A CPT-based design framework for uplifted open-ended piles installed in spatially variable sandy soils. II: Implications to site investigation and pile design for offshore wind farms

Interpreting seabed properties for future offshore wind farm development appears challenging given the requirement to investigate very large areas. Current approaches, where significant numbers of geotechnical boreholes and cone penetration tests (CPTs) are conducted—often at the location of each foundation or anchor—may prove prohibitive given the scale of modern wind farms (typically over 100 turbines). This paper presents a framework for the refinement of the design of piles under axial tension [for example, to anchor floating offshore wind turbines (OWTs)] in seabeds where the spatial variability of soil properties exhibits isotropy or anisotropy in the horizontal and vertical directions. The framework relies on the approach to the rational selection of design lines representing the soil resistance for achieving a target probability of failure (pf) presented in the companion paper. The framework is validated via application to the design of piles for OWTs in both artificially generated (synthetic) and real seabeds using standard deterministic design methods, and then comparing the achieved pf values to the target. The framework is also implemented jointly with a cost model to investigate the overall project cost for different CPT layouts for an example floating wind farm layout anchored in synthetic seabeds. The spatial variability levels of cone tip resistance for these seabeds are assumed to be the same in the vertical direction but different in the horizontal direction. The optimum CPT layouts that achieved the minimum total project cost are shown to depend on the per-CPT cost and the horizontal spatial variability of cone tip resistance, so that a generally applicable “best” CPT layout cannot be identified. However, CPT layouts that include clustered CPTs and/or have small spacings between CPTs and piles (compared to the underlying actual horizontal spatial variability scale of soil properties) resulted both in a lower total project cost and require fewer total CPTs, suggesting significant potential in this approach.