Local Scour, Self-bury And Safety Assessment Of A Submarine Pipeline In Cohesive Soils

DF1-1 submarine pipeline was investigated using an integrated surveying system, including a dual-frequency side-scan sonar, a high resolution sub-bottom profiler, and a swath sounder system. More than a hundred of scour pits under the pipeline were found during the pipeline route survey, most of which have caused the span of the pipeline and threaten the safety of the pipeline.Using methods of field investigations, physical simulations and numerical calculations, the mechanism of local scour around submarine pipelines in cohesive soils was studied, and the maximum scour depth under ordinary weather conditions was estimated; the mechanism of the self-bury of the pipeline was studied, and the relationship between the self-bury and the local scour was discussed; the influences of the local bedform evolution on the hydrodynamic forces experienced by the pipeline were discussed; the maximum allowable free span length (MAFSL) of the pipeline was calculated, and the safety of the pipeline was estimated; Methods for protecting the pipeline were proposed.By analyzing the pipeline field investigation data, the concepts of 'side scour' and 'down scour' were proposed for the local scours around submarine pipelines in cohesive beds, which probably result from tidal currents and hurricane-induced currents respectively. The tidal current in the study area can cause the seabed scours on both sides of the pipeline (side scours), the maximum scour depths of which agree well with the calculated results according to the equations proposed by different researches based on experimental studies with sand or silt beds. The finer sediments were transported mainly in the form of suspended load in the scour process, while the coarser sediments were detained on the seabed, which caused the local sediments becoming coarser. There exists lots of scour pits under the pipeline (down scours), which may be mainly caused by hurricanes.The side scours may be related to the down scours, although they may be caused by different hydrodynamic forces. Experimental studies showed that the down scour process can be divided into four stages: (i) seabed scour on both sides of the pipeline; (ii) piping; (iii) seabed scour under the pipeline; (iv) scour equilibrium. The side scour is the first stage of the down scour process. The study results indicated that the tidal currents in the study area can not make the scour process reach the second stage (piping), which occurs only when the accumulative effects of the side scour reach the critical conditions by hurricanes.The DF1-1 submarine pipeline in the deep water regime (with water depth > 37m) maybe buried itself due to effects of the gravity and the side scours. The maximum burial depth of the pipe was the total effects of that caused by gravity and the side scours respectively, while the burial depth caused by scours corresponds well with the maximum side scour depth. The self-bury may protect the pipeline by reducing the down scours and span height caused by hurricanes.The side scours can either inhibit the down scours by inducing the self-bury of the pipeline or accelerate the down scours by making conditions for piping, the majority effects of which depend on the rate of self-bury, the frequency and intensity of hurricanes. The side scours inhibited the down scours at the early stage after the pipeline was laid, while accelerating the down scours now.Experimental simulation results showed that, depending on wave conditions, soil grain size and initial burial depth of the pipeline, different near-field bed evolution behavior were recorded. The observed behaviors were classified into four different basic regimes, namely: (I) no scour, (II) scour without sand ripples, (III) scour with small sand ripples, (IV) scour with large sand ripples. As long as the heights of ripples are smaller than that of the unburied section of the pipeline, the flow nearby the pipeline is able to destroy the ripples and transport the sand through the gap between the pipe and the seabed, so the moving small sand ripples have very limited effects on the scour process. However, if the heights of ripples are comparable to or larger than the pipeline's unburied section, the flow nearby the pipeline is not strong enough to destroy the ripples. As a result, the scour process is significantly affected by the presence of the ripples with the pipeline being periodically buried either partly or completely by the moving ripples.The influence of scouring progress on wave forces was found to vary significantly in different aforementioned regimes. In regime I, the wave forces were quite stable; in regime II and III, the wave forces generally underwent a gradual reduction and reached their equilibrium force values at rather early stages of the scour process; in regime IV, the wave forces were significantly affected by the ripples activity and usually higher than that without the influence of ripples.Pipelines should be buried to different depths to reduce the wave forces depending on different soil grain size of the seabed. In silt soil, the wave forces on initially half buried pipelines were much smaller than that on initially fully exposed pipelines, but that were not necessarily true in sandy bed. In scour regime IV of sand bed, where the ripples activity can significantly influence the scour process, wave forces (with high fluctuation) on initially half buried pipelines are even higher than those on initially unburied pipelines. These results strongly suggest that the submarine pipeline needs to be buried below the active layer of sand ripples in order to reduce wave forces.Both static and dynamic analyses were used to study the maximum allowable free span length (MAFSL) of the pipeline. The study results show that, the MAFSL under static conditions is 56m. However, the MAFSL is 30m and 20m under ordinary weather conditions and hurricane-induced currents in 100-year return period respectively to avoid Vortex Induced Vibration (VIV) calculated using the highest safety class factor. It is suggested that the spans longer than 20m should be disposed. Additionally eight successive spans which may also threaten the pipeline were proposed in the study. The most hazardous scour pits are in the pipeline section from KP42 to KP51.Methods for protecting free spanning pipelines were analyzed and compared. A so called "artificial grass" technique is suggested for protecting the pipeline section from CEP to KP42, while the re-trenching method is suggested for the section from KP42 to KP51.