Development and Testing of a Novel Subsea Production System and Control Buoy for the East Spar Field Development, Offshore Western Australia
Authors: Casey M.D. , Lawlor C.D.F. 
Published: December 2007

The challenges presented by the remoteness of Australia's North West Shelf can stretch the economic viability of offshore resource development projects to the limit. Accordingly, the East Spar Development has been designed to minimise capital and life cycle costs to ensure the long term viability of this offshore gas field. The offshore facilities comprise a novel, unmanned Navigation, Communication and Control (NCC) buoy linked to a subsea production system including subsea heat exchangers, insert retrievable subsea choke valves, multiphase flow meters and an on-line pipeline corrosion monitoring system. The technological building blocks on which the field development is based are in the main industry proven. However, the novel way in which this proven technology has been arranged into a remotely controlled, self contained, minimum maintenance, unmanned facility is unique and has led to many challenges during the design and testing of the NCC buoy and subsea facilities.

Amongst these challenges has been the formulation of an integration test programme for the NCC buoy and subsea hardware that proves, as far as reasonably possible, the complete functionality of each and every equipment item and interface, subject to constraints imposed by schedule, cost and logistics. Integration testing is particularly important to confirm that the offshore facilities will operate as designed with sufficient reliability and system redundancy to ensure high availability throughout the 20 year field life.

One of the conclusions presented in the paper is that even with tight budget and schedule constraints there are practical ways, prior to committing to costly offshore installation programmes, to design and implement a test programme that will provide an acceptable level of assurance float the offshore facilities will perform as designed.

Introduction

Discovered in March 1993, the East Spar gas and condensate field is located in 95 metres of water 40kms west of Barrow Island, offshore Western Australia (Fig. 1). The field is located some 63 kms from the existing onshore processing infrastructure at Varanus Island, and has confirmed recoverable reserves of 0.5 Tcf of gas and 35 MMbbls of associated condensate. First production is planned for October 1996, a mere 3 1/2 years after it was first discovered.

In order to provide the greatest flexibility in formulating the various development concepts and further optimising these concepts, the East Spar Joint Venture (ESJV, Western Mining Corporation as Operator) decided to form the East Spar Alliance with Kvaerner RJ Brown and Clough Engineering (Fig. 2) in February 1995.

The Alliance, in which operators, designers and constructors work as an integrated team, proved to be enormously beneficial in responding quickly and adapting to opportunities and challenges presented by changing site data and emerging technologies.

Concept Development

The initial plan was based on subsea wells, a piled tripod gathering platform with test separator and future free water removal, a 3 phase 63km pipeline and an onshore gas processing plant.

The tripod platform concept evolved to a guyed tower when site soil conditions were found to be unsuited to piling. Then, warmer than expected seabed temperatures provided the opportunity to implement a lower-risk "all-subsea" scheme, from which evolved the control buoy concept. Each of these evolutions provided successively lower risks to personnel and reduced environmental impact. The production start-up date was retained through each of the evolutions through the use of separate teams studying options in parallel.

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