Subsea Processing and Control System in the GASP Project: Testing of the Prototype System
Authors: Nordvik H.S. , Sarshar M.M. , Taylor Mike 
Published: December 2007

The subsea production and processing system developed under the Goodfellow Assocs. Subsea Production (GASP) project involved two stages of separation thatled to the production of exportable-quality crude oil production ofexportable-quality crude oil by pipeline. The produced gas is transported alonga separate line. This paper describes key elements of the subsea processsystem. A prototype system was developed during the second phase of theproject. The system was tested under dry and submerged conditions in a drydock. Key features of the proto type system and the tests carried out aredescribed. Prototype testing proved the viability of the GASP system and helpedidentify areas requiring particular attention and improvement for futureapplications.

Introduction

The main objective of the GASP project was to develop a subsea productionsystem suitable for the development of satellite marginal fields. This was ajoint industry project supported by the European Community project supported bythe European Community and eight major oil companies. An overview of the systemand the economics are addressed in a separate paper. This paper highlights somekey features of the process and control system and the extensive testing of theprototype system. The base case was a satellite field 30 km from the motherplatform. The field consists of four production wells and four water-injectionwells. A general view of the field scenario is shown in Ref. 1.

Design Parameters.

Table 1 summarizes the main parameters for the design ofthe system. The sensitivity case shows the range of values of interest to theparticipating oil companies and does not reflect the limits for future fieldapplications. The quality requirements for the processed crude oil were notnecessarily the processed crude oil were not necessarily the minimum or optimumfor the system but were set to a relatively more demanding level to allowexport of produced oil directly through an existing trunkline. The high exportpressure specified for the crude oil was to meet the operating pressurespecified for the trunkline. Options for simplifying the process system arediscussed later. process system are discussed later. Process Flow Diagram. Fig.1 shows the Process Flow Diagram. Fig. 1 shows the simplified process flowdiagram (PFD). Two identical production trains were adopted. Each train servestwo wells with an option to divert flow from one train to another through thetest-meter loop. Production testing of individual wells is achieved through abypass valve leading to the test header. The multiphase test meter enablessubsea measurement of individual wells without the need for a test line or aseparate test separator. The separator comprises two stages of gas/liquidseparation to meet the specified true vapor pressure (TVP) requirements. Thisleads to production of gas at different pressures (first- and second-stageseparators). pressures (first- and second-stage separators). An ejector is usedto commingle the gas products from the two sources for export products from thetwo sources for export through a single line. Oil and water are transportedalong separate lines for the base case. Separate booster pumps are used foreach train. This helps to keep the pump duty requirements within the range ofexisting pump designs and improves the availability pump designs and improvesthe availability of the system. A fiscal flowmetering system shown in the PFD, measures the crude oil rate for fiscal PFD, measures the crude oil rate forfiscal metering purposes as the product enter a common subsea trunkline. If theproduced oil is transported directly to the mother platform, this meter is notnecessary. The platform, this meter is not necessary. The first phase of thestudy established that designing a reliable subsea meter that met the desiredaccuracy, proving, and availability targets needed for fiscal metering wouldnot be easy and would require a novel approach. Adapting the existinggeneration of fiscal meters was feasible but would result in a cumbersome andtedious system requiring a costly maintenance and proving program.

Process System Design.

At the outset of Process System Design. At the outsetof work, the aim was to simplify the system as much as possible. Separation ofgas, oil, and water is a well-developed technology. For subsea application, however, the following points affect the design of the separator and may changeits features from those of topside units.Modularization to reduce size andweight if the unit is to be retrieved for maintenance purposes.Capacity tocope with the expected slug and plug flow conditions.Minimum use ofinternals to minimize maintenance requirements.Generous capacity to copewith flow functions, any slowness in control-valve responses, and possiblefoaming.Generous capacity to cope with slackness in the level monitoringand control and to avoid unnecessary system shutdown for minor equipmentmalfunctions.Capability to withstand pressures well above the operatingpressure, although it is feasible to include a pressure-relief system on theseparator to minimize the rating.