Current-induced scour beneath elevated subsea pipelines

The erosion of sediment, or scour, around marine structures is a common occurrence. Scour around subsea pipelines may lead to excessive bending moments and/or vortex-induced vibrations; thus potentially compromising the structural integrity of the pipe. However, when a pipeline is installed along an uneven seabed, certain sections may be elevated with respect to the seabed. Small initial pipe elevations can result in high flow amplification beneath the pipe, which induces a high capacity for scour to occur, and increase the initial gap between the pipe and the seabed. A recent survey of a subsea pipeline revealed multiple incidences of free spanning, whereby a significant number of spans had maximum seabed gaps less than 30% of the outer pipe diameter. Rectification works are challenging and expensive; therefore, this work focused on predicting scour beneath subsea pipelines under steady currents with a particular emphasis on quantifying the influence of the initial pipe elevation. Few existing empirical formulae have included the influence of the pipe elevation. In this work, a combination of experimental, numerical and field investigations has been undertaken; through which the data obtained in this work and from published literature are used to develop new empirical formulae for predicting: (1) the maximum dimensionless seabed shear stress beneath the pipe; (2) the equilibrium scour depth; and, (3) the scour time scale, which includes a suggestion for a new non-dimensional form of the time scale. The key variables of interest are: (1) the pipe-elevation-to-diameter ratio; (2) upstream dimensionless seabed shear stress; and, (3) the pipe Reynolds number. A scalar objective function is used to quantify the influence of the aforementioned variables in deriving a new set of empirical formulae. Upon the validation of a single-phase rigid seabed model with published experimental data, a large parametric study is performed to compute the seabed shear stress beneath the pipe. The numerical data is used to develop an equation for predicting the maximum dimensionless seabed shear stress, which can be compared to the critical shear stress to estimate the initiation of scour beneath the pipe. Subsequently, sediment flume experiments are conducted to investigate the influence of the pipe elevation on the development of scour beneath the pipe. The experimental data from the present study as well as from the published literature are used to derive empirical formulae for predicting the equilibrium scour depth and scour time scale. The recent derivations suggest that the maximum seabed shear stress, equilibrium scour depth, and time scale, are significantly influenced by the pipe elevation and upstream seabed shear stress. However, the Reynolds number effects are small compared to the other parameters. In addition to a review of existing empirical formulae for scour prediction, a comprehensive review of existing numerical modelling techniques has been performed; through which the practical options for modelling scour are identified, employed, verified and validated. A single-phase rigid seabed model is found to be appropriate for performing a large parametric study to predict the maximum seabed shear stress for predicting the initiation of scour. A two-phase Eulerian-Eulerian model is deemed to be practical for predicting the equilibrium scour depth with reasonable accuracy, but not the time scale. This thesis focused on predicting two-dimensional current-induced scour beneath elevated subsea pipelines. The pipe elevation has not been considered in existing formulae for predicting the initiation of scour. Subsequently, the severity of scour can be estimated via the prediction of the equilibrium scour depth and the scour time scale. This would be beneficial when the flow condition may not necessarily result in a deep scour hole, or when the time required for a substantial scour depth to develop is significantly longer than the storm period. Ultimately, this body of work aims to improve the design and management of subsea pipelines.