Development of Operating Envelope for Long Distance Gas Tieback
Authors: Cochran Steven W. , Harun Amrin F. 
Published: October 2008

Long distance tiebacks are now generally considered to be a viable development technique for deepwater gas-condensate fields. The design of these production systems will impose limitations on how the facility can be operated, particularly for transient operations. And it is the operating envelope that we use to describe these operational limitations. This paper addresses the operating envelope of a 90-km deepwater tieback in terms of well operations, turndown, ramp-up, shutdown, and restart. Transient multiphase hydraulic models were used to simulate these operations and to help define operational boundaries.

Introduction

This paper presents the development of the operating envelope for a 90-km deepwater gas-condensate tieback. The design of this production system has been challenging and has limitations on how the facility can be operated, particularly for transient operations.

The operating envelope consists of the boundaries and limitations the system design and environmental factors imposes on the operating parameters. These boundaries can include minimum operating rates, maximum ramp-up and startup rates, maximum water content, etc. For example, much of the focus during the development of this operating envelope has been on the accumulation of liquids in the pipelines at low production rates. Ramp-up procedures have been carefully developed so that the capacity of the onshore slug catcher and liquid handling system is not exceeded. The other principal driver of the operating envelope is the prevention of hydrate formation. Finally, the operating envelope will be used to develop the operating procedures for the production system.

Field Development Overview

The operating envelope presented here is based on the Scarab/Saffron field development. The Scarab/Saffron gas reservoirs are approximately 90 km offshore the Mediterranean coast of Egypt and are part of the West Delta Deep Marine Development. Scarab/Saffron is operated by the Burullus Gas Company, a joint venture comprising BG-Egypt, Edison Gas, and EGPC (Egyptian General PetroleumCompany).

Scarab/Saffron will initially be developed using eight subsea wells in water depths up to 620 m as illustrated in Figures 1 and 2. These wells will be connected via 10-inch infield flowlines, ranging from 2 to 7 km long, to two subsea production manifolds in approximately 400 m of water. Each production manifold hosts four wells and incorporates a connection for a crossover spool linking the two manifolds. Two 20-inch export pipelines link the production manifolds to a Pipeline End Manifold (PLEM) located approximately 20 km nearer shore in 95-meter water depth. A crossover between the two production streams is also provided at the PLEM. ROV-actuated isolation valves allow the production manifold and/or PLEM crossovers to be closed if the operator chooses to do so. The crossovers are normally open during operation. One each 24-inch and 36-inch pipelines link the PLEM to the onshore plant. Figure 3 presents the pipeline profile from the production manifolds to shore.

Future fields will tie in at the PLEM such that total potential export from the PLEM to the onshore plant is approximately triple the maximum Scarab/Saffron production rate.