The Submarine Pipeline as a Structure

ABSTRACT In the past, design of submarine pipelines has not taken full cognizance of the fact that such pipelines are structures and shouldbe designed as structures. The submarine pipeline is, in fact, an extremely complex structure. It is much more complex than many larger and more impressive surface structures. Structural design considerations both during and after construction are discussed. The submarine pipeline is seen to function as a continuous beam, a beam on elastic foundation, a tension member, a compression member, a pressure pipe, an externally loaded conduit, and a suspension element. The system of loads on a submarine pipeline include gravitational, environmental, constructional and operational loads. These can be both static and dynamic. Additionally, when pipelines are buried, the loads from saturated and, in some cases, liquefied soils must be considered. This system of loading is very complex and involves environmental loadings that are extremely variable and location dependent. Recommendations are made for a systematic and rational approach to structural analyses which takes into account the individual nature of each submarine pipeline design problem.. Areas where further research would be beneficial are also discussed. INTRODUCTION The submarine pipeline appears to be a very simple structure. It is, after all, only a single member, usually having the same size throughout. It can be represented by a single line on a drawing. Actually, the submarine pipeline is, in many ways, more complex than a more complicated-appearing building frame. The static and dynamic loadings due to environment, construction methods, and operation are numerous and varied. Additionally, the structural functions of a submarine pipeline during construction and operation are more involved than those of the beams, girders and columns of a building frame. When pipelines first went offshore, there was no particular need to be concerned with the complexity of the structural analysis of the pipelines. The lines were being laid in shallow water, they were small, flexible pipes; they were relatively close to shore, and were usually built with a ductile grade of pipe. Approximate analysis and rules of thumb were adequate for the purpose. However progress in offshore oil development has taken pipelines farther offshore and into more hostile marine environments in many corners of the globe. Pipelines are now faced with deeper water, longer pipelines,' larger-diameter pipelines, and the need to us e higher yield strength grades of pipe. The result is a tremendous increase in the importance of structural integrity, both during construction and during the operating lifetime of the pipeline. This progress has also brought with it new and more complex problems in structural evaluation and analysis. This paper takes a very general look at the pipeline as a structure and, in this way, attempts to provide some insight into the state-of-the-art, the problems, and the directions of effort in dealing with this structure. STRUCTURAL CONFIGURATIONS The possible structural configurations as summed by a submarine pipeline during construction and operation are almost infinite in their variety.