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Collaborating Authors
Influence of Sand Liquefaction On Self-burial of a Pipe Subject to Wave Action
Foray, P.Y. (Laboratory 3S, Institut National Polytechnique de Grenoble, Grenoble, France) | Bonjean, D. (Laboratory 3S, Institut National Polytechnique de Grenoble, Grenoble, France) | Michallet, H. (LEGI (CNRS-INPG-UJF), Grenoble, France)
Coastal or offshore structures such as pipelines installed on the seabed are subject to cyclic horizontal loads either by direct hydrodynamic wave action or through the cyclic movement of risers or flow lines transmitted by floating structures. In fine sandy or silty soils, such cyclic loads may lead to liquefaction of the surrounding bed, which can play an important part in the processes of erosion, trenching or self-burial of the pipes. A large 1-g physical model was built to study the fluid-soil-structure interaction, with special emphasis placed on the conditions in which liquefaction occurs around a pipe instrumented with pore-pressure transducers. The experiments indicate a strong increase in pore pressure at the pipe-soil interface, and lateral visualization revealed the liquefaction of a soil band in the vicinity of the pipe. The penetration of the structure can be related to the phenomenon of liquefaction. INTRODUCTION The process of self-burial of structures resting on the seabed as a result of wave action has been extensively studied by Lyons (1973), Lambrakos (1985), Brennoden et al. (1986), Wagner et al. (1987), Palmer et al. (1988) and Morris et al. (1988), among others. Many of these studies were devoted to specific pipe-soil interaction in order to draw up design criteria for pipeline stability. The first experimental program was conducted at the University of Grenoble by Branque et al. (2001, 2002) to quantify the influence of cyclic amplitude and sand density on pipe penetration and changes in lateral resistance. Transitory liquefaction of the soil close to the pipe was noted in some of the tests, with peak cyclic pore pressures reaching the effective overburden stress. In recent years, increasing attention has been paid to the effect on the stability of coastal or offshore structures of wave-induced liquefaction, in combination with scour effects.
The marine pipelines or Steel Catenary Risers installed in deep offshore sediments are subject to significant cyclic loadings, mainly at the touch down point, where structural damage is often created. Because of this, it is important to study pipe-soil interaction in greater detail. This paper presents the results of experiments carried out using a half-scale model on an artificial soft soil, with emphasis on the influence of the different parameters on the horizontal pipe-soil stiffness. The parameters are the cyclic horizontal amplitude, the number of cycles and the weight of the pipe. INTRODUCTION Steel Catenary Risers or Flowlines installed in deep offshore sediments are submitted to strong cyclic loadings due to the wave action on the floating vessels. These effects are especially concentrated in the so-called touch down point where the pipe takes off and lands repeatedly on the soil. Important studies exist presenting conclusions on the initial penetration of the pipeline into, and its interaction with, the ground (Audibert, 1990; Dunlap et al., 1990; Verley and Lund, 1995; Cathie et al., 2005; among others). Specific interaction models have to be introduced in the structural design of such structures, taking into account the particular mechanical properties of the very soft deep offshore soils, with high sensitivity and very low shear resistance. Simple models of pipe-soil interaction are generally considered as a series of vertical/horizontal linear springs and dashpots, Fig. 1. The objective of the work presented in this paper was to have a better understanding of the effect of cyclic loading on the pipe-soil interaction and to get practical design values of the parameters of the interaction model. In a former research project a physical model was used to study vertical cyclic interaction and to determine the parameters influencing vertical stiffness (Fontaine et al., 2004), and their implementation into a numerical software package to study pipe-soil interaction on fatigue life estimation (Fontaine, 2005).
- Europe (0.69)
- North America > United States > Gulf of Mexico > Central GOM (0.24)
Abstract Flowlines and pipelines installed in deep-sea waters are submitted to axial and lateral loads due to the effects of flow stoppages and starts, thermal influences and internal pressure. To study the phenomenon of soilpipeline interaction, a physical model was used and special emphasis was given to the application of large horizontal loads coupled with vertical loads. This paper focuses on the experimental results of two sideswipetype tests with a pipeline model in a very soft soil with undrained shear strength of ~3kPa. Experimental yield envelopes are also included. The experimental tests revealed that for very shallow pipe embedments the maximum horizontal load is obtained for a value of V/Vmax = 0.5, and that for larger embedments this value is in the order of 0.2. 1. Introduction The design of pipelines installed in deep-sea waters is still a challenge for offshore geotechnical engineering. Flowlines and pipelines can be submitted to combined vertical and horizontal loads, thermal expansion and internal pressure, among other loadings. The pipeline laying installation is not a guarantee of their penetration embedment and, consequently, their stability. The problem of untrenched pipelines has been studied and reported in literature (Murf et al., 1989; Brennodden and Stokkeland, 1992; Cassidy, 2004; Fontaine et al., 2004; Cathie et al., 2005; Zhang and Erbrich, 2005; Cheuk and Bolton, 2006; Dendani and Jaeck, 2007; Bruton et al., 2008; Tian and Cassidy, 2008; among others). A physical model was used to understand the mechanisms of interaction between pipe and soil through a lateral visualisation, and then to simulate the pipe response under combined vertical and horizontal loads. This paper concentrates on the results obtained using sideswipe tests with large and short horizontal displacements, in order to obtain an experimental yielding envelope of the pipe.
- Europe (0.70)
- North America > Mexico (0.29)
Wave-Induced Pipeline On-bottom Stability: Comparisons Between Pipe -Soil And Wave-Pipe –Soil Interaction Models
Gao, Fuping (Institute of Mechanics, Chinese Academy of Sciences) | Wu, Yingxiang (Institute of Mechanics, Chinese Academy of Sciences) | Jeng, Dong-Sheng (Department of Civil Engineering, The University of Sydney) | Jia, Xu (China Offshore Oil Research Center, China National Offshore Oil Company)
ABSTRACT To have a better insight into the mechanism of wave-induced pipeline on-bottom stability, the pipe-soil interaction model (Wagner et al., 1987) and the wave-pipe-soil interaction model (Gao et al., 2003) are compared intensively in this paper. This includes the comparisons of their experimental setups, procedure of tests, phenomena of pipe losing stability etc. The comparison indicates that the critical lines for the instability of anti-rolling pipeline and freely-laid pipeline in the empirical wave-pipe-soil interaction model overall agree with the design values, based on both simplified and generalized methods in DnV standard, respectively. However, with the increase of Froude number, the generalized method in DnV standard becomes more conservative than the wave-pipe-soil interaction model for the on-bottom stability design of pipeline. Therefore, wave-pipe-soil coupling effects should be taken into account when we analyze the on-bottom stability pipeline under wave loading. INTRODUCTION One of the main problems encountered with the use of the pipeline in offshore engineering is the wave-induced pipeline instability (Herbich, 1985). When a pipeline is installed upon seabed and subjected to wave loading, there exits a complex interaction between wave, pipeline and soil. To avoid the occurrence of pipeline on-bottom instability, the pipeline has to be given a heavy weight of concrete coating or alternatively be anchored/trenched. Both methodologies are expensive and complicated from the aspects of design and construction. Recently, considerable efforts have been devoted into the interaction between pipeline and seabed. The state-of-the-art in pipeline stability design has been changing very rapidly recently. Three major investigations have addressed the problem of pipeline-seabed interaction, which include PIPESTAB project (Wagner et al., 1987), the American Gas Association (AGA) project (Brennodden et al., 1989) and a project at Danish Hydraulic Institute (DHI) (Palmer et al., 1988).
Abstract The conventional approach to submarine pipeline stability design considers interactions between water and pipeline (fluid-pipe) and pipeline and seabed (pipe-soil). The seabed is typically assumed hydrodynamically stable in this approach. Interactions between the water and the seabed (fluid-soil) are generally considered only as an afterthought. A new approach for assessing the stability of submarine pipelines is under development and is aimed at including seabed stability (or mobility) as a key aspect of the design analysis. An overview of this approach is presented in this paper. A practical method for utilising this design approach has also been developed, and is based on a combination of numerical analysis and physical model testing. Background On-bottom Stability On-bottom stability design of submarine pipelines is based on assessing the effects of the environment, namely the ocean and the seabed, on the pipeline. In short, a 'stable' pipeline does not displace (or displaces only by a small and allowable distance) when subjected to any environmental loading that may occur - in particular steady-state and oscillatory (wave-induced) on-bottom currents. This approach is known as 'absolute stability' design. As this method has evolved, the criteria for defining pipeline stability have loosened, and now extend to allowing the pipeline to displace a significant predefined distance laterally - up to tens of pipe diameters - under a given loading condition. Whether the distance is arbitrary, based on the operational constraints or the mechanical strength of the pipeline depends on the design approach and the code of practice utilised. This approach is known as 'dynamic stability' design, reflecting that a full dynamic analysis of the structural response is required to predict the displacement of the pipeline during a design storm event. There are three main interactions that affect the stability of a submarine pipeline. They are the interactions between the water and the pipeline (fluid-pipe); the interactions between the pipeline and the seabed (pipe-soil); and the interactions between the water and the seabed (fluid-soil). Fluid-pipe interactions result in hydrodynamic loading of the pipeline. Pipe-soil interactions result in the mobilization of soil resistance - which is often treated as two independent components, arising respectively from ‘friction’ between the pipeline and the seabed, and passive resistance to pipeline movement provided by the soil that is ahead of the embedded part of the pipe. Strictly these two components are not separate mechanisms, but it is common practice, and a reasonable simplification, to consider them in this way. Fluid-soil interactions result in seabed instabilities such as scour, fluctuations and potential build-up of excess soil pore pressure, and potentially liquefaction of the seabed soil. Pipe-soil interactions such as pipeline displacement may also lead to excess pore pressure generation. Each of the interactions outlined above are dependent on the other interactions and their effect on certain parameters. For example, the degree of pipeline embedment is affected by scour and liquefaction (fluid-soil), and in turn affects the hydrodynamic loads acting on the pipeline (fluid-pipe), as well as the passive resistance provided by the soil (pipe-soil). Figure 1 summarises these interactions that affect subsea pipeline stability.