A novel integrated approach to efficiently model viscous effects on wave-structure interaction in extreme sea

Many offshore structures for exploiting oil/gas in ocean and for harnessing marine renewable wave energy, tidal current energy and offshore wind energy have been and will be designed and operated. During the design of these structures, it is essential to consider their responses in the worst situation possibly met(extreme sea). In such situation, the breaking wave impact and the viscous effects are widely recognized to be important. These factors disqualified the well-established linear or second-order wave diffraction analysis based in the frequency domain which has been usually used during the design. However, the Computational Fluid Dynamics (CFD) tools with ability to model the wave impact and viscously may take several days or weeks to produce reliable results for the response of structures in a required large sea area with dimensions at the level of tens or hundreds of wavelengths in 3-D and for many wave periods. Alternative tools based on the fully nonlinear potential theory (FNPT) have relatively higher computational efficiency, e.g. the Quasi Arbitrary Lagrangian Eulerian Finite Element method (QALE-FEM) may complete the simulation within an overnight. However, they cannot deal with the breaking wave impact and take the viscous/ effects into account. Therefore, how to efficiently model viscosity/turbulence and the breaking wave impact associated with wave-structure interaction remains to be a key challenge in offshore and marine engineering.

This project will carry out the research to tackle the challenge by developing a novel approach to efficiently model the interaction between large-domain 3D extreme waves and the offshore structures with consideration of viscous/turbulent effects and breaking wave impact. The new method takes the advantage of the CFD tools and the FNPT based methods by integrating them in a single approach. It is expected to have the computational efficiency at a similar level to the FNPT based QALE-FEM , i.e. simulating wave-structure interaction with viscosity and wave breaking in a required large 3D sea area on modern PCs within an overnight. The new development may make it possible to simulate large floating structures subjected to extreme waves in time domain and so give more realistic results.

A preliminary test has been carried out. The results demonstrate the feasibility and the promising features of the proposed approach.

As mentioned above, this project will develop a novel approach being able to efficiently model the interaction between 3D extreme waves and the offshore structures with viscous/turbulent effects and breaking wave impact. The approach is trying to solve a common problem shared by numerous applications in marine, offshore, coastal, marine renewables and offshore wind energy engineering. The problem is that the existing tools available to them either too simple or too computationally inefficient. Using this new and efficient approach developed in this project, one can carry out extensive numerical investigations and so gain a better understanding of two main effects: local slamming due to wave impacts on structures which can be considerably magnified by breaking waves; and the viscous and turbulent effects on the extreme waves and the responses of the offshore structures. Wide range of industries involved in offshore oil and gas, sea-bound shipping, coastal defences, such as the Lloyds Register, DNV-GL, Saipem, American Bureau of Shipping (ABS), Bureau Veritas(BV), and those related to the harnessing marine renewables and offshore wind energy, e.g. GL Garrad Hassan, will benefit from the efficient numerical tool to accelerate their design, operation, classification and inspection process. The numerical tool will overcome their difficulty in modelling viscous effects and violent wave impact in practices and provide reliable results conform to industrial requirements within acceptable CPU times.

To maximise impact of the project, an advisor group will be formed with members from the two project partners, the Lloyds Register and Saipem (both have provided letters of support and promising to make significant contributions), and potential beneficiaries, such as DNV GL Garrad Hassan(one of the leading consultants in wind and marine renewables) and the BP. The group will meet twice a year and will advise on the research. A final workshop will be arranged to disseminate the project results and promote the knowledge transfer. A wider range of potential users of the developed approach will be invited. The research outcome will also be published in international leading journals, e.g. Journal of Computational Physics and Proceeding of Royal Society,and presented in two major international conferences in the field attracting both academic and industrial interests including in the annual conference of the International Society of Offshore and Polar Engineers (ISOPE, in which the applicant acts as the Secretary of Hydrodynamic Committee) and International Conference on Ocean, Offshore and Arctic Engineering (OMAE).