April 2012, Vol. 239 No. 4

Projects

New Deepwater Gas Pipeline Pressure Retention Concept

Det Norske Veritas (DNV) has developed a new deepwater gas pipeline concept that is designed to “significantly reduce the cost of a deep- and ultra-deepwater gas pipeline while still complying with the strictest safety and integrity regime.”

Called X-Stream, it is said to be able to reduce both the pipe wall thickness and time spent on welding and installation compared to deepwater gas pipelines currently in operation.

The company said the exact reduction in the wall thickness depends on the water depth, pipe diameter and actual pipeline profile. Typically, for a gas pipeline in water depths of 2,500 m (8,202 feet), the wall thickness reduction can be 25-30% compared to traditional designs.

Said Dr. Henrik O. Madsen, DNV’s CEO who announced the news at a press briefing in London, “It’s essential for DNV that the new concept meets the strict requirements of the existing safety and integrity regime.”

Madsen added, “DNV has been instrumental in developing and upgrading the safety and integrity regime and standards for offshore pipelines over the past decades. Today, more than 65% of the world’s offshore pipelines are designed and installed to DNV’s offshore pipeline standard. As the deepwater gas transportation market will experience massive investments and considerable growth over the coming years, new safe and cost-efficient solutions are needed.”

Current deepwater 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. The number of pipe-laying vessels for deepwater pipelines is limited.

New offshore oil and gas fields are being developed in deeper waters and export solutions for the gas are critical. New exploration activities are also heading for ultra-deep waters. The distance to shore is increasing. For such fields, the X-Stream concept can represent an alternative to, e.g., floating LNG plants combined with LNG shuttle tankers.

Concept
By controlling the pressure differential between the pipeline’s external and internal pressures at all times, the amount of steel and thickness of the pipe wall can be reduced by as much as 25-30% – or even more – compared to today’s practice and depending on the actual project and its parameters.

Explains Asle Venås, DNV’s Global Pipeline Director, “By using 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 (internal) pressure starts to fall. The internal pipeline pressure is maintained above a critical level for any length of time.”

The new concept is described as simple and reliable. 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 the collapse pressure – plus a safety margin. In sum – a certain minimum pressure will be maintained in the pipeline at all times.

Says Venås, “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.”

DNV says a team of skilled engineers, headed by DNV in Rio de Janeiro, Brazil, is behind the X-Stream concept. As with the other DNV concepts launched in 2010 and 2011, the X-Stream team was asked to think outside the box.

Detailed Design Still To Come
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 realized on a real project. DNV intends to work further with the industry to refine and test the concept.

Concludes Madsen, “I’m pleased to announce the outcome of this innovation project. At DNV, we feel confident that, by further qualifying the X-Stream concept, huge financial savings can be made for long distance, deepwater gas pipelines without compromising pipeline safety and integrity.” www.dnv.co.uk