Yingxiang Wu

PHYSICAL MODELING OF UNTRENCHED PIPELINE BREAKOUT FROM SAND- BED IN OCEAN CURRENTS

The ultimate lateral soil resistance for pipe losing lateral stability on a sandy seabed under the action of ocean currents is investigated with a newly developed test facility by employing mechanical actuators to simulate hydrodynamic loads on the pipe. Two kinds of constraint conditions, i.e. anti-rolling pipe and freely-laid pipe, are taken into account, respectively. The experimental observations indicate that, the horizontal lateral soil resistance increases gradually to its maximum (ultimate) value when the additional settlement is fully developed. The buildup of the ultimate lateral soil resistance to the anti-rolling pipe benefits from not only the additional settlements but also the sand-particle collections in front of the moving pipe, especially for the anti-rolling pipes. The lateral-soil-resistance coefficient for the anti-rolling pipe is much larger than that for the freely-laid pipe. The pipe surface roughness also affects the lateral stability of anti-rolling pipes. A comparison is made between present mechanical-actuator tests and the previous water-flume tests, indicating the results of two types of tests are comparable and the local scour may reduce the pipe lateral stability in ocean currents.

Forces Acting on the Seabed around A Pipeline in Unidirectional Ocean Currents

In this study, the hydrodynamic forces acting on the seabed around the pipeline in unidirectional ocean currents have been investigated numerically. Two types of seabed are considered, i.e., plane and distorted seabed. The influences of gap between pipeline and seabed on the distribution of forces along the seabed are studied in detail. Computational re- sults show that the pressure at the upstream side of the pipeline gradually decreases and the pressure difference between two sides of the pipeline presents a declining trend with the increase of gap between pipeline and seabed. For the pressure distributions between the distorted seabed and the plane seabed, a double-peaks distribution is observed for the case of distorted seabed, but only one peak exists for the plane seabed. With the increase of gap between pipeline and seabed, the value of peak shear stress along the distorted seabed at the upstream side gradually decreases and the one at the down- stream side varies slightly. When the gap ratio reaches 0.7, the peak shear stress at the upstream side of the pipeline al- ready decreases to a normal level for the case of distorted seabed, while the peak shear stress at downstream side is still a large one for the distorted seabed.