FABRICATION/ASSEMBLY METHODS: Giant floatovers mate Bayu-Undan decks to steel jackets in Timor Gap
Shifting from concrete to steel
The Bayu-Undan installation could be an important reference project for future platform installations.
Aker Marine Contractors' (AMC) contract for the floatover installation of two decks for Phillips Petroleum (91-12) Pty Ltd's Bayu-Undan project in the Timor Gap Zone of Cooperation will draw on the company's experience in the North Sea, where its predecessor, Norwegian Contractors, was responsible for mating the topsides and sub-structures of most of the concrete platforms in the Norwegian sector. All of the offshore, fixed gravity-based platform installations and floating storage and offloading (FSO) vessels contribute to this relevant experience.
The floatover contract was earlier this year awarded to a joint venture between AMC and Clough Engineering. The floatover method means that each of the two decks, which measure some 65 meters by 55 meters, and weigh approximately 14,000 and 11,500 tons, can be installed in a single operation. Each deck will be carried to the field and installed by Offshore Heavy Transport Management (OHTM), which has been subcontracted to perform this task using one of its new 50,000-ton heavy-lift sisterships, Black Marlin or Blue Marlin. These floatover operations will be the world's largest of their kind.
The contract was awarded in the early stages of the project as the Unit operator's decision to use this method of installation must be taken into account in the jacket design, with clearance being allowed for the vessel to enter between the legs and ballast down. There will also be an important interface between AMC/Clough, the Unit operator, and TIGA, through the detail design phase and the supply contractors.
Concrete to steel
Though AMC has experience in floatover installations, the Bayu-Undan assignment is different in that the substructures are steel jackets. But, more importantly, says company President Anne M
To hold the heavy-lift vessel steady during the load transfer operation, mooring lines will be attached to the jacket legs. But because the operation is taking place in the open sea, two tugs attached to anchors will also be connected to the vessel to ensure control over its movements, M
The operation of docking the vessel and installing the decks is expected to take four to six hours. Weather conditions at Bayu-Undan are normally mild. Given the planned orientation of the platforms, north-south-north, a head or quartering sea would usually be expected, but if there is a beam sea, the threshold for postponing the operation would be less than one meter wave height. Model tests of the operation were successfully carried out at the Marintek Institute in Norway last June.
The Bayu-Undan installation could be an important reference project for future platform installations in benign climate regions. In the past, the floatover method has rarely been used for installing topsides on steel jackets. But given the limited heavy-lift crane-barge resources in regions such as Australasia and Southeast Asia, it offers an attractive cost-saving option to install a fully integrated deck with less hookup work prior to completion.
AMC/Clough is bidding for a second such assignment in the region, in conjunction with a supplier, and other projects are under consideration. AMC also has a joint venture with OHTM which could be of significance for future projects.