Design and Operability Considerations of the Gas Flowline at Parque das Conchas (BC-10) Ostra Field
Authors: Atakan Ziya , Chin Y. Doreen , Lang Peter Paul , Iyer Sada 
Published: July 2010

The Ostra field, developed as part of the Parque das Conchas project, located in the deepwater BC-10 block offshore Brazil, is a relatively low-pressure reservoir. To enable production, an innovative technology of subsea separation and boosting has been deployed. The system consists of manifolded subsea separators with ESPs. Separated liquids are boosted by means of ESP's and transported to the FPSO via an " oil?? flowline while separated gas flows to the FPSO via a dedicated gas flowline.

A significant risk to the gas flowline is liquid carry-over (LCO) due to potential inefficient separation from the subsea separators - resulting in liquid loading of the gas flowline and associated slugging. This paper addresses the design envelope, operating strategies and liquid management methodologies that will be used to manage the risk associated with LCO into the gas flowline.

Introduction

The Parque das Conchas development is located in block BC-10, approximately 74.5 mile South West of the coast at the city of Vitoria in Brazil in water depths about 5,900 ft. Shell is the operator with a 50% working interest, in a joint venture with Petrobras (35%) and ONGC (15%). The development includes three fields - Ostra, Abalone and Argonauta B-West, each with different fluid charcateritics. However, only Ostra and Abalone use the concept of subsea separation and boosting - the focus of this paper. Figure 1 shows the subsea field layout for Ostra and Abalone.

It is planned to produce the Ostra field via six horizontal wells. The oil of the Ostra field has an API of 24 and a GOR of 274 scf/sbbl. Abalone has an API of 44 and a GOR of 3,800 scf/sbbl and behaves like a gas condensate. Due to the low, fast decaying reservoir pressure and large water depth, the production from the Ostra field requires artificial boosting, which is provided by an electrical submersible pump (ESP) contained in a Caisson-separator1.

The Ostra/Abalone system has four caisson separators to accommodate the production function from six Ostra wells and one Abalone well. The production fluids entering the caissons are separated as gas and liquid after which the gas from the separators is manifolded and transported via a dedicated gas flowline by natural flow.

Similarly, the boosted liquids from each separator are manifolded into an " oil?? line and transported to the FPSO (see Figure 2).

The caisson separator pressure is controlled by adjusting the gas flowline pressure. The gas flowline pressure can be adjusted by utilizing either a choke on the FPSO turret or by a choke on the artificial lift manifold adjacent to the caisson separators. The operating caisson-separator pressure is expected to be around 900 psi in early life and gradually declining to 500 psia during late life. A service flowline is provided for servicing the Ostra and Abalone fields as well as for use as a production/test line when necessary. The gas and service flowlines are connected by a pigging loop with a pigging valve isolating the two lines.