In an era where global energy consumption has been rising steadily, the demand and supply of renewable energy sources have become increasingly important. Offshore wind is a promising renewable energy source, which has been actively developed in the last decades in the form of bottom-founded offshore wind, bound to shallow water. Floating Offshore Wind Turbines (FOWT) offer a solution for countries without shallow coastal waters with an abundance of wind potential. In a competitive market with different solutions, Bluewater Energy Services (Bluewater) has developed a tension leg platform (TLP) type substructure to support a 15MW turbine. These FOWT installations are subject to the random nature of environmental loading which is highly stochastic in terms of amplitude, frequency and phase angle and is hence considered multiaxial non-proportional (random) variable amplitude loading. Multiaxial loading does not, however, guarantee a multiaxial stress response. One of the considerations for offshore structures subject to repetitive loading is fatigue lifetime estimation, which is predominantly done using uniaxial assessment methods. This is not an issue if the stress response is dominantly uniaxial, however, literature has demonstrated that evaluating a specimen subject mode-{I,III} multiaxial stress response using uniaxial (mode-{I}) assessment methods can lead to a significant overestimation of the fatigue lifetime. To investigate the (multiaxial) stress response of fatigue-critical weld seams in TLP-type FOWT substructures, a new simulation methodology needed to be developed, which allowed for the structure's rigid body motions and elastic deformations to be captured. This methodology was built on an existing hybrid rigid-flexible modelling approach. It was improved upon by considering local (panel-based) hydrodynamic pressure and kinematic structural boundary conditions in order to conduct coupled (aerodynamic-hydrodynamic-mooring) analysis with integrated structural analysis. This approach allowed for the system motion and structural response to a dynamic metocean environment to be simulated using a range of commonly occurring seastates. The integrated coupled analysis was completely conducted within the Ansys environment. Next, time-varying mode-{I,III} stress signals were reconstructed from the structural analysis model using a traction-based structural stress assessment method. In order to assess the extent to which fatigue-critical weld seams are subject to multiaxial stress response, a damage accumulation calculation should be conducted. However, given the added computational expense of such a calculation, a screening method was first considered in order to better understand the characteristics of stress response behaviour. The screening method considered was based on a Minimum-Circumscribed Ellipse method. From this screening method, it was found that stress response behaviour is predictable and can be related to specific loading & response frequencies. Furthermore, it was found that parts of the considered structure were subject to significant levels of multiaxial non-proportional stress response and that applying uniaxial fatigue assessment criteria risks overestimating lifetime estimation and misplacing the governing fatigue hotspot. Finally, it was concluded that fatigue-critical weld seams in a FOWT TLP-type substructure are subject to multiaxial stress to the extent that multiaxial damage accumulation models should be considered. If this is not possible, a multiaxial stress response screening should be conducted to identify fatigue governing locations in the welds, as well as locations subject to multiaxial stress.