Structural integrity assessment of floating offshore wind turbine support structures

Abstract

Floating offshore wind turbines are becoming more attractive to the wind industry due to their capability to operate larger turbines in deeper waters. The floating support structures are anchored to the seabed via mooring chains to impede the structure's unwanted movements. The combination of cyclic stresses and the corrosive marine environment makes the floating support structures vulnerable to corrosion pitting and subsequently fatigue crack initiation and propagation. In this study a framework is proposed to simulate fatigue crack growth from multiple corrosion pits at critical spots of the Spar-type floating support structures to examine the status of the crack during several years of operation. The proposed advanced fracture mechanics based approach provides a methodology to assess the integrity of the structure and subsequently plan for preventive or curative maintenance. The crack growth rate is examined for both singular and multiple cracks at different R ratios and for different stress levels using ABAQUS XFEM. Following numerical simulations, a sensitivity analysis is carried out using Crackwise software for different values of plate thickness, R ratio and initial crack size. The numerical results are discussed in terms of the corrosion pitting effects on fatigue life assessment of floating offshore wind turbines.

On the performance of monopile weldments under service loading conditions and fatigue damage prediction

“…Also, the frequency of occurrence of these wave speeds over the 2‐year period was used to estimate the weight that should be given to each group during fatigue life analysis. For any given moment in time, one of the load cases (or wave force) and its corresponding frequency were assumed to be acting on the OWT structure 33 …”

“…25 The splash zone (at the free surface of sea) is considered to undergo maximum uniform corrosion that could amount to a yearly thickness reduction of 0.2-0.4 mm in the OWT monopile structure. 33 The submerged sections are subjected to a thickness reduction of 0.1-0.2 mm per year. 33 The present study aims to analyze the effect of service loading conditions on offshore wind monopile foundation structure using a full scaled FE model to predict the fatigue life of the structure.…”

“…Seawater is generally considered to be composed of 3.5 wt.% of sodium chloride (NaCl), and its pH ranges from 7.8 to 8.3 25 . The splash zone (at the free surface of sea) is considered to undergo maximum uniform corrosion that could amount to a yearly thickness reduction of 0.2–0.4 mm in the OWT monopile structure 33 . The submerged sections are subjected to a thickness reduction of 0.1–0.2 mm per year 33 …”

“…However, the quantification of corrosion-fatigue mechanism poses an important challenge as the pit-to-crack stage of this process is largely unknown; hence, in most instances, the pit itself is often taken as a crack in the fatigue analysis. 17,33,34 Corrosion damage can be accounted for in the fatigue design process by considering the S-N curves for structural steels in seawater with and without cathodic protection. 19 Nevertheless, such a prediction would have inherent uncertainties due to the variations in corrosion rate with geometric location as well as the water depth.…”

“…Corrosion‐fatigue process is initiated by the formation of localized corrosion pits at certain parts of the wind turbine structure, which are formed as a result of the breakdown of the thin oxide layer on the surface of metals, which then develop into a critical size large enough to initiate a crack. However, the quantification of corrosion‐fatigue mechanism poses an important challenge as the pit‐to‐crack stage of this process is largely unknown; hence, in most instances, the pit itself is often taken as a crack in the fatigue analysis 17,33,34 . Corrosion damage can be accounted for in the fatigue design process by considering the S–N curves for structural steels in seawater with and without cathodic protection 19 .…”