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.

This study aims to optimize geotextile placement depth to enhance subgrade strength and achieve sustainable pavement design. Laboratory tests were conducted to characterize the soil and evaluate the effect of geotextile placement at depths of 3/4D, 1/2D, and 1/4D (where D is the total specimen depth). California bearing ratio (CBR) tests revealed that positioning the geotextile at 0.3D significantly improves subgrade strength, yielding a 78.08% increase in soaked CBR (from 5.84 to 10.4) and a 136.56% improvement in unsoaked conditions (from 3.72 to 8.8). Pavement analysis using IITPAVE software further demonstrated that geotextile placement at 0.3D effectively reduces fatigue and rutting strains, allowing reductions in pavement layer thicknesses-16.67% for bituminous concrete (BC) and dense bituminous macadam (DBM), 38.18% for water bound macadam (WBM), and 25% for granular sub-base (GSB). These optimizations lead to a cost saving of Indian Rupee36,06,610 ($42,430) per kilometer. The findings highlight the practical and economic benefits of placing geotextile at 0.3D depth (150 mm for a 500 mm subgrade), offering improved pavement performance, material savings, and enhanced sustainability. This research benefits pavement engineers, contractors, and transportation agencies by offering a sustainable, cost-efficient design strategy. Additionally, the findings provide a foundation for future research into geosynthetic reinforcement techniques under varying soil conditions, supporting the development of resilient, eco-friendly pavements.

The fatigue life of a monopile-supported offshore wind turbine (OWT) is substantially influenced by the support conditions which inevitably changes during its service life. This study performed fatigue analyses of monopilesupported OWTs with varied support conditions, which not only determine the correlation between the support condition and the fatigue damage of OWT, but also predict fatigue lifetime of OWT for different support condition scenarios. A simplified finite element model of a 10 MW OWT is constructed. Long-term wind and wave data, measured over 25 years in the South China Sea, are utilized to determine the fatigue loads. The results show that the fatigue damage incurred in the parked status of the OWT is negligible, accounting for 0.84% of the total fatigue damage. Both support stiffness and damping have significant effects on the fatigue damage of OWT, both exhibiting a strictly negative correlation with the fatigue damage of OWTs. When stiffness and damping decrease at high rates, the total fatigue damage of the structure exceeds 100%, indicating that fatigue damage may occur in the OWT structure during its life cycle. This study provides a reference for fatigue design and may contribute to the assessment of fatigue damage in OWT.

Several studies have incorporated the trench effect into the steel catenary riser's (SCR) fatigue analysis based on two main approaches: artificial insertion of a trench profile in the touchdown zone (TDZ), and automated trench formation using nonlinear hysteretic riser-seabed interaction models. There have been contradictory results with no coherent agreement on the beneficial or detrimental effect of the trench on fatigue life. The current study has been conducted to resolve existing challenges by proposing a reliable methodology by defining an equivalent stiffness to generate a consistent trench profile entirely compatible with the natural curvature of the SCR in the TDZ.

Large offshore wind turbines (OWTs) may encounter extreme misaligned wind and wave conditions throughout their lifetime, which could trigger side-to-side resonance of the OWTs and thus substantial reduction in fatigue life. This study aims to (1) understand the dynamic response of fixed-bottom OWTs under misaligned wind and wave conditions, and to (2) propose an active torque control algorithm for dynamic loading mitigation. For these purposes, an integrated aeroelastic model, coupled with an advanced soil-monopile interaction (i.e., p-y+M-theta model), is built in OpenFAST for the DTU 10MW OWT supported by a monopile in soft clay. The numerical results show that under misaligned wind and wave conditions, where the wave peak period is likely to approach the tower natural period, the dynamic loading along the side-to-side direction dominates the fatigue design of the OWT. To mitigate the side-to-side dynamic loading, an active torque control algorithm is designed with feedback from measured side-to-side tower vibration to enhance damping, as well as feedforward from measured incoming wave height to counteract external force. Through the use of the feedback-feedforward active torque controller, the side-toside dynamic loading of the 10 MW OWT is significantly reduced, with the fatigue life extended from 19 to 39 years.

The assessment of pipeline free-spans may involve non-linearities, including those arising from the mechanical behaviour of the pipeline, soil properties and hydrodynamic loading; as well as the interaction between these factors. Failure to properly quantify and account for these factors may lead to inadequate design outcomes, which can lead to failure (if unconservative) or result in costly remediation when unnecessary (if conservative). This paper is part of an ongoing study into the vibration response of free spanning subsea pipelines, including the development of a numerical model that highlights the value of modal and full fatigue analyses as tools for understanding span behaviour. For quality control and enhancing the reliability of assessment methodologies, the model has been benchmarked against published data in terms of modal analysis, and against industrial fatigue assessment packages in terms of fatigue analysis - and shows excellent agreement with both. Since verification, the model was used to conduct a sensitivity study on a single free span, to explore how pipeline response and fatigue damage is affected by the value of dynamic soil stiffness and damping.