Determination of flow kinematics close to marine pipelines and their use in stability calculations

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 environmentalmore » 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.« less