MULTI-SCALE TWO-PHASE WAVE-STRUCTURE INTERACTION USING ADAPTIVE SPH COUPLED WITH QALE-FEM

Considerable advances have been made in the modelling of wave hydrodynamics and wave-body interaction in recent years. Wave diffraction analysis based on potential flow theory is now standard with linear and second-order theory in the frequency domain. In the time domain for fully nonlinear analysis arguably the most versatile, robust and efficient method is the arbitrary Lagrange-Euler finite-element method (QALE-FEM) approach for potential flow, capable of covering 3-D domains of say 20x20 wavelengths in plan with 20 wave periods on overnight runs on an 8-core processor. However, the method is single-phase with the physical limitation of an irrotational, inviscid fluid. Extreme loads and impacts or slam generally involve breaking wave conditions where multi-phase (air-water-solid) behaviour is significant. An alternative approach is required for violent flows with complex physics local to a structure or body. Progress with volume-of-fluid (VOF) methods has been applied to wave interactions with columns and coastal structures. However, the most significant advances for violent wave-structure have recently been made using the Smoothed Particle Hydrodynamics (SPH) method. SPH has been an area of promising research for some years due to its versatility in dealing with free-surface flows with overturning, splashing and body interaction. Recent problems with numerical convergence, stability and very noisy pressure have been resolved for single-phase flow through EPSRC funded work through an incompressible, divergence-free, formulation (ISPH). Numerical convergence with high pressure accuracy for several impulsive test cases has been demonstrated with generalised shifting algorithms for eliminating instabilities within the particle distributions. Clearly, predicting high pressure accurately is vital for fluid-body interaction. This has been extended to two-phase flow with the incorporation of a compressible air phase with good results in preliminary tests. The main disadvantage of SPH is the computational time due to the large number of particles required in 3-D, O(10 million - 1 billion), the large number of neighbour interactions per particle, and the relatively small time steps needed. Variable particle sizing and efficient neighbour searching do not enable domain dimensions of many wavelengths in 3-D to be run for many wave periods as generally required for extreme wave-body interaction.

Building on previous work, in this project ISPH and QALE-FEM for wave-structure interaction will be developed in parallel and then coupled achieving efficient computation in two ways:
1. Dynamic adaptive particle sizing, satisfying minimum error conditions or resolving some physical characteristic, e.g. density or vorticity. Promising preliminary results have been obtained in 2-D by the proposers.
2. Coupling an inner SPH domain with an outer nonlinear potential flow domain using an efficient solution method such as QALE-FEM. Dynamic adaptive particle sizing should also be used in the SPH domain.

Offshore wind and marine renewable energy (tidal stream and wave) will make significant contributions to enabling UK targets on CO2 emission. Considerable effort has been made to develop technologies for harnessing this energy. Among all challenges, accurate estimation of loading and motions of support structures due to violent wave motion and slam needs to be improved. Although exploitation of offshore oil and gas reserves has been substantial since the 1960s and the design of platforms is generally quite established, wave slamming on the underside of the deck has become a serious problem which is not well understood. While the structural design for oil and gas structures can afford to be conservative due to the high market value of the product, this is not the case for renewable energy where structures need to be as efficient as possible to keep the resulting cost of electricity to acceptable levels. This requires more accurate prediction of complex extreme phenomena as proposed here.

The main focus of this research is on support structures for offshore wind turbines. For these structures, local slam pressures and loads as well as overall loads and overturning moments will be a problem. This involves different scales of several wavelengths (a few kilometres) for the wave field and several millimetres for the slam/impact process. The water phase is effectively incompressible and the air phase compressible.
This project is to develop a relatively efficient model that can simulate wave fields extending for several kilometres and also resolve the length scales of millimetres associated with impact and slamming, taking into account multi-phase water-air-solid interaction. The computational model produced will be used for analysing extreme loads and slam on support structures for offshore wind turbines: monopiles, jackets, gravity-based and floaters which are strategically important for UK (and worldwide) renewable energy provision. Beyond these the model has numerous other applications in marine, offshore and coastal engineering, from ships and oil and gas platforms to breakwaters. Using the model, one can have better understanding of two main effects: local slam on individual members, such as on the underside of decks or ship hulls, and extreme loading on the overall structure which can be considerably magnified by breaking waves.

The main beneficiaries from this research project will be the emerging offshore wind and marine energy industries, such as Garrad Hassan and utility companies such as E.ON, the existing offshore oil and gas industries and classification societies (such as DNV, Lloyd's register) and coastal engineering consultancies involved in the design of coastal defences. This development will also enable the UK to remain leading in the area of violent wave motion and slam on structures.

In order to make the impact happen, we proposal to carry out the following activities:
1) Involve some key stakeholders (Garrad Hassan and E.ON) and establish an End User Management Group in order that they are directly informed of the project achievements;
2) Organise three meetings with key stakeholders including two project workshops to widen the awareness of these achievements;
3) Organise special sessions at the International Offshore and Polar Engineering Conference & Exhibition to maximise the international of reach of the developments;
4) Create a section on the well known website for SPH community and managed by Manchester to allow to open access to the information produced.