Multiphase and wet gas flow meters are now routinely used in deepwater production systems that are expected to continue operations for as long as twenty to thirty years. There are no meters, either on land, topside offshore, or subsea, that can be expected to reliably perform over these lengths of time without the benefit of calibration or other maintenance. So it is not unreasonable to develop new strategies to enable these multi-million dollar instruments to operate in an acceptable fashion for the long term.

With no other alternatives, first adopters of subsea multiphase meters have been required to periodically retrieve the meter topside so that it can be updated, repaired, cleaned, tested, and otherwise verified for proper operation. Clearly, this is very costly - perhaps costing more than the meter itself, and would cause a loss of information for the period during which the removal took place. Without alternatives, however, this is what is necessary to sustain a subsea meter into its second decade or longer.

A novel approach to verify a subsea meter is to provide a comparison measurement to the in-place meter that will permit the user to know if there is reason to believe the measurement is incorrect, usually in the form of a bias. Several concepts have been proposed.1  Previous investigation has been made to develop a multiphase measurement system that can be clamped on to vertical, bare steel pipe and make a measurement through the pipe walls.2  Because of the pressure integrity requirements for deep water subsea flow lines, the heavy pipe wall thickness presently limits the measurement techniques which can be applied, and the expected measurement accuracy.

In a new investigation, a new clamp-on subsea measurement system has been developed that takes advantage of the recent advances in high-pressure, large-diameter reinforced composite (non-metallic) pipe.3  A non-metallic pipe is electromagnetically transparent. Unlike steel pipe, electromagnetically transparent pipe can host a wide range of through-pipe measurements. In this investigation a subsea flow measurement system using electrical capacitance tomography (ECT) was developed and tested. The technique foresees the use of a remotely operated underwater vehicle (ROV) to clamp an array of electrode sensors on to the electromagnetically transparent pipe in a marine environment. The outputs of these sensors are used to make a two-dimensional image of the flow at two cross-sections in the pipe, and to estimate the flow rates of each phase. Making this happen in the conductive seawater environment is an important step forward. The system was tested underwater to demonstrate the feasibility of subsea clamp-on ECT measurement.

Although ECT measurement has been used on land for liquid and solid particles flow measurement for some time, it has never before been adapted to an underwater environment. The work represents a breakthrough by extending ECT flow measurement to underwater applications. Successful development of a clamp-on subsea ECT multiphase flow measurement would represent a real opportunity for the verification of existing subsea MPFM using an independent measurement operated by standard ROV equipment.

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