Offshore lift ship is said to be ‘world’s largest’
Dag Pike describes the recently completed ‘Pieter Schelte’ which is arguably the most sophisticated ship ever built as well as being one of the most versatile.
With oil exploration companies now under an obligation to remove offshore structures down to the seabed, it was recognised that if the structures could be removed in large sections there would be a considerable saving in the work required offshore thus reducing costs and delays due to weather. Removal of platforms in two sections, topsides and jacket, would also reduce the risk of pollution and allow dismantling and disposal to be carried under controlled conditions ashore.
The original catamaran concept was based on linking two VLCCs together with the recovery system operating between the two hulls. Later, Excalibur Engineering was formed in 1999 to further research into a concept based on a semi-catamaran with the recovery system operating on one side. By 2004 the focus was on a new construction design based on a vessel 360m in length using portal frames for the lift. Three years later the system using lifting beams had been developed and the following year the design had been lengthened to 382m and the catamaran design included a forward section that could handle the removal of topside units in one piece.
In June 2010 the contract for this huge vessel was placed with South Korean shipbuilder Daewoo following the completion of the detail design work by Deltamarin of Finland. The proposed vessel would have a hull length of 382m and a beam of 124m with the length increasing to 477m when the tilting lift beams and the pipe-laying stinger were included. The vessel would have the capacity to handle a topsides lift of 48,000 tonnes as a single unit and a jacket lift capacity of 25,000 tonnes.
The two hulls were built separately in dry dock and after float-out they were connected afloat. The two hulls are separated by a slot at the bow that is 59m wide and 122m long. This size was selected after a detailed study of the existing platforms and the likely size of new platforms that might be required to be installed in the future. The vessel will be able to cope with the installation and removal of around 95% of the existing topside structures in the North Sea utilising the eight sets of lifting beams that can span the bow slot.
At the stern the focus is on the installation and removal of conventional steel jackets and here the tilting beam system can also be used for regular crane lifts such as for the installation and removal of modules, bridges etc. Pieter Schelte is equipped for pipe laying operations with a 170m long stinger (210m including the transition frame) that can be installed between the two hulls at the bow. Pipes up to 68in (1.73m) in diameter can be handled and are welded in 12m lengths with storage space on deck for 27,000 tonnes of pipe and four 500 tonne tensioners. When not in use the stinger can be transferred to a cargo barge.
The vessel is equipped with eight MAN diesel generators providing a total installed power of 95,000kW. The propulsive power is transferred through 12 Rolls-Royce azimuth thrusters which give a maximum speed of 14 knots and which are also used for the fully redundant Kongsberg DP3 dynamic positioning system. Accommodation is provided for 571 people in the offset central superstructure which features a helideck and lifeboats at the main deck level.
On deck there are three pipe transfer cranes of 50 tonnes capacity and a special purpose crane of 600 tonnes capacity. The complex pipe welding station features a double joint factory with five line-up stations, two stations for internal and external welding, six stations for double joints, one non-destructive testing station and six coating stations.
The large scale of the Pieter Schelte gives the vessel a good wave response behaviour, which it is claimed will be better than semi-submersible crane vessels in operational wave conditions. Topsides and jackets will be able to be installed and removed in significant wave heights of up to 3.5m, which is a significant advance over current capabilities and which will maximise the employment of the vessel in harsh conditions such as the North Sea.
The topside removal operation involves the ship moving around the platform and positioning the platform in the slot at the bow. The sets of lifting beams are then positioned under the – previously cut – platform legs and attached using a system of friction clamps. When all clamps are connected, the lift pressure is gradually increased through de-ballasting to transfer the weight of the topsides from the jacket to the vessel. When the legs are finally released, the actual lift is done with a 2m rapid lift-off to eliminate the risk of impact between the topsides and the jacket. To eliminate impact forces on large topsides, due to the enormous weights involved, a motion compensation system is installed.
By removing the topside in one unit the risk of any contamination and the time involved is considerably reduced. This allows for full dismantling ashore under controlled conditions and it would also allow for the of the topsides unit to be refurbished ready for re-use if required.
For jacket removal, the vessel is backed up to the jacket with the aft lifting beams angled outwards at the top. Lifting wires are attached to prepared lift points on the top corners of the jacket and the jacket is lifted vertically to a pre-determined height and then secured and tilted towards the deck. Finally, the jacket is skidded horizontally further inboard and sea-fastened. .
A special barge is used to transfer both jackets and topside units to the shore. This barge is fitted with hydraulic supports that run on rails and these can take the weight of the units as they are transferred onto the barge and then once at the landing point on shore the rails allow the units to be transferred directly to a matching set of rails on land. This allows the transfer of topside and jacket units to be loaded or landed from relatively shallow shore locations.
The whole range of operations of these systems has yet to be tested out on the full scale ship. The initial trials of the vessel’s systems have taken place but the stern lifting beams have not been installed. The Pieter Schelte will finally leave the Daewoo Shipyard at the end of 2014, and the vessel is scheduled to become fully operation early in 2015 with first operations likely to take place in the North Sea.
The development of Pieter Schelte represents a huge investment, with the vessel’s cost estimated in the region of €2.4 billion, making it not only the world’s largest vessel but also the most expensive. It has taken years of planning and development to reach this stage and if everything works as planned it could lead to a significant improvement in the installation and removal of offshore platforms, reducing the risks involved and speeding up operations that have to take place offshore. Much of the initial focus of the operations of Pieter Schelte will be in the North Sea but the vessel is designed to work worldwide with the hull ice strengthened for Arctic operations.
Swiss-based owner Allseas is already looking to the future with plans well advanced to build a second vessel of similar type, but which will be even larger. This ship would be able to handle the installation and removal of virtually all of the existing platforms worldwide.
PRINCIPAL PARTICULARS – PIETER SCHELTE
Length oa: 382m
Total power: 95MW
Speed: 14 knots
Accommodation: 571 persons
Class: Lloyd’s Register
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