On-Bottom Pipeline Stability in Steady Water Currents

Abstract

A recommended practice is established for the on-bottom stability design of a submarine pipeline subjected to steady water currents. A parametric study indicates that the minimum specific gravity required for stability is relatively insensitive to large variations in the value of most parameters in the design equations. The design graphs presented can be used for most pipeline stability design problems. pipeline stability design problems. Introduction The design of an offshore pipeline system must consider the following factors. Operating requirements. such as internal pressure, throughput, and so forth, influence the design greatly. At the same time, the design must provide for the safety and integrity of the pipeline when faced with external and environmental factors during the expected lifetime of the system. Factors influencing the selection, design, and efficiency of the construction system also must be included in the over-all design. The specifications for the line pipe and the pipe-coating system must not exceed those required to pipe-coating system must not exceed those required to assure adequate operating performance and long-term line safety. Unnecessary pipe weight and too-stringent pipe-handling criteria impose restrictive operating limits on pipe-handling criteria impose restrictive operating limits on present construction systems, even in moderate water present construction systems, even in moderate water depths. In deeper water, these criteria may impose impractical requirements on the construction system. The selection of the pipe and specifications for the coating system are affected directly by the criteria established for the maximum environmental conditions expected during the life of the pipeline. Therefore, the environment of the operating pipeline is the starting point of the design. A complete discussion of the environment near the sea-water/soil interface is beyond the scope of this study. However, certain environmental situations must be avoided by careful route study and selection because there is no known method to ensure pipeline stability and integrity when faced with gross instability of the sea bottom. Thus, route selection is the most important step in stability design. Mes gives a general discussion of the ocean bottom needed for design purposes and methods of obtaining the necessary data. Bea et al. in their discussion of bottom movements in the Mississippi Delta present an excellent example of the correlation of present an excellent example of the correlation of bathy-metric surveys, soil borings, sparker data, and the general geology of a given area. Studying the available data is a prerequisite to rational route selection. Once a route has been selected to avoid hazardous environmental factors, the designer then investigates the local, rather than the large scale, environment of the pipeline. First, he asks whether the bottom is locally pipeline. First, he asks whether the bottom is locally stable. Will scour occur? This important question is not considered in this study. Townsend and Farley conducted scour tests in noncohesive soils and concluded that a pipeline would settle and remain in contact with the sand bottom as it eroded. The same conclusion was reached earlier from unpublished tests conducted by the Shell Pipeline Research and Development Laboratory. Of course, if portions of the pipeline are placed on harder or nonscouring materials, then a span can remain suspended over other bottom portions that do scour. Also, this study does not consider the magnitude of the hydrodynamic forces exerted on the pipeline by surface waves. Grace has summarized available wave-force data through 1973 for submarine pipelines. Although wave forces can be as significant as those resulting from steady currents, this investigation only is concerned with the hydrodynamic forces exerted by steady currents. P. 475