The design of steel catenary risers (SCRs) is mainly affected by fatigue performance in the touchdown zone (TDZ), where the riser cyclically interacts with the seabed. This cyclic motion leads to seabed soil softening and remoulding. However, over an extended period of riser operations, the seabed soil undergoes a drainage because of small motion amplitudes of the floating vessel during calm weather or a limited contact with the seabed due to vessel relocation. This may cause recovery of the soil strength associated with excess pore pressure dissipation resulting in an extra fatigue damage accumulation in the TDZ. In the current study, a global SCR analysis has been conducted using a series of coded springs along the TDZ to model advanced SCR-seabed interactions. The instantaneous undrained shear strength of the soil is determined by using a recently developed effective stress framework. The effects of soil remolding and consolidation were integrated during both the dynamic motion of the SCR and intervening pause periods within the critical-state soil mechanics. The model updates the SCR-soil interaction spring at every time increment of dynamic analysis, calculating the cross- stress range while taking into account the overall configuration of the riser on the seabed. The study showed that the consolidation may result in an increased fatigue damage of about 23 %, which is currently neglected by the existing non-linear SCR-soil interaction models.

Dynamic soil-structure interaction (SSI) is an important field in civil engineering with applications in earthquake engineering, structural dynamics, and structural health monitoring (SHM). There is an ongoing need for the development of numerical methods that can accurately estimate SSI parameters to model these systems. In this paper, a Frequency Response Function (FRF)-based model updating method is developed that can estimate the embedded length of foundation piles, in addition to the mobilized soil mass and stiffness, when a lateral impact load is applied. Knowledge of the embedded length of piles is very important for modelling foundation behaviour, and may not be readily available from as-built construction information. For example, if developing reference damage models or digital twins of foundation structures, full knowledge of the pile geometry is required. The work in this paper develops a two-stage iterative model updating method, which utilizes FRF data obtained at the pile ' s head as a result of an applied lateral impact load. The method uses information from the 1st mode of vibration to estimate the mobilised soil mass and stiffness, and subsequently uses information from the 2nd mode of vibration to estimate the embedded length. To appraise the approach, impact tests are numerically simulated on a number of 'piles ' (numerical spring-beam systems) with varying length/diameter ( L/D ) ratios to derive FRFs, whereby the models have known length and dynamic properties. These FRFs are then used as targets in the model updating approach, which iteratively varies the properties of a numerical model of a pile to obtain a match in the FRF data, and subsequently estimates the mobilised stiffness, mass, and embedded length. The results of the analyses illustrate that by minimising the difference in the first and second FRF peaks between the target and estimated FRFs, the method can accurately estimate the mass, stiffness and embedded length properties of the test 'piles ' . The performance of the approach against numerical case applications is assessed in this paper, as the properties of these systems are known in advance, facilitating quantification of the errors and performance. The developed method requires further validation through full-scale testing to confirm its effectiveness in real-world scenarios.

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.