Wave interaction with a submarine pipeline in clayey soil due to random waves
Introduction
Submarine pipelines have been used extensively for many purposes; Viz. to transport crude oil from offshore to onshore, refined products from onshore refineries to ships, seawater intake and outfalls and submarine transportation. For many applications, the pipeline has to cross deep water (where the dynamic forces due to waves are smaller but the hydrostatic loads are significant) and shallow coastal waters (where the wave induced dynamic loads are significant) and different soil conditions. The design of pipeline in the coastal water is a challenge for coastal engineers. In coastal water, the selection of minimum burial depth is one of the important parameter. Higher the burial depth, safer the pipeline from the environmental load, but the cost of installation is prohibitive. The burial of pipeline in coastal water is a cumbersome exercise due to significant dynamics of water and seabed material. Many investigators studied the wave loads on a buried pipeline in sandy soil; typical is by Garrison et al., 1976, Mac Pherson, 1978. But the study in clayey soil is not available. At several places around the world and on the West Coast of India (Kandla and Cochin) and in East Coast of India (Krishna-Godavari basins), vast stretch of marine clays are found with Ic=0.1–0.3. It is not clear what is the effect of burying the pipe in such soil on wave forces and dynamic pressures. Hence a detailed experimental investigation is carried out on uplift forces, dynamic pressures for different hydrodynamic and soil parameters.
Section snippets
Literature review
A lot of knowledge is available on submarine pipelines placed near a plane boundary (both above and below the seabed) and subjected to the action of regular and random waves. Garrison et al. (1976) has conducted experiments to predict the wave forces on a horizontal pipeline resting on the bottom boundary and it was observed that the in line-force, FH varies linearly with incident wave height, H up to a certain height, beyond which it varied non-linearly with H for higher gT2/d and small values
Experimental facility and set-up
The experiments were carried out in a 2 m wide, 30 m long and 1.7 m deep wave flume available at Department of Ocean Engineering, IIT Madras, Chennai. Fig. 1 shows the experimental facility. The flume consists of a piston type wave generator at one end and a rubble type sloped wave absorber at the other end. Random waves from different mathematical spectrum models like PM, JONSWAP, Scot, Bretschneider, ISSC, etc can be generated using the facility. Random waves of required peak period and
General
In this study, concentration has been made on the influence of various input parameters, viz., wave height and periods, water depths, consistency index of clayey soil and different burial depths on the wave-induced pressures and forces on submarine pipeline due to random waves. The time series of the wave elevation and the corresponding wave pressures and uplift forces on submarine pipeline were acquired simultaneously. The random wave data were analysed using the FFT analysis package provided
Conclusions
Based on the present experimental investigations on wave pressures and uplift forces on submarine pipelines in clayey soil, the following conclusions can be drawn:
- 1.
The peak spectral density of wave pressures and uplift force are reduced by about 55 and 65%, when the pipe is just fully buried (e/D=1.0) compared to the pipeline resting on the seabed (e/D=0.0) for Ic=0.33.
- 2.
The peak spectral value of the uplift force has reduced by an extent of about 40% when d/a is varied from 3.75 to 6.25.
- 3.
The peak
Acknowledgements
The authors wish to express their gratitude to the Head, Department of Ocean Engineering, Indian Institute of Technology Madras for providing the necessary facilities to carry out this research work. Thanks to the appropriate authorities of IIT Madras for providing the necessary infrastructure facilities for carrying out this study.
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