Performance Limits for Deepwater Pipelines

ABSTRACT This paper highlights the differences between onshore and offshore pipeline design. It then summarizes the results of combined external pressure, tension, and bending testing on full scale 6 5/8" and 16" pipe, and uses these results to support suggestions for appropriate design formulas for combined external pressure, tension, and bending loads. Finally, purchasing suggestions for pipe subjected to severe deepwater loadings are presented. INTRODUCTION The Offshore Supervisory Committee of the Pipeline Research Committee of the American Gas Association has in the past decade sponsored several research projects in the general area of pipe collapse [1,2]*. This research included theoretical work, small scale testing, and large scale testing. Many things were learned in the research. One object of this paper is to put these things in a convenient form which hopefully will be useful to the pipeline designer. The Pipeline Design Codes have done a good job of providing guidance to designers of onshore pipelines [3,4]*. The rules for internal pressure design, reinforcement of penetrations and connections, and design for wind, seismic loads, weight, and thermal expansion are generally clear and adequate. However, offshore pipelines have some significant additional design requirements. Most of these requirements are due to the remote installation of pipelines under water. The remote installation means that different construction techniques have to be used offshore. Among these are laying pipe over a laybarge stinger with a significant unsupported span to bottom, or laying pipe from a reel barge, or pulling pipe through a J-tube to connect to an offshore platform. The latter two applications result in the pipe undergoing significant plastic deformations during the installation process which could lead to failure of the pipe. Another unique offshore loading is from the water itself which exerts external pressure on the line during construction and operation. This external pressure may cause the pipeline to collapse. There are many cases of underwater pipelines where significant spans occur, such as the unsupported span from a laybarge, or where the lateral loads from weight or current are carried by a combination of tension and large deflections rather than bending moments. This structural behavior LS not accounted for in most piping design/analysis computer programs. Finally, designers of offshore pipelines must consider on bottom stability problems not encountered in onshore pipelines. A pipeline resting on bottom or in a trench must often be coated with concrete to keep it from floating when evacuated. Currents exert lift and drag forces which tend to move the pipe laterally. These forces must be resisted by making the pipe sufficiently heavy. This is a complicated problem, and the Offshore Supervisory Committee has been very active in sponsoring research to define safe pipeline stability design procedures [5]. This paper outlines the offshore pipeline design process, and catalogues in convenient form formulas to consider collapse, collapse plus bending, and collapse plus tension. This paper also presents suggestions for purchase specifications to insure adequate properties in the pipe, both dimensionally and metallurgically.