Challenges of the Britannia Subsea Development
Authors: Smith J.E. 
Published: October 2008

Britannia is a large gas condensate field situated in the central North Sea with reserves of 3 tcf of gas and 140 million barrels of condensate which started production on August 3rd 1998.

The field covers a large areal extent measuring 40kms by 10kms and to be able to develop it economically the west part of the field is produced from a subsea center. Produced fluids return 15kms via subsea flowlines to the PDQ Britannia Platform on the east side of the field. The corrosive nature of the Britannia fluids coupled with the turn up / down requirements of the gas sales contracts and the long 25 year life of the field provided a number of design challenges.

This paper describes the logic behind the design and the work undertaken by the Britannia Subsea Team to provide a cost-effective development. Other papers in this year's conference describe in detail some of the analysis and experimental work required to support the design, these are referenced at the end of this paper.

Introduction

The Britannia Field is located approximately 209kms north east of Aberdeen in the Central North Sea. It straddles four North Sea blocks, 15/29a, 15/30, 16/26 and 16/27a&b and measures approximately 40kms in an east / west direction and 10kms in a north south direction. The offshore facilities comprise a production, drilling, quarters (PDQ) platform located in the east of the field and a 14 slot subsea manifold located in the west of the field in 150m of water. Produced fluids travel 15kms from the subsea manifold via a 14" production flowline and an 8" test line back to the platform. The manifold and associated subsea wells are controlled from the platform via two electrohydraulic control umbilicals.

The facilities are designed to produce at a peak production of 740mmscfd of gas and 70mbpd of condensate with up to 380mmscfd production coming from the subsea facilities. The subsea facilities are required to produce for a period of up to 25 years.

The subsea facilities design was driven by the following main factors:The corrosive nature of the Britannia fluids, containing up to 2.6 mole% of CO2 and 15ppm of H2S.The need to avoid hydrate formation and wax deposition. The wellhead shut in pressure was 303 barg with a minimum seabed temperature of 4°C.Requirements of the gas sales contracts to be able to gofrom zero to maximum production within a 24-hour period.Onstream availability of greater than 97% combined with a long design life of 25 years.Simultaneous drilling and production.Protection from fishing operations.A focus on cost reduction

The Subsea schedule was determined by the Topside and Drilling schedule critical paths. A total of seventeen wells were required to be pre-drilled to achieve plateau production at first gas, it was decided to drill seven of these from the subsea center and 10 from the platform location. To be able to achieve this drilling needed to start early in the project and it was decided to use a manifold and well cluster arrangement rather than a template to retain maximum flexibility. Overall the subsea schedule could be described as generous by Subsea standards which was important to allow the time for the detailed analysis and testing described below.