Nathalie Boukpeti

The Impact of Submarine Slides on Pipelines: Outcomes from the COFS-MERIWA JIP

This paper presents key outcomes of a 3-year Joint Industry Project funded by 6 Operators on the impact of submarine slides on pipelines. This JIP developed new techniques to simulate slide runout, and assess the resulting loading and deformation of seabed pipelines. The work was distilled into guidance for practical application, which has found adoption on projects. The JIP spanned (i) characterization of soils at the solid-fluid transition, (ii) computational modelling of slide runout – via depth-averaged and continuum finite element methods, (iii) physical and numerical modelling of slide runout and pipeline impact, and (iv) analytical studies of pipeline response during slide loading. These elements combine to provide an improved practical basis for quantifying the risk associated with slide-pipeline interaction. To characterize very soft seabed soils, a new geotechnically-based framework was devised based on extensive measurements of different soils. This framework spans the solid-fluid boundary that is crossed as slides evolve into a debris flow and turbidity current. It is shown that the geotechnical link between water content and shear strength extends continuously – with no phase transformation – far into the fluid domain, allowing a single rheology to be applied throughout. Computational modelling of slide runout used a hierarchy of methods, from large deformation finite element analysis (LDFE) (with rate effects and softening at soil element level), through depth-averaged runout, to energy-based analytical solutions. In some regimes of behavior the simpler methods suffice, allowing efficient use of Monte Carlo methods to tackle uncertainty. More complex runout modes can be replicated by newly-developed LDFE techniques. From a runout analysis results, pipeline impact loads can be assessed using new solutions for the bearing capacity and drag forces on pipelines developed from numerical and physical modelling, which again unify concepts from fluid dynamics and geotechnics. Finally, simple analytical methods for assessing the structural response of a pipeline to a known slide loading are provided. These solutions allow rapid assessment of the response of a pipeline to a specified slide loading. These advances improve the methods available for quantitative assessment of slide runout and slidepipeline interaction, allowing better determination of the resulting geohazard risk.

Characterization of the solid-fluid transition of fine-grained sediments

Characterization of the strength of fine-grained sediments as they evolve from an intact seabed material to a remolded debris flow is essential to adequately model submarine landslides and their impact on pipelines and other seabed infrastructure. In the current literature, two distinct approaches for modelling this material behavior have been considered. In the soil mechanics approach, fine-grained soils are characterized by the undrained shear strength, su . The critical state framework proposes a relation between su and the water content, or void ratio of the soil. In addition, rate effects and strain softening effects are described by multiplying a reference value of su by a function of the shear strain rate or the accumulated shear strain respectively. In the fluid mechanics approach, slurries of fine-grained material are characterized by a yield strength and a viscosity parameter, which describes the change in shear stress with shear strain rate. Empirical relationships have been proposed, which relate the yield strength and the viscosity to the sediment concentration. This paper demonstrates that the two modelling approaches are essentially similar, with only some formal differences. It is proposed that the strength of fine-grained sediments can be modelled in a unified way over the solid and liquid ranges. To support this unified approach, an experimental campaign has been conducted to obtain strength measurements on various clays prepared at different water content. The testing program includes fall cone tests, vane shear tests, miniature penetrometers (T-bar and ball) and viscometer tests. Rate effects and remolding effects are investigated over a wide range of water contents spanning the domains of behavior that are usually defined separately as soil and fluid. The present paper focuses on analyzing the results of fall cone, vane shear and viscometer tests. Analysis of the results shows that the variation in shear strength over the solid and liquid ranges can be described by a unique function of water content — suitably normalized — for a given soil. Furthermore, the effect of strain rate and degree of remolding can be accounted for by multiplying the basic strength parameter by appropriate functions, which are independent of the current water content.Copyright

Mechanical response of two carbonate sediments from Australia's North West shelf

The carbonate sediments found offshore on the North West Shelf of Australia can generally be described as silts with varying proportions of clay-sized and sand-sized carbonate particles. This paper investigates the undrained shearing response (as measured under simple shear and triaxial conditions) of two sediments with different grading curves. The importance of the fines content in controlling the density and hence dilatancy and undrained strength of the materials is demonstrated. The paper discusses the importance of fabric to the behavior of these soils and provides guidance on how undrained strengths of soils of this nature may be assessed with a knowledge of a material’s in-situ water content (or void ratio) and composition.Copyright

Strain Accumulation Procedure During Staged Cyclic Loading of Carbonate Sediments

Design of offshore foundation systems requires assessment of the effects of cyclic loading on the soil strength. This paper investigates the applicability of the strain accumulation procedure, which is used to assess the effects of wave loading on the soil strength. Staged undrained cyclic simple shear tests were conducted on a carbonate sediment from the North West shelf of Australia, with varying shear stress amplitude in each stage. The shear strain mobilised at the end of the staged tests is compared with the value predicted by the strain accumulation procedure, using shear strain contours constructed from the results of single amplitude undrained cyclic simple shear tests. It was found that the strain accumulation procedure gives adequate prediction for normalised cyclic shear stress less or equal to 0.3, but largely underestimates the cyclic shear strain for normalised cyclic shear stress greater than 0.3 (the cyclic shear stress being normalised by the effective vertical stress at the end of consolidation).Copyright

Rate Effects in the Superior Sand Model

This paper aims at analyzing the response of the elasto-viscoplastic model Superior sand proposed by Boukpeti et al. for describing rate effects in undrained loading of loosely-packed granular deposits. The equations governing the model in triaxial compression are solved for three loading histories: constant strain rate, constant stress rate, and creep. The resulting stress-strain-time curves are discussed in the light of material instability states in relation to static liquefaction. It is shown that the classical second-order work rate instability criterion is no longer valid, and a new criterion based on the deformation acceleration is presented.