Study On The Scour And Protection Technology For Submarine Pipeline Under Wave Condition

As an important carriers which transport the oil and gas from offshore to land,submarine pipeline are extensively used in the offshore engineering industry. Similarto other offshore engineering structures, the stability and survivability of submarinepipelines are influenced by many uncertain factors in the complicated oceanenvironment. For pipelines laid on sandy seabed, local scour often takes place andinduces undesired pipe span. Such free spanning may lead to the vortex-inducedvibration (VIV) and cause the damage or even destruction of pipelines. As such, localscour around the pipeline and the prevention measures have attracted a great deal ofscholarly and engineers’attention.The instability induced by wave was considered as one of the main problemsfor pipe application in offshore engineering. For the pipelines exposed on the sandybed, the pressure difference induced by wave between the upstream and downstreamof pipeline is generated, such pressure difference will cause the occurrence of seepageunderneath the pipe and it increases with the increment of pressure difference. Assuch, part of the bed in the wake area of pipe is liquefied and the sediment movementstarted, then the piping taking place and the scour below the pipe followed.The analysis of the balance of forces acting on sediment particles on theliquefied bed of downstream gives the critical pressure difference and wave height onparticles over which the movement of sediment and scour take place. Based on theforce analysis on a sand particle in scour hole, the erosion-stopping velocity isformulated. Moreover, the clear water scour underneath the pipe was investigated, andthe formulas for estimating the scour depth under different conditions (with spoilerand gap) are further derived.On the basis of the studies of protection methods using spoiler and the flexible material, the flexible spoiler which is placed between the pipeline and seabed isproposed in this paper. For the shorter flexible spoiler, it will swing with the wave andhas a strong disturb effect on the sand particle, by which the sediment transport andscour is accelerated. Therefore, the pipeline can be buried by itself. For a longerspoiler, the coverage of the spoiler extends and the pressure gradient at the edge of thespoiler decreases, leading to the reduction of the amplitude of swing of spoiler. So, thesediment transport and the scour depth decrease.The pressure distribution under regular wave, the equilibrium scour depth andwidth under regular and irregular wave have been experimental investigated for awide range of the flexible spoiler length/rigid spoiler height, pipe size and waveparameters (e.g. amplitude and frequency). Based on the analysis of the experimentalresults the following conclusions can be made:1. Pressure distribution: for the fixed pipe, the using of rigid spoiler increase thepressure difference acting on the pipe. The flexible spoiler, however, hasadvantage in reducing the occurrence of pipe vibration through reducingthe wave-induced horizontal pressure difference on the pipeline.For the slide pipe: the horizontal pressure difference gradually decreasesas the pipe sinking into scour hole, moreover there is a force downwardwhich accelerate the sinking of pipe.2. Scour depth: for the fixed pipe, the scour depth increases with increasing Kcnumber. The scour depth is deeper for higher rigid spoiler due to theincrease of the pressure difference. The situation for flexible spoiler isslightly different, it initially increases with increasing flexible spoiler length until L=1.5D, after that, it decreases with increasing flexiblespoiler length.For the slide pipe: the scour depth increases with increasing Kc numberand spoiler height. For the flexible spoiler, the scour depth reaches themaximum value at the length of0.7D and then decreases with increasingrubber plate length. When the length is2.0D, the scour depth is smallerthan the case of without spoiler.The scour depth under regular wave is slightly greater than that underirregular wave for both rigid and flexible spoilers for otherwise theidentical conditions.3. Scour width: for the fixed pipe, the scour extents with a spoiler (both therigid and flexible) are greater than that without a spoiler; also it increaseswith the increase of the spoiler height/length and Kc number. It is notedthat for the cases of flexible spoiler there is a drop of the scour width atL=1.5D and the max value occurring at L=2.0D.For the slide pipe, the same trend hold as the fixed pipe, but the scourwidth is smaller than the fixed condition since the pipe sinking.The scour width under irregular wave is slightly bigger than the regularwave condition.After the study of theory and experimental study, the VOF model in CFD wasused to simulate the flow field, the turbulent energy and the pressure distribution around the pipe. The simulation results were compared with the test data and a goodagreement was obtained.