Experimental Study Of Wave Boundary Layer Flows And Wave Forces On Small-Diameter Pipelines

Significant velocity reduction and momentum transfer happen in the near-bed boundary layer flows induced by waves.Seabed roughness will alter the velocity and turbulence distributions in the wave boundary layer,and roughness elements will lead to free spans of the pipeline.These flow and seabed features will noticeably affect the wave forces acting on the small-diameter subsea pipelines(such as,power transmission cables,umbilicals,whose diameter is comparable to the thickness of the wave boundary layer and the seabed roughness).The present thesis aims to develop predicting models of velocity and turbulence in the wave boundary layers,and aim to establish new empirical methods to calculate the wave forces acting on the small-diameter pipelines.A number of physical tests were conducted to investigate the properties of wave boundary layer flows and the pipeline wave forces.Three parts of work constituting the present thesis are described as follows.Firstly,an updated defect function model is proposed to predict the 1DV(one-dimensional and vertical)wave boundary layer flow in the physical and the time space.The updated defect function consists of two terms that dominate the amplitude defect and the phase defect,respectively.Both the amplitude and the phase defects consist of one length scale and one index scale.For the amplitude defect,its length scale is a measure of seabed roughness,and its index scale is similar to the friction factor which is a measure of wall turbulence.For the phase defect,the length scale is a measure of wave boundary layer thickness,and the index scale governs the non-linear development of boundary layer thickness in the phase space.The four model parameters,calibrated against the existing and the present datasets,are correlated with the governing flow parameters.Based on a number of demonstrations,it is found that the present defect function model performs well in predicting the time-dependent wave boundary layer motions.Secondly,the turbulence scaling in the oscillatory boundary layer flows is investigated,focusing on the second moment of the streamwise velocity fluctuation(referred to as turbulence intensity,hereafter).The following features are well observed.In the smooth turbulent flow regime,the amplitude of turbulence intensity is a logarithmic function of the distance to the wall in the overlap region;in the outer region,both the amplitude and the phase shift of the turbulence intensity are linear functions of the amplitude and the phase shift of the velocity deficit(the difference between freestream and the boundary layer),respectively.The former correlation is termed the logarithmic scaling and the latter is called the linear deficit scaling.Based on the linear deficit scaling,the spatio-temporal distribution of the turbulence intensity in the outer region can be well predicted within the smooth turbulent flow regime.In the rough turbulent flow regime,the two scalings are not valid under the disturbance of the bottom roughness elements.Nevertheless,empirical correlations are proposed for predicting the amplitude of the turbulence intensity in the overlap region within the rough turbulent flow regime.Thirdly,a new empirical method is proposed for evaluating the peak wave forces acting on small diameter pipelines under wave conditions.In the present empirical method,the influences of the following aspects on the wave force coefficients are well considered:(1)the velocity reduction induced by wave boundary layers,(2)different turbulence level due to the changing in seabed roughness and(3)different sizes of free spans caused by roughness elements.With the increasing of the KC number,it is found that the velocity reduction induced by wave boundary layer reduces the local velocity across the pipeline diameter and therefore leads to smaller force coefficients.Seabed roughness influences the force coefficients via the following three ways:(1)a larger seabed roughness leads to a thicker wave boundary layer and a smaller local velocity across the pipeline diameter,and therefore leads to smaller force coefficients;(2)a larger seabed roughness enhances the incoming turbulence level and therefore leads to greater force coefficients;(3)larger size of and sparser arrangement of roughness elements underneath the pipeline leads to greater gap flows,and hence,leads to notable decrease of the lift force coefficient.