Lift-installation's latest benchmark

OE Staff
Wednesday, August 1, 2012

All the careful planning and refinement of the Valemon jacket design finally paid off on 18 June when the structure's record-breaking offshore lift-installation was completed. At 03.00 hours, the base of the 160m high steel frame finally touched the seabed, on target and perfectly upright.

The sequence had started with vessel operator HMC first bringing the transport barge into position at the stern of Thialf. Then, with the twin cranes hooked to the jacket's pre-installed lifting slings, the transport barge could be slowly ballasted lower in the water, sea fastenings cut and the jacket raised above its deck.

Once the free-floating cargo barge had been towed away, the bottom end of the jacket could be steadily lowered into the water, with the structure just as steadily rotating from horizontal to vertical as it went.

The procedure did however suffer a late interruption when a mechanical problem stopped one of the main cranes and caused the whole lift operation to halt. At that point the jacket had been fully rotated but still had another 7m to go to reach the seabed 133m below water. So the last stage of set-down was achieved by a combination of booming down the crane and ballasting of Thialf.

Thirteen days later, HMC had stabbed, driven and grouted all 16 foundation piles and finished with the small lifts of five caissons and the 190te wellhead module. On 1 July the vessel was able to sail away from a highly demanding job well done for client Statoil.

After two weeks of waiting on weather, Thialf could at last get to work on the Valemon jacket.

Waiting on weather

The immediate run-up to this, the world's heaviest lift-installation, had been a stark contrast to all that activity. Before they were at last able to swing into action, the team onboard Thialf had to sit out a frustrating 15 days of inactivity, waiting on weather and hoping for sea conditions to calm down enough for them to start the delicate jacket lift.

When that window did open, says HMC project manager Carol Granneman, the weather was excellent. It was very quiet, with seas of less than half a metre and hardly any wind, and that helped us a lot.

The total load on the hooks of Thialf's twin cranes as the jacket rose off its transport barge was 9600te. This included some 800te of temporary equipment such as spreader bars and slings.

Slowly rotating, the lower end of the jacket descends into the water. Right: Landed on the seabed, the jacket awaits the first of the 16 foundation piles in foreground to be stabbed and driven. Below: Hooking up in a calm sea for a record-breaking lift.

Thialf has lifted heavier loads than this a number of times in the past. Largest of these to date has been the 11,800te topside for the Shearwater platform in the UK North Sea, placed in 2000.

But a large jacket is a rather different proposition. It may not be as susceptible to wind forces as a big topside, but it does call for the delicate and complicated task of rotating from horizontal to vertical once it has been raised from the transport barge.

That lift-off is just as delicate, governed by the relative motion between crane vessel and the transport barge brought beneath its cranes. Every lift has its own limits, says Granneman, and this one had probably the most stringent limits we have seen.

Valemon called for sea conditions below 2m significant wave height. In the event we experienced no more than half that, he says. That put us well within crane limits, still with spare capacity left. The movement at crane tips was probably less than 100mm.

Weather during the preceding 15 days of standby time was poor rather than dramatically bad as Thialf waited on location and the jacket stood off about 5km. The most exciting part of the day was the weather forecaster's presentation on what he expected for the next week, says Granneman.

We had one instance where we thought the lift could be possible and got ourselves ready, he continues. But actual conditions turned out a little worse than expected and we had to postpone. You can't do anything about the weather.

When a viable five-day window did come into view HMC could at last spring into action.

Once the jacket had been landed on the seabed, foundation work went faster than expected. About a day was trimmed from the nineday schedule for driving with the big Menck MHU 2100 underwater hammer and grouting the skirt piles.

Installation of the wellhead module also went well. Our colleagues at Heerema Fabrication Group did a terrific job in aligning all those boltholes for connecting to the jacket, Granneman remarks.

The in-house combination of all aspects from design to installation of the Valemon jacket divided between sister companies HFG and HMC bore fruit right from the start.

Once Thialf was chosen as installation vessel, the jacket design could be tailored even more precisely. One difference from rival vessel S7000 is that the blocks of Thialf's main cranes are allowed underwater, even if that does mean a substantial job of cleaning and greasing afterwards. In the procedure chosen, one block went about 30m underwater.

As for the two large trunnion nodes that proved such a headache to design, the jacket turned very smoothly, says Granneman. He also mentions that because the planned operation was so close to Thialf's limits, the option was kept open to place counterweights on the very top of the jacket to modify the load distribution between the two cranes. In the end that was not called for.

Small things can have a great impact on the whole operation, says Granneman. He quotes the example of a flooding pipe whose position could interfere with the piledriving hammer. We just adjusted our work method a little to allow that pipe to stay there. Similarly it was agreed that guide pins on the top of the jacket could be included in the lift and be a helpful influence on centre of gravity.

Valemon has taken jacket lift-installation procedures another impressive step forward with completion of this lump sum contract from Statoil, rumoured to be worth about E150 million.

There may not be any more big jacket lift-installation operations coming up this year, but clearly the technique will continue to be seen on some impressively heavy structures. Right now, Statoil's representative through the whole Valemon jacket EPC Kjell Arvid Tuen is moving to the Dagny project.

Dagny, says Tuen, has a heavier topside, so the jacket is even closer to the maximum lift capacity of the two big crane vessels in the market. We are just about to take a decision on whether we should choose lift-installation or bargelaunch for Dagny. OE

Categories: Europe Installation Engineering

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