An Efficient Wellstream Booster Solution for Deep and Ultra Deep Water Oil Fields
Authors: Birkeland Eivind , Jernsletten Jo , Olsen Geir , Waalmann Jan Gunnar , Nyborg Knut 
Published: October 2008

The paper describes how a deep water field can be developed with a gas-liquid separator and a liquid booster pump located at the riser base. Selection of subsea equipment with an assessment of maturity is described together with the main challenges for the equipment installed in the system.

The business case concludes that the additional investment costs are limited compared to the increased income.

Gas-liquid separation and liquid boosting at the riser base also facilitate a new hydrate prevention philosophy for deep water fields. The pressure in the production system can be reduced to below the hydrate formation pressure by use of the separator and the liquid pump at shut down.

Introduction

An effective energy adding solution is vital for the field economy of subsea developments in deep / ultra deep water. In some cases, it is an absolute requirement to make the development technically feasible. The conventional solution of using gas lift will be ineffective and it may be faced with several technical and operational challenges when the water depth increases. Another solution may be to install subsea multiphase pumps. However, as the water depth increases, the required head to overcome the hydrostatic pressure from the liquid column in the vertical risers will at some stage require two or more pumps in series. Another common problem of these two systems is the slugging in the vertical risers. With gas lift, this problem would be much more apparent.

Gas-liquid separation and liquid boosting may be an alternative to gas lift and multiphase pumping. The system is characterized by a subsea gas-liquid separation followed by a liquid boosting and transport of gas and liquid in dedicated flowlines and risers to one common host (or alternatively to dedicated hosts).

Introduction of a gas-liquid separator and a pump at seabed also make it possibility to depressurize the subsea production system at shut down. This may be a more attractive and cost effective hydrate prevention philosophy than presently used for deep water fields.

The paper will describe a typical deep water field with a traditional development. A modified solution with gas-liquid separation and liquid boosting is proposed and discussed with regards to hydrate prevention and maturity of the system equipment. A business case will compare the production rates and economics for the field by use of gas lift, multiphase pumping, and gas-liquid separation with liquid boosting for energy adding.

Field Description

A typical deep water field has been defined for the purpose of describing the proposed field development solution. A marginal field with 4 production wells shall be developed with a tie-back to an existing processing facility. The water depth is 1500 and the tie-back distance is 5 km. A depletion strategy is selected for the field with no pressure support. The overall field parameters are summarized in Table 1.

Table 1 - Field Parameters (Available in full paper)

Traditional Field Development Solution

The 4 production wells are located close to each other at the same drill centre and are connected to a common production manifold with short production jumpers, see Figure 1. 2 off production headers deliver the well stream to the 10" production flowlines.