Effect of the sloping seabed on 3D soil-spudcan interaction using a material point-finite element(MPM-FEM)model

The sloping seabed affects the bearing capacity and failure mechanism of soil,which may compromise the stability and safety of offshore structures such as jack-up platforms.This paper employs a coupled model combining the material point method and finite element method(MPM-FEM)to analyze the impact of sloping seabeds on the three-dimensional soil-spudcan interaction.The MPM-FEM model implements the B¯approach to solve the challenge of volumetric locking due to the incompressibility constraints imposed by yield criterion.It is validated against the centrifuge results.The effects of sloping seabeds on penetration resistance,soil flow pattern,lateral response,stress distribution,and failure mechanism are discussed.The soil mainly undergoes overall failure when the ratio of penetration depth to spudcan diameter(i.e.D P/D)is between 0 and 0.25.As the slope angle increases,the soil on the side of lower slope is expelled further,resulting in an asymmetric stress distribution and a larger horizontal sliding force of soil.When D P/D increases to 0.75,the soil transitions to localized plastic flow failure,and the range of soil flow affected by the spudcan penetration decreases.The results show that,when the slope angle increases,the lateral displacement and stress distribution on the lower slope of a sloping seabed is significantly larger than that of a horizontal seabed,impacting the spudcan and surrounding soil behavior.The study suggests that the seabed slope significantly affects the range of soil flow and failure at shallow penetration,indicating that the slope angle should be taken into account in the design and installation of offshore jack-up rigs,particularly in areas with sloping seabeds.

Analytical Solution for the Transient Response of A Sloping Seabed Induced by A P-Wave Line Source

Many offshore marine structures are built on the seabed that are slightly or considerably sloping.To study the sloping seabed transient response during marine earthquakes,an analytical solution induced by a P-wave line source embedded in the solid is presented.During the derivation,the wave fields in the fluid layer and the semi-infinite solid are firstly constructed by using the generalized ray method and the fluid-solid interface reflection and transmission coefficients.Then,the analytical solution in the transformed domain is obtained by superposing these wave fields,and the analytical solution in the time domain by applying the analytical inverse Laplace transform method.The the head wave generation conditions and arrival times at the fluid-solid interface are derived through this solution.Through the use of numerical examples,the analytical solution is proved right and the impacts of the sloping angle on the hydrodynamic pressure in the sea,the seismic wave propagation in the seabed,the head wave,and the Scholte wave at the seawater-seabed interface are also addressed.

Accurate Source-Receiver Positioning Method for a High-Resolution Deep-Towed Multichannel Seismic Exploration System

The near-seabed multichannel seismic exploration systems have yielded remarkable successes in marine geological disaster assessment,marine gas hydrate investigation,and deep-sea mineral exploration owing to their high vertical and horizontal resolution.However,the quality of deep-towed seismic imaging hinges on accurate source-receiver positioning information.In light of existing technical problems,we propose a novel array geometry inversion method tailored for high-resolution deep-towed multichannel seismic exploration systems.This method is independent of the attitude and depth sensors along a deep-towed seismic streamer,accounting for variations in seawater velocity and seabed slope angle.Our approach decomposes the towed line array into multiline segments and characterizes its geometric shape using the line segment distance and pitch angle.Introducing optimization parameters for seawater velocity and seabed slope angle,we establish an objective function based on the model,yielding results that align with objective reality.Employing the particle swarm optimization algorithm enables synchronous acquisition of optimized inversion results for array geometry and seawater velocity.Experimental validation using theoretical models and practical data verifies that our approach effectively enhances source and receiver positioning inversion accuracy.The algorithm exhibits robust stability and reliability,addressing uncertainties in seismic traveltime picking and complex seabed topography conditions.

Experimental Study of Pore Water Pressure and Bed Profile Change Under Regular Breaking Waves

There lies a close relationship between the seabed destruction and the distribution of pore water pressure under the action of breaking wave. The experiments were carried out in a wave flume with a 1：30 sloping sandy seabed to study regular breaking wave induced pore water pressure. A wide range of measurements from the regular wave runs were reported, including time series of wave heights, pore pressures. The video records were analysed to measure the time development of the seabed form and the characteristics of the orbital motion of the sand in the wave breaking region. The pore water pressure in the breaker zone showed the time variation depending on the wave phases including wave breaking and bore propagation. The time-averaged pore water pressure was higher near the seabed surface. The peak values of pore water pressure increase significantly at the breaking point. The direction of pore water pressure difference forces in the breaker zone is of fundamental importance for a correct description of the sediment dynamics. The upwards- directed pressure differences may increase sand transport by reducing the effective weight of the sediment, thereby increasing the bed form evolution. The seabed configuration changed greatly at the wave breaking zone and a sand bar was generated remarkably. The amplitude of the pore water pressure changed with the seabed surface. The results are to improve the understanding of sand transport mechanisms and seabed responses due to breaking regular waves over a sloping sandy bed.