Fatigue reliability and calibration of fatigue design factors of wave energy converters

Abstract Target reliability levels, which are chosen dependent on the consequences in case of structural collapse, are used in this paper to calibrate partial safety factors for structural details of wave energy converters (WECs). The consequences in case of structural failure are similar for WECs and offshore wind turbines (no fatalities, low environmental pollution). Therefore, it can be assumed that the target reliability levels for WEC applications can be overtaken from offshore wind turbine studies. The partial safety factors cannot be directly overtaken from offshore wind turbines because the load characteristics are different. WECs mainly focus on wave loads where for offshore wind turbine the wind loads are most dominating. Fatigue failure is an important failure mode for offshore structures. The scope of this paper is to present appropriate Fatigue Design Factors ( FDF ), which are also called Design Fatigue Factors ( DFF ), for steel substructures of WECs. A reliability-based approach is used and a probabilistic model including design and limit state equation is established. For modelling fatigue, the SN-curve approach as well as fracture mechanics are used. Furthermore, the influence of inspections is considered in order to extend and maintain a certain target safety level. This paper uses the Wavestar prototype located at Hanstholm (DK) as case study in order to calibrate FDFs for welded and bolted details in steel structures of an offshore bottom-fixed WEC with hydraulic floaters.

[1]  P. C. Paris,et al.  A Critical Analysis of Crack Propagation Laws , 1963 .

[2]  Torgeir Moan.  Reliability-based management of inspection, maintenance and repair of offshore structures , 2005 .

[3]  F. M. Burdekin,et al.  Engineering critical analyses to BS 7910 — the UK guide on methods for assessing the acceptability of flaws in metallic structures , 2000 .

[4]  Jens Peter Kofoed,et al.  Balancing Power Output and Structural Fatigue of Wave Energy Converters by Means of Control Strategies , 2014 .

[5]  John Dalsgaard Sørensen,et al.  Fatigue Reliability and Calibration of Fatigue Design Factors for Offshore Wind Turbines , 2012 .

[6]  John Dalsgaard Sørensen,et al.  Reliability-Based Structural Optimization of Wave Energy Converters , 2014 .

[7]  John Dalsgaard Sørensen,et al.  Reliability-based optimization and optimal reliability level of offshore wind turbines , 2005 .

[8]  Basic Understanding of Weld Corrosion CORROSION FAILURES OF WELDS , .

[9]  J. Sørensen.  Reliability-Based Calibration of Fatigue Safety Factors For Offshore Wind Turbines , 2012 .

[10]  John Dalsgaard Sørensen,et al.  Framework for Risk-based Planning of Operation and Maintenance for Offshore Wind Turbines , 2009 .

[11]  J D Sørensen,et al.  Safety and inspection planning of older installations , 2007 .

[12]  Niels C. Lind,et al.  Methods of structural safety , 2006 .

[13]  Daniel Straub,et al.  Generic Approaches to Risk Based Inspection Planning for Steel Structures , 2004 .

[14]  Rico Hjerm Hansen,et al.  Modelling and Control of the Wavestar Prototype , 2011 .

[15]  P. Frigaard,et al.  Performance Evaluation of the Wavestar Prototype , 2011 .

[16]  Francesco Ferri,et al.  Validation of a Wave-Body Interaction Model by Experimental Tests , 2013 .

[17]  Inge Lotsberg,et al.  Background for New Revision of DNV-RP-C203 Fatigue Design of Offshore Steel Structures , 2010 .