“Ingenious steelwork is key to this remarkable visitor attraction,” said the judges of the 2013 Structural Steel Awards when handing out one of the top honours to the team behind the restoration of the world famous Cutty Sark.
As the last surviving tea clipper, she epitomises the golden age of sailing.
The biggest overhaul of the Grade I listed landmark in 50 years commenced in 2004 with a comprehensive programme of conservation, but the project was brought to a dramatic halt when a fire in 2007 swept through the wooden structure, causing extensive damage to the centre of the ship.
The project didn’t restart until the end of 2009 when it received an enhanced design brief.
The new design and conservation solution proposed raising the 963-tonne Cutty Sark three metres within the constraints of the dry berth and demanded that the new interventions had to respect, repair and adapt to the original fabric of the ship.
A complex but elegant pre-stressed system hangs and stabilises the ship in its new position. This steel structure will preserve the shape of the ship’s iconic hull and has enabled an additional public space to be created in the dry berth below, allowing visitors to walk underneath and admire the ship’s form.
The Cutty Sark project features innovative use of structural steel for the conservation of a unique Grade I listed ship. In order to achieve this, precise three dimensional surveys were carried out and every new detail was digitally modeled.
The work carried out involved tight tolerances and demanding geometries, which at times were twisted and curved. Every element of the new steelwork was costed throughout the process, allowing optimisation and management of the design.
When the timbers were removed, the true extent of the iron frame’s corrosion due to salty seawater was exposed. This frame was painstakingly removed and sections of new steel ribs were installed. Nowhere was the corrosion worse than at the bottom of the ship where the ribs connected to the keel and the iron had completely dissolved, requiring extensive work.
The vessel has been lifted at regular intervals along its length, to redistribute the weight of the ship and enable 12 new triangulated steel frames to be installed. These frames assume the form of an inverted coat hanger, with two tie rods from the ship’s keel running diagonally up to each end of a horizontal strut that spans the width of the ship immediately beneath the Tween Deck. This frame carries the weight of the ship’s keel and masts back up to the new external support points.
The 12 sets of horizontal beams and diagonal ties form a triangle between the strake plates and new box keelson which encases the ship’s original keel, fixing its vertical position and preserving the ship’s iconic shape from within.
A 13th cradle completes the system by connecting the stern of the ship to the keel. The new steel cradles have been integrated with the existing fabric of the ship wherever possible.
The cradle system is fully adjustable via giant turnbuckles set within the primary ties and struts, which ensures a perfect fit with the existing ship fabric and dry berth. The 24 inclined struts carry the weight of the ship and its visitors down to the mass concrete of the original dry berth.
A process of intense collaboration between designers, specialists and contractors has resulted in a cost effective, robust and elegant solution which will preserve the iconic ship for generations to come.