Foothills Pipe Lines Ltd., Calgary, was scheduled last week to reduce pressure in its gas pipeline system to excavate pipe at the site of line rupture in Saskatchewan.
The aim is to check for further evidence of the cause of the rupture, currently blamed on a rare occurrence of hydrogen induced cracking. The unusual chemical reaction caused a major rupture and fire last February that temporarily disrupted natural gas exports to the U.S. through part of Foothills' Canadian "prebuild" segment of the Alaska Natural Gas Transportation System.
The accident occurred near Maple Creek, Sask., in the system that carries about 13% of gas exports shipped by Nova Corp., also of Calgary.
Foothills' line pressure will be reduced to about 70% of normal for 4 or 5 days this month to check other parts of the system.
Cause of the rupture was investigated by metallurgical consultants and the Canadian Transportation and Safety Board.
A report to Ottawa by Foothills said a number of factors must be present to cause the chemical reaction. They include a source of hydrogen, a hole in the pipe's external coating, and a poisoning agent or block that forces hydrogen atoms into the steel pipe, forming cracks and making the pipe brittle.
The Foothills line is buried below the water table, and there was no oxygen to prevent hydrogen buildup.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
