Client: Conam, Inc.
Owner: Enstar Natural Gas Co.
Cost of Work: $54,000

During a routine pipeline inspection, Alaska Pipeline Company, a subsidiary of Enstar Alaska, Inc., discovered exposure of approximately 300 feet of dual 12-inch-diameter concrete-coated gas pipelines. The pipelines were installed in 1960 in an active Cook Inlet channel near Burnt Island and provide transmission of the primary natural gas supply from the lower Cook Inlet offshore gas field to Southcentral Alaska, mainly Anchorage. Scour appears to have occurred in this area in the past, but had never before caused exposure of the pipelines, although the area settled an estimated 5 feet during the 1964 earthquake.

The pipelines are buried in unusually fine-grained soil comprised of glacial sediments and are subject to a unique flow phenomenon resulting from Cook Inlet tides. Specifically, when the tide drops below -4 feet MLLW (approximately 25 times or 6 hours annually), the incoming tide is delayed from entering this side channel. When the water begins flowing into the channel, it travels at velocities ranging from 12-16 fps. The force of the incoming rush of water results in extreme vibration of the unsupported pipelines, subjecting the exposed pipelines to large amplitudes ranging from several inches to approximately a foot, and frequencies of 40-50 cycles per minute. Each event usually lasts for less than 15 minutes but subjects the unsupported pipelines to large stresses.

Civil contractor Conam, marine contractor American Marine and PND teamed to address both onshore excavation of silts and sand by conventional methods and offshore dredging using a jet sled towed from an anchored barge. The pipelines were partially reburied and further protected by 2-cubic-yard fabric bags filled with native soil, installed over the pipelines by helicopter. Repair results appear satisfactory, and Enstar continues to monitor the site.