Basin Testing of Wave Energy Converters in Trondheim: Investigation of Mooring Loads and Implications for Wider Research

Vladimir Krivtsov,Brian Linfoot

doi:10.3390/jmse2020326

Abstract

This paper describes the physical model testing of an array of wave energy devices undertaken in the NTNU (Norwegian University of Science and Technology) Trondheim basin between 8 and 20 October 2008 funded under the EU Hydralabs III initiative, and provides an analysis of the extreme mooring loads. Tests were completed at 1/20 scale on a single oscillating water column device and on close-packed arrays of three and five devices following calibration of instrumentation and the wave and current test environment. One wave energy converter (WEC) was fully instrumented with mooring line load cells, optical motion tracker and accelerometers and tested in regular waves, short- and long-crested irregular waves and current. The wave and current test regimes were measured by six wave probes and a current meter. Arrays of three and five similar WECs, with identical mooring systems, were tested under similar environmental loading with partial monitoring of mooring forces and motions. The majority of loads on the mooring lines appeared to be broadly consistent with both logistic and normal distribution; whilst the right tail appeared to conform to the extreme value distribution. Comparison of the loads at different configurations of WEC arrays suggests that the results are broadly consistent with the hypothesis that the mooring loads should differ. In particular; the results from the tests in short crested seas conditions give an indication that peak loads in a multi WEC array may be considerably higher than in 1-WEC configuration. The test campaign has contributed essential data to the development of Simulink™ and Orcaflex™ models of devices, which include mooring system interactions, and data have also been obtained for inter-tank comparisons, studies of scale effects and validation of mooring system numerical models. It is hoped that this paper will help to draw the attention of a wider scientific community to the dataset freely available from the Marintek website.

Highlights

Owing to the increasing cost and shortage of energy resources, there has been a growing interest in renewable alternative sources of energy [1]
It can be clearly seen that the wave motion induces a complex motion dynamics of the device, consisting of both wave-frequency and low-frequency components approximating to the natural frequency of mooring system
Comparison of the loads at different configurations of wave energy converters (WEC) arrays suggests that the results are broadly consistent with the hypothesis that the mooring loads should differ

Summary

Introduction

Owing to the increasing cost and shortage of energy resources, there has been a growing interest in renewable alternative sources of energy [1]. An increasing effort should be made towards resolving the problems of extracting energy from the world’s oceans, as they represent a vast potential source of renewable energy. The results of previous research indicate that moorings may have a significant impact on the performance of energy extracting devices, both beneficial and detrimental [2]. These points are relevant to the arrays of wave energy converters, as they have to be installed in a spatially dense manner to make appropriate use of sea space and improve the economics of installation and maintenance. Constraining the “footprint” of the mooring to ensure that the moorings from each device do not interfere with one another may have great significance for the loading experienced by the line [4]

Objectives

Methods

Results

Conclusion