Hydrodynamic Forces From Wave and Current Loads on Marine Pipelines

ABSTRACT The weight coating requirements for assuring the on-bottom stability of several pipelines in the Arabian Gulf have been determined through an integrated design procedure including mathematical modelling for determining extreme wave conditions, field measurements of currents and physical model testing to establish the magnitude of the hydrodynamic forces. The paper describes in detail the hydrodynamic model testing. By using the carriage technique, scales varying from 1: 1 to 1: 4 have been obtained in the laboratory. Further by applying directly measured wave induced bottom velocities from a given sea state the design conditions have been reproduced correctly in the laboratory reducing scaling errors to a minimum. Thereby the uncertainties arising from the application of data from tests in steady current or regular waves to irregular waves or to the combination of waves and currents are avoided. Hydrodynamic coefficients for drag, inertia, and lift forces appropriate for use in common industry formulas for pipelines are determined for different environmental conditions. Reference tests in steady currents and regular waves support the conclusion of this study. The results cover a range of Keulegan-Carpenter numbers from 7 to 100 and Reynolds numbers from 105 to 106. INTRODUCTION On-bottom stability analysis of subsea pipelines is traditionally based on the following well known fundamental formulas for the hydrodynamic forces:(Available In Full Paper) where FD, FM and FL are respectively the drag inertia and lift force per unit length, U and a are the water particle velocity and acceleration (in principle normal to the pipe), D is the outer total diameter and p is the mass density of the surrounding water. CD, CM and CL are the usual coefficients for drag, inertia and lift. However, no firm established method for combining the drag, inertia and lift force exists. In many design procedures the inertia force is neglected since it is considered 90 out of phase with the drag and lift forces. For irregular wave motion this is certainly not always the case, particularly if a steady current is present. For large diameter pipelines with small KC numbers, the inertia force may be dominant. Peak inline forces are commonly combined with peak lift forces. In some design procedures the concept of a wave being a regular sinusoidal function is taken one step further, and inline and lift force are computed for various wave phase angles in order to find the most critical combination. This method, however, cannot be simply applied for irregular waves. The absence of a firm common industry practice for calculating hydrodynamic forces leads to uncertainties in the design which may have dramatic technical and economical consequences for pipeline projects. The need for a thorough review of commonly applied design rules is obvious and several research projects including full scale measurements in the ocean have been initiated over the last years. A drawback of the field measurements is that the results show a considerable scatter and so far no conclusive settlement of the hydrodynamic coefficients has been obtained.