Analysis of the Fatigue Characteristics of Mooring Lines and Power Cables for Floating Wave Energy Converters
Doktorsavhandling, 2018

To reduce greenhouse gas emissions and expand the energy mix, there is a pressing need for the exploitation of renewable sources of energy, such as biomass, hydropower, solar power, waves, and wind. This thesis focuses on ocean wave energy and its applications. Ocean wave energy is abundant and geographically widespread and has one of the highest energy densities among renewable energy sources, presenting a great opportunity for securing an emission-free energy supply. However, one challenge associated with existing wave energy technology is to ensure and verify the reliability and long-term performance of wave energy converter (WEC) systems; these aspects are fundamental to achieving wave energy at a cost that is commercially competitive in the long term.

The main objective of this thesis was to develop a complete numerical analysis procedure for assessing the fatigue characteristics of the mooring lines and power cables used in floating WEC systems. Both the moorings and cables must be designed to survive under cyclic loading and not fail due to fatigue, which would endanger the safety and functionality of the WEC system. However, due to the wide variety of WEC concepts that have been proposed, it remains challenging to identify which numerical methods are most appropriate for the reliable prediction of the mechanical service life of the moorings and cables. Starting from a hydrodynamic and structural response analysis of a WEC system or an array of WECs, the research presented herein contributes to a systems perspective in which the fatigue performance of moorings and cables is predicted, the power performance is estimated, and the levelised cost of energy is calculated with consideration of the interaction effects among WECs in an array.

It was found that a coupled analysis approach should be used to simulate the hydrodynamic and structural responses of WECs because it captures best the mechanical coupling and hydrodynamic interaction effects of WEC systems. A wave-height/wave-period matrix of fatigue was designed as a visualisation tool to illustrate the influence of environmental loads on fatigue damage accumulation in the moorings and cables. Numerical simulations of multi-WEC array farms showed that hydrodynamic interactions among WECs in an array farm strongly affect fatigue damage in moorings, which in turn influence the related cost assessment of a WEC system. Compared with a biofouling-free condition, it was shown that the presence of biofouling on the WEC system not only reduces the power absorption of WECs but also decreases the fatigue life of the moorings and cables. The results obtained from the numerical simulation were validated against a model-scale ocean basin laboratory experiment and compared with measurement data from a full-scale WEC installation. The findings showed that the simulation model can satisfactorily predict the dynamic motion response of a WEC system under moderate sea state conditions and under non-resonant conditions.

mooring line

power cable


coupled analysis

power absorption


wave energy converter

heaving point absorber

computational modelling

marine biofouling effect on fatigue

Lecture hall Hållö in SAGA-building, Hörselgången 4, Chalmers Campus Lindholmen
Opponent: Professor Zhen Gao, Norwegian University of Science and Technology, Norway


Shun-Han Yang

Chalmers, Mekanik och maritima vetenskaper, Marin teknik

A comparison of coupled and de-coupled simulation procedures for the fatigue analysis of WEC mooring lines

Ocean Engineering,; Vol. 117(2016)p. 332-345

Artikel i vetenskaplig tidskrift

Parametric study of the dynamic motions and mechanical characteristics of power cables for wave energy converters

Journal of Marine Science and Technology,; Vol. 23(2018)p. 10-29

Artikel i vetenskaplig tidskrift

Experimental and numerical investigation of a taut-moored wave energy converter—a validation of simulated buoy motions

Proceedings of the Institution of Mechanical Engineers Part M: Journal of Engineering for the Maritime Environment,; Vol. 232(2018)p. 97-115

Artikel i vetenskaplig tidskrift

Yang, S.-H., Ringsberg, J. W., & Johnson, E. Wave energy converters in array configurations—Influence of interaction effects on the power performance and fatigue of mooring lines

To reduce greenhouse gas emissions and increase in the diversity of the energy mix, renewable energy sources, such as biomass, hydropower, solar power, waves, and wind, are a pressing need for all countries around the world seeking to develop a sustainable energy policy. With the ocean covering approximately 72% of the earth’s surface, wave energy is abundant and geographically widespread, thereby holding great potential to satisfy a significant percentage of the worldwide energy supply. To date, however, wave energy has not been utilised on a significant scale. The challenge that currently must be overcome for the wave energy industry to make a commercial impact is to reduce costs and improve the reliability and performance of wave energy systems.

Over hundreds of concepts and devices have been developed to harness wave energy. To exploit the full potential of ocean waves, one trend that has emerged is deploying floating wave energy devices and operating multiple devices together in the same area as an array. Mooring lines and power cables are thus two components that are ubiquitous to all types of floating wave energy devices; the former is used to anchor the device, and the latter is responsible for the energy transmission. Given their continuous environmental loading, both the moorings and cables must be designed to survive a large number of load cycles and not fail due to fatigue, a term used to refer to the progressive structural damage that occurs due to cyclic loading.

This thesis contributes by developing a numerical analysis procedure that can be used for assessing the fatigue characteristics of moorings and cables. The procedure enables the detailed fatigue life assessment of moorings and cables; at the same time, it provides a higher, systems-level perspective of the wave energy device, such as its power performance and energy cost. With the numerical analysis procedure presented in the thesis, we will be able to provide design solutions for moorings and cables with long service lives. Such designs will improve the durability and reliability of wave energy technology—an essential step toward the full commercialisation of wave energy.

Simuleringsmodell för drift- och underhållsstrategi av flytande vågkraftverk – analys av utmattning, nötning, och inverkan av biofouling för effektiv och lönsam energiutvinning

Energimyndigheten, 2016-06-01 -- 2018-05-31.

FoU av dynamiska lågspänningskablage mellan boj och flytande hubb i ett marint energisystem

Energimyndigheten, 2015-12-08 -- 2017-08-01.

Ocean Energy Centre - Mekanisk livslängdsanalys av kablar och förtöjningar för system för havsenergiutvinning

Energimyndigheten, 2012-11-01 -- 2015-12-31.


Hållbar utveckling





Marin teknik

Annan elektroteknik och elektronik



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4454


Chalmers tekniska högskola

Lecture hall Hållö in SAGA-building, Hörselgången 4, Chalmers Campus Lindholmen

Opponent: Professor Zhen Gao, Norwegian University of Science and Technology, Norway

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