PWHT Systems for Record-Breaking Gas Pipeline
“But,” said Antonio Segadelli, the welding and technological department manager at oil and gas contractor Bonatti, “we also have to remember that the gas in this project is highly corrosive. In fact, up to 10% is made up of the worst possible combination of carbon dioxide, sulphurs and hydrogen sulphate, making the impossible even tougher to achieve.”
The project mentioned by Bonatti’s Segadelli is the In Salah Southern Fields project (ISSF) in the southern Sahara in Algeria. The ISSF is phase two of the larger In Salah Gas (ISG) project, which first came on-stream in July 2004. A joint venture between Statoil, BP and Algerian state-owned oil and gas company Sonatrach, ISG is Algeria’s third-largest gas development with a current annual production capacity of around 9 billion cubic meters.
In the first half of 2011, the ISG joint venture signed a $1.12 billion engineering, procurement and construction contract with oil and gas service company Petrofac to develop the four fields in the southern sector. Under the contract, Petrofac will construct facilities such as well pads, manifolds, flow lines and a new central processing facility (CPF) with a gas processing capacity of 17 million cubic meters per day. The CPF will be constructed north of In Salah and tied back to the existing production facilities located in Reg for further transport of the gas to Krechba CPF for carbon dioxide removal and gas export. When completed, the pipeline network will be the world’s longest onshore pipeline made from 13% Cr steel.
Bonatti, the Italy-based oil and gas contractor, was called in to construct pipelines as part of the expansion of the four southern fields.
“We were of course thrilled to be involved,” said Alessandro Bonaschi, a Bonatti welding specialist. “But at the same time we were aware of the complexity and challenges of this project. For example, most people know that the Sahara is very hot. What is less well known is that it is also very cold – temperatures can plunge from 40°C (104°F) to below freezing in one day. There are other extreme weather conditions. Intense sand storms regularly sweep the region, incapacitating all but the toughest men and machines.”
Another major challenge faced by Bonatti was the choice of materials for the pipelines to be constructed in the southern fields.
“The pipelines are made from 13% Cr and duplex steel,” Segadelli said. “These materials offer two great advantages. First, superior strength means reduced wall thickness, which in turn means less weight. Second, duplex is extremely corrosion-resistant, a characteristic that is critical when in sour gas and oilfield environments.”
There is, however, a drawback to using exotic metals such as 13% Cr and duplex – their welding and post-weld heat treatment (PWHT).
“Indeed, joining these heterogeneous materials is already an advanced task,” Segadelli said. “But we must apply in-situ post-weld heat treatment (PWHT) in order to achieve our desired metallurgical microstructure in the heat-affected zones. Luckily, EFD Induction has experience in just this area.”
As Stefano Migliavacca of EFD Induction explained, this experience was based on devising a mobile PWHT system for another In Salah contractor. “Our colleagues at EFD Induction in the U.K. developed three truck-mounted PWHT units that have now been operating in the tough Algerian environment for months.”
According to Migliavacca, the Bonatti project is significant for two main reasons. “Obviously, to be selected by such a prestigious company as Bonatti confirms our ability to satisfy the most stringent criteria. The project also underscores the industry-wide acceptance of our PWHT solution for corrosion-resistant alloys.”
The Bonatti order involves two complete PWHT systems for on-site operation in the Saharan desert. Each system is truck-mounted and comprises an EFD Induction Minac 70/110 power source, specially designed induction coils for 8- to 12-inch and 16-inch pipe, and chiller and quench units. The 13% Cr and duplex flow lines must be heat treated after welding in order to minimize the risk of hydrogen-induced stress cracking.
“Such cracking is caused by the presence of hydrogen in the pipe’s heat-affected zone,” Migliavacca said. “Although precautions are taken during the fabrication and welding stages to minimize hydrogen content, PWHT is still deemed the best way to achieve safe levels.”