickness, and pile spacing. Soil properties, water depth, damping ratio, topside mass, and the rotational stiffness of the topside-pile connection were identified as the most influential parameters governing structural performance. Compared to monopiles, the four-megapile concept exhibits more favourable scaling with increasing water depth and topside size, maintaining practical pile dimensions while avoiding several manufacturing limitations associated with very large monopiles.

To conclude, the study demonstrates that four-megapile foundations represent a promising alternative to conventional monopile foundations, particularly for large offshore topsides and deeper-water applications. The findings provide insight into the governing design parameters and establish a basis for future investigations into advanced soil modelling, installation methodologies, and economic feasibility.