Linking ecosystem services with epibenthic biodiversity change following installation of offshore wind farms

The growing awareness of climate change and the recognised need to secure energy production has been a driving force behind the expansion of the offshore wind industry across the world. Benefits from offshore wind farms (OWFs) may extend further than low CO2 energy production. Wind turbine substructures introduce hard surfaces that are rapidly colonised by epibenthic marine organisms, altering biomass and biodiversity within the local ecosystem. Biodiversity plays a critical role in supporting ecosystem processes and functions that maintain ecosystem services. As offshore wind development continues to grow and modify marine habitats, changes in biodiversity could affect the provision of ecosystem services. In this context, this review sets out to capture the current understanding of epibenthic biodiversity change following the installation of OWFs and attempt to link these changes in biodiversity with marine ecosystem services through the associated processes and functions

Numerical simulation of floating bodies in extreme free surface waves

In this paper, we use the in-house Computational Fluid Dynamics (CFD) flow code AMAZON-SC as a numerical wave tank (NWT) to study wave loading on a wave energy converter (WEC) device in heave motion. This is a surface-capturing method for two fluid flows that treats the free surface as contact surface in the density field that is captured automatically without special provision. A time-accurate artificial compressibility method and high resolution Godunov-type scheme are employed in both fluid regions (air/water). The Cartesian cut cell method can provide a boundary-fitted mesh for a complex geometry with no requirement to re-mesh globally or even locally for moving geometry, requiring only changes to cut cell data at the body contour. Extreme wave boundary conditions are prescribed in an empty NWT and compared with physical experiments prior to calculations of extreme waves acting on a floating Bobber-type device. The validation work also includes the wave force on a fixed cylinder compared with theoretical and experimental data under regular waves. Results include free surface elevations, vertical displacement of the float, induced vertical velocity and heave force for a typical Bobber geometry with a hemispherical base under extreme wave conditions

Critical evaluation of ecosystem changes from an offshore wind farm: producing natural capital asset and risk registers

Offshore wind infrastructure modifies benthic habitats, affecting ecosystem services. A natural capital approach allows risks to nature-based assets and ecosystem benefits to be assessed. The UK Natural Capital Committee produced guidance for conducting natural capital assessments to aid decision making processes. Development of an asset register and risk register are key components of this methodology. The former provides an inventory of NC stocks, and the latter considers the likelihood of changes and the scale of their impact on delivery of ecosystem services. In this study, suitability of the methodology in a marine environment context was critically evaluated. Natural capital stocks before and after installation of Greater Gabbard offshore wind farm were compared and risks to delivery of ecosystem services were assessed. It was demonstrated that incorporating an assessment of impacts on natural capital assets in planning and management decisions (as an extension to traditional environmental impact assessment approaches) could further facilitate sustainable use of marine ecosystems. For example, by preventing access to bottom-trawl fisheries activities, wind farms may promote recovery and increase value of seabed natural capital assets. By also introducing aquaculture systems loss of food provision (from reduced fishing activity) could be offset whilst allowing benthic natural capital assets to recover. Natural capital assessment is relevant to the marine context. However, application of the Natural Capital Committee’s methodology was constrained by the limited coverage of standard benthic sampling tools. Given the scale of wind energy plans across the marine environment it is recommended that these shortcomings are appropriately addressed

A numerical study of a freely floating lifeboat in regular waves

Copyright © 2018 by the International Society of Offshore and Polar Engineers (ISOPE) In the present paper, the open source toolbox OpenFOAM was applied for analysis of the hydrodynamic force and motion of a floating lifeboat in regular waves. The Reynolds averaged Navier-Stokes (RANS) equations were solved and the free surface tracking was achieved by using the volume of fluid method. An overset mesh method was applied for the moving boundary of the lifeboat, in which a body-fitted mesh was generated around the lifeboat using the utility snappyHexMesh and a hexahedral background mesh was produced by the utility blockMesh. The field values were interpolated in the overlapping area between these two layers of meshes. The hydrodynamic forces and the motion of the lifeboat were calculated under the condition that the lifeboat was off-centered in the wave flume to mimic the effects of a larger mother ship. Due to the unsymmetrical condition, full six-degree of freedom (DOF) motion needs to be taken into account. The predicted hydrodynamic force and surface elevation for the fixed lifeboat, and the six DOF motion of the lifeboat were compared to the experimental data. Satisfactory agreement was achieved except the roll moment and motion, for which large discrepancies were observed

A multi-region coupling scheme for compressible and incompressible flow solvers for two-phase flow in a numerical wave tank

We present a multi-region coupling procedure based on the finite-volume method and apply it to two-phase hydrodynamic free surface flow problems. The method combines the features of one incompressible and one compressible two-phase flow solvers to obtain a coupled system which is generally superior to either solver alone. The coupling strategy is based on a partitioned approach in which different solvers, pre-defined in different regions of the computational domain, exchange information through interfaces, i.e. areas separating these regions. The interfaces act as boundary conditions passing the information from one region to the other mimicking the finite-volume cell-to-face interpolation procedures. This results in high performance computing coupled simulations whose functionality can be further extended in order to build a generic numerical wave tank accounting for incompressible flow regions as well as compressibility and aeration effects. We select a series of preliminary benchmarks to verify this coupling procedure which includes the simulation of a hydrodynamic dam break, the propagation and reflection of regular waves, the convection of an inviscid vortex, pseudocavitation, a water column free drop in a closed tank and a plunging wave impact at a vertical wall. The obtained results agree well with exact solutions, laboratory experiments and other numerical data

Numerical simulation of phase-focused wave group interaction with an FPSO-shaped body

Copyright © 2018 by the International Society of Offshore and Polar Engineers (ISOPE) The present paper summarizes the results for numerical simulation of a fixed FPSO-shaped body in uni-directional phase-focused wave groups, which is prepared as a short report for the CCP-WSI Blind Test Workshop on Focused Wave Impact on a Fixed FPSO at the 28th International Ocean and Polar Engineering Conference (ISOPE 2018). The numerical simulations were carried out using the open source toolbox OpenFOAM. An overset mesh method was applied, where two layers of mesh were generated, namely the background mesh and the overlapping body-fitted mesh. The incident focused wave groups were first validated against the experimental data at several positions. With the propagation of the waves, it was found that the waves generated by the numerical model were slightly dissipated due to numerical diffusion. Therefore, smaller wave crest was predicted from the numerical model. Then the simulations were conducted for the same wave conditions with the FPSO structure in place. The surface elevation and the pressure at several locations based on the validation criteria are reported

Numerical investigation of air enclosed wave impacts in a depressurised tank

This paper presents a numerical investigation of a plunging wave impact event in a low-filling depressurised sloshing tank using a compressible multiphase flow model implemented in open-source CFD software. The main focus of this study is on the hydrodynamic loadings that impinge on the vertical wall of the tank. The detailed numerical solutions compare well with experimental results and confirm that an air trapped plunging wave impact causes the vertical wall to experience pulsating pressure loadings in which alternate positive and negative gauge pressures occur in sequence following the first applied pressure peak. The strongest pulsations of the pressure are found to be near the air pocket trapped by the water mass. The instantaneous pressure distribution along the vertical wall is nearly uniform in the area contained by the air pocket. The phases of pulsating pressures on the wall are in synchronisation with the expansion and contraction of the trapped air pocket. The pocket undergoes changes in shape, moves upwards with the water mass and eventually breaks up into small parts. A careful integration of the wall pressure reveals that the vertical structure as a whole experiences pulsating horizontal impact forces. It is found that the average period of pulsation cycles predicted in the present study is around 5–6 ms, and the loading pulsations are quickly damped out in View the MathML source0.1–0.2s. Further exploratory investigation of the fluid thermodynamics reveals that the temperature inside the trapped air pocket rises quickly for about 2 ms synchronised with the pocket's first contraction, then the generated heat is rapidly transferred away in around 3 ms

Improved numerical wave generation for modelling ocean and coastal engineering problems

We introduce a dynamic-boundary numerical wave generation procedure developed for wave structure interaction (WSI) simulations typical of ocean and coastal engineering problems. This implementation relies on a dynamic mesh which deforms in order to replicate the motion of the wave-maker, and it is integrated in wsiFoam: a multi-region coupling strategy applied to two-phase Navier-Stokes solvers developed in our previous work [Mart´ınez Ferrer et al. A multi-region coupling scheme for compressible and incompressible flow solvers for two-phase flow in a numerical wave tank. Computer & Fluids 125 (2016) 116–129]. The combination of the dynamic-boundary method with a multi-region mesh counteracts the increase in computational cost, which is intrinsic to simulations featuring dynamic domains. This approach results in a high performance computing wave generation strategy that can be utilised in a numerical wave tank to carry out accurate and efficient simulations of wave generation, propagation and interaction with fixed structures and floating bodies. We conduct a series of benchmarks to verify the implementation of this wave generation method and the capabilities of the solver wsiFoam to deal with wave structure interaction problems. These benchmarks include regular and focused waves, wave interaction with a floating body and the modelling of a wave energy converter, using different wave-maker geometries: piston, flap and plunger. The results gathered in this work agree well with experimental data measured in the laboratory and other numerical simulations

An overset mesh based multiphase flow solver for water entry problems

This paper extends a recently proposed multi-region based numerical wave tank (Martínez-Ferrer et al., 2016 [1]) to solve water entry problems in naval engineering. The original static linking strategy is developed to enable the dynamic coupling of several moving regions. This permits the method to deal with large-amplitude motions for structures slamming into water waves. A background grid and one or more component meshes are firstly generated to overlay the whole computational domain and the sub-domains surrounding the structures, respectively. During computation, the background mesh is fixed while the small grids move freely or as prescribed without deformation and regeneration. This effectively circumvents the large and often excessive error-prone dynamic deformation of a single-block mesh as well as the complex and time-consuming mesh regeneration. Test cases of dam breaking with and without obstacles are first conducted to verify the developed code by comparing the numerical solution against experimental data. Then the new code is used to solve prescribed and free-fall water entry problems. The obtained results agree well with experimental measurements and other computational results reported in the literature

Pure and aerated water entry of a flat plate

This paper presents an experimental and numerical investigation of the entry of a rigid square flat plate into pure and aerated water. Attention is focused on the measurement and calculation of the slamming loads on the plate. The experimental study was carried out in the ocean basin at Plymouth University’s COAST laboratory. The present numerical approach extends a two-dimensional hydro-code to compute three-dimensional hydrodynamic impact problems. The impact loads on the structure computed by the numerical model compare well with laboratory measurements. It is revealed that the impact loading consists of distinctive features including (1) shock loading with a high pressure peak, (2) fluid expansion loading associated with very low sub-atmospheric pressure close to the saturated vapour pressure, and (3) less severe secondary reloading with super-atmospheric pressure. It is also disclosed that aeration introduced into water can effectively reduce local pressures and total forces on the flat plate. The peak impact loading on the plate can be reduced by half or even more with 1.6% aeration in water. At the same time, the lifespan of shock loading is prolonged by aeration, and the variation of impulse is less sensitive to the change of aeration than the peak loading