Submarine slides are a significant hazard to the safe operation of pipelines in the proximity of continental slopes. This paper describes the results of a centrifuge testing programme aimed at studying the impact forces exerted by a submarine slide on an offshore pipeline. This was achieved by dragging a model pipe at varying velocities through fine-grained soil at various degrees of consolidation, hence exhibiting properties spanning from the fluid to the geotechnical domains, relevant to the state of submarine slide material. To simulate the high strain rates experienced by the soil while flowing around a pipe in the path of a submarine slide, tests were conducted at pipe-soil velocities of up to 4 . 2 m/s. The changing density and shear strength of the samples were back-calculated from T-bar penetrometer test results. A hybrid approach combining geotechnical and fluid-mechanics-based components of horizontal drag resistance was developed. This approach provides an improved method to link the density and strength of the slide material to the force applied on the pipe. Besides fitting the present observations, the method provides an improved reinterpretation of similar data from the literature.
There are situations in offshore energy development where potential impact forces between submarine slides and pipelines need to be estimated. The horizontal slide-pipeline impact force, parallel to the main travel direction of the sliding mass and normal to the pipeline axis, is generally dominant compared to other force components, and hence of particular concern. In practice, pipelines may be suspended at varying distances above the seabed (gap) and existing methods do not consider how this will affect the horizontal slide-pipeline forces. This paper investigates the effects of pipeline-seabed gap and pipeline diameter on the horizontal slide-pipeline impact force via 181 computational fluid dynamics (CFD) simulations at Reynolds numbers of 0.36 - 287. Results show that variation in the pipeline-seabed gap and pipeline diameter alters the slide mass flow behavior as it flows past the pipeline and hence the impact force when the pipeline-seabed gap is below a critical value. A modified hybrid geotechnical-fluid dynamics framework for estimating the horizontal impact force is proposed by considering the effects of the pipeline-seabed gap and pipeline diameter, which is validated with existing experimental datasets.
Geotechnical design considerations for offshore pipelines, foundations, and submarine slides involve assessment of the strength of fine-grained soils and the degradation of that strength with disturbance and remoulding. For offshore pipelines and slides, the relevant strength may be very low (a few kilopascals or lower), relating to near-surface soils and high levels of remoulding including the entrainment of additional water. It is commonly acknowledged that soils exhibit a loss of strength when disturbed, but it is not clear how the degradation properties vary with liquidity index. To address this uncertainty, this paper describes a series of centrifuge tests on kaolin samples consolidated from slurries with an initial voids ratio of 4.0. A total of 81 cyclic T-bar tests were conducted in samples with shear strengths ranging from 0.08 to 1.7 kPa (reflecting different stages of consolidation and in situ total stresses). Large-strain consolidation numerical analyses were used to assist the interpretation of the T-bar test results. The results demonstrate that the soil ductility (a parameter controlling the rate of strength degradation) can be linearly correlated to the liquidity index. The proposed ductility–liquidity index correlation is subsequently coupled with a previously published sensitivity–liquidity index relationship for natural clays to establish a model for the strain-softening behaviour observed in a T-bar test as a function of consolidation. In turn, because the sensitivity is a function of the liquidity index, the intact soil strength is linked to the remoulded strength obtained from laboratory (e.g., fall cone or miniature vane test) and simple index tests. These provide an improved basis to characterize softening effects for inclusion in simulations of submarine slide runout and models for soil–structure interactions that involve intense remoulding.
This technical note describes a simple methodology for reliably measuring the undrained strength of ultra-soft consolidating clay in the geotechnical centrifuge, using the T-bar penetrometer. This methodology relies on the T-bar resistance force owing to soil strength being equal, albeit in the opposite direction, during penetration and extraction when the soil is fully remoulded. The other components of resistance from soil buoyancy, bar self-weight and soil lateral pressure on the T-bar axial strain gauge act in the same direction regardless of the direction of bar movement. The method uses cycles of penetration and extraction to determine the correction required to eliminate these effects. The methodology was validated by comparing the strength inferred from 81 T-bar penetrometer tests with a large strain numerical simulation of consolidation and the resulting gain in soil strength.
Current site investigation practice for offshore pipeline design relies on soil parameters gathered from boreholes or in situ test soundings to depths of 1–2 m below the mudline. At these depths, the fine-grained seabed is very soft and possesses low undrained strength, which can be difficult to measure. This paper describes a centrifuge test programme undertaken to evaluate the feasibility and performance of a novel penetrometer designed to assess the shallow strength of soft seabed over continuous horizontal profiles. This device is termed the vertically oriented penetrometer (VOP). Tests were performed on a normally consolidated kaolin sample, with the VOP translated horizontally at velocities ranging from 1 to 30 mm/s, after embedding the VOP at 30 and 45 mm depths. All tests involved many cycles of VOP forward and backward movement to assess its potential to derive the ratio of intact to fully remoulded strength. Strength determination is achieved by dragging the VOP at a specified embedment depth along the soil surface, and deriving the soil strength from the measured resistance as if the VOP were a laterally loaded pile. The VOP is shown to yield comparable strength measurements to that of a T-bar penetrometer. The VOP is a potentially valuable addition to the range of tools used to characterize soil strength, both in small-scale centrifuge models and, following practical development, potentially also in the field.
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