Characteristics Of Three-dimensional Scour Below Inclined Submarine Pipelines In Steady Currents

Submarine pipelines serve as the lifeline for the oil and gas exploration system in ocean engineering and its stability guarantees the safe transportation of oil and gas from the sea to onshore or floating processing facilities.The local scour below submarine pipelines is induced by the long-termed erosion of currents and waves.The free span propagates along the pipeline and induces structural and fatigue failure of pipelines.A tremendous economic loss and environmental disaster will occur once the pipeline fails.Therefore,a full understanding of the scour development characteristics of the submarine pipeline is of great significance to its design and commission.Based on the engineering background of submarine pipelines’local erosion and recurrence after self-buried,this paper employs a simplified physical model to study the erosion characteristics of inclined pipelines under steady currents,and builds a predictive model of the scour propagation rate.Firstly,the characteristics of three-dimensional scour below an inclined submarine pipeline when the flow incident angle(α)is zero was studied in this paper.The relationship between the shear stressτand erosion rate(η)of the sand surface is obtained by a sand erosion test without structures.And then,the scour pit below a model pipe was recorded using a 3D scanner several times during the scour process to measure the change in the amount of sand.The control body is set to measure the volume of sand under the pipeline and hence the magnification factor(λ)can be calculated.The UDV probe was used to monitor the length of scour hole(L)along the pipe axis and the change of the scour depth S(t)with respect to time at the position where the probe was fixed.Based on results of erosion testing and magnification factorλ,a predictive model of the scour propagation rate was built.Secondly,the characteristics of three-dimensional scour below an inclined submarine pipeline forα≠0~°was studied.By varying flow velocity and the angleαbetween the model and the incoming flow,this paper studied the influence of velocity andαand on the scour characteristics of the pipeline and the results were compared with the situation ofα=0~°.For the characteristics of scour depth,the experimental results show that the initial buried depth ratio(e/D)and flow velocity have a significant impact on the scour depth:1)Forα=0~°,the scour depth at the position where e/D is larger than 0.2 increase with the increase of flow velocity.But the equilibrium scour depth at the position where e/D is smaller than 0.2 has a reverse tendency.2)Forα>0~°,the equilibrium scour depth at the position with the same e/D increase with the increase of flow velocity.However,there is a reverse tendency for the situation ofα<0~°.As for the characters of the scour propagation rate along the pipe axis,the experimental results show:1)The whole scour process includes two stages,i.e.the rapid scouring phase and the slow scouring phase.And the dividing point between the two phases is around the position where the initial buried depth ratio(e/D)is 0.3 forα≥0~°,but it’s position changes to where the initial buried depth ratio is smaller than 0.3forα<0~°.2)Forα≥0~°,the rate of scour propagation increases with the increase of the incoming flow incident angle during the rapid scouring phase and decreases with the increase of the incoming flow incident angle during the slow scouring phase.3)Forα=0~°andα=22.5~°,the increase of flow velocity makes the rate of scour propagation increase during the rapid scouring phase and the slow scouring phase.But forα=45~°,the increase of flow velocity makes the rate of scour propagation increase only during the rapid scouring phase and the rate of scour propagation becomes smaller with the increase of flow velocity during the slow scouring phase.As forα<0~°,the rate of scour propagation decreases with the increase of absolute value ofα.Through the analysis of scour landform,it was found that the sand slope directly below the pipeline is advancing along the pipe axis at an angle of 5~°～20~°,and ripples which expand along the pipe axis existed behind the model pipe.