The King West single well deepwater development is a 2.5 miles, 6" flowline tieback to the existing King system western flowline in water depth of 5,500 ft.
The King West Project Team was faced with high costs and complex methods for hydrotesting and tying this into the King subsea system. The conventional BP GoM Projects subsea flowline hydrotest method is to pump seawater or fresh water, pressurize it, and test for any possible leaks. Upon completion, the water is discharged subsea or displaced using nitrogen.
For deepwater flowlines, residual water from the hydrotest must be inhibited to prevent hydrate formation during initial well startup. However, the bathymetry of the King West flowline indicated that inhibition by methanol displacement alone could not achieve this and complex nitrogen de-watering would be needed.
The Project Team suggested investigating the possibility of using natural hydrate inhibition properties of high salinity inhibited completion brine instead of fresh water for flowline hydrotesting.
This novel approach has major benefits. It reduced environmental impact and personnel exposure, procedure complexity, risks in dewatering operations; process upsets and allows the hydrotest fluid to flow back with well fluids through the host facility process then safely discharged. Moreover, the well startup time was reduced by approximately two days.
In order to fully understand the hydrate inhibition properties of calcium chloride (CaCl2) brine, several tests were conducted at local labs. These test results showed that using a CaCl2 brine with a concentration of 11.2 ppg (33.9 wt%) would be best for King West hydrotesting.
Eight different hydrate prediction softwares were compared with experimental hydrate formation data for high salinity brine-hydrocarbon systems. Comparisons showed only one prediction software matches the experimental data within a close range.
The other key finding observed from the experimental data is that CaCl2 depression of hydrate formation temperature does not reach a maximum at increased concentrations, instead the hydrate inhibition capability continue to increase with further increase in CaCl2 concentration.
The pressurized crystallization point (PCT) for 11.2 ppg CaCl2 brine at 15,000 psi was measured as 15°F, which is well below the seabed temperature for King West flowline.
The King West subsea system was successfully hydrotested on May 29-30, 2003 by using 11.2 ppg CaCl2 brine inhibited with corrosion inhibitor and oxygen scavenger. Production was started with no associated facility upsets, hydrate formation or well downtime.
The single well King West development was designed to be linked by a 2.5-mile long, 6" diameter tieback to the existing King system western flowline at a water depth of 5500 ft. The King system is connected to the Marlin Tension Leg Platform (TLP) which is in 3,500 water depth.
The conventional hydrotesting method of using fresh water or seawater and discharging to the sea or displacement using nitrogen was not feasible for King West due to hydrate formation risks. For deepwater flowlines, residual water from the hydrotest must be inhibited to prevent hydrate formation during initial well startup.
