Though still a concept study, Det Norske Veritas (DNV) says its new pipeline concept X-stream significantly reduces the cost of a deep- and ultra-deepwater gas pipeline while still complying with safety regulations.
X-Stream can reduce both pipeline wall thickness and time spent on welding and installation compared to deep-water gas pipelines currently in operation. The exact reduction in wall thickness depends on the water depth, pipe diameter and actual pipeline profile. Typically, for a gas pipeline in water depths of 2500 m, the wall thickness reduction can be 25 to 30 per cent compared to traditional designs.
Current Limitations
Current deep-water gas pipelines have thick walls and, due to quality and safety requirements, the number of pipe mills capable of producing the pipe is limited. When installing pipelines, the heavy weights are difficult to handle and the thick walls are challenging to weld. And finally, the number of pipe-laying vessels for deep-water pipelines is limited too.
Further and Deeper Exploration
New offshore oil and gas fields are being developed in deeper and deeper waters and gas export solutions are critical. New exploration activities are also heading for ultra-deepwaters. The distance to shore is increasing too. The X-Stream concept can for such fields represent an alternative to for instance floating LNG plants combined with LNG shuttle tankers.
Controlling Pressures
Pipe wall thickness is reduced by controlling the pressure differential between the pipeline’s external and internal pressures. During installation, it is necessary to fully or partially flood the pipeline to control its differential pressure. During operation, the i-HIPPS (inverted High Pressure Protection System) and i-DBB (inverted Double Block and Bleed valves) systems ensure that the pipeline’s internal pressure can never drop below the collapse pressure – plus a safety margin. In sum – a certain minimum pressure will be maintained in the pipeline at all times.
“It will also be important to maintain the minimum pressure in the pipeline during pre-commissioning. This can be done using produced gas separated from the water in the pipe by a set of separation pigs and gel. This technology is not new to the industry. This method has already been initiated as standard practice by several oil companies,” says Asle Venås, DNV’s Global Pipeline Director.
Refining the Concept
A team of mainly young engineers, headed by DNV in Rio de Janeiro, Brazil, is behind the X-Stream concept. The DNV study is a concept study, and a basic and detailed design will need to be carried out before the X-Stream concept is realised on a real project. DNV intends to work further with the industry to refine and test the concept.
