TOT’s safety undergoing testing qualifications. (Image courtesy of Texas Oil Tools, NOV)

Performing multiple completions on high-pressure/high-temperature (HP/HT) gas wells can be an expensive endeavor. The necessity of tripping the riser after each completion to inspect the subsea safety head increases cost — in some cases as much as US $1 million per well. StatoilHydro sought out National Oilwell Varco’s (NOV) wholly owned subsidiary, Texas Oil Tools (TOT), in an effort to develop a “real-world” test.

The operator wanted to use actual condensate from several of its projects in the North Sea to devise a real-world test that could help design a compound capable of withstanding completion operations for a four-batch template, which would reduce costs and maintain safe operations in the field.

Brave new method

NOV TOT manufactures a retrievable subsea safety head that isolates subsea blowout preventers (BOPs) from the workover riser in the event of a failure. The assembly can withstand hot bore fluids up to 295°F (146°C) and cold ambient environments of 34°F (1.1°C). The equipment can handle pressures up to 15,000 psi.

Traditional testing methods for this equipment use water or highly refined oil in place of the condensate. Because gas is harder to seal than liquids, test results can vary greatly from actual field performance. Bore fluids can damage the tool’s elastomeric seals. The need to achieve a bubble-tight seal for this equipment led StatoilHydro to find a test that would resemble the harsh extremes of temperature and pressure experienced at several of its North Sea operations.

According to James Douty, director of engineering, NOV TOT, “StatoilHydro wanted to use actual fluids from one of its projects in the North Sea as a testing medium for the safety head.” Based on its HP/HT gas operations in fields like Kristin and Tyrihans, StatoilHydro wanted to determine if the safety head could be used to complete all wells in a four-batch template.

Steve Deshotels, director of sales for the company, added, “StatoilHydro was also producing through the BOP stack. Submersing the rams in well fluids was adding risk by producing through elastomeric ram seals.” Testing this equipment using actual reservoir fluids was a challenging and groundbreaking event.

Accepting the challenge

The first step to testing HP/HT equipment with actual reservoir fluids was to develop a substantial safety plan. According to Perry McClanahan, a senior design specialist for the company’s Sub-Sea Systems R&D, “The vehicle driving this test was safe operations.”

Performing a simulated live gas test on elastomeric seals had never been done before, Deshotels said. “No other BOPs had passed this type of gas test. We went into an area with a gas test that had never been done before — and passed it.”

Gate valves are the normal means of gas testing because of their sealing quality. Using live fluids under extreme HP/HT conditions meant that NOV TOT had to invest almost four months into developing protocols for performing the test under the safest conditions possible.

“You don’t go into the backyard and heat up gasoline to 295° and put pressure on it without putting some precautions in place,” Deshotels said.

The company pursued this test in a multitiered approach with all of its partners. “This groundbreaking test was too expensive and too dangerous to not take the time to develop safety protocols and proper equipment to make it happen,” McClanahan said.

Setting the goal

NOV TOT and Mohr worked to develop a test plan. Det Norske Veritas (DNV) also contributed to the plan as a third-party witness. The field scenario was driven by HP/HT wells in Kristin, from which StatoilHydro provided actual condensate for the test.

“The testing goal was to see what compounds, with clean backing, would best withstand the time of exposure to an actual HP/HT environment once they were inserted into a BOP,” Douty said. While normal testing procedures call for elastomers to be tested in small batches, using live condensate for this test offered a real-world scenario that could ensure the performance and safety of the equipment once it was onsite offshore Norway.

The design of the test was meant to go beyond industry standards. StatoilHydro wanted to test the equipment over an extended period of time using “live” conditions to represent what it believed to be “real-well” conditions for its fields offshore Norway.

The classifications for HP/HT environments depend on where you stack up in the well bore. Bottomhole pressures and temperatures can increase dramatically compared to those parameters for surface equipment. NOV TOT’s safety head has to be able to handle whatever comes from the reservoir, which for the gas wells offshore Norway can approach temperatures of around 295° and pressures of up 15,000 psi, Deshotels said. “For surface equipment, we’re operating at the high end of what is classified as HP/HT.”

Building the system

In partnership with NOV TOT, Mohr designed a flow system to heat the safety head internally by flowing condensate from the Norwegian field through its center. The sample was submerged in a tank of chilled brine to simulate the extreme subsea environment.

Technicians followed a precise test protocol that included heating the safety head to 295°F with well condensate using a 40 kW circulating heater. Four positive-displacement pumps and two gas boosters were used to apply 15,000 psi bore pressure. A 30-ton chiller/pump circulating system was used to chill, fill, circulate, and evacuate brine solution around the safety head to 28°F (-2°C).

The team designed a special immersion tank because of the volatile nature of the real-world test. It measured 25 x 8 x 8 ft (7.6 x 2.4 x 2.4 m) and was sealed with blast covers per the protocols put in place. The pit was then purged with nitrogen to lower the oxygen level below 13% to abate the likelihood of condensate combustion. Backup thermostat cutoffs and low flow shutoffs also were used to minimize risks.

A new standard?

“Our safety head is the last resort on the well. It has to work when they close it,” Douty said, “Operation of the actuator has never been the issue. The seals become the critical component.”

Success of this initial test prompted NOV TOT to construct its own R&D Lab and test pit where it is currently scheduling its own “real-world” tests.

“Real-world application is the only method of obtaining a viable piece of equipment. Using actual well fluids as a test media is becoming more prominent,” McClanahan said.
Douty added, “Using water as a test media is OK. Moving on to the lab to test slabs reacting with condensate is a little step. Getting into the test pit and running tests with actual fluids flowing through the BOP makes those other kinds of tests seem secondary.”