Previous studies on the lateral instability of the pipeline seldom paid attention to the detailed breakout process at the pipe-soil interface, where the sliding and the rolling mechanism could be coexistent and competitive. In this study, a series of experiments on the steady flow- induced lateral breakout of a partially-embedded pipeline on a silty seabed were conducted in a large fluid-structure-soil interaction flume. The effects of the embedment-to-diameter ratio and the dimensionless submerged pipeline weight on the pipeline lateral on-bottom stability were investigated and the sliding-to-rolling ratio was further analyzed. Experimental observation shows that the values of the critical velocity for pipe breakout are generally small for light pipes and the local scour effect can be ignorable. It is indicated that both the critical Froude number and the corresponding sliding-to-rolling ratio increase with increasing embedment-to-diameter ratio and submerged pipeline weight. Furthermore, comparison of Fr-G relationship is made between the present silty seabed tests and previous sandy seabed tests for the shallowly-embedded pipes.
Pipeline stability on the seafloor is one of main problems encountered for submarine pipeline to transport oil and gas. When a submarine pipeline is installed on the seabed, the seabed must provide enough soil resistance to balance the hydrodynamic loads upon the pipeline to avoid the occurrence of pipeline on-bottom instability. The on-bottom stability of a submarine pipeline involves complex interactions between wave/currents, pipeline and neighboring soil. For pipeline geotechnical engineers, the ultimate soil resistance is one of the fundamental criteria in pipeline on-bottom stability design under various environmental conditions (Wagner et al. 1989).
In the past few decades, numerous experimental studies on the pipeline on-bottom stability have been conducted with 1g mechanical- actuator simulation (e.g., Lyons, 1973; Brennodden et al., 1989), or with Ng centrifuge tests for calcareous sand-pipe interaction (e.g., Zhang et al., 2002). In these experimental investigations, the mechanical actuator systems were commonly employed to simulate hydrodynamic loads upon pipeline. However, for the practical situations, the waves and currents exert loads not only on the pipeline, but also upon the seabed. Therefore, water flume test is a significant supplementary to the traditional mechanical experiment. In fact, various series of water flume tests have been carried out to reveal the flow- pipe-soil coupling effects on the wave-induced pipe lateral instability (e.g., Gao et al., 2003; Teh et al., 2003).
In most of the aforementioned studies, the ocean wave is the primary concern in terms of environmental loads in shallow waters. However, with the oil and gas exploitation moving into deeper waters, ocean current gradually becomes the dominant hydrodynamic load for on-bottom stability of submarine pipelines (Jones, 1985). Until now, studies specifically focusing on the currents-induced pipeline lateral instability, such as the work of Jones (1985), is still quite scare and the underlying physical mechanism has not been well revealed (Gao et al., 2007). In previous studies on the lateral soil resistance, the pipe is mainly constrained against rolling. Nevertheless, the actual scenario is always between the free-laid and rolling-constrained cases, and the sliding and the rolling mechanism at the pipe-soil interface could be coexistent and competitive during the breakout process, which has been seldom examined.
