Subsea Sampling and Sensor Insertion Systems for Improved Accuracy of Production Measurement

Abstract

The increase in deepwater field development in the Gulf of Mexico and other locations has brought an increase in the use of subsea multiphase flow meters (MPFMs) for individual well flow rate monitoring at or near the wellhead. Their justification usually is to obtain improved well and reservoir management, or a more verifiable allocation of production between wells, such as where the ownership or royalty rate differs amongst the wells producing to the same platform. The two most common inadequacies of subsea MPFM installations has been the insufficiency with which one can characterize the produced fluids – for measurement accuracy, and the unavailability of any supplemental measurements, even if only temporary – for measurement verification. Without these, MPFM measurements can be inaccurate, and there may be no means to verify them. Unverifiable and questionable measurements will likely go unused, and the value originally sought with the installation of the MPFM goes out of reach. Although experience with MPFMs on land and topsides, where they are accessible, has demonstrated the importance of characterizing the fluid properties from analyzed fluid samples, and having alternative means to measure flow to validate the MPFMs, subsea multiphase meter installations rarely have any facility for fluid sampling or supplemental measurement. Until recently, the development of sampling techniques has been limited and there have been no systems developed which could deploy a variety of other sensors near the MPFM for the purposes of verification. For subsea MPFMs to fulfill their original purpose of providing long-term reliable subsea wellhead measurement, MPFM installations need to have the facilities for subsea fluid sampling and subsea sensor insertion/removal, which would provide timely and accurate fluid properties at meter conditions, and a means to verify MPFM performance. Topside well tests and samples of commingled well streams cannot meet these needs. This paper describes two technology developments which address these issues: 1) a remotely operated underwater vehicle (ROV) deployed subsea sampling system, which provides a means to capture fluid samples for the purposes of fluid property determination at the multiphase meter or other locations at any time in the life of the well, and 2) an ROV-operated subsea sensor installation/removal system that enables the placement of a variety of sensors in direct contact with process fluids, and extends the flexibility and range of subsea wellhead and subsea process measurement.