Abstract
Sabah block H, located in 1,300 meters of water depth at offshore Sabah, comprises Buluh and Rotan fields. The subsea production system, i.e., trees, jumpers, manifold, umbilicals, risers and flowlines were installed and tied back to an FLNG facility. After Block H achieved 1st gas in Feb-2021, one temperature sensor at downstream of Rotan P3 Subsea XMT failed post 6 days of operation. Replacing the physical transmitter is not practical as it is integrated at an unretrievable part of XMT. A drilling rig is required to pull the upper completion assembly of the well before retrieving the XMT to surface for repair and consider non-economic approach. The objective of the project was to develop a virtual sensor to monitor and control JT cooling effect at downstream of the subsea choke to avoid hydrate plugs during cold start operations.
A Hysys equivalent flowpath model was developed to simulate the thermodynamic process. Gas composition, flow rates from the Multiphase Flow meter, Inlet and outlet pressure at subsea choke, gas mass fraction, Hydrate Inhibitor (MEG and MeOH) mass fraction as well as temperature profiles at the wellhead condition were input and integrated into the model. Four hybrid methods using semi-physics-based approaches were used to correlate the results from the Hysis model within the acceptable range of the energy balance in the expected Gas Mass Fraction (GMF). Other well data were subsequently used to validate the model including fine-tuning before the final acceptance test.
After more than 500,000 data points were correlated with the initial choke model, some errors around 20% were observed in the transient flow period in the first round of validation. Therefore, sets of similar field data from the other nearby wells, i.e., Rotan P1 and P2 were used to further validate the predicted model and adjust some arbitrary constants to correlate the mass and energy balance in the expected GVF mode during the transient and stead state operation. The software acceptance test indicated the accuracy of less than 5% from the Choke model based on final validation data from Rotan P1 and P2. The real-time model was subsequently developed and successfully implemented into the subsea control system at offshore inside the Operator workstation, Integrated Virtualization Manager (IVM), and Master Control System (MCS). This subsea virtual sensor has been critically monitored during the cold start to prevent hydrate formation due to Joule Thompson (JT) Cooling at the downstream of the subsea Christmas Tree. As the lowest temperature was as low as -23C during the cold start, the operator would be able to bean back the choke and inject more hydrate inhibitors to alleviate the effect throughout the end of the field life.
Block H has been inevitably operating in the hydrate formation zone. With high pressures from the deepwater gas well and low ambient temperature around 2C at seabed as well as pre-existing water given by aquifer driven mechanism in reservoir. The virtual temperature sensor is considered as a cost-effective solution to manage the JT Cooling during the cold start and avoid hydrate plugs at the subsea XMT. The field successfully achieved 100% of subsea production uptime in 2022.
