Assessing the Interaction Between Earthquake Faults and Jack-Up Rigs

Abstract

This PhD thesis investigated the interaction between normal and reverse dip-slip faults and jack-up rigs. Finite-element methodologies (FEM) have been developed and validated against geotechnical centrifuge experiments which were specifically conducted by the author for this thesis. The PhD investigated the fault rupture–soil–foundation–structure interaction (FRSFSI) of a complete three-legged jack-up with spudcan foundations in sand.

During an earthquake two events occur, the ground permanently displaces along the fault and dynamic oscillations (i.e. seismic shaking) propagate away from the fault slip-line. Prior to ISO 19905-1 (ISO, 2012), there was no procedure or requirement for jack-ups to be assessed under seismic loading and, the influence of the fault displacement upon the structure is not considered at all. Jack-ups are currently operational, with personnel on-board, in seismically active regions across the world, are we good at avoiding rupturing faults, or are we just lucky?

Several numerical models were developed using the commercially available software Abaqus CAE. A global model containing a full three-legged rig with three spudcan foundations captured the whole structural response as the fault propagated and ruptured at, or near to the spudcans. Further to this, a simplified spring model was developed in order to reduce analysis time. After validating both procedures with centrifuge experiments, parametric investigations were conducted to investigate the influence of rig-fault proximity in order to improve understanding of the structural response. When considering a known fault rupture outcrop, or with the assumption that the outcrop can be predicted (which may be problematic given the uncertainties regarding bedrock location and propagation through soil) non-operation envelopes/zones could be developed, within which significant structural distress would be expected.

Interestingly, the presence of the structure caused significant diversion, bifurcation and diffusion of the fault beneath the spudcan footing, but often this was not enough to divert the fault away from the bounds of the structure. Depending on how close the structure was positioned to the fault, the rig experienced a variable amount of structural distress in the form of hull pitching and displacements, whilst the legs underwent significant changes in the axial force and bending moments.

Understanding the fault rupture-soil-foundation-structure interaction (FRSFSI) is critical when attempting to develop platform design and risk mitigation strategies. The findings may be used alongside other hazard risk mitigation strategies deployed during site investigations and assessments of rig operations. The numerical procedures can be adapted in order to investigate faulting interactions with other infrastructure such as pipelines, cables and wind turbines.

Date of Award	2022
Original language	English
Supervisor	Michael Brown (Supervisor) & Ioannis Anastasopoulos (Supervisor)