The Blackford Dolphin sails up Belfast Lough after leaving Harland and Wolff shipyard following its major refit

The Blackford Dolphin oil rig has been a familiar sight in Belfast's docklands over the last six months

The giant Blackford Dolphin oil rig, which was initially only supposed to be in Belfast for a six-week refurbishment, will finally leave Belfast today after six months dominating the city's skyline.

One of the world's biggest such structures, the 260ft rig, that sailed into the harbour before Christmas, will be heading back to Norway at a snail's pace after her multi-million pound refit at Harland & Wolff, which employed some 400 workers on the project.

The contract price for the work is thought to be over £100m.

While the initial work schedule was only supposed to take 60 days, following the discovery of some structural defects on the rig when it came into the world's largest dock, the structure underwent months more repairs by welders, electricians, painters and engineers from the UK, Poland and Portugal.

The additional problems were only exposed once the rig was out of the water.

The 40-year-old platform, which costs over £330,000 a day to hire from Norway-based owners Dolphin Drilling, was so large that Belfast's famous gantry cranes Samson and Goliath had to be rolled into a corner of the yard to make way – altering Belfast's famous cityscape – and 1.2m tonnes of water had to be removed to accommodate the monster rig.

As it was lit up at night to allow for 24/7 work, it became affectionately known as the city's unofficial Christmas tree over the festive period.

But statements to the Norwegian stock exchange in April and May signified that the work would run on.

There had even been fears that the extended work schedule would mean that the cranes would not be in the correct position for a lifting project for one of Harland & Wolff's renewable energy clients later this month, with the company's sales and marketing manager David McVeigh promising that there would be a "Plan B".

However that job – a delivery of equipment from the yard – looks to be back on track as the workers finally put down their paintbrushes, spanners and pastie baps to watch Blackford Dolphin make its way back out to sea.

On its inbound journey it took more than three months to tow the Blackford Dolphin across the Atlantic from Brazil, and it will finally begin its slow departure at around 3pm today.

The job will involve four tug boats and it could take the rig up to three hours alone to leave the harbour.

Belfast Harbour has issued a notice to mariners stating that no other vessels will be permitted in the main Victoria Channel during the transit.

The rig will be towed to the entrance of Belfast Lough to undergo "sea towage preparations", which will last for another three days.

Harland & Wolff said that the company's work on another rig in 2012 helped clinch the contract.

One thousand workers helped complete the one-month project on the SeaRose floating production, storage and offloading vessel for Canadian firm, Husky Energy.

The floating factory left Belfast four days early, under budget and with no injuries or environmental incidents before sailing to Newfoundland.

But this was not the first time that the Blackford Dolphin had visited Belfast.

It underwent a major upgrade from 2006 to 2008, when Harland & Wolff designed and built accommodation blocks, a power generation module, mud room and additional buoyancy.

Dolphin Drilling is one of the longest established independent drilling contracting companies in the world.

Oil industry analysts were reluctant to comment yesterday about whether the extended work schedule had caused knock-on problems to the overall drilling industry.

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Belfast Telegraph