Author(s): Yee-Meng Chiew
Linked Author(s): Yee Meng Chiew
Keywords: Submarine pipelines; Scour; Vibration; Gap flow; Pounding effect
Abstract: Submarine pipelines constructed in the riverine, coastal, and offshore environments are important engineering structures for a myriad of purposes. In the coastal and offshore environments where such pipelines often are used to convey hydrocarbon across a body of water, their failure not only has severe economic consequences, but also causes serious adverse environmental impact. One of the main causes of submarine pipeline failure is the formation of a scour hole in its vicinity, affecting the integrity of its foundation and sometimes causing failure. Moreover, the scour length sometimes can induce vortex-induced vibration of the pipeline, causing rupture. Scouring generally is attributed to the formation of local vortices which enhance the shear stresses and turbulence, thereby causing soil erosion and scour hole formation. Earlier research studies, e.g., Chiew, 1990, were conducted using a static pipeline to induce the formation of a 2-dimensional scour hole in both laboratory and numerical studies. Very few of these studies had included vibration effects on scour because a 2D rigid pipeline was investigated. In more recent years, researchers, e.g., Wu and Chiew, 2012, began working on 3-dimensional pipeline scour where both the scour depth, width and length are formed and measured in the tests. When the scour hole reaches a certain length, self-induced vibration due to the exposure of the pipeline can occur. Besides the occurrence of self-induced vibrations, forced vibration that is attributed to the movement of the pipeline due to the local hydrodynamic forces also occurs. One of the earliest works on vibrations effects on scour hole formation was that by Li et al., 2013, who examined how a 3-dimensional pipeline vibrating in all 3 dimensions affects pipeline-scour; which somewhat resembles a vibrating catenary riser. Their work reveals that the resulting scour hole profile (Figure) is significantly different from those formed with a rigid, 2-dimensional pipeline. This pioneering work shows, amongst other things, that the effect of vibration, which often is present in the field, should be included to have a better understanding on pipeline-scour and so that failure can be minimized. This paper outlines some of the state-of-the-art laboratory works on vibration effects on submarine pipeline scour. It presents how both self-induced and force-induced vibrations affect scouring, showing that the scour mechanism is markedly different from that of a rigid, 2-dimensional scour with a rigid pipeline where vibrations are absent. The main finding shows that 1-dimensional vertical vibrations can cause the pipeline to move up and down from the seabed, creating a gap to be present which allows a gap-flow between the underside and the pipe and the seabed. This creates a nozzle effects. Moreover, depending on whether the pipeline pounds on the seabed during its descend towards the bed (pounding effect), the result on local vortex formation and seabed erosion can be very different. The extent of the vibration acceleration and frequency also can affect the formation of the scour hole.
Year: 2023