Wave induced forces around buried pipelines

Dynamic response of a porous seabed-pipeline interaction under wave loading: Soil-pipeline contact effects and inertial effects

Abstract: The existing models for the pore pressure and internal stresses within the pipeline under wave loading have mainly based on the assumption of no-slip boundary condition at the interface between pipeline and soil particles. In this paper, soil–pipeline contact effects and inertial forces are considered in the new model. A comprehensive comparison between the experimental data available and the present model is performed and showing good agreements. Based on the numerical results, it is found that soil–pipeline contact effects significantly affect the internal stresses. The maximum difference of internal normal stress can reach 50 times of p0. On the other hand, inclusion of inertial terms will only affect the pore pressure acting on the pipeline. Numerical examples also conclude that the difference of internal normal stresses between the present model (with contact effects and inertial terms) and previous work (without contact effects and inertial terms) increases as the depth (s) of the trench layer decreases, but as the width of the trench layer (l) increases. Finally, we compare three different types of trench shapes, rectangle, trapezoid and triangle trench layers, and found that triangle trench layer will reduce the pore pressure, but increase the internal stresses.

Non-Linear Wave-Induced Transient Response of Soil Around a Trenched Pipeline

Abstract: Submarine pipelines are always trenched within a seabed for reducing wave loads and thereby enhancing their stability Based on Biot’s poroelastic theory, a two-dimensional finite element model is developed to investigate non-linear wave-induced responses of soil around a trenched pipeline, which is verified with the flume test results by Sudhan et al [Sudhan, CM, Sundar, V, Rao, SN, 2002 Wave induced forces around buried pipeline Ocean Engineering, 29, 533–544] and Turcotte et al [Turcotte, BR, Liu, PLF, Kulhawy, FH, 1984 Laboratory evaluation of wave tank parameters for wave-sediment interaction Joseph H Defree Hydraulic Laboratory Report 84-1, School of Civil and Environmental Engineering, Cornell University] Non-linear wave-induced transient pore pressure around pipeline at various phases of wave loading is examined firstly Unlike most previous investigations, in which only a single sediment layer and linear wave loading were concerned, in this study, the influences of the non-linearity of wave loading, the physical properties of backfill materials and the geometry profile of trenches on the excess pore pressures within the soil around pipeline, respectively, were explored, taking into account the in situ conditions of buried pipeline in the shallow ocean zones Based on the parametric study, it is concluded that the shear modulus and permeability of backfill soils significantly affect the wave-induced excess pore pressures around trenched pipeline, and that the effect of wave non-linearity becomes more pronounced and comparable with that of trench depth, especially at high wave steepness in shallow water

Study of Scour around Submarine Pipeline with a Rubber Plate or Rigid Spoiler in Wave Conditions

Abstract: This paper presents results from laboratory experimental model studies investigating scour by waves around a pipeline attached to a flexible rubber plate or rigid spoiler. The rubber plate was placed between the submarine pipeline and bed, while the rigid spoiler is attached to the top of the pipe. The scour around pipelines with and without a rubber plate or a rigid spoiler under regular and irregular waves was observed and measured for a range of pipe sizes, wave amplitudes and frequencies, and the length of the plate/height of the spoiler. The experiments reveal that although the rigid spoilers can enhance the scour depth and extent (thus accelerating the self-burial of the pipe), they also have significant influence on both the upstream and downstream bed topography as sand ripples and dunes are formed. On the other hand, the rubber plates cannot only significantly increase the scour depth but also have little effect on the upstream and downstream bed. The experiments show that when the length of the plate is about 1.5 times the pipe size, it provides the optimum performance in terms of the largest scour depth while restricting the impact on nearby beds for the parameters investigated in this study.

Bedform evolution around a submarine pipeline and its effects on wave-induced forces under regular waves

Abstract: The paper presents an experimental investigation of seabed evolution behavior around a submarine pipeline and the hydrodynamic forces on the pipeline under regular waves. Unlike the previous flume tests that have largely used beds with median sands, this study focuses on fine sediments such as sandy silt and silt. The primary objective of the study was to investigate: (i) the scour process under different wave conditions and with different sediments and (ii) the influence of the bedform evolution on the hydrodynamic forces experienced by the pipeline. In terms of scour and ripple formation, four distinct regimes of the near-field bed evolution behavior are identified which are: (I) no scour, (II) scour without ripples, (III) scour with small ripples and (IV) scour with large ripples. The influence of bedform evolution on wave forces was found to vary significantly in different regimes. In regime I, the wave forces were quite stable; in regime II and III, the wave forces underwent a gradual reduction before reaching their equilibrium values at fairly early stages of the scour process; in regime IV, the wave forces were significantly affected by the migrating ripples and can be rather unsteady throughout the testing period.

Experimental and Numerical Investigation of Local Scour Around Submarine Piggyback Pipeline Under Steady Current

Abstract: As a new type of submarine pipeline, the piggyback pipeline has been gradually adopted in engineering practice to enhance the performance and safety of submarine pipelines. However, limited simulation work and few experimental studies have been published on the scour around the piggyback pipeline under steady current. This study numerically and experimentally investigates the local scour of the piggyback pipe under steady current. The influence of prominent factors such as pipe diameter, inflow Reynolds number, and gap between the main and small pipes, on the maximum scour depth have been examined and discussed in detail. Furthermore, one formula to predict the maximum scour depth under the piggyback pipeline has been derived based on the theoretical analysis of scour equilibrium. The feasibility of the proposed formula has been effectively calibrated by both experimental data and numerical results. The findings drawn from this study are instructive in the future design and application of the piggyback pipeline.

Wave-induced pore pressures and effective stresses in a porous bed

Abstract: A general theory for the pore pressures and effective stresses induced in a porous bed by ocean waves is developed. The pore fluid as well as the soil skeleton are considered compressible and the f...

Wave-Induced Pressures in Beds of Sand

Abstract: Measurements of the variation of the fluctuating component of pressure have been made in two different beds of sand in a wave channel. The sand was laid from an overhead hopper, and it is shown that the stratification of the bed produced by this method causes the permeability to be different in the vertical and horizontal directions. It is suggested that a similar situation exists in the sea bed. Analytical relationships for the pressure and velocity distributions in such a situation are developed, and the agreement between theory and experiment is found to be quite good. As far as is known, this is the first time that the applicability of traditional porous-media analytical methods to wave-induced flow in beds of sand has been verified experimentally.

Seepage force on a pipeline buried in a poroelastic seabed under wave loadings

Abstract: The water wave induced seepage force on a pipeline buried in the seabed is investigated. The seabedis modeled as a porous and elastic medium containing a nearly saturated pore water, which is generally known as the Biot model. The pipeline is assumed to be rigid. It is not supported by any anchoring force. The governing equations describing soil stresses as well as pore water pressure under periodic wave loadings are solved numerically using a Boundary Integral Equation Method. Numerical results of pore water pressure amplitude around the pipeline are compared with laboratory data. Agreement is fairly good. Sensitivities of pore water pressure response to different soil and fluid parameter are also examined. It is found that with realistic parameters the uplift seepage force on the pipeline can be as much as 60% of the displaced water weight if the pipeline is located in the pore water pressure boundary layer.

Wave Induced Pressures in Permeable Seabeds

Abstract: A theoretical investigation on the transmission of wave induced pressures into a permeable sea bottom has been performed. Assuming that the soil water is compressible, while the grain skeleton is assumed rigid, the resulting theoretical model describes an effective mechanism by which the soil permeability affects the pressure transmission.

Wave-induced uplift force on a submarine pipeline buried in a compressible seabed

Abstract: A two-dimensional finite-element simulation of the wave-induced hydrodynamic uplift force acting on a submarine pipeline buried in sandy seabed sediments subject to continuous loading of sinusoidal surface waves is presented. Neglecting inertia forces, a linear-elastic stress-strain relationship for the soil and Darcy's law for the flow of pore fluid are assumed. The model takes into account the compressibility of both components (i.e., pore fluid and soil skeleton) of the two-phase medium. The results of numerical analysis are presented and discussed with respect to soil and pore fluid parameters where special attention is paid to the question of soil saturation conditions. The meaning of the results is also related to surface wave conditions. As a general conclusion, the practical, engineering recommendation is given in order to make a realistic, safe and economic estimation of the wave-induced uplift force acting on a buried submarine pipeline.