Design and Analysis of Mooring System for Semi-submersible Floating Wind Turbine in Shallow Water

Abstract

Floating wind turbines are necessary to produce power from wind in deeper water where bottom-fixed wind turbines are not economically competitive. However, it is still not clear to which water depth a floating wind turbine is cheaper than a bottom-fixed one. The semi-submersible is an attractive floating concept with wide applicable range of water depths because of its small draft. The thesis deals with the challenge of designing mooring system for semi-submersible floating wind turbine in shallow water, including a study of different hydrodynamic load models for shallow water conditions, especially with respect to the higher order wave effects based on second-order and fully nonlinear wave theories.

While significant efforts have been made on design of mooring systems for offshore structures in deep water, there has been quite limited study on mooring system design for floating wind turbines in shallow water especially when the water depth is around 50 m to 80 m. Therefore, one of the focuses in this thesis is to study the mooring line behavior in different water depths and propose feasible mooring design concepts for semi-submersible floating wind turbines in shallow water. In this connection, different mooring line materials like chain and synthetic fibre rope, mooring components like clump weight and buoy, anchors like drag embedment anchor and suction anchor are used. The nonlinear material behavior of synthetic fibre rope is described with the state-of-the-art numerical model - 'Syrope'. Based on numerical analysis in serviceability and ultimate limit state conditions, the performance of different mooring designs are compared from a number of perspectives: static performance, dynamic response as well as capital costs etc. Second-order difference-frequency wave effect is considered while sum-frequency effect is neglected in view of the relevant large natural periods of the mooring systems considered. For taut mooring made of fibre rope, the mooring line tension increases linearly as offset increases. Meanwhile, the tension increases nonlinearly for catenary mooring made of chain as offset increases and the nonlinear increment is more significant in shallow water than deep water. It is found in this study that a taut mooring consisting of pure fibre rope, and a catenary mooring consisting of chain with additional clump weight and buoy are recommendable concepts.

Calculation of second-order difference-frequency wave load effect in shallow water based on Newman’s approximation method and full quadratic transfer function (QTF) method are compared and it is found that it is important to apply the full QTF method in shallow water. While the effect of nonlinear wave kinematics is not significant for small waves and deep water, it becomes important for large wave and shallow water. Due to the difficulty and excessive computational costs of analyzing fully nonlinear wave-structure interaction which deals with fully nonlinear free surface conditions and body boundary conditions, the state-of-the-art method is to generate the wave trains in a separate numerical wave tank, which neglects the effect of the presence of the structure on the wave field. In this study, the wave realizations are generated in a 2D Harmonic Polynomial Cell (HPC) wave tank with horizontal flat seabed. The coupled dynamic response analysis is then performed based on the imported wave kinematics data in HAWC2 where the hydrodynamic load is calculated using Morison's equation and the wave kinematic is integrated up to the instantaneous wave surface. To increase the computational efficiency, the wave kinematics data generated from 2D HPC wave tank is approximated by polynomial fitting method before being imported into HAWC2 for determination of wave loads. The link between wave kinematics database and HAWC2 is extended from one dimensional case which is limited to bottom-fixed structure to two dimensional case which is applicable for floating structure as well. The whole numerical code package has been verified against stream function wave and linear Airy wave. In the end, fully nonlinear irregular wave trains are applied in the dynamic analysis to study the effect of wave nonlinearity on the floater motion, internal structural tower force and mooring line tension for different load cases. The results not only demonstrate the importance of considering wave nonlinearity effect in hydrodynamic analysis in shallow water but also prove the applicability and efficiency of the proposed numerical tool.