Integrated Geohazards Evaluation and Risk Assessment for Subsea Facilities
Authors: Fenton Clark , Jayson Daniel , Gillies Murdo , Parkin Anthony 
Published: October 2008

Offshore oil and gas developments in geologically active regions face a number of geohazards, including earthquake shaking, fault rupture, liquefaction, slope failure, and fluid escape. The magnitude and frequency of these hazards are often difficult to quantify; leading to large uncertainties in the risk analyses carried out for engineering development. Traditional deterministic hazard assessments often conclude that such regions are too problematic for development. This paper presents a recently developed probabilistic approach that integrates geohazard evaluation with engineering limit state analysis, which in turn allows a dynamic, streamlined approach to risk assessment. This Failure Modes and Effects Analysis (FMEA) ensures end user focus, with the drilling, subsea and pipeline, infield compression facility, and reservoir teams driving the process through quantification of the consequences of failure. It also enables reality checks and establishes a process for asking the right questions at the right time. This approach has the benefit of rapidly identifying the real geohazard and engineering issues. This, in turn, allows rapid decision making to be undertaken, not just for adjustments to offshore site survey programs, but also, in efficient allocation of engineering resources. It also provides a quantified risk based set of engineering decision tools suitable for life of field appraisal of both HSE and operational issues. The rapid assessment of CAPEX risk allows decisions to be made regarding reassessment of field design and/or identifying defensive measures requiring CAPEX reallocation.

This integrated probabilistic approach to geohazard assessment and subsequent risk analysis significantly streamlines the decision processes required for field design. By employing an integrated team of engineering and geoscience experts, potential "project stopper" problems are swiftly identified and solved/avoided by rerouting/resiting facilities or enhancing engineering design. This approach has the potential to lead to significant cost savings and a reduction in time required for field development.

Introduction

The Terang/Sirasun Development Area is located on the Kangean shelf on the northern margin of the Bali basin. The Bali basin is part of the East Java basin, a back arc basin developed on the margin of the Sunda platform, to the north of the Sunda volcanic arc (Figure 1). The Sunda volcanic arc, represented by the islands of the Indonesian archipelago, developed along the leading edge of the Sunda platform as a result of subduction of the Indo-Australian oceanic plate beneath the Eurasian plate (Hamilton, 1979). Subduction along the arc has been continuous during the Cenozoic (Letouzey et al., 1990).

The East Java basin is part of a system of backarc basins developed around the margin of the Sunda Shield (Southeast Eurasian plate). These basins and their sub-basins are defined by a series of faults that strike approximately eastwest (Bransden and Matthews, 1992; Basden et al., 1999). These basins initially formed as late Paleogene terrestrial rift basins, and later subsided until they were finally subject to marine transgression during the Neogene.