April 2014, Vol. 241 No. 4

Features

FPSO Decommissioned Off Tunisia

Unique Seaflex, a Unique Maritime Group, recently carried out a complex underwater lift project in 300 meters of water off Tunisia for Lundin Tunisia BV in the Oudna field. Lundin Tunisia BV is a 40% stakeholder in and operator of the Oudna field located 80 km off Tunisia.

Although abandoned by Shell in the mid-1980s when relatively small discoveries in deepwater were considered unviable due to the lack of pipeline infrastructure, the advent of the floating production storage and offshore (FPSO) vessel has made such fields viable.

Oudna field was de-mothballed a decade or so ago and serviced until recently by the Ikdam FPSO.

At the end of the field’s viable lifecycle, a problem arose on how to decommission the FPSO, and in particular, how to handle the delicate operation of disconnecting several hundred tons of moorings without incident.

After a series of discussions between Lundin and Unique Seaflex, it was determined that a combination of 41 different-sized parachute-style Air Lift Bags (ALBs) up to 50 tons in size would be installed on the moorings to reduce their tension to a manageable level. The deck-mounted linear winch for the job was rated to 450 tons, but because the three mooring anchors were extremely embedded in the sandy seabed, the effective tension on the chafe chain would have increased by an estimated 1.5-2 times the original 400 tons.

The use of Unique Seaflex’s ALBs allowed the deck-mounted linear winch to pull on the chafe chain to slacken the connection to the Ikdam deck hook. The connecting shaft was then pulled out from the wren hook and by doing so, freed the chafe chain, allowing the mooring system to be lowered on a wire, thereby de-tensioning the system. Once released, the chafe chain was transferred to a nearby anchor-handling towing vessel which held the de-tensioned mooring system while the Ikdam FPSO disconnected.

Along with the fleet of ALBs, Unique Seaflex supplied the heavy-duty shackles and lifting slings as well as an air inflation/deflation system and a topside technician to oversee the operation and ensure the safest and most efficient use of the package of equipment.