The world's largest cargo barge – Heerema's H-851 – announced its arrival on the offshore scene in 1988 when installing the 44,785t jacket for Shell's Bullwinkle platform in a then daunting 412m of water in the Gulf of Mexico. Now, following extensive redesign work by Groningen-based RR Maritime Engineering, it is being readied for upcoming large-deck floatover challenges off Australia and Russia. Ron Rosenbrand reports.
The H-851 was purpose-built – in 1987 by Daewoo Shipbuilding & Heavy Machinery of Korea – for large jacket launches lining up at that time in the Gulf of Mexico and offshore California. But even with its unprecedented dimensions – 260m long, 63m wide, 15m deep and with a deck thickness of over 50mm – the giant launch barge was dwarfed by the Shell Bullwinkle jacket, its first assignment (OE June 1988). As OE's Adrian Cottrill reported at the time, when loaded out at the Gulf Marine Fabricators assembly yard at Ingleside, Texas, the top of the jacket projected 120m beyond one end of the barge – an unsupported cantilever of 12,000t. The base stuck out 20m beyond the other end. And at each side, the jacket overhung the barge by up to 42m.
The following year H-851 (so named because it was Heerema's first of 850ft length) was deployed for two big Exxon jackets in Californian waters, but neither Harmony (launch weight: 40,140t) nor Heritage (32,250t) came close to its 54,000t jacket weight capacity. And with FPUs making major market inroads, big fixed structure jacket launch opportunities were becoming few and far between.
The barge would later serve in various other capacities, for example as a large transportation barge, a launch barge for two Chevron compliant towers in Angola and a construction site for the dismantling of Shell's Brent Spar.
Rankin redesign
Then along came the North Rankin B (NRB) project on Australia's North West Shelf (see page 41). In 2008, operator Woodside Petroleum awarded Heerema Marine Contractors (HMC) this installation contract, involving a large jacket launch and a 24,000t deck floatover with a deck support frame of 6000t. The work is scheduled to start in December 2011 and complete in January 2012. HMC elected to build a new cargo barge (H-591) for the jacket and to use the H-851 for the deck floatover.
However, the entrance gap for the NRB deck floatover was based on a 42m-wide barge – not enough to allow the barge the necessary stability to transport the deck from Korea to Australia. HMC decided to turn the H-851 into a 'T'-shaped barge – by reducing the width over its first 100m from 63m to 42m – and brought in startup consultants RR Maritime Engineering (RRME) to prepare the engineering for the necessary modifications.
The topsides will be supported by the deck support frame, which will be skidded onto the bow of the barge during loadout. The barge needs to have enough stability and strength to accommodate the skidding of the deck support frame.
In July 2008, RRME and its Polish partners Genfer Lloyd started preparing a detailed finite element model of the barge to determine the additional strengthening required for transporting the NRB deck. The scope included the removal of the ballast tanks on either side of the barge for a length of 100m from the bow, reducing the width to 42m. It was thought initially that the longitudinal bulkheads at 21m from the centre line would form the new side shell, but calculations showed that it would have a major impact on the strength of the barge. The design was therefore amended to include a double side construction with a breadth of 1.5m.
Sea-keeping analysis and strength, stability and construction calculations had to be made in order to implement the modifications. Based on HMC's conceptual FEM models, RRME performed the detailed finite element analyses and designed the required reinforcements to comply with the strength requirements. All barge systems had to be modified and the vent-piping rerouted. Bending moment, shear forces and torsion also required structural analysis and detailed engineering. Transferring the loads from the narrowed front end of the barge to the wider middle part also called for detailed analysis, and strengthening of the structure was expected. The possible strengthening required for stern and bow loadouts, transport and installation also needed to be considered, and RRME updated all drawings and documents, including the operations manual and stability booklet.
Sakhalin success
Following completion of the initial design, including the design appraisal from Class surveyor Lloyd's Register, HMC learned that it had also secured the installation contract for the Arkutun Dagi development off Sakhalin Island in the Russian far east. This work, scheduled for May 2012, involves a larger deck of 38,000t with a 7000t support frame. The additional weight of the Arkutun Dagi deck necessitated considerable changes to the H-851 redesign to make it suitable for a stern loadout and transport and installation on the bow of the barge. RRME and Genfer Lloyd were also tasked with preparing the scope for these additional modifications.
In addition to the adaptations to accommodate the installation of the Arkutun Dagi topsides floatover, HMC was also looking to redesign the H-851 in such a way that it would not only be suited for the upcoming NRB and Arkutun Dagi assignments but would also enable it to become more competitive in the market for large-deck floatovers. To that end it was decided to install a free-flooding rapid-ballast system and purchase barge-mooring winches.
The global structural finite element analysis made use of two models: a 3D finite element global model, with a relatively fine mesh in the transition region, covering the full length, depth and breadth of the vessel, to determine the global structure response, and two local models to ensure proper stress evaluation and assessment. The results were used in a local analysis concerning the transition area and the bulkhead frame there. The most unfavorable load scenarios were looked at using local models to ensure final stress assessment.
Final design
The design on which HMC finally settled involved:
The design of the integrated rapidballast system with a capacity of 40,000m3/hour included the installation of 44 inlets for 22 tanks in the pipe passage to the designated tanks. In each inlet two valves are installed (one for safety purposes). For additional safety, two accumulators were installed, each able to operate the entire system, and two power packs were installed, capable of closing all the valves in case of emergency. The entire rapid ballast system is operated from a control container on deck and can also be operated from the control room as a back-up.
In April 2010, HMC contracted China's Yiu Lian Dockyard (Shekou) to carry out the barge modifications, with the work scheduled for completion at the end of March 2011. Then the new-look H-851 will be fitted with purpose-built mooring and docking winches and made ready for the North Rankin B project. OE