The long-term settlement of subsea pipelines on a clayey seabed is crucial for the on-bottom stability of the pipelines, especially in deep waters. In this study, a poro-elasto-viscoplastic finite element analysis is performed for predicting long-term settlement of subsea pipelines by incorporating a rheological constitutive model. A method for identifying the creep-settlement (Sc) from the total-embedment (Sk) is proposed on the basis of the obtained linear relationship between the secondary consolidation coefficient (C alpha e) of the clayey soil and the total-embedment (Sk) of the pipe. The identifying method is validated with the existing theoretical solutions and experimental data. Parametric study is then performed to investigate the key influential parameters for long-term settlement of subsea pipeline. A non-dimensional parameter Gc is introduced to quantitatively characterize the soil rheology effect on pipeline settlement. The relationship between the proportion of creep-settlement in the total-embedment (Sc/Sk) and Gc is eventually established for identifying whether the proportion of creep-settlement in the total-embedment is remarkable.

Waves can cause significant accumulation of pore water pressure and liquefaction in seabed soils, leading to instability of foundations of marine hydrokinetic devices (MHKs). Geostatic shear stresses (existing around foundations, within slopes, etc.) can substantially alter the rate of pore pressure buildup, further complicating the liquefaction susceptibility assessments. In this study, the development of wave-induced residual pore water pressure and liquefaction within sandy seabed slopes supporting MHK structures is evaluated. Unlike most earlier studies that excluded the impact of shear stress ratios (SSR) on the residual pore pressure response of sloping seabeds, asymmetrical cyclic loadings are considered herein for a range of SSRs. To obtain wave-induced loading in the seabed (and cyclic shear stress ratios, CSRs), the poroelasticity equations governing the seabed response, coupled with those for fluid and structure domains, are solved simultaneously. Utilizing an experimental model based on anisotropic cyclic triaxial test data that includes CSR and SSR impacts, an equation for the rate of pore pressure buildup is developed and added as a source term to the 2D consolidation equation. Numerical investigations were performed by developing finite element models in time domain. The models were calibrated using particle swarm optimization method and validated against wave flume experimental data. The results indicate that the consideration of static shear stresses has led to sudden rise in residual pore pressures followed by fast dissipations at early and late time steps, respectively, beneath the structure. The exclusion of SSR is shown to cause significant overestimation of pore pressure accumulations at late cycles, potentially causing significant overdesign of MHK foundations. The impact of proximity to the free drainage boundary, CSR amplitude, and loading frequency on the accumulation of residual pore pressure is illustrated. The residual liquefaction susceptibility of the seabed is shown to decline by increase of the seabed slope angle.

To address the challenges of extraction difficulties and penetration risks associated with traditional spudcan jackup platforms, a new jack-up platform featuring a pile-leg mat foundation is proposed. The horizontal bearing capacity of hybrid foundations under the influence of dynamic loads is a critical factor that requires close attention. This research numerically examined the dynamic response of a hybrid foundation to horizontal cyclic loading on a sandy seabed. A user-defined subroutine was employed to incorporate the Cyclic Mobility (CM) model within Abaqus, facilitating the analysis of sand response under different densities. The horizontal cyclic bearing capacities of the foundation were investigated considering the effects of different loading conditions, sand density, and pile-leg penetration depth. Simulation results indicate that the cyclic loading amplitude, frequency, and load mode significantly influence the generation of soil excess pore water pressure (EPWP), subsequently affecting foundation displacement and unloading stiffness. Under cyclic loading, the loose sandy seabed shows the most pronounced fluctuations in EPWP and effective stress, leading to surface soil liquefaction. While surface soil in medium-dense and dense sand conditions remains non-liquefied, their effective stress still varies significantly. Increasing the pile-leg penetration depth enhances the foundation's horizontal bearing capacity while affecting its vertical bearing capacity slightly.

Seabed instability is one of the important reasons for offshore structure damage. Unlike most previous studies that treated the oscillatory and residual response separately, a coupled model for wave-induced response in non- homogeneous seabeds is proposed in the present study. Effects of spatial derivative terms in seabed parameters are introduced into the accumulation of pore pressure. Model validations are conducted by comparing the present simulation with the previous analytical solutions, wave flume tests, and numerical simulations. The validated model is applied to investigate the effects of grain size, non-homogeneous distribution of seabed parameters, and non-linear wave conditions on the wave-induced seabed dynamic response and liquefaction. It is found that (1) the oscillatory mechanism in pore pressure variation dominates in the coarser seabed, while the residual mechanism becomes obvious with the decreasing grain size, (2) consideration of the non-uniform permeability and Young's modulus would promote and suppress the pore pressure accumulation and liquefaction, respectively, and (3) the simulation error in pore pressure between homogeneous and non-homogeneous seabeds increases with the increase of the wave nonlinearity.

Liquefaction and shear sliding (i.e., slides) are common failure modes of cohesionless seafloor under ocean waves. However, existing research has rarely focused on shear-sliding failure, especially when considering wave- induced residual pore water pressure. Additionally, the relationship between shear-sliding failure and liquefaction is not well understood. In this study, a slice method is developed to assess the shear-sliding failure in cohesionless seafloor under non-linear waves, incorporating the effect of both oscillatory and residual seabed responses. The applicability of various liquefaction criteria is discussed, based on the interrelation between the shear-sliding and liquefaction zones. The results indicate that the seabed soil is more prone to shear-sliding failure than liquefaction under wave-induced pore water pressure. When only oscillatory pore water pressure is considered, the liquefaction criteria, assuming the initial vertical effective stress vanishes due to the excess pore water pressure, better identify the liquefaction zone, which is enveloped by and overlaps with the shear- sliding zone at a factor of safety of zero. In cases where both oscillatory and residual pore pressure coexist, the unified liquefaction criterion, which also assumes onset of liquefaction at zero vertical effective stress, provides more reliable predictions of the liquefaction zone. As residual pore pressure accumulates, the difference between shear sliding and liquefaction depths becomes more pronounced. A sensitivity analysis of shear-sliding depth with varying soil parameters indicates that relative density exerts the most significant influence, followed by the effective internal friction angle, while the shear modulus has the least effect. The effect of variations in soil parameters on shear-sliding depth diminishes to some extent with prolonged wave action.

Seabed foundations consisting of interbedded layers of saturated soft clay and sand deposited during the Quaternary period are widely distributed in the coastal areas of Southeastern China. These soil foundations are prone to significant settlement under seismic loading. The study of the seismic dynamic response characteristics of saturated foundations with interbedded soft clay-sand and the development of rapid prediction models are essential for controlling settlement and ensuring the service safety of marine structures. A total of 4000 sets of seabed foundation models are randomly generated, with layers of saturated soft clay and sand and with a random distribution of layer thickness and burial depth. The mechanical behavior of saturated soft clay is described using the Soft Clay model based on the boundary surface theory, and the generalized elastoplastic constitutive model PZIII is used to characterize the mechanical behavior of sandy soil. The finite element platform FssiCAS is employed for a computational analysis to study the characteristics of seismic subsidence in saturated seabed foundations with interbedded soft clay-sand. A machine learning model is implemented based on the Random Forest algorithm, in which 3200 sets of numerical simulation results are used for model training, and 800 sets are used for validating the model's reliability. The results show that under seismic excitation, the pore water pressure within the saturated seabed foundation with interbedded soft clay-sand accumulates, effective stress decreases, and the seabed foundation softens, to a certain extent. During the post-seismic consolidation phase, significant settlement of the seabed foundation occurs. The fast prediction model based on the Random Forest algorithm could reliably predict the settlement characteristics of submarine foundations. This research provides a new technological avenue for the rapid prediction of the seismic settlement of submarine foundations, which could be of use in engineering design, assessment, and prediction.

The sinkage of underwater landing robots deployed on loose seabed sediment over extended periods poses a significant challenge. Excessive sinkage can significantly reduce the locomotion performance of robots or even cause them to become trapped in the sediment. The study investigates the long-term sinkage behavior of underwater landing robots on seabed sediments using numerical simulations. Focusing on varying pressure source geometries and sediment mechanical properties, this research identifies distinct stages of sediment sinkage: transient, viscoelastic, and stable creep. A time-dependent pressure-sinkage model is established to predict sinkage behavior over extended durations, accounting for the effects of pressure magnitude, pressure source geometry, and sediment characteristics. Results demonstrate significant differences in sinkage depth between cylindrical and plate pressure sources, with maximum disparities reaching 158 mm under high-pressure conditions. The study further reveals that sediment parameters such as density, cohesion, and internal friction angle significantly influence sinkage, particularly in later stages, whereas loading rates affect initial but not long-term sinkage. The findings provide valuable insights for optimizing underwater landing robot designs and operational strategies, especially for long-term seabed deployments on complex sediment types. This research offers guidance for mitigating excessive sinkage risks and improving stability in underwater environments with varied sedimentary conditions.

This study proposed a novel experimental platform to conduct dynamic loading tests of a truncated model steel catenary riser (SCR) within the touchdown zone (TDZ). The facilities of the platform, including a soil tank, a loading system, and a soil stirring system, are introduced in detail. A steel pipe with the same diameter as the in situ SCR has been used in the laboratory tests to investigate the vertical motion of the pipe and the effect of the trench on the lateral motion. As the amplitude of the vertical motion increases, the depth of the trench deepens, the bending moment range increases, and the excess pore water pressure at the bottom of the pipeline first accumulates and then dissipates during loading. The development trend of the trench depth and the influence of the soil strength on the SCR bending moment are also studied. During the test, a seabed trench develops, and its shape is similar to that of the in situ trench.

Despite the increasing number of space launches, growth of the commercial space sector, signing of the Artemis Accords, maturation of space mining technologies, the emergence of a regulatory environment through domestic legislation, and a comprehensive body of international law, an intergovernmental governing authority has yet to be established to manage mining activities on the Moon. We developed a Lunar Mining Code and mapping tool to regulate and manage prospecting and exploration activities for water ice at the Moon's poles. The Lunar Mining Code is composed of a notification system to manage prospecting, a contract system for issuing exploration licenses to allotted areas on the Moon, and best mining practices and principles to promote equal access and safeguard the lunar environment.

Offshore wind turbines (OWTs) are gaining prominence worldwide, and the hybrid pile-bucket foundation, which combines a monopole and a bucket, has emerged as a noteworthy development. In this study, a 3-D numerical model for the 5-MW OWT was constructed utilizing the OpenSees platform. The dynamic characteristics of the sand was modeled with the PDMY02 constitutive model and the soil was discretized using brick up elements. To investigate the dynamic behavior of the OWT in an actual marine environment, the coupled model was subjected to dynamic loadings, encompassing waves, wind, and earthquake. Two seismic motions with different frequency components were considered, respectively. The study focused on exploring the impacts of key influencing factors on the OWT rotation, tower-top acceleration development and spatiotemporal distribution of excess pore water pressure ratio (EPWPR). These factors include dynamic load combinations, earthquake intensity, soil relative density, wind speed, angle between load directions, and pile length. It is revealed that the inclination angle of offshore wind turbines (OWTs) may exceed the allowable threshold under specific conditions of load combinations, seismic motion inputs, and seabed conditions. Thus, it is suggested to appropriately consider the effects of wind and wave actions in the seismic analysis of OWTS.