Underwater pipelines are vital to the oil industry. Extending the service life of these pipelines is a key issue in improving the sustainability of oil transportation. A geotextile mattress with floating plate (GMFP) is a novel and sustainable countermeasure for scour and erosion control and is herein introduced to protect a partially buried pipeline from local scour in steady currents. A series of experiments was designed to verify the protection capabilities of the GMFP and investigate its parametric effects on protection. The average seepage hydraulic gradient under the pipeline was adopted to depict the protection effects of the GMFP, and was calculated with the pore pressure readings under the pipeline. The test results show that the GMFP is capable of protecting a pipeline from the onset of local scour in a unidirectional current. The average seepage hydraulic gradient below the pipeline decreases remarkably after a GMFP is installed. The average hydraulic gradient shows a descending trend with increased sloping angle α when 0.64 < sinα < 0.77. The hydraulic gradient hits a nadir at sinα = 0.77 and climbs with the increasing sloping angle when sinα > 0.82. The hydraulic gradient ascends when the bottom opening ratio δ increases from 0.167 to 0.231, due to the decreased intensity of the bottom vortex. The hydraulic gradient drops with a rising plate height, except for a fluctuation at H_p = 0.12 m. An approximate negative correlation is found between the obstruction height of the floating plate and the average hydraulic gradient under the pipeline. This could be partially attributed to the extension and amplification of the bottom vortex on the leeside of the pipeline due to the increased plate obstruction height.

A geotextile mattress with sloping curtain (GMSC) was introduced to protect underwater pipelines from scour in steady currents. GMSC is a newly developed countermeasure against river and estuarine scour. To better understand the beneficial effects of GMSC on seepage flow under pipelines, dye was injected to determine the hydraulic gradients across the pipeline in a series of verification tests. Hydraulic gradients across the pipeline with and without a GMSC were quantified for different curtain angles, curtain heights of the GMSC, and distances between the pipeline and the GMSC, and installing GMSCs with or without sand-pass openings, which are openings to allow bottom flow with a high concentration of sediment to pass. The results of the experiments show that a GMSC is capable of protecting a pipeline against scour in a steady current by effectively reducing the hydraulic gradient in the bed soil below the pipeline. The hydraulic gradient across the pipeline decreases nonlinearly with the increase of curtain height and angle of the GMSC. The effect of sand-pass openings on the hydraulic gradient across the pipeline is not significant. The hydraulic gradient decreases nonlinearly with decreasing distance between the pipeline and the GMSC. When the distance decreases below a critical point, the seepage flow under the pipeline reverses. The GMSC may provide an optimum protection effect on the pipeline when the distance between the pipeline and the GMSC is about 6 times the obstacle height of the GMSC ($H_c\sin \alpha$ ) by reducing the flow velocity approaching the pipeline close to zero.

A detailed literature review on the protection of local scour beneath a submarine pipeline is presented. The review covers two basic parts of countermeasures against the pipeline scour, namely preventing the onset of scour and stimulating the self-burial of a pipeline. The research progress on the methods of the two sections is discussed in detail separately. The methods preventing the onset of scour have been extensively studied, but the understanding on their mechanisms is yet to be improved. The progress in stimulating the self-burial of a pipeline mainly focuses on a spoiler attached to a pipeline, which is investigated comprehensively with both experiments and numerical simulations. Both parts of countermeasures have been applied in some practical engineering projects and the protection effects are generally satisfying.

The three-dimensional scour beneath a partially-buried pipeline in regular waves was visualized using a miniature camera installed in a transparent pipeline. The scour mechanism was analyzed based on the results. Scour development was observed to start at the upstream edge of the span shoulder when the flow in the span headed downstream. The nearby sediment scoured quickly, and a new scour front formed, which can be attributed to the deflected flow entering the scour hole. The new scour front retreated gradually. The end of the original scour front deformed and moved downstream, probably due to the enhanced seepage flow near the edge of the span shoulder. After that, the new scour front extended to the downstream interface of the sediment and the pipeline, and continued to retreat until the first half of the scour process ended. In the second half of the scour process, the sediment transportation occurred in a similar but mirror-imaged manner. The scour hole propagation rate was also determined based on visualization. The results show that the scour hole propagation rate under a pipeline decreases with an increasing pipeline embedment ratio and rises with the KC (Keulegan–Carpenter) number, which is similar to the result of a previous study.

Rigid flow deflectors are usually used on water flow beds to protect engineering structures such as breakwater in coasts and to regulate flow routes in open channels. To reduce its side-effects, i.e., local scour at the toe of deflectors, a flexible flow deflector is proposed, and the corresponding local scour was investigated in this study. A flume experiment was conducted to investigate local scour. To show the advantage of flexible deflectors, a control experimental test was also conducted using a traditional rigid deflector under the same blockage area configuration and the same flow conditions. The flow field near the flexible deflector was also measured to reveal the local flow field. The results show that the bed-scour develops near the toe edges of both flexible and rigid deflectors, but the maximum and averaged scour depths for the flexible deflector are smaller. This advantage of flexible deflector in scour depth is mainly caused by its prone posture, which induces the upward stretching and enlarging horizontally rotating vortex and the upward shifted vertically rotating vortex. The former dissipates more turbulent energy and the latter results in smaller bed shear stress, which lead to smaller scour depth directly. In addition, the up- and down-swaying movement of the flexible deflector can also assistant to dissipate more turbulent energy, thereby damping the intense of the horseshoe vortices and thus weakening scour depth as well. The results of this study provide an elementary understanding on the mechanisms of a flexible flow structure and an alternative deflector-device to reduce scour depth.

The intersection of the span shoulder and the scour hole under a pipeline, also called the scour front, can affect the scour hole extension rate and the bearing capacity of the span shoulder, which is associated with pipeline spanning and failure. Studies on scour front evolution are limited due to technical restrictions. In this study, visualization experiments on three-dimensional scour under a pipeline in steady currents were performed to reveal the evolution of the scour front at the beginning of scour process, and the parametric effects on the scour front pattern. The angle of scour front witnessed a steady drop at the start of the scour development process and fluctuated gently thereafter. This can be attributed to the decrease in the blockage effect over time. A gravitational movement was observed on the downstream end of the scour front, which is associated with the variation of fluid pressure along the scour front. The scour front pattern was described by the angle between the scour front and the streamwise direction. Parametric analysis showed that the averaged scour front angle climbs as the Froude number increases and the pipeline embedment drops. This phenomenon can be expounded by the intensified flow deflection near the span shoulder due to the enhancement of the pipeline blockage effect, caused by the ascending Froude number and descending pipeline embedment. The results of this study can be adopted in future mechanism investigations on the scour hole propagation rate and the failure analysis on the span shoulder.