A hydrodynamic analysis of deep-water moorings

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2015
Thesis identifier
  • T14033
Person Identifier (Local)
  • 201160721
Qualification Level
Qualification Name
Department, School or Faculty
  • The move by the offshore oil & gas industry to deep water has an impact on the selection of mooring system configuration and design method. Methods of analysis need to be re-evaluated as water depth increases. The primary purpose of this thesis is to study the hydrodynamics of deep water moorings and the nonlinear dynamic response of the mooring line, which is representative of a Spar platform and a floating offshore wind turbine (FOWT). Emphasis is placed on the coupling effects between the floating body and mooring line and the nonlinear dynamic response of elastic mooring line. For a Spar platform, this thesis studied the behaviour of a 4-point mooring system in water depths from 300m to 3000m, using an indirect time-domain method. A panel method was applied for the hydrodynamics of the floating structures and a lumped mass and spring method for the dynamic response of the mooring lines. Coupled analysis results for intermediate water depth were compared with experimental data to check the validity of the numerical modelling. The results from coupled low frequency (LF) and fully coupled analysis are compared and discussed. Results from parametric studies are compared to offer guidance to mooring system designers on the suitability of particular approaches. For a floating wind turbine, three water depths-300m 600m and 900m were simulated in the time domain under both operational and shutdown conditions. A fully-coupled analysis was carried out to study the motion response of the FOWT under wave only and wave-plus wind condition. The aerodynamic modelling was based on the blade element momentum theory, while the mooring system global performance was simulated by the indirect time-domain method. By performing a comprehensive parametric study, the effects of the second-order wave drift force and the aerodynamic turbine thrust force on the motion response of the FOWT are studied and discussed. The performance of a polyester mooring line is non-linear and its elongation plays a significant role in the dynamic response of an offshore moored structure. Unlike chain, the tension-elongation relationship and the behaviour of elastic polyester ropes are complex. In this thesis, by applying a new stiffness model of the mooring line, the traditional elastic rod theory is extended to allow for large elongations, which are appropriate for simulating the static and dynamic response of both polyester lines and traditional chains. Galerkin's method was applied to discretise the equation of motion for the rod. One beneficial feature of the present method is that the stiffness matrix is symmetric; in non-linear formulations the element stiffness matrix is often non-symmetric. The static problem was solved by Newton-Raphson iteration whereas a direct integration method was used for the dynamic problem. The mooring line tension based on the enhanced model was validated against the proprietary software OrcaFlex. Results of mooring line top tension predicted by different elongation conditions were compared and discussed. The present method was then used in a time-domain simulation of a Spar-type platform, typical of those used for offshore wind turbines, moored by three taut lines in waves and currents. From a comparison between linear and non-linear formulations, it is seen that a linear spring model under-estimates the mean position when the turbine is operating, but over-estimates the amplitude of the platform response at low frequencies when the turbine has shut down.
Resource Type
Date Created
  • 2015
Former identifier
  • 1229790