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Changing priorities

Milford Haven - Preserving a calcified seaweed dramatically complicated a major jetty refurbishment at a south Wales port.Ruby Kitching donned her souwester to report.

Rejoicing broke out briefly last week when the first 1,300t section of a crumbling 1960s reinforced concrete jetty was lifted out of the water at Milford Haven, south Wales.

The lift was a tense moment for joint venture contractor BesixKier, because no material could be allowed to fall into the water.

Just offshore from the Haven's sprawling, oil refinery complex, lies a 1,000 year old coral bed.

Though little thought was given to environmental conservation when the South Hook jetty was built 40 years ago, the methodology for its refurbishment is entirely tuned to minimising impact on the marine environment.

The former oil tanker jetty had been out of use for 13 years and is now being strengthened and partially reconstructed under a £70M, three-year contract, in readiness for the first shipments of liquefied natural gas (LNG) next year.

Chloride attack had severely weakened the jetty's concrete and eaten away reinforcement - it is possible to see right through some piers where concrete has disintegrated. Furthermore, oil tankers 40 years ago were much smaller than today's gargantuan LNG tankers.

'The easiest construction solution would have been to knock the jetty down and start again, ' says Besix-Kier deputy project manager Philip Miles. But with the corals to protect this was never a possibility.

Repair and strengthening work involves refurbishing piers along the jetty's entire 1km length.

Repairs are also needed to the transverse beams that connect pairs of piers to form trestles, or bents, supporting the deck. And the deck itself is being replaced with new precast concrete units.

Of the four berths at the jetty's end, two are being brought back into use. To achieve this, one must be demolished and rebuilt, while another will be upgraded with new mooring and fendering.

A third berth will be retained, awaiting possible conversion for LNG ships in future, with the fourth berth removed.

The scope of work also includes installing supports to carry pipework linking the LNG tankers to regasification plant on land. Supports for pipework expansion loops, the rebuilt berth, mooring, turning and collision dolphins require new piles to be installed. Any redundant infrastructure dating from the jetty's oil import days is simply being removed.

Project director Mathieu Dechamps says that design has been concurrent with demolition since there are no drawings for the original jetty. Cutting into the structure to expose the reinforcement is the only way to grasp exactly how strong the original design was.

Enclosed temporary platforms cling like outlandish barnacles to the jetty. These steel 'houses' measure 15m long by 5.2m high and 4.8m wide, and allow workers to carry out hydrodemolition, welding and concreting.

Erecting the houses is a complicated task. First, steel bracing is installed between bents just above the high water mark and at deck level. The existing cast insitu deck slab is then sliced up using hand tools and lifted off the bents by barge mounted cranes. This then allows access platforms to be threaded over the bent. The house structure is then lowered on to the access platform while further cross bracing and scaffold platforms are erected.

The houses are leapfrogged along the jetty as refurbishment work is completed. When repair of a stretch of bents is complete, precast roadway deck sections are installed.

Overhauling the bents involves hydrodemolition - blasting them with water and grit to remove the concrete cover and reveal the reinforcement. Visual inspection tells engineers whether they need to replace the bars or cut out highly corroded sections and weld on new ones. Dechamps says there is a constant process of redesigning based on the results of the inspections.

After the reinforcement hasbeen repaired, it is sprayed with concrete and wrapped in a titanium mesh before a further layer of concrete is applied.

Once full concrete strength has been achieved, an electric current is applied to the titanium mesh which prevents chlorides from the seawater reaching the reinforcement.

Where old reinforced concrete piers are being permanently removed, divers sever them using diamond wire sawing equipment. Removal will be completed using hydrodemolition to cut the piers down to 1m below sea bed level.

Last week's successful extraction of berth one followed weeks of diamond cutting the existing piers allowing berth sections to be plucked out of the water.

Concrete from demolition of the berth has already been crushed on an 'on site' barge.

'We're selling the crushed material to a company which will collect it by water. This cuts down on lorry movements on our project and theirs, ' says Miles.

Although not the cheapest option, it is more environmentally friendly, says Dechamps, in line with the project's objective of having as little impact as possible.

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