A Fluid-pipe-soil Approach to Stability Design of Submarine Pipelines

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.