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.Local scour below pipelines has been identified as a key factor that impacts the stability of pipelines.The objective of the present study is to investigate spatio-temporal variations of scour below a partially embedded and inclined pipeline in the longitudinal direction,subject to oblique steady currents and provide a reliable evaluation method for the stability of pipelines.Local embedment around a pipeline can form due to either installations pressure or sediment transport,whereas the longitudinal inclination can be due to either natural sagging of pipeline into a scour hole or artificial interventions.First of all,the research background is introduced in Chapter 1,together with the research purpose and the layout of the thesis.Chapter 2 presents the experimental setup,including the O-Tube water flume,model pipeline,test equipment,experimental parameters and the validation of the setup.In Chapter 3,the effect of the pipeline inclination on the scour propagation rate is investigated.A contact image sensor was employed to record the time history of the scour propagation and thus the characteristics of the propagation rate was determined.Results indicate that the propagation rate decreases with increasing pitching angle(?)for the pipelines at the same yaw angle(?).At ?=1° and 2°,the scour propagation demonstrates 2 stages with 2 significantly different propagation rates,i.e.the fast development stage and the slow development one.For a larger ?,the scour tends to reach equilibrium faster with a smaller axial propagation extent.At the same ?,the propagation rate increases with the increase of the flow velocity and the scour reaches equilibrium faster under increasing flow velocities.At ?=2°,the effect of the flow velocity on the axial propagation extent is negligible.When the pipeline is yawed,both the propagation rate and axial extent increase with increasing ? at the same ?.However,when ? is larger than 22.5°,both the propagation rate and axial extent decrease with increasing ?.The measured propagation rate and axial extent are correlated with various affecting parameters and a prediction model of the scour development below an inclined pipeline is proposed.Chapter 4 examines the effect of the pipeline inclination on the scour depth.An ultrasound Doppler velocimetry was used to record the temporal variation of scour depth during the whole scour process and the characteristics of the scour depth was discussed.Firstly,the equilibrium scour depth decreases while the time-scale of scour increases as ? increases.At ?=2°,a phenomenon of scour-burial-scour has been observed at a higher velocity during the equilibrium stage.Secondly,for the same ?,the equilibrium scour depth is found to increase with increasing |?|.However,when |?| is larger than 22.5°,the equilibrium scour depth shows a decrease trend as |?| increases.The variation of the scour time-scale with respect to ?,? and flow velocity is discussed as well.The last chapter summarizes the findings of the present study and provides recommendations for future studies.