Simplified finite element analysis of wave-induced effective stresses and pore pressures in permeable sea beds

Author(s):  
Xiaoqing Teng

Primary structural components of a spar hull are designed to resist lateral hydrodynamic and hydrostatic pressure and global loads. The scantlings of each primary component are usually determined based on the largest pressure it may encounter in various phases such as wet-tow, upending, and in-place operational conditions. The effect of global bending moment and shear force on the spar hull is often evaluated much later via laborious finite element analysis. This paper proposes a simple analytical tool for quickly assessing global hull strength of classic spars in the in-place condition. A spar platform undergoes steady, low-frequency, and wave-frequency motion of comparable magnitude at the same time in a storm event. The present approach separates the wave-frequency component from the steady and low-frequency response. A closed-form solution is developed for wave-induced motion and loads by taking advantage of the simple cylindrical shape of classic spar hulls. The theoretical solution is verified by comparing to numerical WAMIT results. The low-frequency response is approximated as one part of the steady response, since its dynamic effect is weak. The steady structural response is mainly a function of the heel angle. It is demonstrated that local effect of wind pressure and current load is not significant. The total response, as a summation of the wave-induced loads and the steady solution, is represented by global bending moment and shear force envelopes along the spar hull for a given sea state. Global bending and shear stresses of primary structural components can be further calculated and checked against code requirements. This procedure has been implemented through Matlab scripts. A comparison with global finite element analysis for a classic spar is made showing very good agreement. The present simple procedure allows us to evaluate primary structural components of a spar hull without resorting to expensive finite element modeling. It can help the scantling design by providing the global load. The approach can also be used to identify critical environmental conditions and structural components for detailed finite element analysis.


Author(s):  
Jiang Tao Yi ◽  
Fook Hou Lee ◽  
Siang Huat Goh ◽  
Yu Ping Li ◽  
Xi Ying Zhang

The numerical modeling of spudcan penetration involves technical challenges posed by large soil deformation coupled with significant material non-linearity. The Lagrangian approach commonly used for solid stress analysis often does not work well with large deformations, resulting in premature termination of the analysis. Recently, the Arbitrary Langrangian Eulerian (ALE) and the Eulerian methods have been used in spudcan analysis to overcome problems caused by the soil flow and large deformation. However, most of the reported studies are based on total stress analysis and therefore shed no light on the excess pore pressures generated during spudcan installation. As a result, much remains unknown about the long-term behaviour of spudcans in the ground, which is affected by the dissipation of excess pore pressures. This paper reports an effective-stress finite element analysis of spudcan installation in an over-consolidated (OC) soft clay. The Eulerian analysis was conducted using ABAQUS/ Explicit, with the effective stress constitutive models coded via the material subroutine VUMAT. The results demonstrated the feasibility of conducting effective-stress finite element analysis for undrained spudcan penetration in OC clays. The paper discusses the flow mechanism, stable cavity depths and bearing capacity factors when spudcan installation occurs in various OC soils. It was found that the pore pressure build-up concentrates in a bulb-shaped zone surrounding the spudcan. The size of the pore pressure bulb increases with increasing penetration. The maximum excess pore pressure, which is generated near the spudcan tip, is predominantly controlled by the undrained shear strength at the tip level.


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