Optimisation of Offshore Wind Farm Jacket Foundation

Abstract

Jacket foundations are increasingly popular for supporting offshore wind turbines and their cost is a significant part of the overall construction cost of an offshore wind farm. An effective way of reducing the construction cost is to reduce the structural weight of the supporting structure, while ensuring structural safety and serviceability. The multiplanar KK circular hollow section (CHS) joints, also known as the double K joints (DKK), are most widely adopted for offshore wind turbine jackets construction. Their structural design is commonly governed by fatigue resistance which is assessed with the hotspot stress vs. fatigue strength curve method (S-N curve). The existing method of predicting hotspot stress is very conservative. Furthermore, there is no accepted S-N curve for high strength steel whose use could reduce the weight of offshore jacket structures. This thesis details the development and validation of an accurate, yet simple and computationally efficient method of calculating hotspot stresses in DKK CHS joints under arbitrary loading conditions enabling reduction of jackets self-weight. The approach was developed in two stages. First, an arbitrary internal loading case in any DKK CHS joint is decomposed into symmetric and anti-symmetric axial, in-plane bending and out-of-plane bending unit load cases. Next, distributions of stress concentration factors (SCFs), including carryover stresses, under these unit load cases and around all four brace-chord connections’ weld perimeters are constructed using 11 key SCFs. An extensive parametric study using ABAQUS was performed covering many external loading conditions and joint geometries, to provide data for regression analyses to derive formulae for the 11 key SCFs. The application of the new formulae showed savings of up to 10.43% of jacket self-weight using normal steel grades and 18.72% using high steel grades (>700 MPa) against using the widely accepted industrial method (Efthymiou 1985; 1988). A new S-N curve was derived for high strength steel. Potential sites (Seagreen and East Anglia) for offshore wind farm construction at the North Sea were considered for the study.

Date of Award	31 Dec 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Yong Wang (Supervisor) & Tim Stallard (Supervisor)