Subsea Processing and Control System in the GA-SP Project: Testing of the Prototype System

Abstract

Abstract The subsea production and processing developed under the "GA-SP" project involved two stages of separation, leading to production of exportable quality crude oil by pipeline. The produced gas is transported along a separate line. Key elements of the subsea process system are described in this paper. A prototype system was developed during the second phase of the project. This system was tested under dry and submerged conditions in a drydock. Key features of the prototype system and the tests earned out are described. Prototype testing has proved the viability of the GA-SP system and has helped to identify areas requiring particular attention and improvement for future field applications. 1. INTRODUCTION The main objective of the GA-SP project has been to develop a subsea production system suitable for the development of satellite marginal fields. This was a joint industry project supported by the European Community and eight major oil companies, An overview of the system and the economics are addressed in a separate paper (OTC 6766). In this paper, some of the key features of the process and control system are highlighted with reference to the extensive testing of the prototype system. The "Base Case" relates to a satellite field 30 km away from the mother platform. The field consists of four production wells and four water injection wells. A general view of the field scenario is shown in Figure 1. Design Parameters The main parameters for the design of the system are summarised in Figure 2. The sensitivity case shows the range of values of interest to the participating oil companies and does not reflect the limits for future field applications. The requirements for the quality of the processed crude oil was not necessarily the minimum or optimum for the system but was set as a relatively more demanding case in order to export produced oil direct via an existing trunkline. The options for simplifying the process system are discussed in sections which follow. The high export pressure specified for the crude oil was to meet the operating pressure specified for the trunk line. Process Flow Diagram (PFD) Figure 3 shows the simplified process flow diagram. Two identical production trains are adopted. Each train serves two wells with an option to divert flow from one train to another via the test meter loop. Production testing of individual wells is achieved via a by-pass valve leading to the test header. The multiphase test meter enables subsea measurement of individual wells without the need for a test line or a separate test separator. The separator comprises two stages of gas/liquid separation in order to meet the specified TVP requirements. This leads to production of gas at different pressures (first and second stage separators). An ejector is used to commingle the gas products from the two sources for export via a single line. Oil and water are transported along separate lines for the Base Case. Separate booster pumps are used for each train.