PLATFORM TRANSPORTATION Contractors tackle logistics of large scale, long distance tow-outs
Over the past year, a number of giant structures have been ferried around the world to major new field developments. Some of these projects required long-term planning and special ingenuity on the part of the transporters and installers.
Between March and June last year, Dockwise shipped out four of the topsides `super-modules' for the Hibernia project to the construction site in Bull Arm, Newfoundland. The contract had been awarded in 1993 to allow the company to mobilize all at once its three largest vessels, the Mighty Servants 1, 2 and 3, as well as the Russian vessel Transshelf, for which Dockwise acts as commercial and operational manager.
Last year's transport, with the modules weighing between 5,800 and 8,000 tonnes and measuring up to 36 metres high, involved four individual shipments to the assembly pier in Bull Arm. The mud module (M30) and utility module (M40) were shipped from Belleli's fabrication yard in Taranto Italy.
The M10 process module and the M50 service module were brought over from Hyundai Heavy Industries' facility in Ulsan, South Korea. The latter, described as ten stories tall, will accommodate up to 280 offshore personnel.
Dockwise designed the logistics plan such that every two weeks, a module was discharged at the assembly pier near St John's. To reduce friction and ensure smooth movement of this operation, all the vessels were fitted with special skid-beams and the modules were hydraulically suspended during skidding.
Because of the tidal movement during loading, each ship's position was controlled and held stable by a unique water ballasting system. After loading, the skid-beams were removed to allow the module to rest on four separate supports during transportation.
All modules were shipped to the assembly pier in Bull Arm and discharged by means of multi-wheel trailers. As the pier was too narrow to accommodate a full-length trailer, a barge was moored opposite the vessels to allow landing.
Discharging via a pier from and to an independently ballasted tail barge/heavy lift vessel combination had never previously been attempted anywhere, according to Dockwise. The complexity of keeping two vessels perfectly in equilibrium during such a transfer had, however, been simulated many times.
Next year, Hibernia's assembled topsides modules will be mated with the gravity base structures, and the completed platform will then be towed to the drilling and production location 315 km offshore. First oil is then expected in late 1997.
TPG 500 voyage
Dockwise's ability to ship modules around the world means that oil companies are no longer restricted by geographical (though not political) factors in their choice of construction yard.
Another example was BP's Harding Field in the UK North Sea. Development was considered uneconomic through a conventional platform, so a TPG 500 production jack-up was selected instead, with Hyundai in Ulsan again the appointed construction yard.
The Transshelf was contracted to handle the tow-out of the completed unit late last year via a 12,000 mile journey to Stavanger. Altogether, the production, drilling and quarters facilities weighed 22,750 tonnes - the heaviest offshore load-out ever attempted from a quay or vessel onto a barge, according to Dockwise. This was made possible through a specially developed, four-track skidding concept.
A hydraulic-jack skidshoe system was designed to align the loads on the skidways to prevent overstressing of both jack-up and vessel. A special solution was required for the link-beams linking the quay and the vessel, mainly because each side of the vessel was fitted with four sponsoons for transport stability.
These sponsoons could not take any load, so the link-beams had to bridge their 3.5 metres of width. With the TPG 500 being pulled onboard by means of strand jacks, the vessel would be pushed against the quay during loading. To prevent damage to the sponsoons, eight compression struts were fitted to keep the vessel away from the quay.
Ballasting was complicated, as the cargo moving on board would impact the trim, heel and draft of the vessel. These movements, and the tide, had to be compensated by the ship's ballasting system, which was employed at full capacity during its outward journey.
Elf Exploration Angola's COB 1 dentral production platform deck, following placement on the jacket in January using the Unideck TPG installation method.
The jack-up arrived in Stavanger after a smooth, 54-day voyage, despite the fact that the cargo protruded 36 metres on each side of the vessel, making it the widest transport ever to transit the Suez Canal.
Early this year, the TPG 500 sailed from Norway to its final destination in UK block 9/23b, where it was to be mated with the Harding gravity base that was already in place on the seabed.
Once on site, the first step was for one leg of the hull to be lowered until it reached the docking pile on the base tank's first tower. Then the TPG 500 was spun round until the second and third legs could be lowered, in sequence, in the same way. This operation lasted one hour. When the legs were lowered, the hull lifted itself up until there was an air gap beneath it.
The jacking system is composed of 18 frames of four or six reducers situated along the hull where the legs pass. They can raise 22,000 tons at the rate of 45cm/min. Racks are aligned on the chords and guide the jacking. Then the hull is locked in its final position. The locking system consists of nine double units with an equal number of counter-racks and clamping plates, giving a total locking capacity of 72,000 tons.
Six days after installation, the TPG 500 was in safety configuration. The drilling team was onboard four days later. As the hook-up at sea is reduced to a minimum, first oil is expected to be produced in April.
Cobo floatover
Around the same time (January 16), Technip-Geoproduction was successfully installing the Cobo Field central production platform deck COB P1 for Elf Exploration Angola, approximately 200 km northwest of Luanda.
This was the first time a 9,500 ton deck had been installed on open seas with a long swell, using the Unideck TPG float-over method. This technology allows the deck to be completed onshore, towed to its site on an ordinary transport barge and installed on its jacket by ballasting and jacking.
Elf awarded the turnkey contract to Technip-Geoproduction for the transport of the deck from Eiffel's yard in Fos-sur-Mer, southern France, to the Cobo Field. Once there, the deck was placed on the jacket in a water depth of 90 metres. This maneuver lasted six hours.
Unideck works as follows: the barge supporting the deck enters between the jacket legs and is then moored. Next, the barge is ballasted until the deck piles meet the jacket legs (a two-hour operation). At that point, the deck is jacked down 1.8 meter in one minute, transferring the main part of its load from the barge to the jacket. All that remains then is to keep ballasting the barge until final transfer of the load onto the jacket is accomplished. The barge can then be disengaged.
According to Technip-Geoproduction, Unideck allowed the client a greater flexibility in scheduling Cobo operations offshore, with reduced offshore hook-up and commissioning work kept to a minimum. The platform should start producing oil this May.
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