ABSTRACT:

Experiments have been made in laboratory flumes to investigate the interaction of a mobile sand bed with a cylinder under currents and under waves. The cylinder is free to move vertically under its own weight. The presence of a cylinder always caused incipient bed motion and ripple formation to occur at lower Shields numbers. The cylinder axis always dropped and forces were always reduced, sometimes substantially, as a result of bed motion. Hydrodynamic force was measured through the surface pressures by a novel device.

INTRODUCTION

The stability of pipelines on the seabed when exposed to waves and currents has been the subject of much investigation. To assess instability, hydrodynamic forces are derived from wave/current kinematics with appropriate force coefficients and Coulomb friction has been assumed, again with an appropriate coefficient. However penetration of the pipeline into the bed is now considered to enhance stability. There has been considerable investigation of the lateral cylinder movement produced by wave loading causing the pipeline to dig itself in under its self weight, taking into account many factors, e.g., Soteberg et al. (1988) and papers in the Offshore Technology Conferences (OTC) of 1987 and 1989. There has also been considerable investigation of scour around pipelines (cylinders) where the cylinder is fixed in position in currents (Bijker and Leeuwestein, 1984; Kjeldsen et al., 1973; Sumer et al., 1988); waves (Bijker and Leeuwestein, 1984; Sumer and Fredsoe, 1990; Zdravkovich, 1986) and for a vibrating cylinder (Sumer et al., 1988; Zdravkovich, 1986). The time scale for the development of scour has been investigated for a fixed cylinder in currents and waves (Fredsoe et al., 1991) and the onset of scour around buried pipes has been investigated in currents (Chiew, 1990) and waves (Sumer and Fredsoe, 1991).

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