Multi-phase meters make gas drilling safer
15 Jan 2000
High pressure, high temperature (HP/HT) fields in the Central Graben region of North Sea 150 miles to the east of Aberdeen will play a vital role in the UK's gas supply in the next century. Projects valued at £4 billion, including Erskine, Elgin, Franklin, Shearwater, and the BP-operated Eastern Trough Area Project (ETAP), are currently underway in this region.
The first of these, Erskine, is about to start commercial production. This is the first HP/HT field to be developed in the North Sea, and its performance will be watched with keen interest when it comes onstream later this year. The Texaco-operated development is also one of the first to use multi-phase meters for sound commercial reasons.
An HP/HT development must have an extremely reliable pressure protection system to prevent an uncontrolled release of hydrocarbons from the reservoir. It would be uneconomical to construct process systems and pipelines to contain the enormous reservoir pressures. Under the Safety Case Regulations, the pressure protection system will be subject to formal safety assessment (FSA).
Pressure protection involves the process control system, the emergency shutdown (ESD) system, Christmas trees and sub-surface safety valves, as well as relief and flare systems. These safety systems act as layers within a hierarchy, and the Erskine field contains an extra layer known as a High Integrity Pressure Protection System (HIPPS). This involves an extra shutdown valve downstream of each Christmas tree. To ensure high reliability two or three pressure sensors, and an independent shutdown logic system, are used to close the valve. Pipework upstream of the HIPPS shutdown valve will be rated to contain the well shut-in pressure. Lower rating, say ANSI 2500, is normally used downstream.
When Erskine comes onstream later this year, it will act as a proving ground for technology which may then be used in the Elf-operated Elgin and Franklin fields, and the Shell Expro-operated Shearwater field. These projects are also close to the BP-operated ETAP scheme. The £1.6 billion ETAP budget brings the overall value of work in the central North Sea to around £4 billion.
ERSKINE:A HIGH-PRESSURE SITE
Development of Erskine started in 1995. Texaco said that advances in drilling technology, based upon its experience in the Gulf of Mexico, together with deregulation of North Sea gas sales and marginal field concepts related to Cost Reduction in a New Era (CRINE), provided the impetus for the development.
Erskine is being developed at a cost of £290 million by a 50:50 joint venture between Texaco and BP. The estimated recoverable reserves are 330 billion cubic feet of gas, and 75 million barrels of condensate. The gas lies in Jurassic sandstones 15000m deep, with bottom hole pressure of 14000 psi and 350 C. The depth and pressure of the Erskine reservoir are more than twice that of a typical North Sea field.
Minimal processing facilities have been provided on the normally unmanned Erskine production platform. Production fluids will be piped, unprocessed, to the Lomond platform for separation and export through existing pipelines. A processing module has been installed on Lomond to handle the Erskine fluids.
MULTI-PHASE METERS
Erskine is even simpler than previous minimum facility platforms because the multi-phase meters have been used for well testing in place of a test separator.
The traditional test separator is a large pressure vessel connected by a manifold, pipes and valves to each of the wells. To perform a traditional well test, a well is switched from the main production manifold to the test separator. Flows of oil, water and gas are then measured by their own meters on the outlet of the test separator.
By eliminating the test separator on Erskine, the pressure system was made smaller and less complex; thus improving the 'inherent safety' of the platform. The multi-phase meters also require less maintenance than the separator. This was important, because visits to Erskine will be less frequent than other normally unmanned platforms.
The Erskine multi-phase meters have been supplied by the Norwegian company Framo Engineering A/S. The meters are designed for an 800 bar operating pressure with maximum operating temperature of 175 C. The Framo design incorporates a patented flow mixer to provide stable and homogeneous flow through the measuring section. The measuring section has a venturi meter to measure fluid velocity, with a dual energy gamma fraction meter to measure oil, water and gas fractions.
Framo, Fluenta, Multi-Fluids International and Kongsberg are the best known manufacturers of multi-phase meters. Most of the R&D has been supported by oil companies. Around half-a-dozen other companies, including the UK-company Jiskoot, have plans to compete in this rapidly expanding market.
Most of the meters have limits to their working envelop so that they must be carefully tailored to their applications. However, trials have shown that the meters provide better than 10 per cent accuracy over a range of working conditions. This level of accuracy is adequate for well testing.
Subsea multi-phase meters have been used in the North Sea, Australia and Brazil. They allow well tests to be carried out on the seabed, far from host platforms. This saves the cost of providing an additional pipeline for well testing. Multi-phase meters have also been used to allocate production when two fields are tapped by the same facility.
