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Old boats are a bit like buses, nothing comes along for ages then three turn up at once. In mid-April we had the centenary of the Titanic disaster and the opening of a dedicated museum in Belfast. Two weeks later the Queen was admiring the newly restored tea clipper, the Cutty Sark in Greenwich. And mid-May marks a milestone for the most venerable and ancient ship of all - the Tudor warship Mary Rose, which sank in the Solent in 1545.

That milestone is the handing over of the completed Mary Rose museum to the client ready for fit-out. This coincides with a second equally significant milestone. The remains of the timber hull of the Mary Rose have been sprayed constantly for the past 17 years with a chemical called polyethylene glycol (PEG). The ship was underwater for 437 years and the wooden hull would fall to pieces if it was allowed to dry out normally. The PEG is allowed to displace all the water in the timber by soaking right through and will stop the wood splitting and twisting, plus it will give it more body once dried out under very carefully controlled conditions. The sprays are just about to be turned off, enabling the four-year drying out process to commence.

As the frame surrounds the Mary Rose Warings had to be careful not to drop anything during construction. The biggest risk was erecting the main beams over the ship

All this is taking place in a dry dock at Portsmouth Historic Dockyard next to another historic ship, HMS Victory. The dry dock is enclosed with a tent-like building called a Weymss structure. This is double skinned with the air between the two skins maintained at a constant 16潞C. This acts as a buffer zone to enable the air around the Mary Rose to be maintained at a constant 28潞C in a space called the hotbox.

The PEG is sprayed onto the hull through hundreds of spray nozzles as a liquid, but will turn into a wax if the temperature drops to room temperature. 鈥淚f wax forms inside the nozzles you would need to unscrew all these individually and put these into a bucket of hot water to remove the wax,鈥� explains Tim Lloyd, associate engineer with Ramboll, which is responsible for the structural and service engineering. He adds that the PEG would also solidify on the surface of the Mary Rose and would have to be washed off.

Incredibly, contractor Warings has built the museum around the Weymss structure while ensuring temperatures are maintained inside the hotbox and the spraying operation is uninterrupted. It has even had to modify the Weymss structure as part of the work. Warings senior construction manager Nicolas Beausseron and senior building services manager John Bowser are on 24-hour standby via mobile phone if anything goes wrong. 鈥淚f the temperature drops below 16潞C in the thermal buffer we will have to come to site to see if it is okay,鈥� says Bowser. 鈥淚n the winter you could guarantee a low level alarm every other day.鈥�

He adds that the PEG spraying plant can only switch off for an hour a time and for no more than four hours in total a day. It has been very reliable but is now getting old after 17 years of near constant operation. 鈥淲e are now going through a phase when there have been lots of problems,鈥� he says.

The team had another challenge to contend with because the dry dock housing the Mary Rose is a scheduled ancient monument.

The temperature drops were an ongoing problem, as the project developed we had to put in additional heating

John Bowser, Warings

This meant piling through it to support the steel frame of the museum was out of the question. Ramboll devised a frame which was supported on just four points well away from the dock rather like a table. But the huge trusses and cantilevers necessitated by this solution weren鈥檛 going to come cheap. 鈥淓arly indications showed the steel would cost 拢8,000 a tonne which is double the market price,鈥� explains Beausseron. 鈥淭he big sections could only be rolled in Japan and would take up to 36 weeks for delivery. On an 80-week project that is quite a challenge.鈥�

Warings calculated the job would cost 拢21.5m which was more than the client could afford. Keen to win the work, the contractor proposed an alternative steel frame solution. 鈥淲e made a few simple load path changes and reduced the steel cost to 拢2,000 a tonne,鈥� says Beausseron. Investigations showed the dry dock was founded on solid ground so Warings proposed using a multitude of concrete pad foundations and using these to support a comparatively simple portal type frame which saved 拢4m.

According to Beausseron Ramboll wouldn鈥檛 take responsibility for the alternative design. 鈥淲e suggested to the client they went to design and build and we would take the risk,鈥� he says. The two pavilions on either side of the main museum have been piled conventionally.

As the frame surrounds the Mary Rose Warings had to be extremely careful not to drop anything on it during construction. The biggest risk was erecting the main beams over the ship. To minimise this risk the beams were split into three sections with two of these fixed to the columns at the sides first then the central section positioned last. This was lifted into position using a 20% oversized mobile crane using a sling that supported the beam at both ends. Cherry pickers were used to bolt this section to the two side sections - the cherry picker baskets were encapsulated with netting and all tools tethered with lanyards to reduce the risk of falling objects. Metsec steel framing was fixed to the frame and black stained timber reminiscent of a ship鈥檚 hull was attached to this frame.

The Weymss structure had to be reduced in width as this was in the way of the new steel frame. A temporary, insulated curtain was put up to maintain temperatures around the ship. New columns to support the roof beams of the Weymss structure were erected closer to the Mary Rose, the loads transferred and the sections of Weymss structure roof beams now extending beyond the new columns cut off. Steel frame elements that pass through the Weymss structure were sealed up afterwards. 鈥淚f the seal didn鈥檛 work for any reason we would get a warning call,鈥� says Bowser.

Temperature control

The M&E installation has been even more complex than the steel frame. Not only are there the challenges of maintaining the temperatures inside the hotbox and keeping the PEG spraying plant running, there are exacting temperatures that need maintaining inside the finished building. When the drying process starts conditions inside the hotbox must be maintained at a constant 19oC with a tolerance of +/-0.75oC and a relatively humidity of 54% with a tolerance of +/-4%. This temperature will be maintained after the Mary Rose has dried out. The museum is well insulated and has a target airtightness of less than 3m3/hr@50pa. The missing half of the Mary Rose hull has been recreated in glass reinforced plastic and is placed parallel to the original hull so visitors can walk between the two.

This fake hull, known as the context gallery, is being used to display the hundreds of artefacts found with the wreck. It will be one of the largest display cases in the world and has an exacting airtightness of 0.1. This type of precision doesn鈥檛 come cheap - the case has cost 拢0.5m.

Maintaining the temperatures in the hotbox during the winter hasn鈥檛 been easy. 鈥淵ou are trying to maintain temperatures of 28oC when it is -5oC outside,鈥� says Bowser. 鈥淭he temperature drops were an ongoing problem, as the project developed we had to put in additional heating.鈥� The hotbox was initially heated with boilers located inside the Weymss structure, but these had to be moved outside the building to a temporary plantroom as it was in the way of the context gallery. A permanent plantroom has been built and now heats the hotbox.

The exacting tolerances for the air supply to the Mary Rose necessitated three enormous air handling units with a throughput of 30m3 a second. These have been shoehorned into the dry dock under the museum. 鈥淥ne of the biggest ponderables we have had to deal with is there isn鈥檛 the space for the normal amount of back up plant for this sort of conservation work,鈥� says Lloyd. He says the air drying plant should be backed up by twice the capacity of the existing units. But there wasn鈥檛 any spare space for back up plant. 鈥淚n our case there is no standby,鈥� says Lloyd, adding a novel solution had to be found to take care of this problem.

Each air handling unit has three sets of heating and three sets of cooling coils, and six fans rather than the normal one. Only two out of the three coils and five out of the six fans are used for normal duties. If a coil fails there is a minimal amount of back up from the unused coil and because the three air handling units are linked coils from the other units can be called on in emergency. If the fans in one air handling unit fails the speed of the fans in the other two units can be temporarily cranked up beyond their normal duty while the defunct unit is repaired. These units are installed and ready to go.

When the PEG sprays are turned off there is a three-week window to thread the ductwork needed to precisely direct the conditioned air around the hull of the Mary Rose. Special flexible, fabric ductwork will be used to make the job easier and because the atmosphere inside the hotbox is corrosive. This, combined with bacteria friendly temperatures, means workers need protective suits and breathing apparatus to work inside the hotbox. The work will be done by Mary Rose Trust workers and includes the installation of lighting so visitors can see the hull properly.

The dangerous environment means the hotbox will be retained during the drying out process. A separating wall runs the length of the context gallery and visitors will be able to see the Mary Rose through windows inserted into this wall when the museum fitout completes at the end of this year. When the hull is fully dried in 2016 this wall will be taken down and people will be able to properly appreciate the Mary Rose for the first time in 471 years.