With new fields regularly being developed in ultra-deep waters and increasingly remote locations, long-distance export solutions for natural gas that cannot be flared are becoming project-critical. A new concept has been unveiled that could dramatically cut pipeline construction and installation costs while also representing a viable economic alternative to existing transportation options such as floating LNG units.

According to DNV, its deepwater gas pipeline concept could enable huge savings on the costs of constructing and installing such lines by reducing the amount and thickness of steel used without compromising pipeline safety and integrity.

The X-Stream concept allows this reduction in pipeline wall thickness by controlling the differential pressure, therefore reducing the time currently spent on difficult welding and installation work carried out on deepwater gas pipelines. It also uses only established and field-proven technologies.

State-of-the-art gas pipelines for deepwater and remote locations generally have large diameters and high external hydrostatic pressures that require thick walls to prevent them from collapsing. This wall thickness, according to DNV, makes it extremely difficult to produce, with only a limited number of pipe mills capable and available to do such work due to quality and safety requirements. The heavy weight of these lines also makes their handling and logistics difficult, while the thick pipe walls are difficult to weld.

The pipes also require extremely thick and costly buckle arrestors, and by their nature they can only be laid by a limited number of pipelay vessels with prohibitive day rates. For example, for very large diameter pipelines (around 32 in. to 36 in.), the cost is estimated at more than US $1 million/day.

Brainstorming session

Last year DNV — with more than 65% of the world's offshore pipelines designed and installed to its classification standards –gathered its young engineering talent for a brainstorming session in Rio de Janeiro, Brazil, specifically to tackle this kind of challenge. The result was the X-Stream concept, designed particularly with booming areas such as Brazil's presalt province, West Africa, the US Gulf of Mexico, and Western Australia in mind.

DNV is a global provider of knowledge for identifying, assessing, and managing risk, not an engineering or construction house. As a result, it maintains the study and solution are purely conceptual at this stage and will need to go through basic and detailed design processes before the concept can be undertaken on a real project.

But the company is introducing the idea within the industry domain to spark feedback from both pipeline contractors and oil and gas operators with the intention of potentially working further in partnership with interested parties to refine it, test it, and eventually hand over full control to them.

The exact reduction in the wall thickness of deepwater pipelines will depend on the water depth, pipe diameter, and actual pipeline profile.

Controlling the pressure differential

The crucial aspect of DNV's concept is to control the pressure differential between the inside and outside of such pipelines at all times. By controlling this pressure differential, the amount of steel and thickness of the pipe wall can be cut by 25% to 30% and possibly more. This would make it easier and cheaper to manufacture and install a typical pipeline in approximately 2,500 m (8,202 ft) water depth.

The concept also is reliant on an innovative slant on the established high-pressure protection system (HIPPS), of which there are already more than 20 in use worldwide.

Asle Venas, DNV Global Pipeline director, said, "By utilizing an inverted high-pressure protection system (i-HIPPs) and inverted double block and bleed valves (i-DBB), the system immediately and effectively isolates the deepwater pipe if the pressure starts to fall. In this way, the internal pipeline pressure is maintained above a critical level for any length of time."

During installation, it is necessary to fully or partially flood the pipeline to control its differential pressure. During operation, the i-HIPPS and i-DBB systems ensure that the pipeline's internal pressure can never drop below a safety margin higher than the collapse pressure.

The DNV X-Stream solution maintains and controls the differential pressure using multiple i-HIPPS installations. The main i-HIPPS would be installed above surface at each end of the pipeline, while two further secondary i-HIPPS would be installed on the seabed on either side of the main collapse-critical stretch of line. According to the company, this could be anywhere up to 1,200 km (746 miles) in length, although the concept is focused primarily on a 400-km (250-mile) case study. (Image courtesy of DNV)

There are several other benefits if the X-Stream concept becomes reality, according to DNV CEO Henrik O. Madsen, costs would be saved through the ability for operators to choose more competitively from a wider range of pipelay vessels, while there also would be a resulting rise in lay rates and a quicker welding process.

The competition element also would be improved because operators would have a wider choice of pipe mills capable of producing the deepwater pipe. Other bonuses include not having to use extremely thick and costly buckle arrestors in some cases, he said.

Madsen outlined the thinking behind DNV's initiative. "Deepwater oil and gas developments today are not allowed to flare, so we need to find a gas solution. Floating LNG is one solution, but it's still a relatively new solution. So we have come up with some new ideas, and we want to challenge companies with these. This is the issue we challenged our young engineers in Brazil with, to find a solution to bring presalt gas to shore. We hope that designers in this industry will take this idea and see if they think it is a good one."