Delft Offshore Turbine (DOT) is developing a new offshore wind turbine, including, amongst other innovations, a slip-joint that connects the turbine tower and monopile foundation. The slip-joint is a mechanical, friction-based connection that provides many advantages over a bolted or grouted connection, such as the ability to support high loads, reduced maintenance costs and rapid installation. However, for the slip-joint to be accurately designed and to have its performance guaranteed, it is necessary to know the contact behaviour within the slip-joint.

Heat transfer has been selected as the optimal measurement basis over magnetic, electrical and other techniques, as heat transfer through the slip-joint wall is correlated with both contact pressure and air gap size (in non-contact regions). A continuous-heating thermographic point measurement method has been developed to measure the contact pressure and contact gaps in the slip-joint connection. This requires the use of an induction heater to heat one side of the slip-joint wall while, on the other side of the wall, a thermal camera measures the temperature rise. This method was tested on a sample slip-joint from a decommissioned onshore wind turbine.
Prior to full scale testing, the method was validated through comparison with a finite element model. This comparison shows that the method is sensitive to variations in contact and pressure, especially in regions where contact pressure is close to zero. Additional small-scale experiments were conducted with two 8mm steel plates separated by a known gap. This demonstrated the sensitivity of the method to gap size changes of less than 0.1mm and gap sizes greater than 1mm. Finally, single-ply validation testing shows the precision of the measurement method to be ±13% (at 90% confidence interval).

Following validation, the method was used to measure the contact distribution across 207 grid points on the sample slip-joint. The results show thin (0.3-1m), longitudinal low-pressure regions extending from the top and bottom of the joint towards the centre. Physical verification with feeler gauges indicates that 34% of the joint is not in contact (with 95% confidence interval of [23% 51%]). Feeler gauge testing also shows that the correlation between heat measurements and actual gap size is in agreement in both full-scale and small-scale validation measurements. Finally, feeler gauge measurements show there is a ±13% error in the thermal measurements.

The results show that the thermographic method for slip-joint contact area measurement is suitable to resolve pressure and gap size variations in the slip-joint. Additionally, the measurement results show that it is necessary for engineers to consider up to 50% non-contact during the design process and joint contact should be assessed in all slip-joints post-installation. Finally, future development of the method through the use of more sensitive equipment and performing additional single-ply measurements to establish a clearer error estimate is recommended. This should be augmented by a comparison to residual stress measurements using the Magnetic Barkhausen Noise (MBN) method, as well as acoustic resonance stiffness measurements, to verify contact results and examine the cause of such contact variations in a sample joint.