Abstract
The purpose of this paper was to investigate the impact of residual stresses on fatigue damage of offshore wind turbine monopiles by numerical analysis approach using ABAQUS software, a finite element analysis (FEA) tool. Three monopile models with the same dimension (within standard range) have been developed in ABAQUS and partitioned circumferentially into equal rings. Longitudinal partitions have been rotated through 180° as obtainable in practice. Characteristic loads typical of a real life offshore wind turbine environment have been applied to all three models, with tensile and compressive residual stresses applied as additional loads at the critical weld region to the first and second models while the third model had no additional load. With zero boundary conditions applied in all six degrees of freedom, the simulation has been run for 107 cycles of wind and wave loads as recommended in standards in each case. Stress results obtained from the critical weld region in the three models showed that the presence of tensile residual stresses equal to the material yield stress contributed a maximum 0.05% to fatigue damage of the monopile when compared with results from the model with no residual stress while the presence of compressive residual stresses with the magnitude of the yield stress of the material caused a gain of 0.06% in fatigue life by similar comparison, indicating negligible contribution of residual stresses to the stress build up in the critical weld region, thus suggesting that the magnitude of the residual stress as high as the yield stress of the material of the monopile is not large enough to cause the monopile to open up in the axial direction.
Highlights
Climate change, a global phenomenon arising from the increase in levels of carbon dioxide (CO2) and other greenhouse gases due to human activity, has threatened human existence over the years
A numerical analysis which examined the effect of residual stresses on fatigue damage of offshore wind turbine monopile foundations has been carried out in this work
A monopile model within the range of established monopile geometry has been built in ABAQUS where the best/worst case of residual stress have been applied in the critical weld region as concentrated force along Z-direction, the dominant stress direction
Summary
A global phenomenon arising from the increase in levels of carbon dioxide (CO2) and other greenhouse gases due to human activity, has threatened human existence over the years. While there are several foundation types used in shallow waters, the monopile is the most widely used as it accounts for about 75% of offshore wind turbine (OWT) installations in water depths up to 40 m [2] [3] These monopiles are a single large-diameter manufactured steel tubes which penetrate the seabed and have outer diameters of up to 7.5 m, a wall thickness of up to 150 mm and penetration depths ranging from 15 to 30 m [3]. The monopile bears the loads (forces) of the rotor-nacelle assembly, the tower, work platform as well as the transition piece All these constitute the vertical loads (axial loads) acting on the monopile. These lateral loads are generally larger than the axial loads, cause bending and more critical than the vertical loads in monopiles as they control the serviceability limit state of the whole structure [3]
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