Fatigue damage accumulation for offshore wind turbines is highly dependent on damping of the structure and particularly on the soil damping contribution during idling conditions. This paper presents an efficient method to estimate soil damping specific to wind turbine design and offshore environmental conditions. The method assumes a narrow-banded response, suitable for turbines in low damping conditions. The proposed method demonstrates that soil damping varies significantly with load level and that predictions can be improved over the common industry practice of assuming a constant soil damping value. The fatigue damage predicted using this proposed method is calibrated using a multi-surface plasticity foundation model. The structural response with this plasticity model is also compared to linear elastic soil modelling without any soil damping. This analysis shows that significant reductions in damage may be observed when soil Masing behaviour is incorporated. Overall, this paper shows that soil plasticity, and its variation with load level, should be included within a design framework and suggests a straightforward way of doing so.

Active-layer thickness (ALT) is one of the most robust measures used to assess the impact of climate change on terrestrial permafrost. Testing of a handheld dynamic cone penetrometer showed that it was capable of measuring ALT with the same level of accuracy as conventional methods in boreal and tundra sites in eastern Siberia. The penetrometer also characterised the vertical structure of ground hardness within the active layer. The vertical profile of penetrometer measurements corresponded closely with soil plasticity and the liquid limit in high-centred polygons produced by thermokarst subsidence in dry grassland areas at a boreal site at Churapcha. The ALT was markedly deeper (>70cm) at gravelly slope points adjacent to a wet tundra plain (<50cm) in a CALM grid (R8) at Tiksi. Overall, the penetrometer is considered to provide an accurate and informative proxy for rapidly assessing the spatial heterogeneity and interannual changes in ALT. Copyright (c) 2016 John Wiley & Sons, Ltd.