Recently, the Pipeline Research Committee of the American Gas Association (A.G.A.) has sponsored several research projects in the area of submarine pipeline on-bottom stability. This coordinated research effort has focused on, and resulted in, development of simulation software for design, and the preparation of design guidelines.
Analytical models for both the hydrodynamic and pipe/soil interaction forces have been developed and implemented into the pipe dynamic analysis software. This software is designed to predict the motions of a pipeline exposed to current and an irregular sea-state. Based upon results of the model tests and computer simulations, pipeline on-bottom stability design calculation procedures have been prepared in a design handbook.
This paper describes a research program in pipeline on-bottom stability, sponsored by the A.G.A., which has been underway since 1983. The research program has been conducted as a series of separate projects coordinated in "building block" fashion. Although the target product was defined in general terms, the full extent of the work was not initially conceived as one large, single project. Instead, the research was executed as a series of tasks involving numerous organizations and researchers. Throughout the work, results of earlier tasks were used to define new tasks, required to reach the desired objectives. These objectives were:
to obtain an accurate assessment and verification of the forces which govern pipeline stability,
to develop analysis tools capable of predicting the governing forces and determining their effect on pipeline stability, and
to produce practical calculation procedures.
This research effort was planned, coordinated and monitored by an ad-hoc committee composed of representatives from A.G.A. member companies with assistance from the consultants used to perform the various research projects.
The major portion of the research has focused upon hydrodynamic forces. Large scale model tests on stationary submarine pipelines exposed to currents, waves and combined waves and currents have been conducted for pipelines resting on a seabed, partially buried pipelines, and pipelines resting in shallow trenches. Similar tests have also been conducted to determine the reduction of hydrodynamic force which is realized if the pipe moves under the influence of the wave or wave and current loadings. The subsequent analyses of the measured forces have resulted in force coefficients applicable in common industry formulas and in force coefficients for use in more refined and accurate force calculation procedures.
In addition to the hydrodynamic force program, model tests on pipe/soil interaction forces have been conducted on loose sands, dense sands, and soft clays. The effect of pipeline oscillation on these interaction forces was also analyzed to prepare empirical formulations of the lateral soil resistance developed by oscillatory pipe movements.
On-bottom stability design of submarine pipelines has traditionally been based on the static balance between applied hydrodynamic forces and resisting soil forces as depicted in Figure 1.