Keywords: pore pressure, cover layer, pipeline, wave, finite element modelling.

This paper shows how a cover layer may affect the wave-induced pore pressure and internal stresses around/within a buried pipeline. A two-dimensional finite element model proposed by the authors [1] will be adopted in this study to achieve the aims. The pipeline will be considered as an elastic deformable material in the model. The excess pore pressures will be examined to the extent of showing how they directly influence the stresses placed on and within the pipeline. Then, the effects of a cover layer will be examined.

Submarine pipelines have been extensively used in the transportation of fluids across large stretches of water. They will undoubtedly continue to be used in numerous wide-spread applications, thus the protection of an offshore pipeline must therefore be implemented. Pipelines have been being damaged from a number of causes ranging from wave and current interaction through to anchor dropping and dragging. To protect the pipeline, it is customary to bury the pipeline in the seabed with a certain range in the cover layer. Since the cost in artificially trenching and refilling is high and the expenditure often covers a large portion of the total budget of a pipeline project. Thus, it is necessary to investigate the effects of a cover layer on the wave induced seabed response around the pipe.

It is well known that ocean waves propagating over the ocean exert fluctuations of wave pressure on the sea floor. These fluctuations further induce excess pore pressure and effective stresses, which have been recognised as a dominant factor in analysing the instability of a seabed. Once the wave-induced seabed instability occurs near a pipeline, the phenomenon of pipe floating may occur [2]. An inadequate design that leads to pipe floating will to be a costly exercise as well as an environmental disaster. Thus, the evaluation of wave-induced soil response, including pore pressure, effective stresses and soil displacement must be taken into careful consideration for marine geotechnical engineers involved in the design of foundations for offshore pipelines.

Although the importance of the wave-soil-pipeline interaction phenomenon has been addressed in the literature, this problem has not been fully understood because of the complicated soil behaviour and geometry of the pipeline. Based on Biot's model [3], the wave-induced pore pressure around a buried pipeline has been studied through a boundary integral equation method [4] and a finite element method [5]. Among these, Cheng and Liu [4] considered a buried pipe in a region that is surrounded by two impermeable walls. Recently, the authors developed a finite element model to investigate the wave-seabed-pipe interaction problem[5]. However, all the aforementioned models have assumed the pipeline is a rigid non-deformable material. The deformation and stresses within the buried pipeline cannot be examined through the above models. Recently, a new finite element model has been established by removing the rigid non-deformable assumption for the pipe [1].

All the above investigation have only investigate the mechanism of wave-induce dpore pressure distribution around a pipeline, rather than teh protection of teh buried pipeline. In engineering practice, a cover layer of a coarser material is often used in the protection of a buried pipeline in order to minimize the potential of wave-induced seabed instability.

The numerical model was first verified by against the previous experimental results [6]. The comparison shows an overall agreement between teh propose dmodel and experimental data, and a better prediction of pore presusre than the previous model [4]. Based upon the numerical results, the following conclusions can be drawn:

1. a cover layer of coarser material (such as coarse sand and gravel) than the surrounding soil significantly increases the soil stability in the vicinity of the pipeline and induces higher pore pressures;
2. the excess pore pressure along the pipeline is reduced two-fold with the introduction of a cover layer;
3. compared with the pore pressures, the existence of a cover layer insignificantly affects the internal stresses within the pipeline.

1

Jeng, D. S., Postma, P. F. and Lin, Y. S., Stresses and deformation of a buried pipeline under wave loading", Journal of Transportation Engineering, ASCE, accepted. doi:10.1061/(ASCE)0733-947X(2001)127:5(398)

2

Clukey, E. C., Vermersch, J. A., Koch, S. P. and Lamb,W. C., "Natural densification by wave action of sand surrounding a buried offshore pipeline", The 21st Offshore Technology Conference, 291-300, 1989.

3

Biot, M. A., "General Theory of three-dimensional consolidation", Journal of Applied Physics, 12, 155-164, 1941. doi:10.1063/1.1712886

4

Cheng, A. H. D. and Liu, P. L. F., "Seepage force on a pipeline buried a poroelastic seabed under wave loadings", Applied Ocean Research, 8, 22-32, 1986. doi:10.1016/S0141-1187(86)80027-X

5

Jeng, D. S. and Lin, Y. S. "Response of in-homogeneous seabed around a buried pipeline under ocean waves", Journal of Engineering Mechanics, ASCE, 126, 321-332, 2000. doi:10.1061/(ASCE)0733-9399(2000)126:4(321)

6

Turcotte, B. R., Liu, P. L. F. and Kulhawy, F. H., "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, 1984.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £122 +P&P)