Development of excess pore-water pressure within seabed caused by the waves propagating on the surface of sea may threaten the safety of an offshore foundation. Very few studies have considered effect of wave specifications on residual wave-induced liquefaction around caissons. In this study, a comprehensive elasto-plastic soil model for large liquefaction-induced shear deformation of sand is adopted to capture changes of pore water pressure and effective stresses within the sandy seabed. The caisson is expected to behave under the linear elastic law and considered as a single-phase medium. OpenSees, the FEM framework initially programmed for soil-structure interaction under seismic loadings, is used for the problem of Wave-Foundation-Seabed Interaction (WFSI). Biot’s consolidation theory, linear wave theory and the advanced soil model are eventually combined to simulate the soil response accounting for the hydrodynamic pressure of wave imposed on the surface of seabed in the presence of an offshore foundation. This finite element numerical model is validated by a well-documented centrifuge experimental model, upon which the gravitational analysis is conducted on whole domain before performing the dynamic analysis.
It is demonstrated from numerical results that caisson’s skirt tip is a potential zone for early development of liquefaction due to static shear-stress concentration beneath it which acts together with contact pressure to develop 3D hydraulic gradients in the vicinity of caissons which are all found to be prominent in assessing a WFSI problem. It was either identified that inclusion of skirts interrupts the dissipation pattern of accumulated pore-pressure because of longer drainage path which is another scope of the current study.

Drag embedment anchors combined with catenary mooring systems are widely used for temporary and permeant station keeping of the offshore floating facilities. By growing exploration and production of offshore reserves, the number of mooring failure incidents in floating facilities has been increased. This implies the significance of the reliability assessment of the mooring system components and particularly drag embedment anchors as one of the key elements. The currently used anchor design codes consider only homogeneous seabed soil conditions. It is publicly accepted that the presence of the layered seabed may significantly affect the ultimate holding capacity of anchors. Therefore, it is expected that layered seabed condition affects the reliability indexes of these anchors as well. However, there are only a few published studies that have investigated homogeneous seabed soil conditions ignoring the effect of layered soil strata. In this study, the reliability of drag embedment anchors was comprehensively investigated in the layered seabed (clay over sand). An advanced calculation tool was developed to obtain the holding capacity of the anchors by combining a series of iterative limit state and kinematic analysis. Time domain dynamic mooring analysis was conducted by assuming a semisubmersible platform to obtain the dynamic line tensions. The uncertainties of the environmental loads, metocean variables, seabed soil properties were incorporated into a first-order reliability analysis (FORM) to obtain the failure probabilities. A probabilistic model established for determination of holding the capacity for nominated drag anchor families. The study revealed a significant effect of the layered soil condition in reliability assessment by lowering the magnitude of reliability indexes. The improvement of the recommendations provided by design codes by incorporation of the complex seabed condition was found necessary for a safer and cost-effective anchor design.

Monopile-supported offshore wind turbines are vulnerable to both scour and earthquakes in seismically active regions. Scour changes not only the damping and natural periods of the monopile-soil system but also the seismic and hydrodynamic loads on the pile. In general practice, the estimated maximum scour depth is often used for the design of the marine foundations. Nevertheless, unlike the pile static capacities, the highest seismic demand of pile may not be associated with the maximum scour depth employed in the routine design. Such combined effects of scour and earthquakes on the monopile are not well understood, particularly for a live-bed condition involving a changeable scour depth. This study aims to fill this gap through parametric analyses considering various scour depths and seismic inputs. An open-source finite element model was developed following the dynamic-beam-on-nonlinear-Winkler-foundation method via OpenSees, where pile-water interactions were represented by hydrodynamic added masses. Moreover, pile static lateral responses under various scour depths were analyzed in OpenSees using the conventional nonlinear Winkler foundation method. Through the parametric analyses, scour effects on both the static and seismic responses of the monopile in soft clays were investigated, and the critical scour depths corresponding to the peak static and seismic demands were obtained. In the end, recommendations for selecting proper scour depths were made for the design of the monopile under static and seismic loading.