Integrated Design Assessment Optimizes Layout and Engineering of Timor Gas Field
Authors: Carpenter Chris 
Published: March 2020

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 19391, “An Integration of Geology, Geohazards, Geophysics, and Geotechnics for Pre-FEED Site Layout and Engineering Design of an Offshore Gas Field in the Timor Sea,” by Pattarapong Chullabrahm, Korn Saranyasoontorn, and Maythus Svasti-Xuto, PTTEP, et al., prepared for the 2019 International Petroleum Technology Conference, Beijing, 26–28 March. The paper has not been peer reviewed. Copyright 2019 International Petroleum Technology Conference. Reproduced by permission.

The complete paper presents an integration of geology, geohazards, geophysics, and geotechnical assessments for a design of an offshore gas production facility and an associated export pipeline. The gas field described in the paper is off the northwest coast of Australia in the Timor Sea at a water depth of approximately 130 m.

Introduction

The gas field is envisioned to include a subsea production system, a fixed central processing platform (CPP) for separation and treatment, and a floating storage and offloading (FSO) vessel for condensate. Processed gas will be transported by a new gas export pipeline (GEP) that connects the CPP with a tie-in point on an existing export pipeline. A schematic of the infield facilities is shown in Fig. 1.

Regional Geology and Geohazards

To understand the seabed geology, the associated geohazards, and their effect on the proposed development, an initial desktop study into the seabed and shallow subsurface geology was undertaken. The main output from the desktop study was a preliminary geological ground model that helped determine the most suitable geophysical and geotechnical survey techniques to obtain accurate information needed to model the ground conditions further in preparation for engineering design.

Regional Geology. The field development lies within the Vulcan sub-basin of the Bonaparte Basin, which contains sediments to a depth of 15 km. The sub-basin is a southwest/northeast-trending extensional depocenter comprised of a series of fault bound horsts, grabens, and terraces and is active tectonically. The resulting deformation is dominated by extensional fault structures (e.g., normal faults).

During the late Pliocene epoch and early Quaternary period, sedimentation of the Bonaparte Basin was dominated by shallow water carbonates in a tropical ramp setting, including the development of isolated, intrashelf carbonate buildups that are often referred to as pinnacles or shoals. Avoiding these steep-sided shoals is an objective in selecting a preferred pipeline route; however, acknowledgement of other, unmapped seabed features such as terraces, banks, and valleys must also be considered.

The base-case GEP route, called the Southern Bypass route, has a length of approximately 347 km and is proposed to link the field to the tie-in point. A more-direct route between the field and the tie-in point could shorten this route significantly (up to 70 km). This provides an opportunity for a significant expense savings for GEP construction but only if a technically feasible route through a major subsea canyon—the Lambert Shelf Valley (LSV)—can be found. This canyon is up to 100 m deeper than the surrounding seabed and has steep-sided slopes ranging between 30 and 50°.

Geohazards. The LSV has the potential to influence the proposed seabed infrastructure. The geohazards associated with this canyon have a direct effect on the GEP. The following geohazards required investigation:

Slope instability

Turbidity currents

Hard ground

Mobile bed forms

Pockmarks and shallow gas

Soft soils

Liquefaction