Virtual Sensor Development Aims To Help Avoid Hydrates in Deepwater Gas Field

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 24621, “Virtual Sensor Development To Prevent Hydrate in Deepwater Gas Field,” by Suphawat Kiertkul and Syed Mohd Halim Syed Nasir, PTTEP. The paper has not been peer reviewed. Copyright 2024 International Petroleum Technology Conference. _ Sabah Block H consists of the Buluh and Rotan fields in 1300 m of water. After Block H achieved first gas, one temperature sensor downstream of the Rotan P3 subsea tree failed after 6 days of operation. Replacing the physical transmitter was not practical because it was integrated into an unretrievable part of the tree. The objective of the project described in the complete paper was to develop a virtual sensor to monitor and control the Joule-Thomson (JT) cooling effect downstream of the subsea choke to avoid hydrate plugs during cold-start operations. Introduction The operator has developed the gas fields in Block H to supply gas-capacity rates of 300 MMscf/D to a floating liquefied natural gas (FLNG) facility capable of producing 1.5 million tons/year of LNG. Normal operating flow rates during production plateau were expected to be 270 MMscf/D. The surface inlet separator pressure was expected to be approximately 65 barg during the high-pressure mode, with a capability to reduce production pressure to 17 barg in late field life. Phase 1A of Block H deepwater gas development consisted of three wells from the Rotan field and one well from the Buluh field. Gases mainly are methane with a low condensate/gas ratio of 0.5–2.0 bbl/MMscf and low CO2 content. No hydrogen sulfide (H2S) was expected from the reservoir, but the design could handle H2S up to 50 ppm. The produced-water rates also were expected to be low. Three gas-production wells, RT-P1, RT-P2, and RT-P3, were connected in a daisy-chain configuration and tied back with flexible well jumpers to a main 10-in. flexible flowline. The furthest well, RT-P3, was tied back to the production flowline with a pipeline end termination (PLET) and the remaining two wells, RT-P1 and RT-P2, with inline sleds. Similarly, Well BL-P1 was connected to the PLET by flexible jumper and to an 8-in. flexible flowline. The flowlines from both fields terminate at the hub manifold with dual headers to allow commingled production to the FLNG host facility by dual 10-in. flexible risers. Background After Block H achieved first gas in February of 2021, one temperature sensor downstream of the Rotan P3 subsea tree went to the fault state. The sensor was showing 172.5°C, representing temperature values outside of the calibrated range (between –30°C and +150°C). Based on the reported data and previous experience, the most likely cause was an open circuit to the Pt1000 element that lost contact with the sensor element (Fig. 1).