Bayu-Undan Substructure Foundations: Geotechnical Design and Analysis
Authors: Neubecker S.R. , Erbrich C.T. 
Published: October 2008

This paper will discuss the geotechnical design of the foundations adopted for the Bayu-Undan Central Processing Platforms (CPP), which are located in the Timor Sea. The two substructures are characterised by heavy topsides and relatively low environmental loading which, when combined with the cemented calcarenite soil conditions, proved ideally suited to the selection of relatively small but highly stressed steel plated gravity based foundations. Four separate plated foundations were provided per jacket with each foundation comprising a long but narrow strip footing between two of the eight jacket legs.

Deck installation was achieved using a floatover method, which for the assumed design conditions imposed temporary tensile loads on the foundation. A small pile was therefore drilled and grouted at each corner of the jackets solely to resist these loads. Both jackets were installed in mid 2002 while the topsides were successfully floated over in June and August 2003.

This paper describes the geotechnical aspects of the foundation, and the analysis techniques that were employed during the detailed design stage to address the difficult design considerations for this novel foundation.

Introduction

The Bayu-Undan field is located approximately 500 km west of Darwin, Australia and 250 km east of the Democratic Replublic of Timor-Leste (formerly, East Timor) in an area known currently as the Joint Petroleum Development Area (JPDA - formily the Zone of Cooperation between East Timor and Australia). The estimated ultimate recoveries for the field are 3.4 TCF of gas and roughly 400 million barrels of condensate and LPG (e.g. propane and butane). The field lies in 80m of water and is operated by ConocoPhillips with Eni, Santos, INPEX, Tokyo Electric Power Company, Incorporated and Tokyo Gas Co., Ltd as co-venturers.

The first phase of development, approved in February 2000, focuses on gas recycling with condensate and LPG recovery. The field facilities consist of:A single wellhead platform (WP1) located 7.4 km east of the main central processing platforms (CPP)A Drilling, Production, and Processing (DPP) platform (with a dry topsides weight of 14,600t)A Compression, Utilities, and Quarters (CUQ) platform (with a dry topsides weight of 11,200t)A flare bridge and cantilevered flare towerA 172,500 dwt FSO that stores and offloads condensate, propane, and butane as separate products The field is set to come onstream early in 2004 with full production of rich gas and liquids anticipated mid year. The facilities design capacity is 1.1 BCFD of rich gas and 115,000 BPD of extracted liquids.

Phase II of the project, which was approved in June 2003, involves building a 26" gas export pipeline to Darwin to feed a 3.52 mtpa LNG plant (design capacity) located south of Darwin on Wickham Point. Phase II of the project is well underway and is scheduled for start-up in early 2006.

Design of the Bayu-Undan foundations proved to be a very challenging aspect of the project in the early days due to the remote location and the nature of the geotechnical properties in this region of the world.