Fig. 1 Schematic showing (a) extraction location for preparing cross-weld specimens, (b) specimen geometry (all dimensions are in millimetres)

Strain controlled cyclic load tests were performed using a 100 kN Instron servo-hydraulic machine at room temperature condition. Three strain amplitudes of ±1%, ±2% and 0-3% were selected to capture the cyclic deformation behaviour of the cross-weld and base metal specimens. Two tests were performed at each strain amplitude, with one test capturing the cross-weld material behaviour while the second test giving the base material behaviour which acts as a reference for comparison purposes. In each test, the specimen was loaded at a frequency of 0.1 Hz and the measurements were recorded at 40 Hz to capture the variations in stress levels in the test specimen. The tests were conducted for 200 cycles or until failure, if the cycles to failure was less than 200 cycles.

For the purpose of determining the service loads acting on an OWT, online monitoring data for the wind and wave characteristics was taken from an offshore wind farm located in the North Sea. The data was collected from a 6 MW capacity OWT which had a rotor diameter of 154 m and hub height of 106 m. The measurement and recording of the wind profile was carried out using supervisory control and data acquisition (SCADA) system. The data was collected for a period of 2 years, from beginning of 2016 to the end of 2017. The data included main shaft rotational frequency (RPM), wind speed (minimum, maximum, mean and standard deviation), wind direction (monitored by the yaw position sensor), ambient temperature and turbine power generation, recorded every 10 minutes over the 2-year duration. Similarly, the wave characteristics including the maximum wave height, wave period, mean wave spectral direction and water temperature were monitored using a wave buoy (SEAWATCH midi model) at an interval of every 30 minutes over the 2-year period. The wind speed and wave height variations exhibited a random behaviour with respect to time as shown in Fig. 2(a) and Fig. 2(b) respectively.