Survivability of Wave Energy Converter and Mooring coupled system

Director of Studies: Professor Deborah Greaves

Funded by: EPSRC - UKCMER

Student: Edward R.

Background

​​A history of international wave energy converter (WEC) and mooring failures demonstrates that considerable uncertainty exists around the fully coupled behaviour of any WEC, its mooring and the environmental factors including wave, wind, current and bathymetry. Mooring design technology has a considerable history and is well established in the offshore industry. However, there are significant differences between mooring systems for conventional floating offshore structures (such as tension legged platforms; floating production, storage and offloading vessels; spar buoys) and wave energy converters. Mooring arrangements for WECs cannot simple be adopted from the offshore oil and gas industry: the differences between installation locations, installation arrangements (especially in the case of wave energy farms), and motion requirements necessitate new mooring designs. Vessel motion requirements are a particularly acute differentiating factor. Unlike conventional offshore structures, WECs are designed to develop specific motions under wave loading. So, while a WEC must constrain unwanted motions it must also permit those motions which produce power. In terms of survivability, WECs are often designed such that the wave induced loads are large. In extreme events these designs can place an abnormal requirement on the mooring structure. The research aim is to develop a computational tool that can be applied in the design process to yield a thorough understanding of moored WEC’s behaviour in the offshore environment and in particular under survivability conditions. To drive the system simulations for extreme case of steep and breaking waves, the hydrodynamic interaction with the WEC hull will be simulated using a fully nonlinear computation fluid dynamics approach for the free surface. This will be combined with nonlinear dynamic structural analysis for the mooring in a fully coupled dynamic analysis. The method will be verified against experimental and field data, where possible, and the relationship between waves, currents, WECs and mooring systems investigated. Insight will be​ gained into the impact of mooring design on energy extraction of point-absorption devices.

The ultimate aim is to produce a simulation tool to improve understanding of the coupled dynamic behaviour of the systems, leading to greater operational efficiency and ultimately reduce the number of physical prototypes required during the design of a WEC.

Objectives

  • Develop numerical methodology for extreme wave interaction with offshore WEC.
  • Develop numerical methodology for coupled nonlinear dynamic analysis of WEC and mooring.
  • Verify numerical models with available physical tank test data and field measurements.
  • Investigate survivability of WEC and mooring systems in extreme wave conditions.
  • Provide guidance for WEC and mooring system optimisation in extreme wave loading.