Elsevier

Ocean Engineering

Volume 29, Issue 5, May 2002, Pages 533-544
Ocean Engineering

Wave induced forces around buried pipelines

https://doi.org/10.1016/S0029-8018(01)00012-9Get rights and content

Abstract

This work refers to an experimental investigation carried out to analyze wave induced pressures on a pipeline buried in a permeable seabed. In this investigation, the model tests were performed on a pipeline buried in the soil test bed. The wave flume used was 30 m long, 2 m wide and 1.7 m deep, 96 number of tests were conducted with waves generated for different wave heights. A pipeline 200 mm in diameter was buried in the sandy bed at different burial depth ratios. The pipeline was laid perpendicular to the wave direction, pressure was measured with 12 transducers along the outer circumference of the pipeline. The results show that wave induced pressures are significantly controlled by the wave period analyzed in terms of the scattering parameter (ka). Higher pressures were recorded at the top and the lower pressures were recorded at the bottom.

Introduction

In shallow waters, waves passing over a seabed can induce substantial pore water pressures in the top layer of the seabed, and there can be fluctuations in these induced pressures depending on the wave conditions and soil characteristics. Water waves propagating, over a porous bed exert a positive pressure under the crest and negative pressure under the trough, on the fluid–bed interface. These seepage flow patterns are generated in the sea bed whereby water moves from relatively higher pore water pressure regions under the crests to the relatively lower pressure regions under the troughs. The wave induced flow in the porous bed experiences a flow resistance and transmits a force, which is a seepage force onto the buried pipeline. From a hydrodynamic point of view, the flow induced by waves in the porous bed is of interest since it is involved in energy dissipation. Putnam (1949) is one of the first few researchers who presented analysis for these type of problems.

Lifting of pipelines under the induced uplift forces from waves and the lateral shifting and bending of pipelines under the combined action of waves and currents can cause serious stability problems in pipelines and these account for large number of failures. The proper design and the maintenance of pipelines require thorough understanding of the pipe–soil interaction under the complex conditions of wave and current environment. Various models have been developed in the study of wave and sea bed interactions. Most of the analyses were based on the assumptions of incompressible pore fluid and soil skeleton and with the flow in the porous bed governed by Darcy's law. Using the Darcy's flow assumption, Sleath (1970) demonstrated that the theoretically predicted pore pressures induced by waves propagating over a porous bed compared favorably with the results obtained from model tests in the laboratory. Moshagen and Torum (1975) considered the wave-induced flow in a porous medium under the assumption of compressible pore fluid and incompressible soil below and found that the inclusion of pore fluid compressibility in the analysis of wave induced pore pressures in a porous soil significantly altered the vertical seepage forces acting on the soil around the pipeline. However, the work of Prevost et al. (1975), suggest an alternative in which soil was considered to be a compressible material and the pore fluid to be incompressible. For a soil of low permeability they presented a solution for pore pressure distribution and this distribution compared well with the solution obtained from analysis based on incompressible pore fluid and soil. Madsen (1978) presented a general theoretical analysis of flow, pore pressures and effective stress induced in a porous bed by waves and discussed the nature of the solution as well as some of the geotechnical implications.

MacPherson (1978) studied the wave-induced pore water pressure in the vicinity of a pipeline buried in a permeable seabed. It was shown that, under monochromatic progressive waves, the magnitude of the seepage force acting on a single pipeline was large enough and should be included in design calculations. Liu and O'Donnel (1979) extended MacPherson's (1978) work to problems concerning the multiple pipeline installation and the effects of the finite thickness of seabed. From the work of Cheng and Liu (1986), it could be observed that the seepage force was independent of hydraulic conductivity, pore water compressibility and soil elasticity. The integrated pressure force was of constant magnitude, but its direction circles the pipe once with the passage of each wave (McDougal et al., 1988). Magda (1995) chose a numerical method to perform the analysis of two phases interaction and the generation of pore pressure was completely dependent upon the relative stiffness of the components of the system and the boundary conditions formed by a submarine pipeline buried in seabed sediments. Magda, 1996, Magda, 1997 performed comprehensive numerical studies of the hydrodynamic force acting on a submarine pipeline buried in compressible seabed sediments using the boundary integral equation method. Magda (1999), mainly focused on the problem of pore pressure perturbation effects, affecting the wave induced pore pressure field by the presence of a stiff and impermeable body of the submarine pipeline. In this analysis, the hydrodynamic uplift force perturbation ratio, the contribution of pore pressure perturbation effects in a global solution for the hydrodynamic uplift force acting on a submarine pipeline buried in seabed sediments were studied.

From the aforementioned review, it can be observed that there is a good interest shown in evaluating the forces on submarine pipelines and in many of the theoretical analyses presented, there is a certain amount of disagreement with respect to the assumptions made for the compressibility of pore fluids and soil skeleton. A few investigators employed methods using model testing in the laboratory and in these laboratory-testing programs; the tests were mainly limited to one set of conditions. In the present paper, the results of an experimental program carried out in a wave flume to investigate the hydrodynamic characteristics of a pipeline buried in a permeable soil bed due to monochromatic progressive waves are reported. From the results obtained from 96 tests conducted under various combinations of burial depths and wave parameters, the results are analyzed to bring out the variations in the dimensionless forces and pore pressures around the pipeline with the scattering parameter and angle of placing of the pressure transducer.

Section snippets

Experimental work

An experimental study bringing out the hydrodynamic effects on soil–pipe interaction was made in the Ocean Engineering center at IIT Madras. A brief description of the test flume used, model pipe used, soil bed formed and the instrumentation used is presented in the following paragraphs.

The tests were carried out in a 30 m long, 2 m wide and 1.7 m deep wave flume. Approximately at mid length, there is a test soil pit in which a pipe was placed on the soil bed formed. Tests were conduced at

Results and discussions

In order to evaluate the forces, it is required to compute the wave lengths and mud line pressures, P0 and this has been done by using linear wave theory (Cheng and Liu, 1986).

From the measured pressures (in terms of m of water) of these gauges, the non-dimensional pressures (P/P0) are worked out based on the computed values of P0. P/P0 plotted against the angle of placement of the pressure transducer (θ) for different values of scattering parameters (ka) and relative buried depths (e/D) are

Conclusions

The following conclusions can be made based on the tests conducted. The wave induced pressures around the pipelines buried at the shallow depths are significantly controlled by the time period of the waves, which can be analyzed in terms of the scattering parameter (ka). The results confirm that the pressures are at a maximum at the top and at a minimum at the lower regions. Within the shallow zone with respect to the depths, the pressures increased with the burial depths up to an approximate

References (12)

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