Investigation of a Subsea Separation Station Operating Envelope Using Subsurface-to-Topsides Integrated Dynamic Simulations
Authors: Costa David , Vu Van Khoi , Barnay Gilles Charles , Larrey Dominique , McClimans Ole Thomas , Sund Eirik Bjerve 
Published: October 2008

Abstract
TOTAL and FMC Technologies have performed a technology evaluation of a possible gas/liquid separation system for West Africa (1500 - 2000 m WD). The production system includes one separator station with combined flow from two fields, the subsea separator/booster station will have a capacity of 110 000 bpd liquid and 2.5 MSm3/d associated gas.
A method has been devised by FMC Technologies and Kongberg Maritime (former Fantoft Process Technologies) to check the system response to transient conditions of a subsea gas/liquid separation system for the selected field. An integrated dynamic model including wells, flowlines, subsea separation unit and risers have allowed investigating the system response to a wide range of operating conditions such as: well flow variations, pressure variations, slugging, shut-down followed by the depressurization and liquid evacuation, start-up, accidental stop of one pump, gas blocked outlet and gas blow by.
This approach is a first as in previous projects the flowlines, subsea separation and risers were simulated independently for transient conditions and did not take into account any potential interactions between the wells, flowlines, separator/booster and risers. The design of the subsea separator/booster station has been validated as well as its ability to handle a whole range of transient operating conditions and the operability of the subsea gas/liquid separation has been checked.
This methodology is easily transferable to other deep or ultra-deep fields with subsea equipment including control loops in interaction with the flowlines and/or the risers.

Introduction
Subsea gas/liquid separation is an attractive and economical solution to develop deep offshore fields producing fluid without wax and for which gas lift alone is neither not suitable nor sufficient for activation. Such scheme allows the following:
- To simplify the gathering system (one flowline only instead of the classical loop to collect well effluent, hydrate risk during shut down being mitigated by depressurisation),
- To increase the production rate in low energy reservoir by adding subsea liquid pumping, simplifying also topsides (no gas lift).
Subsea separation after many years of development and pilot testing is today entering into a more commercial product phase since the award of the Tordis Subsea Separation Boosting and Injection (SSBI) contract in 2005 [1] to be installed at 200 meters water depth in 2007.
For other types / enhanced performances of subsea separation system and for applications into deeper waters (eg 1500 - 2000m), there is no fully field proven system existing. Hence, it is necessary to have a robust design in order to enhance the availability compared to surface systems and to reduce to the minimum manu al remote intervention from topsides (to limit wear on equipment).
During the Girassol (Angola) deep offshore project [2], operated by TOTAL, with conventional subsea production loops (no subsea processing), OLGA dynamic multiphase flow simulator was used.