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.

Influence of soil–structure modelling techniques on offshore wind turbine monopile structural response

“…Comparison of the 3D mass soil and API‐RP springs‐supported models performed as part of the PISA project supports the findings of this study. Ratio of horizontal load at the top of the OWT structure to the pile head displacement (force/displacement) was reported to be 19.38 for the 3D mass soil model and 8.75 for the API‐RP model 5,17 . This represents a 121% increase in 3D mass soil model stiffness on the API‐RP spring‐supported model stiffness.…”

“…The springs supported models can still be applied in the design of OWT monopiles, but this would require significant deviation from current springs force–stiffness calculation and instead rely heavily on calibration using both the mass soil model and measured monitoring data to verify, investigate, and correct uncertainties in the design 19 . Engineering justification and benefit for undertaking the design and analysis using this method can be made on the grounds of reducing computational costs and improving efficiency considering the sheer number of modelling iterations, improvements, loading conditions, and combinations required for the complete structural design of OWT monopiles 17 …”

“…Ratio of horizontal load at the top of the OWT structure to the pile head displacement (force/displacement) was reported to be 19.38 for the 3D mass soil model and 8.75 for the API-RP model. 5,17 This represents a 121% increase in 3D mass soil model stiffness on the API-RP springsupported model stiffness.…”

“…The PISA project for soil-structure modelling and interaction is an example of research progress aimed at addressing some of the modelling issues in the offshore wind turbine industry, including the influence on the structure fatigue life which is outside the scope of this study. 5,17 E. Understanding the upper bound capacity limits of OWT monopile is an important and interesting question presently facing the industry.…”

Anisotropic Stiffness and Shear Strength Characteristics of a Stiff Glacial Till

“…The study formed part of the PISA Joint Industry Project described by Byrne et al (2017) and Zdravković et al (2020a) which included numerical analyses and lateral loading tests on instrumented monopiles, with diameters up to 2m, at the Cowden site, that were designed to improve design for monopiles driven to support large turbines at many North and Baltic Sea glacial till offshore windfarm sites, see Schroeder et al (2015Schroeder et al ( , 2020 or Manceau et al (2019). The laboratory research described herein aimed to resolve the significant mismatch noted by Zdravković et al (2020b) between stiffness trends found from triaxial undrained compression tests on Cowden till and those inferred from field and laboratory geophysical testing and observed in the PISA pile tests.…”