Slimming down for the deeps

A new thinner-walled pipeline concept developed by DNV is claimed capable of significantly reducing the cost of a deep- and ultra-deepwater gas pipeline while still complying with the strictest safety and integrity regime. Meg Chesshyre reports.

Tasked with thinking outside the deepwater pipeline design box, a small team of DNV engineers headed by the class society's Rio de Janeiro office came up with X-Stream. Although based on a concept study, with basic and detailed design still needed before it can be taken to the project stage, X-Stream is already reported to be generating interest among operators and contractors.

X-Stream is based on established and field-proven technologies innovatively arranged and complies with common pipeline codes such as ISO and DNV-OS-F101. DNV is now looking to further refine and test the concept with industry support.

According to DNV chief executive Henry Madsen, X-Stream can reduce both the pipeline wall thickness and time spent on welding and installation compared to deepwater 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 2500m, the wall thickness reduction can be 25-30% compared to traditional designs, he says.

‘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,' adds Madsen, noting that 6% of DNV's annual revenues are devoted to R&D work.

By further qualifying the X-Stream concept, DNV feels confident that ‘huge financial savings can be made for long distance, deepwater gas pipelines without compromising pipeline safety and integrity', declares Madsen.

Export solutions

Quality and safety requirements for current deepwater gas pipelines require thick walls, and the number of pipe mills capable of producing them is limited. When installing pipelines, the heavy weights can be difficult to handle, the thick walls are challenging to weld and the number of vessels capable of laying them in deepwater is limited.

With new offshore oil and gas fields being developed in deeper and deeper waters, export solutions for the gas are critical. The extension of exploration activity into more remote, ultra-deep waters is inexorable. DNV believes X-Stream will offer an alternative to developments such as floating LNG plants combined with LNG shuttle tankers for such fields.

‘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,' says DNV. ‘This will of course make it easier and cheaper to manufacture and install the pipeline.'

Asle Venås, DNV's global pipeline director, explains: ‘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 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.' He adds that the HIPPS concept is proven technology, with more than 20 such systems in use worldwide, and that the i-DBB system ensures ‘a 100% internal leak tight valve'.

Simplicity and reliability are key features of X-Stream, says Venås. ‘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. As a result, 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,' adds Venås. ‘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.'

The main benefits claimed for X-Stream are listed as: