Three-dimensional scour at submarine pipelines

The significance of pipeline-scour has been well recognized because this phenomenon and the resultant vortex induced vibration (VIV) are closely related to the damage or even failure of pipelines. Although numerous studies have been conducted during the last few decades, knowledge on pipeline-scour is still quite limited, especially considering that most of the published research was performed with a 2-dimensional model while pipeline-scour is actually 3-dimensional in field conditions. To improve the understanding of 3-dimensional pipeline scour and explore the mechanics behind the phenomenon, this study focuses on the free span formation, or the propagation of the scour hole in the transverse direction of flow under unidirectional steady current in clear-water conditions. The entire research work is arranged in two stages. Stage I comprises the scour experiments in which the general characteristics of pipeline-scour in the transverse flow direction are observed and a parametric study on the effect of four non-dimensional parameters on the propagation velocity of the free span is then conducted. Observations and data from more than 70 tests are collected under different combinations of environment and pipeline conditions to examine the characteristics of pipeline-scour in the transverse direction. The results show that the development of the scour hole in the transverse direction may be divided into a rapid and a slack phase of development. The former is characterized by a higher but constant propagation velocity, while the latter is characterized by a lower and reducing propagation velocity. Four non-dimensional parameters are identified from dimensional analysis and their effects examined systematically with careful variable control. Stage II deals with the study of the mechanics of scouring at the span shoulder of the pipeline. Based on the observations and data from Stage I, the hydrodynamic force and differential pressure at the span shoulder are considered the two driving forces of pipeline-scour in the transverse direction of flow. These two quantities are measured and their effect on the propagation velocity is analyzed from which a semi-empirical equation is proposed. In this part of the study, the 3-dimensional flow velocity profiles adjacent to the span shoulder as well as the middle portion are first measured in order to investigate the flow structure and its effect on the development of pipeline-scour in the transverse direction. A naturally formed 3-dimensional scour hole around a modeled pipeline is immobilized using adhesive, giving a steady flow field around a fixed boundary. A 3-dimensional down-looking ADV is then used to measure the flow velocities at several critical locations. The characteristics of the flow field and its effect on the scouring process are discussed. Second, the pressure gradient at the span shoulder between the upstream and downstream sides of the pipeline is measured during the scouring process with wet-wet differential pressure sensors, which are placed in the modeled pipeline. The data confirm the significant effect of the pressure gradient, which greatly enhances the propagation velocity in the transverse direction and differentiates the rapid from the slack phase of development identified earlier. It also establishes a major dependent relationship between the non-dimensional propagation velocity in the transverse direction and the pressure gradient at the span shoulder, which implies that the effect of the hydrodynamic force is secondary. Finally, the effects of the pressure gradient and hydrodynamic force at the span shoulder on the propagation velocity are expressed in terms of a semi-empirical equation, which may be used to evaluate the propagation velocity in clear-water conditions.Doctor of Philosophy (CEE