Non-linear wave-induced transient response of soil around a trenched pipeline

Abstract

Submarine pipelines are always trenched within a seabed for reducing wave loads and thereby enhancing their stability. Based on Biot's poroelastic theory, a two-dimensional finite element model is developed to investigate non-linear wave-induced responses of soil around a trenched pipeline, which is verified with the flume test results by Sudhan et al. [Sudhan, C.M., Sundar, V., Rao, S.N., 2002. Wave induced forces around buried pipeline. Ocean Engineering, 29, 533–544] and Turcotte et al. [Turcotte, B.R., Liu, P.L.F., Kulhawy, F.H., 1984. Laboratory evaluation of wave tank parameters for wave-sediment interaction. Joseph H. Defree Hydraulic Laboratory Report 84-1, School of Civil and Environmental Engineering, Cornell University]. Non-linear wave-induced transient pore pressure around pipeline at various phases of wave loading is examined firstly. Unlike most previous investigations, in which only a single sediment layer and linear wave loading were concerned, in this study, the influences of the non-linearity of wave loading, the physical properties of backfill materials and the geometry profile of trenches on the excess pore pressures within the soil around pipeline, respectively, were explored, taking into account the in situ conditions of buried pipeline in the shallow ocean zones. Based on the parametric study, it is concluded that the shear modulus and permeability of backfill soils significantly affect the wave-induced excess pore pressures around trenched pipeline, and that the effect of wave non-linearity becomes more pronounced and comparable with that of trench depth, especially at high wave steepness in shallow water.

Introduction

Wave-induced forces upon submarine pipelines have attracted extensive attention from coastal engineers, for the ever-increasing engineering activities in offshore and coastal regions. To reduce wave-induced forces and thereby to enhance their stability, offshore pipelines are always trenched into seabed. The trenches are generally backfilled either by in situ sediment or by pouring selected backfill material over the pipeline from bottom-dump barge. When water waves propagate over a porous seabed, cyclic excess pore pressures can be generated within seabed with accompanying decrease in effective stresses, which have been recognized as dominant factors for the instability of a buried pipeline. Thus, a proper evaluation of the wave-induced excess pore pressures around pipeline is important for offshore geotechnical engineers involved with the design of foundations for offshore pipeline.

There have been various analytical investigations of the problem of seabed response to wave loading, based on different assumptions of the rigidity of soil skeleton and compressibility of pore fluid, including Madsen, 1978, Yamamoto et al., 1978, Mei and Foda, 1981, Jeng, 1997. Besides the development of analytical solutions, numerical simulations have been widely applied to examine such a problem in recent years, such as the finite difference method (Zen and Yamazaki, 1990), the finite element method (Thomas, 1989, Gatmiri, 1990, Jeng and Lin, 1996). Most previous investigations of the wave–seabed interaction have been reviewed by Jeng (2003). However, all aforementioned investigations have only examined the soil response under the action of two-dimensional progressive waves in the absence of a marine structure.

The importance of wave–soil–pipeline interaction phenomenon has ever been addressed in the literature (Clukey et al., 1989). To date, this problem has not yet been fully understood because of the complicated behavior of soils and the special geometry of pipeline. Design of marine pipelines regarding their stability is a rather complicated problem. Based on the potential theory, the hydrodynamic uplift forces on the buried pipelines have been studied (MacPherson, 1978, Spierenburg, 1986, MacDougal et al., 1988). However, the potential theory is somewhat far from the realistic conditions of soil as a two-phase medium. Based on Biot's consolidation theory (Biot, 1941), the wave-induced pore pressure around a buried pipeline has been studied through a boundary integral equation method (Cheng and Liu, 1986) and a finite element method (Magda, 1997). Among these, Cheng and Liu (1986) considered a buried pipe in a region that is surrounded by two impermeable walls. Magda (1997) considered a similar case with a wider range of the degree of saturation. Jeng (2001) proposed a 2D FEM model to investigate the wave-induced pore pressure, effective stresses within soil around buried pipeline. However, most previous investigations of the wave–seabed–pipe interaction problem have been concerned only with a single sediment layer and linear wave loading. In engineering practice, pipelines are always trenched and covered with coarser materials, especially in shallow water. Moreover, the actual waves in shallow water zone are always characterized as non-linear. However, the non-linear wave-induced responses of backfill soils around trenched pipeline have not been revealed yet.

This paper is aimed at developing a finite element model for investigating the effects of backfill soil properties on non-linear wave-induced excess pore pressures upon a trenched pipeline, which will be verified with existing experimental results. The effects of non-linear component of wave loading are examined firstly. Moreover, the influences of the physical properties of backfill materials and the geometry profile of trenches are thereafter numerically evaluated, respectively, by varying shear modulus, permeability coefficient of backfill soil, slope angle and depth of trenches, etc.