Structural fatigue assessment and optimisation of offshore wind turbine jacket foundations.

Offshore Wind Turbine (OWT) is an expensive type of renewable energy system, and there is a continuous effort to lower the capital and operational costs. Jacket foundations are increasingly used in offshore wind due to their relatively light weight and adaptability in deep waters. However, the contemporary jacket designs are based on conservative practices from the oil and gas industry. There is still substantial room to optimise the jacket designs for offshore wind usage. This research aims to generate innovative jacket designs by applying a topology optimisation algorithm. The research demonstrates the use of advanced computational techniques to improve existing designs by enhancing structural integrity and fatigue life whilst reducing the mass. The research presents a comprehensive investigation of different parameters and design modifications that impact the design life of an existing jacket. An OC4 jacket foundation is employed, modelled in industrial software from DNV, and transformed into a super element model. The time-series loads obtained from Bladed are used to assess fatigue damages experienced during the foundation's service life. Furthermore, the research presents a topology optimisation method to retrofit an existing jacket foundation design by finding the optimum load path on the structure to enhance fatigue life and lower costs. The jacket's structural optimisation is performed by considering its dynamic response while adhering to the relevant international design standards. In particular, time-domain fatigue simulations were performed to assess the structural integrity of the topology- optimised jacket for the first time. As a result, a range of optimised models with various thickness and diameter options are presented, which are shown to be rational and verify the optimisation procedure. The research contributes a unique integrated topology optimisation framework, dynamic analysis, and time-domain fatigue simulations using industry-standard software tools, and achieved a mass reduction of 35.2% and simultaneously realized a 37.2% better fatigue life compared to the baseline model. The overall environmental load calculation, optimisation procedure and results provide useful practicalities for designing offshore wind turbine foundations and potentially facilitate the relevant industry's structural integrity and cost reduction.

Haart, Phil - Associate Supervisor

EngD in Renewable Energy Marine Structures (REMS)