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5486 results in Thermal-fluids engineering

Page 68 of 275

  • 1 - Overview

  • Dong-Sheng Jeng, Griffith University, Queensland
  • Book:
    Mechanics of Wave-Seabed-Structure Interactions
    Published online:
    28 April 2018
    Print publication:
    26 April 2018, pp 1-33

  • Summary

    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.

  • 8 - Physical Modelling: One-Dimensional Compressive Tests

  • Dong-Sheng Jeng, Griffith University, Queensland
  • Book:
    Mechanics of Wave-Seabed-Structure Interactions
    Published online:
    28 April 2018
    Print publication:
    26 April 2018, pp 285-315

  • Summary

    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.

  • 7 - Seabed Instability around Offshore Wind Turbine Foundations

  • Dong-Sheng Jeng, Griffith University, Queensland
  • Book:
    Mechanics of Wave-Seabed-Structure Interactions
    Published online:
    28 April 2018
    Print publication:
    26 April 2018, pp 253-284

  • Summary

    Introduction

    Ambitious wind power development programs were launched by many countries to promote the exploitation of renewable energy and a significant part of the expansion will occur in the form of offshore wind farms (Danish Energy Authority 2005). In Denmark, for example, 28 per cent of the electricity is now produced from wind, and the figure is predicted to reach 50 per cent by 2020 which means the current capacity will be extended by 2000 MW wind power before 2020, and 1500 MW will come from offshore wind farms.

    The foundation of an offshore wind turbine, which transfers the forces from the structure to the surrounding soil, is a critical part in the design of a wind turbine structure and it may account for up to 35 per cent of the total installed cost (Byrne & Houlsby 2013). The foundation options usually are gravity-based foundations and monopiles for shallow water, suction caissons and multiple-footing foundations for relatively deep water, and floating options for much deeper water (Breton & Moe 2009).

    Linton & Evans (1990) made major simplifications to the calculation of forces and free-surface amplitudes and exploited the exact theory for the scattering of waves by N vertical circular cylinders developed by Spring & Monkmeyer (1974). Subsequently, many theoretical studies in terms of wave–vertical pile interactions were carried out (Evans & Porter 1997; Malenica, Taylor & Huang 1999; Li, Zhang, Guan & Lin 2013). Recently, Liu, Li & Li (2007) experimentally investigated the group effectiveness and the effect of superstructures on wave current forces. Research on the response of offshore foundations to cyclic loads such as wave loadings has been conducted over the past few decades, and a series of two-dimensional (2D) and three-dimensional (3D) models have been developed to obtain a better understanding of the interactions between soil and pile foundations (Randolph 1981; Yang & Jeremic 2002). However, pore-water pressures have been left out of the preceding works and the soil was considered a material without voids. In addition, engineers tend to increase pile stiffness or soil ultimate bearing capacity to avoid foundation failure. As a matter of fact, pore-water pressure that accumulates in the wave-action process may lead to a decrease of effective stress and eventual collapse of the upper structure.

  • 3 - Soil Response in Marine Sediments under Combined Loading of Waves and Currents

  • Dong-Sheng Jeng, Griffith University, Queensland
  • Book:
    Mechanics of Wave-Seabed-Structure Interactions
    Published online:
    28 April 2018
    Print publication:
    26 April 2018, pp 109-128

  • Summary

    Introduction

    The co-existence of waves and currents in offshore areas is a common physical phenomenon and their interaction is an important topic in coastal and ocean engineering. The presence of a current in a propagating wave will change the original characteristics of wave. For example, the following current will elongate the wavelength, and the opposing current will shorten the wavelength. However, most previous studies of the soil response in marine sediments have considered wave loading only. In fact, the pressure acting on the seabed is significantly different when there is a current in the flow field, according to the potential flow theory. In Hsu et al.'s (2009) work, the thirdorder approximation for wave-seabed interactions was used to determine the dynamic pressures acting on the seabed surface for the porous seabed model. Later, the same analytical solution for wave-current interactions (Hsu et al. 2009) was used to develop analytical solutions based on the quasi-static approximation (Zhang, Jeng, Gao & Zhang 2013), the u–p approximation (Liu et al. 2014) and the u–U approximation (Liao, Jeng & Zhang 2015). A similar approach was adopted to examine the case of a fully buried pipeline (Wen, Jeng & Wang 2012).

    In this chapter, the theoretical models of wave-current interactions are outlined first. Then the effects of currents on the oscillatory seabed response are investigated numerically by adopting the ‘u–p’ approximation (Biot 1956a; Zienkiewicz et al. 1980), in which the inertial terms of solid and pore fluid are both considered as the governing equations for a porous seabed. Based on the numerical model, the effects of current on seabed response are investigated; a parametric study is carried out to investigate the effects of wave and soil characteristics on the seabed response, as well as the liquefaction under combined loading of non-linear waves and currents.

    Flow Models for Wave-Current Interaction

    Analytical Solution: Third-Order Approximation of Wave-Current Interactions

    In this chapter, to obtain more accurate results of seabed response under combined wave and current loadings, the third-order solution of wave-current interactions is used to determine the dynamic wave pressures acting on the seabed. For the problem of the third-order wave-current interactions, some works are available in previous literature. The interaction between a linear wave and a uniform current was studied by Thomas (1981). Baddour & Song (1990b) further investigated the interaction of linear waves and currents.

  • Preface

  • Dong-Sheng Jeng, Griffith University, Queensland
  • Book:
    Mechanics of Wave-Seabed-Structure Interactions
    Published online:
    28 April 2018
    Print publication:
    26 April 2018, pp xi-xii

  • Summary

    The phenomenon of the wave-seabed-structure interaction has attracted extreme attention among coastal and geotechnical engineers in recent years. Intensive research activities in the area have been carried out by numerous research groups across the world. Understanding of the mechanisms and processes of the wave-seabed-structure interaction problem is particularly important for marine geotechnical engineers involved in the design of foundations around marine infrastructures. The aim of this book is to provide readers with a comprehensive theoretical background for the wave-induced soil response in marine sediments around marine infrastructures, covering various aspects.

    This book consists of eight chapters. The first chapter sets out the background to the topic, describes recent advances in the area and explores possible future research agendas. This chapter is a useful starting point for postgraduate students, and provides junior researchers and readers new to this discipline with an overall picture of the research topic. Chapter 2 presents detailed mathematical formulations for the wave-induced soil response, including pore pressure, effective stresses and soil displacements. This chapter summarises the key processes from my first book (Porous Models for Wave-Seabed Interactions, published by Springer in 2013) and some additional new results. Chapter 3 presents analytical and numerical models for the seabed response subject to combined wave and current loading. In Chapters 4 and 5, two-dimensional marine infrastructures are considered. Among these, seabed response around caisson-type breakwaters is presented in Chapter 4. The cases of a submerged breakwater and a breakwater on a sloping seabed are included in the chapter. In Chapter 5, offshore pipelines are considered, including those fully and partially buried in a trench layer. In Chapters 6 and 7, three-dimensional model are presented for two different types of marine infrastructures: breakwaters and offshore wind turbine foundations. Chapter 6 provides an intensive study for the soil response around breakwater heads, while Chapter 7 presents the case of conventional offshore wind turbine foundations such as monopiles and a case study at Donghai offshore wind farms. Finally, Chapter 8 presents results of the recent laboratory experiments on the wave-induced oscillatory pore pressures, including sand-clay mixtures.

  • 2 - Basic Seabed Mechanisms

  • Dong-Sheng Jeng, Griffith University, Queensland
  • Book:
    Mechanics of Wave-Seabed-Structure Interactions
    Published online:
    28 April 2018
    Print publication:
    26 April 2018, pp 34-108

  • Summary

    Introduction

    Considerable efforts from researchers in the field of marine geotechnics have been devoted to the phenomenon of the wave-induced soil response since the 1970s. One reason for the growing interest is that many coastal/offshore structures (such as vertical walls, caissons, offshore monopiles and pipelines) have been damaged by the waveinduced seabed response, rather than from construction deficiencies (Christian et al. 1974; Smith & Gordon 1983; Lundgren et al. 1989).

    As mentioned in Chapter 1, two mechanisms of the wave-induced soil response have been reported in the literature. Both oscillatory (transient) and residual mechanisms of the wave-induced soil response have been intensively studied since the 1970s with two different approaches: the Yamamoto–Madsen model (Madsen 1978; Yamamoto et al. 1978) for the oscillatory mechanism and the Seed–Rahman model (Seed & Rahman 1978) for the residual mechanism. Following these approaches, numerous models have been proposed in the literature. In this chapter, the basic concepts and some key results of the previous models for wave-seabed interaction will be summarised.

    Oscillatory Mechanism

    Basically, previous studies of the wave-induced oscillatory soil response can be classified into three categories: the Yamamoto–Madsen model, the boundary-layer approximation and the dynamic model. We outline the contribution of several key publications available in the literature here.

  • (a) Yamamoto–Madsen model (YM model, or Biot's consolidation model): Yamamoto et al. (1978) proposed an analytical solution for an infinite seabed with hydraulic isotropy, while Madsen (1978) derived an analytical solution for a similar problem but with hydraulic anisotropy (i.e., permeabilities in all directions are different). Different seabed conditions, such as a seabed of finite thickness (Yamamoto 1977) and a layered seabed (Yamamoto 1981), were considered. Based on the plane stress conditions, Okusa (1985b) further reduced the sixth-order governing equation in the YM model to a fourth-order governing equation. Later, the YM model was further extended to more complicated wave conditions such as three-dimensional (3D) short-crested wave systems (Hsu et al. 1993, 1995; Hsu & Jeng 1994) or seabed conditions such as cross-anisotropic soil behaviour (Jeng 1997b) or a non-homogeneous seabed profile (Jeng & Seymour 1997a; Kitano & Mase 2001). Some numerical models have been developed with this framework, which were reviewed in Chapter 1.

  • (b) Boundary-layer approximation: Based on the mixture theory, Mei & Foda (1981) proposed the boundary-layer approximation for the wave-induced soil response.

  • 4 - Integrated Model of Wave-Seabed Interactions around Caisson-Type Breakwaters

  • Dong-Sheng Jeng, Griffith University, Queensland
  • Book:
    Mechanics of Wave-Seabed-Structure Interactions
    Published online:
    28 April 2018
    Print publication:
    26 April 2018, pp 129-176

  • Summary

    Introduction

    Breakwaters are commonly adopted to protect and enhance the utility of coastlines. For example, the total length of all breakwaters in Japan is 4,143 km – one-fifth of its coastline (Hsu, Uda & Silvester 2000). In countries such as the United Kingdom and Japan, coastline protection is a national priority. The construction of new breakwaters and the expansion of existing breakwaters involve major investment. Worldwide, the combined costs for building new breakwaters and maintaining the existing ones are on the order of tens of billions of pounds a year.

    Breakwaters are vulnerable to liquefaction of the seabed foundation, a process that can often lead to significant degradations of the foundation in as little as a few years after construction and can sometimes even result in total collapse (Zen et al. 1985; Lundgren et al. 1989; Franco 1994; Zhang & Ge 1996; Sumer & Fredsøe 2002; Chung, Kim, Kang, Im & Prasad 2006). Inappropriate design or maintenance of breakwaters can lead to catastrophic coastal disaster. A recent example of coastal tragedy caused by the failure of breakwaters is that of New Orleans during Hurricane Katrina, which caused deaths and personal and economic chaos (Travis 2005).

    The phenomenon of wave-seabed-structure interactions (WSSIs) has a major bearing on this issue and is central to the design of coastal structures such as breakwaters, pipelines and platforms. There have been numerous investigations of wave-seabed interactions around marine structures based on Biot's poro-elastic theory. Among these, Mase et al. (1994) developed a finite element method (FEM) numerical model to investigate wave-induced pore-water pressures and effective stresses under standing waves in a sand seabed and in the rubble-mound foundation of a composite caisson-type breakwater based on Biot's consolidation equations. Later, Mizutani & Mostafa (1998) and Mostafa et al. (1999) developed a boundary element method–FEM combination numerical model to investigate the wave-seabed-structure interaction. In their models, Poisson's equation is used to govern the irrotational wave field for an incompressible, inviscid fluid, and Biot's poro-elastic consolidation equations are used to govern the porous seabed and structures. Jeng, Cha, Lin & Hu (2001) developed a two-dimensional (2D) generalised FEM numerical model (GFEM-WSSI) to investigate the wave-induced pore pressure under a linear wave around a composite breakwater located at a finite, isotropic and homogeneous seabed.

Page 68 of 275