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Expresso Bonga

Technical Shipyards

The largest ship ever to sail up the Tyne is due arrive on Saturday. Diarmaid Fleming reports on what has had to be done to accommodate her.

Tyneside shipyards have lain idle for decades as Far Eastern yards captured the world market. This week, however, the mammoth 300,000 tonne floating production storage and operation (FPSO) vessel Bonga reverses the eastward trend as it reaches the end of its journey from the Samsung yards in South Korea.

Amec will fit out the Bonga before it goes into service for Shell off the African coast.

While the arrival is expected to bring hundreds of thousands of sightseers to the banks of the river, site preparations by civil engineers have been under way since the end of July to provide suitable berthing facilities.

This is a major challenge. The Bonga is 300m long with a beam of 75m. From keel to main deck it measures 32m, the same height as a 10 storey building - or Nelson's Column. Not surprisingly, there is no modern berth on the river suitable to take a craft of this size, so substantial onshore civil engineering works were needed to accommodate the Bonga.

The new berth must provide adequate fendering systems to absorb the energy of the docking vessel, without causing damage to the ship's hull or the onshore structures. Mooring structures were also required to enable the vast hull to tie up.

Contractor Harbour & General, with headquarters near the site, had recently completed the construction and installation of the spectacular Gateshead Millennium Bridge, almost within a shadow's reach of Amec's Hadrian's Yard and the Bonga's destination. With local knowledge and contacts and a skilled direct labour force, the firm was chosen to work with project consultant Mott MacDonald to build the new berthing facility.

Speed may be of the essence as the cliche goes on most construction sites, but on this one, the need to meet the deadline is impregnated on the minds of all. 'The ship is coming on 12 October, and there is no flexibility regarding that deadline.

Whatever might happen on site, the ship is coming and all solutions are driven by that, ' says Harbour & General marketing manager Eddie Tattersall.

The rigidity of the deadline is due to a number of factors.

Amec's contract is for £400M, which would be seriously affected by any delays. The date chosen was governed by tidal conditions on the river: missing the tidal window would mean a delay of two weeks before the right conditions reoccur. This would have huge cost implications, delaying everyone on the project from the specialist towing team bringing the vessel from Korea to all those due to work on the fit-out.

A neap tide - when the difference between high water and low water levels are at a minimum - will provide the best possible conditions for the arrival of the Bonga. 'With neap tides, there is a longer period of slack water, making it easier to control the vessel, ' explains Harbour & General contracts manager Andrew Harding. 'The effects of currents on a ship of that size must be phenomenal.'

Dredging work has been carried out for the Port of Tyne Authority to ensure a safe navigation up the river to Amec's yard at Wallsend, its location around 9km downstream of the Tyne and Gateshead bridges avoiding the need to pass under these spans. Harding says some tricky calculations had to be made to balance the depth of the river, the state of the tide and the air draught of the unwieldy vessel.

'Deballasting can reduce draught to a minimum, but this means a lot of hull above the waterline', he explains. 'With a ship this big, high winds could be a major problem, so you don't want it sitting any higher out of the water than you have to.'

The ideal water conditions last no more than two hours, Harding adds. While there might be some opportunity to berth on the 13th October, after that the conditions shorten dramatically, making the operation too risky for another fortnight.

Harbour & General's work was initially only anticipated to involve piling work associated with the fendering systems. Two berthing points were required, along with three mooring points.

An array of tubular steel piles forms the berthing points. Vertical 914mm diameter fender piles 16mm thick and 32m long are supported by an arrangement of 30m long, 508mm diameter circular hollow section struts with walls12mm thick. The struts are in a v-shaped configuration on plan, each bearing against a pair of massive concrete 'thrust blocks'.

Each strut runs through a concrete block cast near mid-length to provide restraint against buckling, with the ends welded to a plate which bears directly against the thrust block.

'These blocks are around 9m by 7m on plan, and 4m deep, consisting of heavily reinforced concrete, completed in one pour, ' says Harding. There is no fancy formwork: for speed the blocks were cast within a temporary sheetpiled box, giving an interestingly functional finish.

The blocks are founded on an array of 32m long tubular steel raking piles, generally 508mm diameter 12mm thick sections, providing both compressive and tensile resistance. The soil consists of between 5m and 10m of made ground onto clay, and some contains contaminants from the site's old shipyard days.

Forces in the piles are massive as can be expected given the size of the vessel, with working loads in the thrust block piles of 200t provided by skin friction, giving over 2,000t per block.

No rock anchors are provided.

'The piles were designed to a 1 in 2.5 rake: 1 in 3 would have been easier. We used a 16m rig with vibrating hammer to drive the first 12m length of pile. Once that was in, we just welded the extensions on and drove with an S90 hammer without the need for temporary works, ' says Harding.

The bollard blocks are similar to the thrust blocks, each housing three 200t mooring bollards anchored by 50mm diameter holding-down bolts cast into the reinforced concrete block.

These are unpiled, relying on the blocks' mass alone to provide anchorage. The vertical fender piles, good for 90t in compression, support a spectacular 10m by 4m rectangular steel fender panel covered with a proprietary rubbing surface against which the Bongo will berth.

To lessen the energy of the berthing impact, the vessel is to be winched into place rather than hit the berth at speed. The berthing energy will be absorbed by rubber elastomeric fender units bolted to the back of the fender panel, and fixed to the fender pile.

Inspections during the course of the fendering work revealed concerns about the existing quay wall. Harding's team is therefore installing a sheet-piled combiwall in front of the quay wall, consisting of Larssen LX32 piles forming box sections with a pair of straight LX32s in between.

The wall is anchored by tie bar cast into a new concrete block behind the existing capping beam at the edge of the existing quay.

Total value of the works under the ICE 6th Edition form of contract is £1.7M, well in excess of the initial scope of work intended. Harding and his team expect to be busy right up to the day of the arrival.

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