Giant Air Lift Bags ease FPSO decommissioning

JANUARY 29, 2014 — Cowes, U.K., headquartered Unique Seaflex has facilitated a complex underwater lift project in some 300 m of water — using parachute style Air Lift Bags.

The project was carried out for Lundin Tunisia BV, operator of the Oudna field 80 km offshore Tunisia.

Abandoned by Shell in the mid-80s when relatively small discoveries in relatively deep water were still considered unviable due to lack of pipeline infrastructure, Oudna was demothballed a decade or so with the advent of  FPSO technology and serviced until recently by the FPSO Ikdam.

The end of the field’s viable lifecycle presented the problem of how best to decommission the FPSO — in particular, how to take care of disconnecting several hundred tons’ worth of moorings without incident.

After a series of technical 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 t in size would be installed on the moorings to reduce their tension to a manageable level.  

The deck-mounted linear winch was rated to 450 t, but because the three mooring anchors were extremely well embedded in the sandy seabed, the effective tension on the chafe chain would have increased by an estimated 1.5 to 2 times the original 400 t.  

The use of Unique Seaflex ALBs allowed the deck-mounted linear winch to pull on the chafe chain so as 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 thus allowing the mooring system to gently be lowered on a wire, thereby de-tensioning the system. Once released, the chafe chain was transferred to a nearby AHTV which held the de-tensioned mooring system whilst the Ikdam FPSO disconnected.

Along with the fleet of ALBs, Unique Seaflex also supplied all 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.

Graham Brading, Managing Director of Unique Seaflex who was instrumental in leading the Unique Seaflex effort on the project  said, “Whilst we do supply standard buoyancy products to solve standard buoyancy problems, Unique Seaflex is always dedicated to bring out world-class solutions when clients describe their operational challenges and ask us to work with them to develop methods which will deliver them the best possible technical, operational and commercial outcome.”

Unique Seaflex manufactures air-filled bags for salvage and installation operations and water-filled bags for testing or ballasting purposes. Acquired by Unique Maritime Group of Sharjah, UAE in 2011, Seaflex has a presence and a stock-position on every continent except Antarctica – including most of the main oil and gas markets around the world.