Pipelining – Equipment, Methods and Materials - Drag and Lift Forces on a Submarine Pipeline Subjected to a Transverse Horizontal Current

Design of a submarine pipeline system is governed by many factors, one of which is the effect of transverse horizontal currents on the pipeline structure itself Although this feature alone can be of utmost importance to the life span and stability of the underwater pipeline, it is usually the least recognized, with these hydro-dynamic forces being either completely overlooked or grossly underestimated. In an attempt to supply the methods and means for stabilization of sub-marine pipelines, much capital has been spent by pipeline companies. Design criteria incorporated by these organizations extend over a broad spectrum of engineering with bases varying from pure theory to cut and dry methods. A history of numerous failures has substantiated the postulate that many of these submarine pipelines have been underdesigned from the stability standpoint. By the same token, equally as many have been excessively overdesigned with resultant excess expenditures. An attempt has been made in this paper to establish the magnitudes of these hydrodynamic forces by measuring the lateral current-induced differential pressure distribution around the periphery of the submarine pipeline. INTRODUCTION The expenditure of money for research by corporations obviously requires beneficial results to justify the investment. One of the functions of the engineer is first to make the problem known and then to provide a means of solution at the least possible cost. The initial procedure is to investigate all pertinent published technical articles on the subject to determine if this problem has been previously solved. If resolution of this problem is not obtainable, the next course of action is to evaluate through experiments these forces and their magnitudes. On the specific subject of drag and lift, the accumulation of data is provided from three separate and distinct sources: from experience, from theoreticians and through experimental observations. The experience source has proved costly with money wasted either because of underdesign and failure or excessive expenditures for overdesign, and has not been sufficient to form a definite conclusion on these hydrodynamic forces. The pure theoretician is often not completely cognizant of the situation and his works bear little adaptation to the real problem. The experimentalist has his own problems with regard to obtaining results from small-scaled models and successfully adapting them to the larger prototypes. Each method has its own advocates who diligently adhere to their own separate routes as being the only means of resolving this problem. However, a review of the various methods of analysis leads one to the conclusion that if the advantages of all three are combined, a reasonable solution to this problem can be reached. Briefly then, the problem is to obtain the magnitudes of the forces on the submarine pipeline subjected to a lateral current by investigations of previous design considerations, by a study of existing theoretica1 data and by appropriate experimentation. This paper deals mainly with obtaining experimental data and the interweaving of theoretical and experience factors for resolution of these forces. APPARATUS AND PROCEDURE These tests were performed in 1961 at Collins Construction Co.'s main office near Port Lavaca, Tex. Test equipment consisted of a steel tank 3 ft high by 4 ft wide by 40 ft long, with several pumps circulating the water through the tank via suction lines from the downstream portion of the tank and discharging from the pumps directly into the upstream end of the tank. For reduction of turbulence, a wire section was constmcted in the upstream portion and straightening vanes in the form of pipes, screens, etc., were used to reduce the vortex and eddy action in the tank (Fig. 1). A venturi was built at the test specimen for obtaining velocity sufficient to simulate actual conditions. This full-scale test was initiated to eliminate the error introduced in modeling and obtain a truer picture of forces actually encountered. Six- and 10-in. specimens were prepared by