Z. Atakan, D. Chin, Peter Lang, Sada Iyer, Shell Exploration &
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
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.