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.