In order to characterise the fatigue crack growth (FCG) behaviour of the welded components used in offshore wind turbine monopiles, a number of tests have been performed on compact tension, C(T), specimens extracted from typical monopile weldment sections with the notch tip located in the HAZ region. The preliminary results from these tests have shown a ?bi-linear? FCG trend which is thought to be due to the compressive-tensile residual stress profiles available in the samples. It is therefore proposed that Neutron Diffraction (ND) measurements are performed on a slice of S355ML Steel welded plates and also a C(T) specimen extracted from the welded plates. The ND measurements from this experiment will facilitate the interpretation of the FCG behaviour in monopile welded structures and will contribute to the development of an optimum design for the offshore wind monopile structures.