Increased fatigue service life and inspection intervals of welded offshore foundations

Project Number 387

Offshore wind park owners are required to inspect the structures of the foundations at specified intervals (usually every fourth or fifth year). These inspections, in particular for the sub-sea structure, are very expensive. Moreover, the weather window to safely inspect the structure is limited. Optimised inspection intervals without jeopardizing safety will reduce the O&M costs.

In addition offshore wind structure designers and manufacturers are faced with increasing turbine power and increasing water depth, both leading to higher loads on the substructure. On the other hand, there is a requirement to reduce structural weight and capital costs. The design standards with respect to fatigue are known to be conservative. A proven, less conservative design standard will allow designers and manufacturers to build lighter structures and the service life of existing support structures could potentially be extended.

The technical life of support structures of offshore wind turbines is significantly affected by degradation due to fatigue. Welded joints, being the locations with high local stresses that may have initial defects, are the critical parts of the structure from fatigue point of view.

Fatigue damage can, when not properly monitored, result in sudden collapse of the structure. The design challenge is to balance fatigue performance (lowering local stresses) and structural weight (lower installation and manufacturing costs). Standards and guidelines provide methods to predict the fatigue life. These are conservative, i.e. the real fatigue life is often longer than predicted. This is due to the required safety, uncertainty in load and resistance variables, and simplifications in the calculation procedures. Minimisation of the failure probability by fatigue within the design life and minimisation of inspection are design drivers as they affect the operating expenditure.

The aim of the project is to develop a substantiated accurate model for fatigue crack growth that results in a reduced conservatism of the fatigue life prediction. The inspection intervals could be extended compared to the current state of the art.

The project is substantially funded by the Dutch Topconsortium Knowledge and Innovation (TKI) Offshore Wind. Among others, TKI comprises the partly governmental-financed TNO. Participation of VGB would ensure that demands of the wind turbine operators represented in the VGB-European Working Group “Wind Energy” will influence the project execution.