Drag And Inertia Coefficients For Partially-Buried Offshore Pipelines
Authors: Parker Michael E. , Herbich John B. 
Published: October 2008

This model study was designed to simulate more accurately the actual design conditions for most offshore pipelines. The data were analyzed by two approaches:presentation of results in terms of a dimensionless force that can be used to give prototype force estimation without requiring the calculation of the wave kinematics, andpresentation of drag, lift, and inertia coefficients that can be used in the Morison equation in combination with. Stokes' third-order wave theory.

All results have been presented in a dimensionless form. Assuming that the laws of hydraulic modeling are followed and that there are no scale effects, these results can be used for determining the wave-induced forces on a partially buried pipeline.

INTRODUCTION

Since the late 19th century, men have been constructing various types of pipelines in the oceans The first constructed were the submarine sewage outfalls, which provided a simple "out of sight, out of mind" solution to a sewage disposal problem. Until World War II, this was the extent of submarine pipeline engineering. During the Second World War, after the Normandy Invasion, twenty 3-in. (7.6 cm) submarine pipelines were laid to provide the Allied forces with fuel. Since World War II, there have been great strides made by the offshore petroleum industry and others in the commercial uses of the sea. The submarine pipeline has proved itself to be an economical and safe method of transportation of fluids from offshore locations. The petroleum industry uses pipelines for the transportation of crude oil and natural gas, utilities for extraction of cooling or not water; and factories and cities for dumping wastes though under strict environmental control.

Since all structures placed in the ocean are subject to wave and current forces, it has always been important to estimate wave forces adequately. Until a theoretical method for calculation of such forces is developed, designers are forced to rely on experimental methods for force determination. These experimental methods may be either model studies or full-scale studies. Model studies are the most common method since full-scale studies are prohibitively expensive. Model studies also are convenient since nearly every design condition has its own peculiarities; i.e., mass production of offshore structures is not feasible.

When a pipeline is laid in an underwater environment, there will be some immediate settlement of the pipeline. The degree of this settlement depends on the bearing capacity of the sediments beneath the pipeline. If the pipe subsequently is buried, the wave forces are reduced to those caused by pore-water pressure variations. If the pipeline remains exposed to any degree, it is subject to forces caused by waves and currents. The amount of the pipe exposed can range from a fully exposed section such as a pipeline spanning between two points to having only a fraction of the diameter exposed due to settlement and sedimentation. There is no accurate way to predict the degree of settlement in other than very general terms.