Introduction

Submarine pipelines play an extremely important role in the transportation of offshore energy resources, which is one of the main concerns of offshore engineering. In general, the fluctuating pressures acting upon the seabed due to the progressive motion of ocean waves will further induce excess pore pressure and reduce the effective stress within the seabed soil. When the excess pore pressure increases, the vulnerability of underwaterlaid pipelines may be exposed due to wave-induced liquefaction of underlying seabed soil layers. Therefore, the evaluation of the wave-induced soil response is particularly important for offshore engineers involved in the protective design of offshore pipelines.

Because of the existence of embedded pipelines, the generation of pore pressure in adjacent zones is quite different from that in the far field. Some experimental and numerical studies have been carried out to investigate the characteristics of oscillatory pore pressure near the pipeline, as well as the effects of wave and seabed parameters. Among these, Turcotte, Liu & Kulhawy (1984) conducted a series of wave tests to investigate the wave-induced oscillatory pore pressure around a buried pipeline. In their experiments, the pore pressures along the pipeline were measured. Cheng & Liu (1986) proposed a boundary-integral method to investigate the dynamic response of a seabed with a buried pipeline. In both of these approaches, the pipeline was buried in a rectangular trench layer with impermeable walls. Later, Madga (1996) proposed a one-dimensional simplified finite element model to investigate the wave-induced uplift seepage around a pipeline, which was further applied to investigate the seabed instability (Madga 1997). Based on Biot's (1941) poro-elastic theory, a two-dimensional finitedifference model was established for wave-seabed-pipe interactions in an isotropic homogeneous seabed (Jeng & Cheng 1999), which was further used to investigate seabed instability caused by liquefaction and shear failure around a pipeline (Jeng & Cheng 2000).

Based on the principle of repeatability (Zienkiewicz & Scott 1972), a finite element model was established for a Gibson soil (Jeng & Lin 1999b), a general non-homogeneous seabed (Jeng & Lin 1999a) and a cross-anisotropic seabed with a cover layer (Wang et al. 2000). With the same framework, the model was further extended by considering the pipeline as an elastic material to examine the internal stresses of the pipeline for different conditions (Jeng 2001c; Jeng, Postma & Lin 2001).

Background

Marine geotechnics is a multidisciplinary research theme covering conventional civil engineering disciplines such as fluid mechanics, coastal engineering, geotechnical engineering and structural engineering. This research area has attracted great attention among coastal and geotechnical engineers due to the growing activities in marine environments worldwide. An appropriate design for the foundations of marine infrastructures such as breakwaters, offshore pipelines, platforms and offshore wind turbine systems plays an important role in the success of offshore engineering projects. The evaluation of the soil response to hydrodynamic loading such as waves and currents around foundations of marine structures and the resultant seabed instability is one of the key factors in the design of foundations.

When a coastal structure is installed in a marine environment, the presence of the structure will alter the flow patterns in its immediate neighbourhood. The flow condition around the structure not only affects the wave force acting on the structure, but also induces seafloor instability. The former has been the main concern in the design of coastal structures, and has been intensively studied by coastal and structural engineers in the past. However, the latter involves the foundations of the structure, and has attracted attention from coastal geotechnical engineers in recent years.

In the past few decades, considerable efforts have been devoted to the phenomenon of wave-soil-structure interactions. The major reason for the growing interest is that many coastal structures (such as vertical walls, caissons, offshore monopiles and pipelines) have been damaged by the wave-induced seabed response, rather than from construction deficiencies (Christian, Taylor, Yen & Erali 1974; Smith & Gordon 1983; Lundgren, Lindhardt & Romold 1989). It has been reported in the literature that concrete armour blocks at the toes of a marine structure subsided into the seabed, and wave-induced liquefaction and shear failure have been identified as the culprit for this problem (Silvester & Hsu 1989). Another reason is that poro-elastic theories for wave-soil interactions have been applied to field measurements, such as the determination of the shear modulus of soil (Yamamoto & Trevorrow 1991) and the directional spectra of ocean surface waves (Nye & Yamamoto 1994), as well as acoustic waves propagating through porous media (Yamamoto & Turgut 1988).

When water waves propagate in the ocean, they generate significant dynamic pressures on the seafloor. These dynamic pressures further induce pore-water pressure and effective stresses within the seabed.

Introduction

Theoretical studies of wave-seabed interactions have been reviewed in the literature (Jeng 2003c). Among these, based on Biot's consolidation theory (Biot 1941), Yamamoto et al. (1978) proposed an analytical solution for the wave-induced soil response in an infinite seabed. This framework was further extended to threedimensional short-crested wave-induced seabed response in a seabed of finite thickness (Hsu & Jeng 1994) and a layered seabed (Hsu et al. 1995). Later, many theoretical studies of more complicated wave and seabed conditions were reported, for example, cross-anisotropic soil behaviour (Jeng 1997b; Kitano & Mase 1999), non-homogenous seabeds (Jeng & Seymour 1997a; Kitano & Mase 2001), inertial effects (Jeng et al. 1999; Jeng & Rahman 2000; Ulker et al. 2009), full dynamic soil behaviour (Jeng & Cha 2003; Ulker et al. 2009) and combined wave and current loadings (Ye & Jeng 2012a; Wen et al. 2012; Zhang, Jeng, Gao & Zhang 2013).

In addition to the theoretical studies, several laboratory studies have been reported in the literature. Experimental studies mainly include two-dimensional wave-flume tests (Tsui & Helfrich 1983; Sumer et al. 1999) and one-dimensional compressive tests (Zen & Yamazaki 1990a, 1990b; Chowdhury, Dasari & Nogami 2006) and centrifuge tests (Sassa & Sekiguchi 1999; Sassa et al. 2001). The purpose of two-dimensional flume experiments was mainly to capture the pore pressure build-up, while the purpose of one-dimensional tests was generally to capture the response of soil to oscillatory pore pressure. In addition, the drawback of two-dimensional experiments (including waveflume tests and centrifuge tests) was the limited number of measurable points in a shallow soil layer (about three or four measurement points in a cross section of 10 cm). On the other hand, the advantages of one-dimensional laboratory experiments was the thick soil layer, which allows us to have more measurable points in the vertical profile of pore pressures, especially in the region near the seabed surface. Thus, a one-dimensional facility was used in this chapter to resolve the vertical profile of pore pressure distributions. All the aforementioned one-dimensional compressive tests (Zen & Yamazaki 1990a, 1990b; Chowdhury et al. 2006) were performed with 500 cycles. This limitation was overcome by the author in his recent work (Liu et al. 2015).

A few experimental studies about clayey soils have been performed in the past few decades.