Wave pressures and uplift forces on and scour around submarine pipeline in clayey soil

The role of scour in shipwreck site formation processes and the preservation of wreck-associated scour signatures in the sedimentary record – evidence from seabed and sub-surface data

Abstract: Fully submerged shipwreck sites act as open systems, with the exchange of material (sediment, water, organic and inorganic objects) and energy (wave, tidal, storm) across system boundaries. Formation processes at wreck sites are driven by some combination of chemical, biological and physical processes, with physical processes dominant in initial phases of site formation. Scouring and associated depositional patterns that form in response to hydrodynamic forcing are commonly the dominant physical processes acting at shipwreck sites. Scour is initiated by the introduction of a shipwreck to the seafloor, leading to increases in flow velocity and turbulent intensity around the structure. Near-field and far-field scour pits form at wreck sites due to the interaction of horseshoe and lee wake vortices with the mobile substrate. The morphology of resultant scour signatures are controlled by the orientation of the wreck structure in relation to the prevailing hydrodynamic regime, the morphology and size of the wreck and individual site components, the hydrodynamic regime (currents, waves or combined waves and currents), bathymetry and the geology of the site (seafloor and sub-surface conditions). Individual objects or artefacts may act as nuclei to promote scour at a local scale. Under high-energy conditions, groups of artefacts and/or disarticulated structural components emanating from a wreck may compound natural scour processes by rolling or sliding. Under suitable environmental conditions, wreck-associated scour features can be preserved in the sedimentary record.

Pipeline–Seabed Interaction

Abstract: A review of the existing research on the interaction between a pipeline and an erodible bed exposed to waves and/or currents is presented. The review covers three topics: scour, liquefaction, and lateral stability of pipelines. The basic mechanism that leads to scour in two-dimensional (2D) and three-dimensional (3D) cases is first described, as deduced from small-scale laboratory experiments. The onset of scour from piping and the developing tunnel erosion are among the processes described. The lateral expansion of the scour hole along the pipe is described, also based primarily on small-scale laboratory experiments. The state of the art of the mathematical/numerical modeling of the scour processes is presented. The associated self-burial of the pipe is described and compared to field observations. In addition to scour, liquefaction may also constitute a risk for pipeline stability. The cause of liquefaction and the resulting consequence for pipeline stability in a natural environment are discuss...

The impact of scour processes on a smothered reef system in the Irish Sea

Abstract: The Irish Sea, like many marine areas, is threatened by anthropogenic activities. In particular the Pisces Reef system, a series of smothered rocky reefs are subject to fishing pressures as a result of their position within a Nephrops norvegicus fishery. In an area of sediment deposition and retention the reefs modify the environment by increasing the energy of near-bottom currents which results in localised scouring. This is the first study to attempt to characterise and investigate the ecological functioning of the Pisces Reef system. A multidisciplinary approach was essential for accurate investigation of the area. To facilitate more effective management of the benthic habitats of the Reef system, this study integrates acoustic, seismic, grab sampling and video ground-truthing methods for benthic habitat discrimination. Orientation of the scour hollows also suggest that seabed features could be used to infer dominant flow regimes such as the Irish Sea Gyre. The data revealed significant geology–benthos relationships. A unique biotope was described for the reef habitat and it was demonstrated that scouring may influence community composition through disturbance mechanisms. This study provides preliminary information required for management of a unique habitat within a uniform region.

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.

Sediment Transport Circulation Pattern through Mesotidal Channels System

Abstract: There are three modes of sediment in motion, which are recognized as rolling and/or sliding; jumped and suspension particles motion. These particles motion include two essential sediment transport modes, which are bedload and suspended load. The first corresponds to the part of the total load which is moving very close to the bed. On this opportunity, the sediment is transported by rolling and/or sliding along the floor or jumping immediately above the bed. The suspended load comprises the particles that are carried away at suspension in the moving fluid. Bedload transport occurs in low velocity flow and/or large grain sizes, while suspended load take place at high velocity flow and/or small grain sizes. The sediment transport theory to point out that sediment capacity flow and sediment availability in channels regulates the motion of sediments. The sediment transport capacity is the maximum load that can be transported by flow and the sediment availability is the material supply that the flow can put in motion. In general, the sediment load that moves in a rolling or saltating mode is smaller compared with the suspended load transported in a natural flow. However, the bedload sediment is important for its contribution to the morphological change of the channel. Moreover, it is the most important factor in determining the stability of natural channel, the bed topography, and therefore, the cause of the hydraulic resistance and some properties of the flow. Accordingly, on this chapter we focus on the bed sediment transport pointing to the pathway migration rather than the sediment transport rate itself. The sediments transport as bedload in estuarine environment controls the bottom morphology. Therefore, estimating the transport of non-cohesive sediments, mobilized by the channel floor, has relevance to the understanding of many pressing environmental problems including eutrophication, contaminant transport, sediment bed erosion, siltation and waste disposal, which impacts on seabed stability and at times on ecosystem distribution. Also, sediments that are moving on the bed generate different bedforms such as shoal, dunes, point bar, etc., which can be hazards to navigation and to affect the flow conditions and the flanks channel stability.

Erosion of mud/sand mixtures

Abstract: The prediction of sediment erosion is an important issue in coastal engineering projects. There are methods for predicting the erosion of cohesive sediment (mud) and non-cohesive sediment (sand), but there are presently no relationships for mixed sediments. However, natural sediments rarely consist of only mud or sand and the erosional properties of combined mud and sand sediments are required so that the whole spectrum of natural sediment size combinations can be modelled. This paper attempts to characterise the erosion behaviour of mixed sediments in a way that can be used for predictive models. In this paper mud (or fines) is defined as clays and silts, which pass through a sieve of size 62.5 μm, and sand is defined as the fraction retained. The collaboration between European researchers in the framework of the MAST G8M project has resulted in the accumulation of an extensive amount of data on the erosion of mud/sand mixtures. The data, which originate from both laboratory and field experiments, has been used to examine the physical processes behind the erosion behaviour of mud/sand mixtures. It was found that adding sand to mud, or vice versa, increases the erosion resistance and reduces the erosion rates when the critical shear stress for erosion is exceeded. The highest values for the erosion shear stress of homogeneously mixed beds occurs at a maximum in the region 30 to 50% sand by weight. The most significant effect on erosion resistance occurs on the addition small percentages of mud by weight to sand. The mode of erosion also changes from cohesionless to cohesive behaviour at low mud contents added to sand, with a transition occurring in the region 3% to 15% mud by weight. The erosional properties are also strongly dependent on the history of the bed and it is common that mud and sand segregate under typical deposition conditions owing to their different settling velocities in water which creates discrete layers. The erosion of these segregated beds should thus be modelled as a sequence of mud and sand erosion “events”.

Scour Below Pipelines in Waves

Abstract: This paper presents the results of an experimental investigation on scour below pipelines exposed to waves. The effect of leewake of the pipe is the key element in the scour process, and it is demo...

Cohesive Material Erosion by Unidirectional Current

Abstract: The initiation of motion of consolidated cohesive sediments under a unidirectional flow of clear water was studied. Experiments were performed in a flume‐tunnel capable of providing a bed shear stress up to 26 Pa and a velocity of 3.5 m/s, 3 mm above the bed. Samples were prepared in a specially designed press using a carefully controlled consolidation procedure. Critical shear stress and velocity were found to increase with compressive strength, vane shear strength, plasticity index, clay content, and consolidation pressure.

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 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.