波流共同作用下海底子母管线水动力的物理模型试验研究

通过物理模型试验研究海底子母管线分别在规则波加流和不规则波加流作用下的水动力特性。基于Morison方程,采用"等效直径法"分析得到子母管线拖曳力系数C_D,惯性力系数C_M和升力系数C_L(C_D⁺,C_L^-)。试验分别考察了流速比U_c/U_w,母管与海床间隙比e/D及子母管间的相对缝隙G/D对海底子母管线水动力系数的影响。结果表明水动力系数随U_c/U_w的增大而减小;当e/D<0.5时,海床对子母管线受力的影响比较明显,C_D,C_M及C_L⁺均随e/D增大而减小,|C_L^-|随e/D增大而增大;对子母管间的相互影响也不可忽略,C_D,C_M和|C_L^-|均随G/D增大而减小,C_L⁺值随G/D增大而增大。     

畸形波作用下双层水平板防波堤压力分布特性研究

基于物理模型试验,考虑畸形波参数、相对板宽、相对波高等影响因素,就畸形波对平顶双层水平板防波堤作用进行研究。首先对畸形波作用下双层水平板的波浪力分布特征进行了讨论,然后就最大波动压强、结构最大总垂向力与不规则波作用进行了对比分析。结果表明,畸形波作用下,双层水平板最大波动压力出现在前端迎浪区域附近,向尾端逐渐递减。双层水平板4个受力面的压力分布不同且有相位差,4个受力面的最大波动压力时间差约在0.1T_p~0.4T_p范围内变化。与不规则波作用比较,畸形波作用没有显著改变波压包络分布特征,但增大了波压包络强度值。试验范围内,就最大总力而言,两者最大总浮托力比值在1.06~2.45间变化;向下的最大总垂向力比值在1.22~2.07之间变化;就波动压力而言,其增大的幅度与畸形波参数α₁相关性最强,随α₁的增大而增大,在α₁=2.04~3.1试验范围内,畸形波作用时的最大压强比不规则波作用时可约增大20%~80%。就最大波吸力而言,两者的比值与畸形波参数α₄相关性最强,随α₄的增大而减小。在α₄=0.62~0.75试验范围内,最大波吸力强度的比值在1.61~0.87范围内变化。当α₄≤0.72时,畸形波作用时的最大波吸力大于不规则波作用时的最大波吸力;当α₄ > 0.72时则刚好相反。     

潮流及单向流作用下海底沙波形成和运移试验

基于量纲分析理论进行水槽试验,研究了潮流以及单向流作用下海底沙波的形成和发展过程。通过分析 海床地形数据,对海底沙波的特征尺度和发展过程进行定量描述,得出了潮流流速、周期、水深以及叠加单向流等因素对沙波特征尺度的影响。结果表明,潮流作用产生的海底地貌由大尺度的沙波和小尺度的沙纹共同组成,大尺度沙波在地貌形态塑造中占主导地位。从平坦海床开始,沙波波高和波长随水流作用逐渐增大,增长速度越来越慢,最终达到动态平衡。沙波特征波高和特征波长随流速和水深增大而增大,同时随往复流周期的增大而增大,并不断趋近于单向水流的情况。进一步对小尺度的沙纹地貌进行分析,得出了沙纹特征尺度随水流条件的变化规律。     

非线性管土耦合作用下钢悬链式立管触地区敏感性分析

浮体运动和海床土刚度是引起钢悬链式立管(steel catenary riser,简称SCR)管土相互作用的关键因素,将导致SCR触地区的疲劳损伤。以工作水深为1500 m的浮式平台上生产立管SCR为研究对象,基于法向抗力模型和侧向阻力模型建立管土作用模型,在环境载荷和浮体运动作用下,开展SCR与浮式平台的整体分析,研究海床土参数对SCR触地区动态响应和疲劳寿命的敏感性。通过改变海床土的不排水抗剪强度Su0、强度梯度ρ、吸力因子f_suc、吸力衰减参数λ_suc以及再贯入系数λ_rep等,得到不同参数对触地区动力响应、疲劳寿命的影响规律。研究结果表明:①基于软黏土海床,随着不排水抗剪强度Su0的增加,触地区立管疲劳寿命减幅达到33.23%,敏感性最高;②吸力因子f_suc越大,立管疲劳寿命越小且减幅达23.77%,其敏感性较高;③随着再贯入系数λ_rep增大,触地区立管疲劳寿命增幅达到15.48%;④海床抗剪强度梯度ρ和吸力衰减参数λ_suc对立管疲劳寿命影响较小。研究结论能为SCR设计分析及安全服役提供重要参考。     

斜向波作用下斜坡海床上管线三维冲刷不均衡性研究

由于海床起伏不平,斜坡的存在必然改变波浪对管线及海床的作用特性,进而影响管线三维冲刷。基于波浪港池实验,考虑规则波的作用,采用中值粒径为0.22mm的原型沙铺设与波浪传播方向成45°夹角的斜坡,研究斜向波作用下斜坡上海底管线的三维冲刷特性。通过测量管线下方冲刷坑宽度和深度的差异,分析管线三维冲刷的不均衡性。实验表明:管线的存在使斜坡上的波高有所降低;斜向波作用下管线三维冲刷的不均衡性表现为深度不均衡性和宽度不均衡性,宽度不均衡性主要是管后淤积泥沙的后移引起的,周期对三维冲刷不均衡性的影响比波高对其的影响程度大;管线自深海向近岸延展时,随水深的减小,冲刷深度分为缓慢发展阶段和快速发展阶段。     

急性CO2酸化对菲律宾蛤仔钙壳和呼吸作用的影响

基于渤海表层典型的碳酸盐系统,通过实验室密闭培养实验,分析急性CO₂酸化条件对菲律宾蛤仔(Ruditapes philippinarum) CaCO₃形成速率(G)和CO₂呼吸速率(R_C)的影响,以探讨局部海域CO₂酸化的底层海水在潮流或者风海流等因素的驱动下,脉冲式影响贝类栖息地时养殖贝类可能的响应。结果表明,菲律宾蛤仔在急性CO₂酸化条件下发生轻微的钙壳溶解和显著的呼吸抑制。CO₂酸化和菲律宾蛤仔的呼吸作用共同驱动钙壳溶解,溶解速率随Ω_文石下降而升高,G(μmol/(FWg·h))=0.14 × Ω_文石-0.49 (n=12, r=0.95, p<0.01)。活体菲律宾蛤仔钙壳保持稳定的Ω_文石临界值为3.5,而在Ω_文石=1.0的条件下,每天溶解的钙壳相当于贝壳总重的2‰。相较于钙壳溶解,Ω_文石改变对菲律宾蛤仔呼吸作用的影响更大,R_C(μmol/(FWg·h))=0.27 × Ω_文石+0.90 (n =12, r=0.82, p<0.01)。由于呼吸代谢决定了摄食等各种耗能行为的效率,因此本研究的结果表明,尽管菲律宾蛤仔可以通过摄食等自身调节机制来抵御造成钙壳溶解的环境胁迫,然而这一机制本身就可能受到酸化环境的不良影响。     

海流作用下子母管结构横向涡激振动

基于量纲分析,设计模型试验研究稳态海流作用下子母管结构的横向涡激振动。通过对结构横向振动位移、水动力载荷和流场速度的同步测量分析,研究子母管结构横向涡激振动幅值和频率随约减速度的变化规律以及母管的水动力特性。实验结果表明,子母管间距比和质量比对管道横向振动和水动力有较大影响。在管间距比为0.1～0.5范围内,子母管结构涡激振动存在明显的不对称性。随着子母管间距比的减小,结构横向最大振幅增大,涡激振动的约减速度范围变宽;随着质量比的增大,涡激振动的约减速度范围变窄。母管的平均阻力系数随子母管间距比的增大单调递减,而平均升力系数则呈现非单调变化的特征。     

波浪作用下沙丘状海底地形对溶质迁移影响特征研究

基于海床土体变形、渗流和溶质运移的耦合数值计算模型,考虑波浪沿沙丘状海底地形传播的浅水效应,本文分析了沙丘状海床内部孔隙水渗流特征和海水中溶质进入沉积物的运移特征,并通过与平底海床计算结果的对比,分析了沙丘状海底地形对溶质迁移的影响特征。结果分析表明,相对于平底海床,沙丘状海床一定深度内孔隙水渗流呈现明显的二维特征,进一步加速了溶质在沉积物中的迁移速率;对于沙丘状海底,对流和水动力弥散均有效促进了溶质向沉积物中的迁移,其中,对流作用相对稳定,而水动力弥散作用随着波浪作用时间的增加而不断减弱。参数分析表明,沙丘地形对溶质运移过程的影响程度基本不受波高的影响,但其随着波浪周期、水深和坡角的增大而增大,随着剪切模量和渗透系数的增大而减小,其中坡角的影响最为显著。     

波浪作用下粉质土海床液化界面波压力的研究

波浪作用下粉质土海床的液化是影响海上平台、海底管线等海洋构筑物安全的灾害之一。在进行构筑物设计中应考虑海床液化的深度问题,而液化土体对下部海床的界面波压力是计算海床孔隙水压力增长以及液化深度的重要参量。本文基于波致粉土海床自上而下的渐进液化模式,利用双层流体波动理论,推导了考虑海床土体黏性的海床界面波压力表达式,并与不考虑黏性时的界面波压力进行了比较分析。结果表明,计算液化后土体界面波压力时,是否考虑液化土体的黏性对结果影响较大,进而可能影响粉质土海床液化深度的确定。     

内孤立波浅化破碎过程斜坡沉积物孔压响应特征实验分析

观测资料显示内孤立波沿斜坡浅化过程对海底沉积物的作用犹如一台水中吸尘器，在破碎转换阶段达到最强，甚至会触发一系列地质活动，引发地质灾害。为界定此过程中沉积物的动力响应特征和影响因素，在大型重力式分层流水槽中模拟不同振幅内孤立波和不同类型沉积物斜坡连续作用过程，利用孔隙水压力采集系统实时记录孔隙水压力变化，对比分析不同水动力、坡度、沉积物类型情况下沉积物中超孔压变化特征。分析结果表明，内孤立波破碎过程，破波位置海床表层波压力和不同深度超孔隙水压力都存在相似的"U"型负压力变化过程；破碎波经过位置沉积物表现为和表面波压力正相关的孔压响应特征。破碎点沉积物中超孔压幅值随深度减小，约在6%波长深度位置减少到坡面压力的50%。超孔压幅值和内孤立波振幅、沉积物类型和斜坡度密切相关，坡度由0.071变化到0.160时，波压力幅值可增大至1.6倍。内孤立波振幅变化不影响不同类型海床土动力响应规律，只与超孔隙水压力值大小有关，内孤立波对海床的动力作用可认为弹性作用。     

Scour depth under pipelines placed on weakly cohesive soils

We here study the scouring processes that evolve around a submarine pipeline placed on a weakly cohesive seabed. We first analyze some laboratory tests carried out by Vijaya Kumar et al. [21], Xu et al. [25] and Zhou et al. [28] that focused on the scouring around a horizontal cylinder lying on a cohesive bed, subject to waves and currents. The specific purpose is that of finding a new formula for the prediction of the equilibrium scour depth under submarine pipelines. After a theoretical analysis of the main parameters, the sought formula has been found to be a function of: (i) the hydrodynamic forces acting on the cylinder (through the Keulegan–Carpenter parameter KC), (ii) the clay content of the soil C_c, and (iii) the burial depth e₀/D. In the presence of small amounts of clay (C_c< 5 %), the scour depth depends directly on KC (as confirmed by many literature works for pipelines lying on sandy soils, e.g. [18]) and inversely on C_c (as already seen for bridge abutments on cohesive soils, e.g. [1]), the best-fit law being characterized by a coefficient of determination R² = 0.62. If some burial depth is accounted for, this being a novelty of the present work, a more general formulation can be used, valid in the presence of weakly-cohesive soils and with burial depths of the pipe smaller than 0.5 (R² = 0.79). For large clay-content ranges (2% < C_c < 75 %), the scour depth depends directly on both KC and C_c, this giving R² = 0.79 (no burial depth) and 0.91 (some burial depth). However, this finding is at odds with the main literature, because, for large amounts of clay, it is fundamental to consider the liquidity index LI, which accounts for some important clay properties, like the plasticity. We argue that the absence of LI is balanced by the direct dependence of the scour depth on C_c. Notwithstanding the small number of available data, a formula for the prediction of the scour depth under pipelines lying on cohesive soils is fundamental for several engineering applications. The present contribution represents the first attempt to build such a formula, when the pipeline is subject to the wave-current forcing and the seabed is characterized by a relatively small clay content.     

Wave forces on large offshore pipelines

A laboratory investigation of wave forces induced by a regular train of waves on a large pipeline resting on the bed and at various clearances from the bed is presented. From considerations of dimensional analysis horizontal and vertical components of wave forces acting on the pipeline are expressed as force coefficients which are shown to be functions mainly of H/2a, gT²/2a, d/a and e/2a. A simple unseparated flow model based on potential flow theory and Morison's equation is presented for evaluating the maximum forces on the pipeline. The experimental results are com3ared with the theoretical results and data from existing literature. Based on the experimental results, hydrodynamic coefficients C_M and C_L have been evaluated     

Current-induced scour beneath initially elevated subsea pipelines

When a subsea pipeline is laid on an uneven seabed, certain sections may have an initial elevation with respect to the far-field seabed, e_o, and thus potentially affecting the on-bottom stability of the pipeline. This paper focuses on quantifying the effects of the upstream dimensionless seabed shear stress, θ_∞, and Reynolds number, Re, on: (1) the maximum dimensionless seabed shear stress beneath the pipe, θ_max, to be compared to the critical shear stress in order to determine whether scour would occur and progress towards an equilibrium state; and, (2) the dimensionless equilibrium scour depth beneath the pipe, S_eq/D. Using a 2-D Reynolds averaged Navier-Stokes (RANS) approach along with the k-ω Shear Stress Transport (SST) turbulence model, a parametric study involving 243 computational fluid dynamics (CFD) simulations was conducted. The simulation results were used to develop a closed-form equation for the prediction of θ_max. Subsequently, experimental measurements of S_eq/D have been compiled from published literature, to develop a new closed-form equation for the prediction of S_eq/D with a high correlation to the experimental data. In summary, we present two closed-form equations for the prediction of θ_max and S_eq/D for pipelines with an initial e_o/D, which are applicable for both clear-water and live-bed conditions. The effects of θ_∞ and Re have been included, albeit Re having a small influence as compared to the other parameters.     

A lateral global buckling failure envelope for a high temperature and high pressure (HT/HP) submarine pipeline

Submarine pipelines are the primary component of an offshore oil transportation system. Under operating conditions, a pipeline is subjected to high temperatures and pressures to improve oil mobility. As a result, additional stress accumulates in pipeline sections, which causes global buckling. For an exposed deep-water pipeline, lateral buckling is the major form of this global buckling. Large lateral displacement causes a very high bending moment which may lead to a local buckling failure in the pipe cross-section. This paper proposes a lateral global buckling failure envelope for deep-water HT/HP pipelines using a numerical simulation analysis. It analyzes the factors influencing the envelope, including the thickness t, diameter D, soil resistance coefficient μ, calculating length L_f, imperfection length L and imperfection amplitude V. Equations to calculate the failure envelope are established to make future post-buckling pipeline failure assessment more convenient. The results show that (1) the limit pressure difference p_max (the failure pressure difference for a post-buckling pipeline when it suffers no difference in temperature) is usually below the burst pressure difference p_b (which is the largest pressure difference a pipeline can bear and is determined from the strength and sectional dimensions of the pipeline) and is approximately 0.62–0.75 times the value of p_b and (2) thickness t has little influence on the normalized envelopes, but affects p_max. The diameter D, soil resistance coefficient μ, and calculating length L_f influence the maximum failure temperature difference T_max (the failure temperature difference for a pipeline suffering no pressure difference). The diameter D also significantly affects the form of the normalized envelope.     

WAVE-CURRENT FORCE ON VERTICAL PILE

The drag and the interia coefficients Cd,CM of wave-current force on vertical pile are well related to the redefined Keulegan-Carpenter number N' Kc by the test data of the authors. The relation could be also used for irregular wave-current force to calculate in time domain. A simplified method for the calculation of cumulative probabilistic distribution of peak value of irregular wave-current force is also recommended in this paper. These methods were justified by the model test of the authors.     

Physical modeling and swirling strength analysis of vortex shedding from near-bed piggyback pipelines

The vortex shedding from near-bed piggyback pipelines in a steady flow has been investigated experimentally in a large water flume. A specially arranged PIV system with upward-illumination of pulsed laser arrays from the flume bottom was employed for the flow visualization and quantitative measurement of the lee-wake flow in a sub-critical regime around the piggyback pipelines in the proximity of a plane boundary. Based on dimensional analyses, a dimensionless maximum swirling strength (W_m) is used for analyzing the vortex shedding intensity and its frequency. Time-averaged swirling strength analyses indicate that the lee-wake patterns for the near-bed piggyback pipelines are dependent on the configuration factors, including the gap-to-diameter ratio (e/D), the spacing-to-diameter ratio (G/D), and the diameter ratio of two pipes (d/D), etc. The swirling strength in the lee-wake is obviously asymmetric for piggyback pipelines with bed proximity. For the fixed values of G/D and d/D, the maximum swirling strength decreases with the decrease of e/D. Moreover, for the examined G/D range (0 ≤ G/D ≤ 0.5), minimum values of W_m and corresponding VIV amplitude for the piggyback pipelines are evidently within the same range of spacing-to-diameter ratio G/D ≈ 0.05–0.20.     

Ionic medium effects in sea water — A comparison of acidity constants of carbonic acid and boric acid in sodium chloride and synthetic sea water

It is shown that the values of pK_1C and pK_2C for carbonic acid, pK_B for boric acid and the ionic product of water, pK_w, in sea water may be explained on the basis of their determination in 0.7 M_w sodium chloride and the formation of the following ion-pairs: NaSO₄^?, MgSO₄, CaSO₄, MgCO₃, CaCO₃, MgHCO₃⁺, CaHCO₃⁺, MgOH⁺, HSO₄^?, MgB(OH)₄⁺ and CaB(OH)₄⁺. On the whole the calculated stability constants are lower than those given by Garrels and Thompson (1962).     

How tides and river flows determine estuarine bathymetries

For strongly tidal, funnel-shaped estuaries, we examine how tides and river flows determine size and shape. We also consider how long it takes for bathymetric adjustment, both to determine whether present-day bathymetry reflects prevailing forcing and how rapidly changes might occur under future forcing scenarios.Starting with the assumption of a 'synchronous' estuary (i.e., where the sea surface slope resulting from the axial gradient in phase of tidal elevation significantly exceeds the gradient in tidal amplitude ), an expression is derived for the slope of the sea bed. Thence, by integration we derive expressions for the axial depth profile and estuarine length, L, as a function of and D, the prescribed depth at the mouth. Calculated values of L are broadly consistent with observations. The synchronous estuary approach enables a number of dynamical parameters to be directly calculated and conveniently illustrated as functions of and D, namely: current amplitude Û, ratio of friction to inertia terms, estuarine length, stratification, saline intrusion length, flushing time, mean suspended sediment concentration and sediment in-fill times.Four separate derivations for the length of saline intrusion, L_I, all indicate a dependency on (U_o is the residual river flow velocity and f is the bed friction coefficient). Likely bathymetries for `mixed' estuaries can be delineated by mapping, against and D, the conditions L<sub>I</sub>/L<1,E<sub>X</sub>/L<1 (E<sub>X</sub> is the tidal excursion) alongside the Simpson-Hunter criteria D/U<sup>3</sup><50 m<sup>−2</sup> s<sup>3</sup>. This zone encompasses 24 out of 25 `randomly' selected UK estuaries.However, the length of saline intrusion in a funnel-shaped estuary is also sensitive to axial location. Observations suggest that this location corresponds to a minimum in landward intrusion of salt. By combining the derived expressions for L and L_I with this latter criterion, an expression is derived relating D_i, the depth at the centre of the intrusion, to the corresponding value of U_o. This expression indicates U_o is always close to 1 cm s⁻¹, as commonly observed. Converting from U_o to river flow, Q, provides a morphological expression linking estuarine depth to Q (with a small dependence on side slope gradients).These dynamical solutions are coupled with further generalised theory related to depth and time-mean, suspended sediment concentrations (as functions of and D). Then, by assuming the transport of fine marine sediments approximates that of a dissolved tracer, the rate of estuarine supply can be determined by combining these derived mean concentrations with estimates of flushing time, F_T, based on L_I. By further assuming that all such sediments are deposited, minimum times for these deposition rates to in-fill estuaries are determined. These times range from a decade for the shortest, shallowest estuaries to upwards of millennia in longer, deeper estuaries with smaller tidal ranges.     

Computational study on near wake interaction between undulation body and a D-section cylinder

We present a numerical study on the hydrodynamic performance of undulation NACA0012 foil in the near wake of D-section cylinder. Computations are conducted using unsteady incompressible Navier-Stokes equations with a moving adaptive mesh based on laminar flow. Investigations are focused on the effect of distance ratio between foil tip and centre of cylinder (L/D≤2.0) on the thrust/drag performance of foil and cylinder at various foil undulation frequency (St). We found that, foil thrust coefficient (C_t) increases considerably with the appearance of cylinder and an optimal distance exists at which C_t reaches maxima. The maximum increment is about eleven times that of its counterpart of single foil, which is obtained at St=0.23 and L/D=0.5. Our results for the cylinder drag coefficient (C_d) observed the existence of optimal parametric map, combined with various gap ratios and foil frequencies. With these parameters, insertion of an undulation foil can significantly lead to the drag reduction indicating that undulating foil could work efficiently as a passive vortex control device for cylinder drag reduction.