• Energy Research

Abstract For Marine Renewable Energy (MRE) to become a viable alternative energy source, it must encompass large arrays of devices. Arrays may include 1000s of devices. The associated foundations or anchors may encounter a range of seafloor sediment types and geotechnical properties. Wave and tidal energy convertors induce unique loads on foundations and anchors that are different from other seafloor engineering applications. Thus, there is a need for a combination of advanced site analysis and performance assessment. Geotechnical engineering plays the vital role of ensuring that foundation and anchor systems perform successfully for MRE devices. Our paper reviews the unique frequency and magnitude of loading regimes experienced by MRE arrays. We examine potential loading conditions on the foundation-anchor systems. Loading regimes include environmental and system loads from single devices or arrays of devices. We present specific load examples from field data. We explore the applicable geotechnical approaches to address these conditions, including constitutive models that may or may not adequately capture the response of the seafloor sediments to the MRE loads. Partially to fully dynamic constitutive model formulations may be necessary to properly model sediment-fluid hydromechanical response to MRE loading. Spacing of full MRE arrays and spatial variability in sediment properties may require multiple foundation types.

The research leading to these results has received funding from the European Community's Seventh Framework Programme for DTOcean Project.

The growing marine renewable energy sector has led to a demand for increasingly compliant mooring systems. In response, several innovative mooring tethers have been proposed demonstrating potential customisation to the stiffness profile and reduced peak mooring loads. Many of these novel systems utilise materials in a unique application within the challenging marine environment and their long term durability remains to be proven.This paper presents a multifaceted investigation into the durability of a novel polyester mooring tether with an elastomeric core. Laboratory based functionality tests are repeated on tether assemblies following a 6 month sea deployment. Results show a 45% average increase in dynamic axial stiffness. This is supported by high tension laboratory based fatigue endurance tests showing a peak increase in dynamic axial stiffness of 42%. Sub-component material tests on the core elastomer support the assembly tests, separately demonstrating that certain aspects of tether operation lead to increased material sample stiffness. The average increase in material radial compressive stiffness is 22% and 15% as a result of marine ageing and repeated mechanical compression respectively; these are the first results of this type to be published.The performance durability characterisation of the tether establishes the mooring design envelope for long-term deployment. This characterisation is crucial to ensure reliable and effective integration of novel mooring systems into offshore engineering projects.

Abstract Many studies have been published concerning the influence of the immersed shape (in still water) of a floating body on its response and power capture from ocean waves. With a few notable exceptions, much of this analysis has assumed small amplitude motion and linear models have been employed to predict response. The form of the upper surface of such a body has received little attention. Here, we show how the shape of the upper (top) surface of a floating body can be designed to ensure that the response amplitude of the body is within a specified value. This is of considerable importance to the survivability of wave energy devices. The approach used is to achieve a large increase of both natural period and hydrodynamic damping for only a small change of float mass. These two factors impose a hydrodynamic limit on the displacement which may be exploited to avoid the ‘end-stop’ problem often encountered in wave device design. To demonstrate the change of response, experimental measurements are presented of the response of an axisymmetric float with rounded base and conical upper surface with rounded perimeter due to a range of regular, irregular and focused wave conditions. Power extraction is not considered since the mechanically undamped response represents the worst case. In contrast to a simple, straight-sided axisymmetric float, a smaller change of mass is required to satisfy a particular response amplitude limit. Although a significant reduction is not expected, hydrodynamic damping may reduce with increasing physical scale, and this remains to be quantified.

Much of the published work concerning the response and power output of closely spaced arrays of heaving wave energy devices concerns behaviour in regular waves only and is based on numerical analysis. To date, limited experimental work has been published and it remains unclear how device interactions predicted in idealised models relate to the response of proposed devices in realistic irregular wave-fields. Experimental measurements of the power absorbed by a small two-dimensional array of heaving devices in both regular and irregular waves in a wide flume are reported. In regular wave conditions, positive interactions (where the average power output of the array exceeds the same number of isolated devices) are measured. These tests indicate that the occurrence of positive interactions is largely dependent on the incident wave period and the performance of adjacent devices. Preliminary tests indicate that float responses tend to be smaller when subjected to short period irregular waves of matching peak frequency and standard deviation of surface elevation. The data presented provide an insight into interactions within irregular wave conditions and forms a basis for evaluating numerical models.

The design of wave energy mooring systems is challenging: overdesign incurs a significant cost penalty, underdesign may lead to a premature failure and incorrect design could reduce the power production. Consequently, compliant mooring systems are being developed for wave energy applications.This paper presents tank test results for a scale model of the buoy and mooring used at the SouthWest Mooring Test Facility (SWMTF), an offshore facility developed to conduct long-term sea trials for wave energy device moorings. A compliant three leg catenary mooring system using Nylon ropes in the water column is investigated. Preliminary static, quasi-static, decay, regular and irregular wave tests were conducted on the 1:5 scale model, using the Ifremer basin in Brest. A corresponding numerical model was developed with a time-domain mooring modelling tool, inputting hydrodynamic data from a radiation/diffraction potential modelling program.After the calibration of several hydrodynamic parameters, the numerical model demonstrated good agreement with the experiment. However, numerical results show large differences with the field test results, mainly because of unknowns in the anchor position. The methods and procedures presented will allow the effective validation of numerical models to enable the development of appropriate mooring systems in wave energy applications.

a bstract The interaction between fibre rope and steel parts on vessels (fairlead and roller) is technically well understood but not commonly published in codes or practised by mariners. What appears to be a smooth steel surface to the naked eye can still be abrasive medium to synthetic mooring components. There are very few reports of external rope abrasion tests in the literature. The surface finish at the contact between the rope and the steel guide can cause damage and consequently prematurely degrade the exposed yarns of the rope and thus reduces the overall load bearing capacity of the rope. The standard ISO 18692 [1] recommends that prolonged cycling of a rope around rollers should be avoided, however it is specified that occasional bending and running over rollers are allowable. There are two guides to specify surface roughness. MEG 3 [2] states that steel fairleads should be polished to Ra 10, but in practise this may be difficult to achieve or obtain with carbon steel. The US Navy guide also states that the surface of steel should have better than 125 μi or 3.2 Ra [3] . The study presented here discusses the bending of a synthetic rope around a roller during transportation. It relates the motion behaviour of the vessel to rope wear and provides a detailed numerical simulation correlated with post analysis of the rope after the failure. The investigations show that the roughness of the steel roller caused the abrasion of the rope which was exacerbated through the vessel dynamics, resulting in the rope having an estimated residual strength of 14% MBL before rupture. The experimental tests have established a linear relation between strength loss and surface roughness and it was observed that the abrasion mainly occurs in the early stages of load cycling. The presented work recommends the use of lubricated nylon instead of carbon steel rollers to limit abrasive rope wear. The paper also devises a methodology to carefully assess and quantify potential rope abrasion to ensure that the residual rope strength withstands the required load capacity.