Positive energy – using seabed ambient pressure to generate hydraulic power

By Michael Rogers

OBS Technology lab’s test bed to demonstrate the depth compensation function of a DGP accumulator prototype – from left, Lorents Gran, Technical Manager; Odd Rønning, R&D Senior Engineer; and Kåre Tyldum, R&D Design Engineer
OBS Technology lab’s test bed to demonstrate the depth compensation function of a DGP accumulator prototype – from left, Lorents Gran, Technical Manager; Odd Rønning, R&D Senior Engineer; and Kåre Tyldum, R&D Design Engineer (photo: OBS)

Innovating to meet the challenges of subsea operations is in OBS Technology’s DNA. Based in Husøysund, Norway, south of Oslo, the company has provided innovative technologies to the oil and gas industry for more than 30 years.

Beginning in the mid 1980s, OBS designed and manufactured breathing systems for the deep-water divers who installed and maintained offshore oil facilities. By the early 1990s, OBS had developed and certified a conditioned gas breathing system that would allow divers to work at 400-metre depths.

The early 1990s also marked a shift to using remotely operated vehicles (ROVs) to work with the increasing amount of equipment that was being moved from platform topsides to the seabed. As ROVs enabled oil and gas development in deeper waters, displacing the need for diving, OBS applied lessons learned from working with breathing pressure regulators and flow controllers, evolving to become a developer of subsea system support components designed to meet the extreme requirements for reliability.

The latest OBS innovation, Depth Generated Power (DGP), takes this expertise one step further, to make use of the ambient pressure at the seabed to produce hydraulic power.

With funding from Oslofjordfondet and Innovation Norway, OBS has been working with TechnipFMC for several years during the development of the DGP concept. To find out more about the design, Energy Northern Perspective spoke with Nils Terje Ottestad, founder of OBS Technology, who now serves as the company’s R&D Manager. A nuclear physicist, Ottestad’s early work included ensuring health and safety in a number of disciplines, including scientific diving, which paved the way to his launch of OBS.

Discussing the application of DGP with Brede Thorkildsen,Chief Engineer R&D and a 17-year veteran at TechnipFMC, revealed that company’s perspective. Thorkildsen, with a background in Mechanical Engineering and Process Design, has spent his entire career developing technology for turbines and pumps as well as for subsea systems and intervention.

Subsea power
Subsea mechanical operations rely on hydraulic power. Currently, this hydraulic energy is stored in accumulators employing compressed gas. At sea level ambient pressure, the accumulator solution works well, but the compressibility of gas means that in subsea applications, the greater the depth, the larger the accumulator needs to be to compensate for the higher pressure at the seabed.

Three chambers – Depth Generated Power
Three chambers – Depth Generated Power (illustration: OBS)

“We thought it must be possible to generate power at deeper depths in a different way,” says Ottestad. “Then we had the idea that the only thing that increases when you go down is the ambient pressure. So, if we create a zero-pressure chamber – a void – we then have the possibility to generate power by using the difference between the ambient subsea water pressure and the void.”

Fixed pressure at any depth
According to Ottestad, the DGP accumulator is basically a hydraulic cylinder with a piston that divides the internal volume into three separate chambers. “The middle chamber – what we call ‘VOID’ – contains neither gas nor liquid. A near vacuum, for practical purposes this chamber is considered as pressure free,” he says.

On one side of the piston, the “energy transfer chamber” contains liquid at ambient pressure that is regulated by means of a depth compensating valve. The opposite side of the piston, the “HP chamber”, interfaces with the hydraulic system.

“The liquid at ambient pressure in the energy transfer chamber generates force against the pressure sensing area of the piston, and because the VOID chamber is pressure free, all the generated force is transferred to the hydraulic fluid stored in the HP chamber,” Ottestad explains. “And because DGP accumulators are gas free, hydraulic power is applied very quickly.”

In addition, the depth compensating valve allows the DGP to generate a fixed hydraulic pressure once the unit reaches a specific depth, and the pressure remains fixed, even as the unit is lowered deeper. Therefore, a DGP accumulator can provide roughly four times more hydraulic power than an equivalent weight traditional piston accumulator at 400 meters water depth – a trait that improves as the depth is increased.

Ottestad continues, “the basic DGP concept was outlined in 2012 and has been considerably improved since then. The depth compensating ability of the current DGP accumulators is probably the most important improvement. In the current state of development, the DGP accumulators provide very advantageous performance in relation to traditional, compressed gas accumulators.”

“We are now in the process of preparing a program for having the first type of DGP accumulators approved for operational use,” adds Ottestad.

Subsea application
TechnipFMC’s Brede Thorkildsen says that DGP will find its place within his company’s entire range of subsea applications, from production systems to intervention equipment. “DGP has numerous applications. Wherever there is a need for subsea storage of hydraulic power, it can be used to our advantage – for deepwater applications in particular.”

From left, Nils Terje Ottestad and Brede Thorkildsen
From left, Nils Terje Ottestad and Brede Thorkildsen (photo: OBS)

“For TechnipFMC, the DGP enables us to get more out of existing hydraulic systems,” Thorkildsen continues, “The DGP can effectively extend capabilities of current system solutions, and as such, this technology enables better use of ‘our technology portfolio’.”

Not only does the DGP’s smaller size make it less costly and more competitive, the hydraulic power generator is safer by design. Thorkildsen explains, “It’s safer equipment to operate as there is no high-pressure gas, which also leads to improved reliability, as there are no associated issues from gas leakage.”

When asked about how TechnipFMC might use the DGP in existing fields, Thorkildsen says, “Remember that one of the largest markets that is growing every year is the installed base,” describing a possible scenario:

“Imagine that you have an existing field and you want to add one or two Xmas trees. The umbilical system with the hydraulic power supply may not be able to cope with more trees. So, in that case, you could add the DGP system locally and use electric power from the control system to provide the energy needed. Therefore, the DGP can be connected with the hydraulic system for the needed fluid and then it provides the required pressure.”

The future
Thorkildsen envisions a bright future for DGP: “Looking forward, there will be a lot of the existing, ‘legacy’ products, which are qualified subsea today – and they will need hydraulics.”

“A qualified DGP solution should become a standard feature for most subsea systems for improved capabilities: So, in the world of ‘core hydraulic products’ such as pressure control valves, accumulators, etc. the DGP technology can take its rightful place to enhance these systems,” he adds.