Xiaowei Feng

Optimal shear key interval for offshore shallow foundations

Embedment of offshore shallow foundations is typically achieved by ‘skirts’, i.e. thin vertical plates that protrude from the underside of a foundation top plate and penetrate the seabed confining a soil plug. Skirted shallow foundations are often idealized as a solid, rigid element for geotechnical analysis of the foundation, on the assumption that sufficient skirts, or ‘shear keys’ will be provided to ensure that the deformable soil plug displaces as a rigid body. Should too few shear keys be provided, failure mechanisms involving deformation within the soil plug may occur, leading to a reduction in load-carrying capacity.There is currently no formal guidance regarding the optimal spacing of shear keys to ensure rigid body displacement of the soil plug. The absence of guidance may lead to unconservative designs if the number of shear keys is under estimated to save on fabrication or to conservative designs if additional shear keys are provided to minimize the risk associated with the uncertainty. Either case is undesirable and clear benefit is to be gained from a better understanding of shear key spacing.This paper presents guidance on the minimum number of shear keys required to achieve optimal capacity of square and rectangular skirted foundations (i.e. equivalent to that of a solid rigid foundation) under undrained generalized six degree-of-freedom loading in soft soils with linearly increasing shear strength with depth.Copyright

A method for predicting six degrees-of-freedom ultimate limit state of subsea mudmats

Rectangular mudmat foundations are extensively deployed in deep waters to support subsea infrastructure leading to renewed interest in optimizing the design of offshore shallow foundations. Offshore industry guidelines (e.g. API RP2GEO and ISO 19901-4) are based on classical bearing capacity theory of a plane strain strip foundation resting on the surface of a uniform Tresca material. More realistic conditions are accounted for through a range of superposed empirical modification factors and the effective width principle.In practice, subsea foundations experience complex loading in six degrees-of-freedom (vertical load, biaxial horizontal load, biaxial moment and torsion), due to expansion and contraction of connected pipelines and jumpers; they may be able to mobilize transient tensile capacity; and they are typically three-dimensional in plan, shallowly embedded and founded on soft, normally consolidated, soils with linearly increasing strength with depth. Accurate determination of the ultimate limit state of subsea mudmats is best achieved by considering the relevant foundation, soil and loading boundary conditions explicitly.In this paper, a simplified approach for predicting the ultimate limit state of mudmat foundations under six degrees-of-freedom, based on failure envelopes, obtained from extensive finite element analyses, is compared with the traditional bearing capacity methods as recommended in industry guidelines.Copyright

A Toolbox for Optimizing Geotechnical Design of Subsea Foundations

This paper presents a toolbox for optimizing geotechnical design of subsea foundations. The geotechnical design challenge of subsea shallow foundations is to withstand greater dead and operational loads on soft seabeds without increasing the footprint size or weight. The motivation is to reduce costs associated with installation - for example eliminating the need for a heavy-lift vessel to place foundation units alone if handling limits of pipe-laying vessels are exceeded - whilst providing acceptable in-service reliability. The tools presented focus on prediction of undrained seabed response and are intended for deep water developments on fine grained seabeds, as this scenario presents a significant challenge in terms of minimizing subsea foundation footprints. The toolbox addresses optimization of geotechnical subsea foundation performance through four aspects: (i) optimizing the analysis methodology, (ii) modifying the foundation configuration, (iii) improving the site characterisation data as input to the design, and (iv) altering the basis of design. The research presented derives from a combination of physical model testing in a geotechnical centrifuge, numerical analysis and theoretical modelling. The methods, procedures and processes are presented in terms of design equations, theoretical frameworks or design charts, many of which are freely available as web-based applications. Worked examples throughout the paper demonstrate the efficiencies in terms of footprint area to be realized through adoption of these tools.

Correlating vane shear results with undrained strength parameters for slope stability analysis.

When using vane strength to evaluate the stability of soft clay slopes in Tianjin Harbor area, the calculated safety factors are often much less than 1.0 for stable slopes. A reasonable explanation is given to this phenomenon and a new way of using vane strength is presented for slope stability analysis. A correlation between the in-situ vane shear strength and undrained shear strength parameters is proposed based on the considerations that the vane strength is anisotropic and that the vane strength increases with depth linearly. Stability analysis was performed for several existing slopes. The results indicate that the safety factors calculated using the correlated shear strength parameters are more reasonable than those calculated with the vane strength directly.