Fauzan Sahdi

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.

Experiments using a novel penetrometer to assess changing strength of clay during remolding and reconsolidation

The process of remolding and subsequent recovery of soil shear strength is a common geotechnical problem, especially in the offshore environment, where foundations and pipelines are subjected to intermittent operational and environmental loads. A novel vertically oriented penetrometer (VOP) was used in a centrifuge testing program to study the changing strength of normally consolidated kaolin from successive disturbance and reconsolidation events. The VOP is used to determine soil shear strength via an interpretation akin to the laterally loaded pile. The test series involved cyclic movement of the VOP at velocities ranging from 0.3 to 3 mm/s, with a corresponding 100-fold variation in the duration of each cycle. For the VOP tests at low velocities with high cyclic periods, the soil resistance initially reduces, but then shows an increase that scales with the elapsed time, indicating reconsolidation. Ultimately, if the cycling continues for sufficient time, the recovery in strength from reconsolidation can exceed the weakening from remolding. A previously published framework using critical-state soil mechanics concepts is shown to capture the changing resistance well.

Evolution of Riser-Soil Stiffness in a Soil Crust Layer

Site investigation data have shown that a crust layer may occur at the surface of many seabeds, with the crust exhibiting undrained strengths up to an order of magnitude higher than that of underlying normally consolidated sediments. This may affect the fatigue life assessment of steel catenary riser (SCR) as a result of riser-soil interaction. The paper presents data from centrifuge model tests, investigating the changes in riser-soil stiffness during cyclic motion of a rigid model pipe (simulating a riser element) within a model crust layer. The paper reports data from a cyclic test with 2000 cycles at a cyclic amplitude of 0.01D. The soil stiffness decreased initially for the first 300 cycles, but increased thereafter, indicating the importance of quantifying the long-term stiffness changes in this type of soil layer. The results are discussed in relation to current guidelines for estimating suitable values of seabed stiffness for SCR fatigue design.

Variable penetration rate testing for shear strength of peat – a review

Abstract This paper presents a review of the advances in the variable penetration rate testing methods on inorganic soils and its potentials to investigate the appropriate penetration rate and resistance factors for penetrometer testing of peat. A partially drained condition, which often leads to misinterpretation of test results, has been observed in peat penetration testing when the standard rate of 20 mm/s is used. Although the impact of rate-effects on penetration resistance measured with CPTu, T-bar and ball penetrometer have been investigated extensively in various intermediate soils, research is limited on how penetration rate controls drainage conditions and affects consolidation behaviour in peat. This review synthesises research developments in using variable penetration rate tests. The objective is to evaluate the transition of drainage conditions and consolidation behaviour of inorganic soils while focusing on its adaptability for peat. The review provides guidance on the investigation of the penetration rate testing in peat.

Temperature change and the total stress anomaly in paste backfill

This paper presents preliminary results from a laboratory backfill model test in order to explain the effect of temperature change during the cement hydration on the total stress within cemented paste backfill. It is conducted via temperature control test in the absence of the cement. This investigation is an attempt to resolve an anomalous behavior reported in recent full scale monitoring reports, where the total vertical stress at the stope base shows a progressive increase after backfilling is terminated. The result in this paper shows that the total vertical stress is not affected by the level of the temperature but rather by the temperature gradient. The empirical relationship between the temperature gradient and the change in the total vertical stress is proposed. The total stress anomaly found in the full scale monitoring of paste backfill could be explained by the finding.

Measurement of Amorphous Peat Shear Strength in the Direct Shear Box at High Displacement Rates

The ASTM standard provides guidelines for the drained direct shear test (DST) and requires the samples to be sheared at rates estimated from time to failure,

Centrifuge modelling of active slide–pipeline loading in soft clay

t_{f} \ge 50t_{50}

Strength properties of ultra-soft kaolin

tf≥50t50 (where