Mark Randolph

Considerations on the Design of Keying Flap of Plate Anchors

AbstractOne of the critical issues associated with plate anchor performance and design relates to the reduction of the loss of embedment during the keying process. As deep water offshore sediments typically exhibit an increasing soil strength with depth, the loss of embedment results in a reduction in anchor bearing capacity. A keying flap hinged to the main plate has been developed and adopted by industry with the aim to reduce the loss of embedment by limiting the vertical motion of the anchor. However, uncertainties remain regarding the behavior and the performance of the keying flap. This paper presents a series of numerical analyses performed to investigate the flap rotation mechanism and the condition of activation of the flap. They are compared with existing centrifuge modeling. The numerical results validate the centrifuge observations and demonstrate the nonactivation of the keying flap for typical anchor pull-out conditions.

Hybrid Subsea Foundations for Subsea Equipment

A hybrid subsea foundation (HSF) is defined here as the combination of shallow and deep foundations, designed such that both shallow and deep foundation elements contribute to the total foundation capacity. In this project the type of HSF investigated is a shallow mat foundation connected to short piles, with pinned connections made close to the mat corners. The motivation for this research is the potential economic benefit to using HSFs in deepwater subsea developments, where mat sizes to resist typical subsea foundation design loads (e.g., from pipeline end terminations, manifolds, and riser bases) require ever larger installation vessels with higher associated cost. This paper assesses the design feasibility of a HSF for subsea facilities in soft clay and provides recommendations for design. The focus here is on analytical and numerical modeling, although corroborating results from physical modeling are provided. Numerical and physical modeling has shown that the proposed analytically based design approach is suitably conservative, with deformations well within typical serviceability limits at the calculated capacities. Most significantly, the paper demonstrates that significant reduction in the foundation footprint can be realized by supplementing a mat with short piles at each corner.

Measuring Radial Total Stresses on Model Suction Caissons in Clay

Miniature total pressure transducers (TPTs) were incorporated on model suction caissons in order to measure changes in radial stress during installation and loading of the caissons. The accuracy of TPTs in clay was first assessed by calibration tests at both 1g and in a geotechnical centrifuge at up to 120g. Calibration tests in a modified triaxial apparatus showed that the accuracy of the TPTs under loading, unloading, cyclic loading, and sustained loading was better than 92.5%. The initial readings of the TPTs were also found to change only slightly on transfer among air, water, and soil, in spite of differences in the heat exchange environment. The cross-sensitivity of TPTs to an axial load of 300 N on the caisson was insignificant. The accuracy of TPTs in water in the centrifuge was better than 96%. The TPTs also gave reasonable measurements of radial stresses generated on the outer wall of suction caissons during penetration in clay at 120g, as assessed by comparison with stress changes predicted using the strain path method.

Experimental Study of Suction Installation of Caissons in Dense Sand

Physical models have been developed to investigate suction installation of caisson foundations in fine-grained dense silica sand. The main controlling factor, pumping rate, was found to have significant influence on the differential pressure result across the caisson base. Rapid pumping is unlikely to bring any adverse effect to the installation process, even applied at very shallow initial wall penetration depth. Quick installation, with the exception of caissons with wall-to-diameter (t/D) ratio larger than 1%, appeared to be beneficial in reducing the excessive sand heave. Observations showed that caisson geometry and surcharge also affected the installation performance. Although requiring only marginal increase in suction pressure to install, caissons with thicker walls created substantially higher sand heave during installation. Varying the absolute caisson size did not seem to affect the suction pressure for a given t/D. Seepage flow was also calculated during each test, and found to increase with deeper skirt penetration.Copyright

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.

The design of Subsea foundations subject to general cyclic loading using a massively scalable web based application

Subsea developments require the design of large numbers of shallow skirted foundations to support structures such as manifolds, pipeline and umbilical terminations and in-line tees. Safe and economic design relies on the accurate assessment of foundation capacity against thousands of load-combinations. Performing these design calculations is a significant computational task. The objective of this paper is to demonstrate how new developments in cloud computing can be utilized to optimize foundation design. Engineering design is no longer limited by computing power thanks to the introduction of low-cost on-demand cloud computing platforms. This paper describes a massively scalable cloud based application for rapidly assessing the vertical-horizontal-moment-torsional capacity of shallow skirted foundations against thousands of cyclic load case combinations that arise from numerous environmental and service conditions. The detrimental effect of cyclic loading and the beneficial effect of consolidation on soil strength are incorporated within a single workflow. It is shown that cloud technologies can radically improve traditional engineering design procedures, allowing engineers to focus on the innovative and creative aspects of their work, while the tasks of preparing, executing and documenting calculations become near instantaneous and more easily assessed for quality assurance. More critically, the technology allows rapid and rigorous optimization of the foundation dimensions to achieve the most cost-effective solution that satisfies all load cases. The scalability of the application allows multiple users to run large numbers of calculations simultaneously across a virtually unlimited number of computer nodes. The system can be accessed through a standard web browser and can run simulations on any internet-connected device. Results are saved in the cloud and can be accessed anywhere and shared among colleagues, enhancing collaboration and quality assurance. The approach results in demonstrably superior design outcomes, achieved more quickly. This paper presents what is believed to be the worlds first web based application for shallow foundation design that exploits the availability of low cost on-demand cloud computing services. The paper will explain some of the challenges in implementing such a system and provide examples. We believe this type of technology represents the future for geotechnical design work, providing better design in a more efficient manner.

Evaluation of Elastic Stiffness Parameters for Pipeline–Soil Interaction

AbstractThis paper focuses on elastic stiffness parameters for axial, horizontal, and vertical motions of a pipeline relative to the seabed, with the aim of expressing these parameters in terms of fundamental elastic properties of the soil. Limited information exists in the literature on the axial elastic response of on-bottom pipelines, particularly for nonhomogeneous soil. Therefore, an approximate analytical approach was developed for axial stiffness, focusing on the case of shear modulus proportional to depth. The solution was then verified through numerical analysis. Further numerical analysis was carried out to obtain relationships for horizontal and vertical elastic stiffnesses of on-bottom pipelines. Finally, relationships among elastic stiffnesses were developed. Here recommendations are made for the selection of proper elastic stiffnesses in all three directions of motion. These recommendations allow consistent and rigorous modeling of elastic pipe–seabed interactions with application to the ana...

Numerical investigation of diving potential and optimization of offshore anchors

AbstractThis paper reports numerical modeling results investigating the diving potential and optimization of offshore anchors. A simplified two-dimensional plate anchor model and a complex three-di...