For the first time a topside deck has been installed onto a pre-installed jacket by using a semisubmersible heavy transport vessel.

Recently the Mighty Servant 3 successfully installed the AMP1 fully integrated topside for TotalFinaElf onto a jacket in the Amenam Kpono field, offshore Port Harcourt, Nigeria.

In October 2000, Dockwise Shipping in Breda, the Netherlands, which operates 14 semisubmersible heavy transport vessels, was awarded a contract by Upstream Constructors International FZCO of Jebel Ali, in the United Arab Emirates (UAE) for transportation and installation of the TotalFinaElf AMP1 topside. Transportation included a skid-on loading operation at J. Ray McDermott's Middle East construction yard in Jebel Ali and subsequently transportation - via the Cape of Good Hope - to offshore Nigeria. The installation comprised a float-over and mating operation using a combination of the carrier's ballast system and the so-called 'UniDeck concept.'

The AMP1 topside has a weight of 11,200 tons, a length of 246 ft (75 m), width of 229 ft (70 m) and a height of 106 ft (32.5 m).

Feasibility

Transport feasibility studies commenced in 1999 by Dockwise in close consultation with clients Upstream Constructors International FZCO, Technip and SaiBos.

Our engineers were particularly involved in the following aspects:

• checking vessel's strength based on the skid beam and grillage design of the client;
• design, purchase, logistics and operation of the mooring and associated equipment;
• detailed ballasting procedures for both the load-out and installation.

The 27,720 tons deadweight tonnes (dwt) heavy transport vessel Mighty Servant 3 loaded the AMP1 topside at McDermott's construction yard in Jebel Ali on 25 January 2003 by skidding-on over the stern. To facilitate this operation the vessel's removable buoyancy casings were positioned in the forward recesses while two longitudinal skid beams were installed on deck from the stern to the forward part of the main deck.

The topside, supported by eight skid shoes, was pulled on board the vessel using strand-jacks. Once the AMP1 topside had reached the final stowage position on board the vessel, it was lowered onto a steel grillage and the skid equipment located aft of this grillage was removed.

Even before the topside was loaded, preparations had already started for the discharging operation. On four double drum winches, 2.56 miles (4.1 km) of 48 mm steel wire was installed on the main deck for the float-over and mating operation. In addition, two casing winches with reeved wires were also used for the mooring system. A total of 20 deck sheaves, four multi-sheave blocks and two balanced head fairleads were installed to guide the wires from the winch drum to their respective connection points in the final mooring arrangement. Eight tension measuring pins integrated in the deck sheaves provided line load information during the discharging operation. Two 600 kVA generators were installed to provide uninterrupted power supply with 100% redundancy to the installation jacks of the UniDeck concept. Apart from the vessel lighting system, an additional 24 floodlights were installed for possible discharging operations during night-time.

Apart from protection guards, platforms and steps over the winch wires, a specially designed and built boarding ladder, as well as two additional life rafts were installed for safety.

After a voyage of only 23 days covering 7,000 nautical miles (12,964 km) via the Cape of Good Hope, the Mighty Servant 3 arrived at the discharge location offshore Port Harcourt, Nigeria.

Float-over and mating

Upon arrival at the Amenam Kpono field location, environmental measuring instruments and monitoring systems were installed on board the transport vessel. All equipment, including mooring winches and jacking system, was prepared, checked and made ready for the discharging operation. Once the installation of the jacket was completed, the ship was positioned in front of the jacket and connected to four pre-installed anchors. Four approach/mooring wires were connected between the vessel and the jacket, ready for the float-over and mating operation.

The installation operation was based on the UniDeck concept, developed by Technip. This concept uses a jacking system that is able to lift the entire topside and can be lowered rapidly in order to minimize impact between topside and jacket. Eight pairs of hydraulic jacks lifted the AMP1 topside while the ballast system of the Mighty Servant 3 was used to match the required vertical position of vessel and topside in relation to the jacket.

Once environmental and operational conditions were acceptable the vessel maneuvered inside the jacket using bow anchor winches and mooring winches on deck and propellers on stand-by. Less than 8 in. of spacing remained between the ship and jacket fender on both sides. At the moment the vessel and topside reached the correct position inside the jacket, ballasting of the vessel reduced the clearance between the AMP1 topside shock absorbers and the jacket stabbing cones. UniDeck jacks were then used to lower the raised topside 70.86 in. 1 minute transferring about 7,000 tons of topside weight onto the jacket. The remainder of the topside weight was still supported by the Mighty Servant 3. Ballasting of the ship continued while the UniDeck jacks were extended without changing the load on ship or jacket.
A second load transfer resulted in the topside being supported completely by the jacket and a clearance being created between the ship and the AMP1 deck. The information of the ballast panel of the ship was closely monitored on a computer display in a control room on deck from where the jacks were also operated.

All mooring wires between the jacket and the ship were then removed and the vessel was moved out from between the jacket. The float-over and mating operation of the AMP1 topside was successfully completed on March 14, 2003.

Traditionally barges, using tug assistance, performed float-over and mating operations.

But, the advantages of using a self-propelled semisubmersible heavy transport vessel are:

• Shorter transportation time compared to a tug-barge combination due to higher transit speed and less weather sensitivity,
• Reduced accelerations, lower motion responses in comparison with barges,
• Less risks relating to damage and/or total loss during transportation, which results in lower costs for marine cargo insurance coverage,
• Availability of an integrated, more advanced and controllable ballasting system for more accurate and faster ballasting, and
• Ability to build at distant fabrication locations.

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