Solving subsea compression with technological innovation

September 30 2014 - Drew Robb

Ending a manned mission to Mars would represent an engineering feat for the ages. But not far behind in terms of technological innovation is the subject of subsea compression. As the quest for raw materials drives oil and gas companies farther offshore into deeper and deeper waters, at some point it becomes nearly impossible to push hydrocarbons to the surface with the existing degree of natural pressure.

“Some hydrocarbons become non-recoverable due to the loss of pressure during production,” said Rick McLin, Development Manager for Turbomachinery Control at Rockwell Automation, a company involved in subsea control systems.

More recovery, less expenditure

Moving production to the seabed lowers the amount of pressure that is required to extract raw materials. McLin estimates that the use of subsea innovation could allow oil and gas companies to recover almost 20 percent more from producing fields. Additionally, capital expenditure could decrease between 30 to 40 percent, and overall operating costs could drop by nearly 50 percent due to decreased topside staff and supplies.

As subsea facilities feed directly to onshore production facilities, the need for fewer personnel and elimination of topside structures also reduces risk. Further, exploration can be expanded into smaller, deeper fields and into harsher environments, helping companies access the resources needed to meet worldwide demand.

Making subsea a reality

The technological challenges are indeed staggering. But many companies are moving ahead with innovative projects that will soon make subsea a reality. Statoil, for example, recently used an underwater welding robot to install a tie-in point on a live gas pipeline at the Åsgard field in the North Sea at a depth of 265 meters without the pipeline being prepared in advance.

"For a subsea engineer, this can be compared with landing on Mars," says Kjell Edvard Apeland, project manager of the remote-controlled development in Statoil and head of the operation on the Åsgard field. "When the compressor module and the manifold for Åsgard subsea compression are installed, we will connect the pipeline from these to the tie-in point.”

Instead of taking three months to shut down the pipeline (at high cost) to execute the weld, Statoil managed it in 10 days. Statoil is one of several companies involved in a race to be the first to land a centrifugal compressor on the ocean floor as part of oil & gas operations. Others in the race include Dresser-Rand, GE Oil & Gas, Man Diesel & Turbo, FMC Technologies, Sulzer Pumps, Cameron and Schlumberger.

It is all about exploiting ever deeper and more inaccessible oil and gas resources. Traditionally, natural gas resources use topside compressors which are expensive — the cost of a rig, its construction and deployment, daily operation, processing, ongoing maintenance, human safety and the environmental impact cost.

Compression capabilities

Subsea alternatives are a way to provide separation and compression capabilities to bypass the topside rig and deliver natural gas to onshore processing facilities via underwater pipelines. But to achieve that, the unit must be capable of continuous, trouble-free operation for many years.

“The biggest challenge is that a subsea compressor has to be maintenance free in a harsh environment,” said Klaus Brun, Program Director, Southwest Research Institute. “The casing needs to provide perfect sealing in a high-pressure, corrosive salt water environment and the process and cooling gas may contain contaminants and liquids that the compressor must be capable of handling.”

Technological challenges

Subsea compression requires three principal technologies: high-speed, direct-drive motors; magnetic bearings (with catcher bearings); and process gas recycling for motor cooling. These technologies have been commercial for several years, said Brun, but they are still not fault free. Any maintenance issue or failure of the compression system would require expensive lifting of the compressor to the surface, and in most cases these costs would be prohibitive.

The compressor would just be abandoned on the bottom of the ocean. “The only feasible compressor technology for subsea at this time is a direct-coupled, multi-stage centrifugal compressor that is hermetically sealed and driven by a high-speed electric motor using magnetic bearings,” said Amin Almasi, a turbomachinery consultant based in Australia. “There should not be a gear unit, lubrication or oil seal system, or dry gas seal system.”

Technological barriers have to be overcome, Almasi explained. On a subsea compressor, the compressed gas (often the gas after the first impeller) can be used for cooling the bearings and the driver. However, untreated or dirty gases for this job could cause problems. External cooling is one way to avoid dirty gas issues. But newer designs are finding ways to get around this challenge.

Magnetic bearings are being used to eliminate oil systems, but the relatively low-load limits of magnetic bearings pose an engineering problem. Roughly speaking, the load limit of magnetic bearings is around 50 percent of that of conventional oil-lubricated bearings.

You can read the remainder of this article in the September/October edition of Turbomachinery International