Determination of Flow Kinematics Close to Marine 'Pipelines and Their Use in Stability Calculations

Abstract

ABSTRACT Most of the expressions describing the hydrodynamic forces on marine pipelines are based on free stream kinematics (undisturbed flow) and it is expected that these formulae will fail when oscillatory flow fields are considered. The presence of the pipe will locally change the flow field. A wake is formed in each half period and is then washed over the pipe in the following half period giving rise to relatively high local flow velocities, which differs significantly from the flow generated by the free stream. Through measurements of the flow velocities in the vicinity of the pipe the effect of the wake has been quantified. Furthermore, a hydrodynamic description of the flow field around a cylinder resting on a plane boundary exposed to a regular oscillating flow field is developed. By combining the measured hydrodynamic forces, the near pipe measured flow velocities and the theoretical developed expressions for the wake effect the principle for establishing simple formulae describing the hydrodynamic forces is presented. The advantage of this approach is that the expressions have been developed maintaining a direct relation between the hydrodynamic forces and the underlying physical phenomena. This means that results can be extrapolated more easily to cover other environmental situations without having to conduct extensive model investigations. Thus, the establishment of a rational procedure for calculating the hydrodynamic forces to be used in on-bottom stability design of marine pipelines exposed to natural sea states seems to be within reach. INTRODUCTION On-bottom stability design procedures vary considerably from project to project. Detailed project reviews reveal that no general code of practice exists for the individual activities in the stability design procedures. Each company has its own procedure for establishing water kinematics at the sea bed, calculating hydrodynamic and friction forces, as well as its own philosophy behind applied safety factors. The result is that the overall safety of a specific pipeline against excessive lateral movements is difficult to evaluate. The risk for unduly conservative design or for having to perform remedial work after pipeline construction is therefore present. The conclusive settlement of methods for calculating hydrodynamic forces on marine pipelines represents a major step towards the establishment of a rational design procedure for on bottom stability, and will have significant economical impact on pipeline projects. The following expressions are commonly used to determine the hydrodynamic forces on marine pipelines exposed to time varying flow fields:(Mathematical equation 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 coefficients for drag, inertia and life. The coefficients are in general determined through model tests.