Application of the Findings of the PISA Joint Industry Project in the Design of Monopile Foundations for a North Sea Wind Farm

Abstract

Monopiles are the most common foundation type in the offshore wind industry. Their design is largely dependent on the ability to accurately model the soil-structure response of the foundation, with more refined modelling approaches enabling significant reductions in required embedment depth, fabrication cost and installation risk. The PISA joint industry project (JIP) has been completed in recent years with the objectives of developing a more refined soil-structure response modelling method compared to other available methods such as the API p-y curve approach. The scope of this paper is to detail how the PISA recommendations have been implemented on a real offshore wind farm project located in the UK North Sea, identifying how the findings can be incorporated into a combined geotechnical and structural analysis approach to enable efficient serial design of multiple foundations for wind turbines. The paper presents how existing design processes and criteria can be modified to take into account the recommendations of the PISA JIP for use in design. Discussion will be provided on the following procedures: calibration of the PISA 1-D soil response formulations to site specific conditions; the combination of the homogeneous sand and clay formulations to accurately model soil-structure response in layered soil profiles; and, consideration of the effects of cyclic loading in conjunction with the use of the PISA monotonic soil response formulations. Results will be presented to demonstrate the calibration of the PISA 1-D soil response formulations to a layered soil site. Discussion will also be provided on the significant monopile lengths savings achieved when using a PISA approach compared to an API p-y curve approach. The monopile mass reduction will be illustrated against trends derived from installed monopiles. Observations will be provided on how the use of a PISA based approach can affect the governing design cases and how this is likely to impact on monopile design for future projects. Discussions and conclusions will also be presented on the challenges of implementing the PISA recommendations in monopile design for real projects and what additional work is required to enable further costs savings in implementing the new design approach. The PISA JIP recommendations are the cutting edge in monopile foundation design. The paper will provide discussion on how these recommendations can be effectively implemented in design based on experience from the foundation design for a real offshore wind farm. The wind farm in question will be one of the first constructed for which foundations have been designed using a PISA based method, demonstrating the significant CAPEX savings possible using the PISA approach.