SS: Geohazard Risk Assessment - Vulnerability of Subsea Structures to Geohazards - Risk Implications
Authors: Parker Eric J. , Traverso Chiara , Del Giudice Tania , Evans Trevor , Moore Roger 
Published: July 2009

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: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;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;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.

Introduction

The past decade has seen a considerable advance in the industry with respect to the identification and quantification of geohazards (Moore et al 2006, 2007; Galavazi et al 2006). There has been less attention, however, to understanding of the ability of subsea infrastructure to survive geohazard processes and events. This paper summarizes the results of recent work on the subject that have been used in risk-based geohazard engineering. A basic concept for this approach is the formal definition of risk:Risk = Probability (of Geohazard Event) x Consequence (1)

In this definition, probability refers to the likelihood of occurrence of a specific adverse geohazard event, while consequence refers to the result of the event. Consequence is usually measured in terms of societal, environmental or economic losses. Quantifying the consequence of an event even in economic terms can be quite complex. For example, damage to a subsea structure would imply repair or replacement, loss of production and considerable delay, all of which have an economic impact.