Geohazard Risk Assessment—Vulnerability of Subsea Structures to Geohazards—Some Risk Implications

Abstract

The past ten years have seen a considerable advance in the industry with respect to the identification and quantification of geohazards. One issue which is lagging and neglected in the technical literature is the ability of subsea infrastructure to survive geohazard processes and events. This paper summarizes our recent work on the subject that is used in a risk-based approach to foundation design and, where practicable, to improve the geohazard resistance of structures.The paper discusses the vulnerability of subsea infrastructure to impact of geohazards. A case study is presented for a deepwater manifold subject to landslides and turbidity currents. The vulnerability of the structure is evaluated comparing available resistance of foundation and jumpers to the anticipated loadings. The results give guidance that informs early field planning and risk mitigation strategies. Possibilities for geohazard resistant design are explored.Results and conclusions: 1) Critical issues for structural performance are the capacity of the foundation to resist geohazard loads without failure or excessive deflection, and of the connections (jumpers and flying leads) to withstand deflections or distributed loads caused by the geohazard event; 2) Geohazard resistant design for ground movements should increase foundation capacity and increase flexibility of connections. In this case, driven piles are likely to be more effective than suction piles given the smaller area exposed to soil forces. However , for loads impacting above the mudline foundation capacity may be increased using either suction piles or driven piles. The strength of connections should also be increased. The conflict between connection strength and flexibility is a design challenge; 3) The best mitigation strategy for geohazards is avoidance. This is generally possible for manifolds and other fixed structures. Location of wells and flowlines is dictated by reservoir constraints and relocation is generally not an option; so geohazard design design should be concentrated on accepting and controlling repairable damage. Export pipelines and umbilicals are critical, as loss or damage results in an interruption of production. The main geohazard mitigation strategy in this case is careful routing.

Direct monitoring of active geohazards: emerging geophysical tools for deep‐water assessments

“…Turbidity currents are a potential hazard to seafloor infrastructure (Bruschi et al 2006;Clare et al 2015b) as they can transport large volumes of sediment (up to hundreds of cubic kilometres) at high velocities (up to 19 m/s; Piper, Cochonat and Morrison 1999) over thousands of kilometres (Talling et al 2007). Longitudinal structures are most vulnerable to impacts including drag, loss of stability, undermining due to scour, or rupture (Parker et al 2009;Clare et al 2015b parameters (e.g., measuring high concentration flows with acoustic instruments; Hughes Clarke 2016); and (iv) the often-destructive nature of geohazards that we wish to measure (e.g., Khripounoff et al 2003). Geotechnical monitoring of offshore sites is becoming more commonplace, however, including the deployment of in-situ piezometers and tiltmeters to understand slope stability issues at specific locations (e.g., Richards et al 1975;Prior and Suhayda 1979;Bennett et al 1982;Sultan et al 2004;Kvalstad et al 2005;Strout and Tjelta 2005;Stegmann et al 2011).…”

Strength of fine-grained soils at the solid–fluid transition

“…Recent review articles related to submarine slide behaviour are presented by Locat & Lee (2002) and Masson et al (2006). Zakeri (2009) andParker et al (2009) review the limitations of current engineering practice related to slide-infrastructure impact behaviour, and highlight the difficulties associated with the quantification of slide strength and the resulting impact forces.…”

Analytical modelling of the steady flow of a submarine slide and consequent loading on a pipeline

“…Reviews of the prevalence, behaviour and modelling of submarine slides are presented by Locat & Lee (2002) and Masson et al (2006). Submarine slides present a significant hazard to seabed infrastructure, yet calculation methods to assess the run-out of slides and the consequent loading on pipelines and other infrastructure are poorly developed (Jeanjean et al, 2005;Parker et al, 2009;Zakeri, 2009).…”