Well testing of subsea step out fields can be more economic and effective if a metering unit is installed near the wellhead. Such a unit has been developed from the BOET Subsea Separation programme and uses control system elements already certified and demonstrated offshore. It is developed for small gas fields and comprises a single separation vessel with ultrasonic and Coriolis metering principles for the separated gas and liquids. The unit layout is based on the size, weight and guidebase installation of a typical subsea Xmas tree.
Many future UKCS gas developments are likely to be small isolated accumulations. These may be developed as subsea well clusters, with the production commingled at a subsea manifold and transported by infield pipeline to a host platform.
For accurate and regular well testing a second test pipeline will be neededwith associated platform riser and test separator. These are costly facilities, particularly the-pipeline, which can be-avoided if a test separation and metering unit can be located on the seabed at the subsea field instead of - being on the platform topsides.
This technical paper describes a subsea metering unit based on the BOET subsea separation experience. It will be located, as a seabed installation, in the export flowline route from either a single subsea gas well, or a subsea test header in the case of a manifolded cluster.
The unit is intended to measure the flow rate of hydrocarbon gas and liquids and any associated water, to well test accuracy of about 5%. It is designed and developed with the potential to measure to allocation metering accuracy of 2–3%, after 6–18 months field experience.
The first unit in service will therefore be for well test with the option that it can be uprated in service to allocation metering duty as the gas field, and adjacent fields are developed further. It will also be the first step in the longer term development of a subsea fiscal meter for gas wells. Fiscal metering will require 1% accuracy and a higher standard of self calibration and sampling to meet gas customer and government requirements. The development of such a fiscal meter is proposed as a longer term programme, based on the service experience of the well test/allocation meter.
In principle the unit will take the form of a single stage gas/condensate separator with flow measurement systems in the gas and liquid pipework. After measurement (and sampling where necessary) the product phases are recombined in the export flowline (although they could remain separate if the extra flowline for liquids is justified). The unit is designed to fit on a standard API 4-post guidebase either in a template wellbay or as an isolated seabed unit. It will usually be supported on 30 inch casing piled into the seabed and run and secured by normal wellhead equipment and procedures. Autonomous control is provided continuously by a subsea PLC (programmable logic controller) in a dry subsea module.