Propelling subsea innovation

Jerry Lee

June 1, 2016

Jerry Lee takes a look at Bastion Technologies’ PYRO-Accumulator, which could potentially help meet requirements laid out in BSEE’s new well control rules.

Two 7gal, 10,000psi rate PYRO-Accumulators  Photos from Bastion Technologies.

Gas charged accumulators are widely used as the hydraulic power source for subsea blowout preventers (BOPs). To compliment these conventional accumulators, Bastion Technologies has developed a PYRO-Accumulator, which uses a solid fuel source, like the booster rockets used to send vehicles into space, instead of gas.

Conventional accumulators use pressurized gas to deliver the hydraulic power used to actuate a BOP. Bastion’s technology burns a solid propellant charge made of fuel and an oxidizer, PYRO, to produce a gas that provides hydraulic power.

The PYRO-Accumulator is composed of a gas generator (pyrotechnic section), a piston, and a hydraulic section. The propellant charge and igniter are housed in the pyrotechnic section, and a discharge port is located at the end of the hydraulic section opposite the piston. To provide hydraulic power, the igniter initiates a controlled burn of the propellant. As the propellant burns, hot gas is rapidly produced, which drives the piston and compresses the hydraulic fluid held in the hydraulic section. The fluid is exhausted through the discharge port, which is connected to the BOP, to actuate the BOP.

Cut pipe from the February demonstration.

PYRO-Accumulators are capable of producing over 10,000psi of pressure, and over 100 gal/min flow rates. The propellant charge can be reloaded, has a service life of 10 years, and if greater pressure is required, more propellant can be loaded. In addition, the critical systems, including the electronic control unit, are dual string, which can achieve a reliability of 99.99% and a Safety Integrity Level of 4 (1 being lowest). Higher reliability can be achieved as more strings are added.

The technology can be used in failsafe subsea or land applications as a powersource for BOPs or operating valves.

Conventional vs PYRO

For decades, nitrogen accumulators have sufficiently functioned as a subsea hydraulic power source. However, as operators develop fields in deeper waters, the efficiency of conventional accumulators decrease; they are capable of delivering less of its hydraulic fluid capacity. As depth increases, the subsea pressure the rams need to overcome increases, and the temperature decreases.

Propellant section being loaded.

“Since conventionals are charged with gas on the surface, where temperatures may be over 100°F, when the accumulators are lowered to seabed, where temperatures can be 32°F or even lower (due to pressure of seawater), the gas cools, reducing the gas pressure available for use by as much as 20% or more,” says Nazareth Bedrossian, Bastion Technologies’ director for oil and gas. “Also, because BOP’s must be closed quickly, conventionals undergo a rapid adiabatic discharge that reduces its temperature, and the pressure of the gas available to pressurize the hydraulic fluid. In deepwater, it is not uncommon for a 15gal capacity conventional accumulator to only provide 0.5gal of usable fluid.”

In addition, because the pressure is stored in a gas, they need a hydraulic capacity that is multiple times the usable fluid volume they can deliver subsea. In order to deliver the hydraulic fluid required by the BOP, more accumulators need to be added, which increases the weight of the BOP stack.

PYRO-Accumulators, on the other hand, are designed to always deliver 100% of their hydraulic capacity regardless of depth. Because of the 100% efficiency, the PYRO-Accumulator weighs 70% less and has a smaller footprint than comparable conventional accumulators, Bedrossian says. As energy is stored in a chemical reaction it can generate 200% or more hydraulic pressure. It also provides built-in water depth compensation.

Conventional accumulators also require topside equipment to operate. Not only does this take up valuable space on the rig, but a technician is required to operate it.

The PYRO-Accumulator does not require surface equipment, umbilicals, or a subsea support system. This, and the capability for direct connection to the shear rams, removes two sources of hydraulic losses, which allows the rams to shear pipe 50% faster, Bedrossian says. Additionally, the power source is self-contained and on-demand, and only requires a 25V subsea battery to remain operable. The system can be activated by acoustic signaling, a pre-programmed pressure switch, or an electronic command for fail-safe applications or when intermittent power is needed.

RPSEA

In 2014, Bastion presented the technology to the Research Partnership to Secure Energy for America, (RPSEA), of which it is a member, for funding and was rated highly by the ultra-deepwater program advisory committee.

“Shear ram BOPs are the toughest to work, and require a lot of energy. Having the energy source at the wellhead instead of coming from the surface was something we were very interested in,” says Bill Head, ultra-deepwater program manager, RPSEA. “And, if the PYRO-Accumulator was not being used as an energy source, it could be used as a safety release for possible casing gas buildup. This thing is a safety device that needs to be developed,” he says. “It’s smaller, stronger, and can be autonomous.”

Bastion continued the development of this technology, going to Seattle, Washington, for testing earlier this year. Using two 7gal PYRO-Accumulators to actuate a 13.625in blind shear ram with 3000psi working pressure, the objective was to shear a 20.8lb/ft pipe with a 5.56in outer diameter, and .375in thickness. Within 9 seconds, the PYRO was ignited, sheared the pipe, and the rams were at 100% stroke. The test was also witnessed by three BSEE representatives.

“Now we know it will work, so we (Bastion and RPSEA) are trying to do research to make it better, more autonomous, stronger, and safer,” Head says. “We’re developing a JIP (joint industry project) with Bastion to try to get the prototype testing funded.”

Offshore applications

Many BOPs are older and have been in operation for years (although they have to be inspected once every five years per API 53). For operators interested in redundancy or automation, the PYRO-Accumulator is focused on providing enough energy for the shear rams when it is needed.

“With some modifications, it could be put on everybody’s equipment,” Head says.

Earlier this year, the BSEE released its new well control rules, three of which apply to accumulators. The long-awaited post-Macondo revisions now require a dedicated subsea accumulator with the capacity for autoshear and deadman. The PYRO-Accumulator meets these requirements because it does not require a control system, and it can be connected directly to the shear rams.

Within five years, BSEE will require the use of dual shear rams in subsea BOPs to shear currently unshearable elements, such as a tool joint. However, with the capacity to generate pressures in excess of 10,000psi, PYRO-Accumulators could address the issue of unshearable elements.

Also within five years, BSEE requires greater accumulator volumes to enable the well control system to close and hold, unassisted, against the maximum anticipated surface pressure while remaining a minimum of 200psi above the pre-charge pressure in the bottle.

“Given the inefficiencies of the conventional accumulators, meeting the new rules with conventionals would be costly both from weight and footprint perspective due to space limitations on BOPs, crane capacity, BOP and rig structural integrity,” Bedrossian says. “The efficiency of PYRO-Accumulator to supply 100% of its hydraulic capacity results in BSEE’s requirements being met, while providing a cost effective solution as it reduces weight and footprint by 70%.”

With one technology, all three rules pertaining to accumulators in BSEE’s new rules are met. The PYRO-Accumulator is one of BSEE’s STAR technologies; STAR No. 28.

“This technology is on the road to becoming a failsafe device we can rely on as an additional tool, not a substitute,” Head says. “We need more testing, and we need to get the industry to see that this is an innovative alternative, which can provide an increased safety margin and a whole lot more reliability.”