TPi April 2012

products & developments

Deep-water concept to reduce cost but not safety

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 utilising an inverted High Pressure Protection System – i-HIPPS – and inverted Double Block and Bleed valves – i-DBB – the system immediately and effectively isolates the deep-water pipe if the pressure starts to fall,” explained Asle Venås, DNV’s global pipeline director. “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 the collapse pressure – plus a safety margin. 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,” said Mr Venås. 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 utilised on a real project. DNV intends to work further with the industry to refine and test the concept.

“It’s essential for DNV that the new concept meets the strict requirements of the existing safety and integrity regime, and I’m pleased to confirm that this concept does,” said Dr Henrik O Madsen, DNV’s CEO. “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 deep-water gas transportation market will experience massive investments and considerable growth over the coming years, new safe and cost- efficient solutions are needed.” 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. The number of pipe-laying vessels for deep-water pipelines is also limited. New offshore oil and gas fields are being developed in deeper and deeper waters and export solutions for the gas are critical. New exploration activities are also heading for ultra-deepwaters. The distance to shore is also increasing. For such fields, the X-Stream concept can represent an alternative to floating LNG plants combined with LNG shuttle tankers.

DNV has developed a new pipeline concept, called X-Stream, that it claims can significantly reduce the cost of a deep- and ultra-deepwater gas pipeline while still complying with the strictest safety and integrity regime. X-Stream is based on established and field- proven technologies that have been innovatively arranged. X-Stream can reduce both the pipeline wall thickness and time spent on welding and installation compared to deep-water gas pipelines currently in operation. 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 2,500m, the wall thickness reduction can be 25 to 30% compared to traditional designs.

dnV – UK www.dnv.co.uk

Dr Henrik O Madsen, DNV’s CEO

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