Offshore wind turbines are often supported on monopiles and are always subjected to long-term cyclic loading during their service life. This cyclic loading induces changes in the damping, stiffness and permanent accumulated rotation of the monopile foundation. The main purpose of this paper is to investigate the effect of these three changes occurring simultaneously on the dynamic response and fatigue life of offshore wind turbines in sand. To this end, an integrated methodology is presented based on time-domain finite element model and small-scale model tests of rigid piles. Three states of the monopile foundation are selected and defined based on the operational time of the turbine. The results show that these three changes have a slight effect on the dynamic response of offshore wind turbines, but have a significant effect on the fatigue life. The fatigue life decreased from 23.3 years for the initial state to 20.99 years for the medium state and 19.45 years for the ultimate state, a decrease of 10% and 16.5%, respectively, indicating that these changes should be addressed in the design of the fatigue life calculation of offshore wind turbine structures. The systematic parametric analysis shows that soil damping has the greatest effect on the dynamic response and fatigue life, followed by soil stiffness, which is less affected by permanent accumulated rotation.

Pipeline free-spans are sections of the pipeline that are not supported by the seabed, and can occur either due to natural processes such as seabed undulations and scour, or exist as engineered sections including pipeline crossings or buckle initiators. Free-spans can leave the pipeline susceptible to inline (horizontal) or cross-flow (vertical) Vortex-Induced-Vibrations (VIV) that could potentially result in fatigue failure. For spans deemed to have a high probability of failure, i.e. critical spans, or for spans evolving over time due to seabed mobility, expensive mitigation can be required. To assess the remaining fatigue life of a free-span, the system response must be quantified, including interactions with the seabed at the span shoulders, which are usually defined in terms of dynamic soil spring stiffness and soil damping values. The paper presents findings from an experimental study to determine soil stiffness and damping during simulated VIV motions for pipeline 'elements' (representing parts of the free-span 'shoulder') at varied pipeline vertical pressures and installation modelling scenarios. Testing was performed in a carbonate silty sand sample. The paper focuses on in-line VIV, although selected results for cross-flow VIV are also presented. The results indicate that the initial embedment, amplitude of the cyclic motion and pipeline self-weight significantly affect the soil response. These findings will improve understanding of Pipe-Soil-Interaction (PSI) for free-span shoulder sections, resulting in increased confidence in the selection of non-linear soil springs and dashpots, and contributing to more informed assessment of free-span behavior.