Air transfer remains the obvious choice when shuttling crews to and from offshore facilities. But developments in the large crew transfer vessel sector mean cost-effective alternatives are now available
Large catamaran crew boats coupled with heave-compensated gangways can provide a cost-effective and viable alternative to costly helicopter transfers for offshore workers, believes Austal global sales manager, offshore, Chris Pemberton.
He made the same point at this publication’s Asia conference last September but in an exclusive analysis for this report, Mr Pemberton has provided a detailed assessment of the two options, focusing on the Browse basin in the northwest of Australia, where both Shell’s Prelude and Inpex’s Ichthys will soon be operating.
They are within 11 nautucal miles of each other, he said, but the nearest port of Broome is 250 nautical miles away by air. “With Ichthys nearing the end of its hook-up and commissioning phase and Prelude still going through it, many hundreds of engineers are required on both vessels on a daily basis,” he said. Two other fields, Chevron’s Gorgon and Wheatstone, are also offshore North West Australia, although less remote than the two new projects.
Prelude alone can accommodate up to 340 people during its commissioning phase, but this has been augmented by the semi-submersible POSH Arcadia floatel, which has 750 beds, moored alongside. “This has made Broome’s Browse Basin Helicopter Support Base extremely busy with numerous 19-seat EC-225 and Sikorsky S-92 helicopters providing transfers many times a day,” he said.
A complicating factor in air transfers is that helicopters must take enough fuel for a round trip in case the platform landing location is unusable, which reduces their capacity to 14 people because of the extra fuel weight. “This has meant that a new airfield location has been developed to support Browse in a remote Aboriginal community on Cape Levique,” he said. Its location is called Lombadina and an intermediate stop between Broome and the offshore facilities.
Now, however, both Shell and Inpex “are actively considering what marine back-up can be put in place to provide cover,” for example if the helicopters were grounded, Mr Pemberton said.
He compares the 14 seats available in a helicopter with the 100 in an Austal-Incat Crowther 57 m large crew transfer vessel (LCTV), which is designed for 100 seats or more. The journey to Browse and back (500 nautical miles in total) will take longer – about 12.5 hours compared with about 3.3 hours of air time for the helicopter. Allowing time for refuelling in Lombadina, a helicopter can make about three journeys and carry 42 people in the same time that an LCTV will transport 100 people.
The LCTV option also costs less per person to operate, according to Mr Pemberton. The purchase price of an LCTV is about the same as a helicopter, but the LCTV costs about US$3,000/hr to operate, compared with US$10,000/hr for a helicopter. So the round-trip cost is slightly less for the helicopter, but because it carries fewer people, the cost per seat on an LCTV “is at least 50% cheaper”, he said, adding that it can also carry cargo.
Transfers to the offshore location can be done safely by fitting an Ampelmann Walk to Work (W2W) active heave-compensated gangway, which can be operated safely in significant wave heights of up to 3 m, which Shell’s data indicates is only exceeded on 2% of occasions annually.
“Technology exists with these large catamaran crew boats, coupled with the W2W gangways, to provide a cost effective and viable alternative to the costly helicopter transfers offshore,” Mr Pemberton said.