Article - Issue 13, August 2002
The D154 project – Redevelopment of the Submarine Support Facilities at Devonport Royal Dockyard
The Devonport Royal Dockyard provides the British submarine fleet with the facilities to carry out maintenance and refits, including work on the nuclear reactors that power these vessels. In order to meet the latest requirements for this task an upgrade of the existing facilities, known as the D154 project, had to be carried out. This proved to be a major engineering and management exercise involving a number of companies with a broad range of skills and experience. Malcolm Smith describes the work undertaken.
The Royal Navy currently operates Swiftsure and Trafalgar class attack submarines and the larger Vanguard class ballistic submarines that carry Trident missiles. All the Royal Navy’s submarines are now powered by nuclear reactor systems.
During the lifetime of a submarine a number of engineering maintenance refits are required. In refit, the pressure hull – which houses the crew, power plant and weapons, and is constructed to resist the pressure of the water when the submarine is submerged – and all the key systems are inspected and repaired or replaced as necessary. Spent nuclear fuel may be removed and replaced with new fuel and selected systems will be updated with new technology to enhance operation or safety performance. A major refit and refuel takes approximately two years to complete and requires specialised facilities and expertise specifically developed for the task of handling and working with radioactive substances. Submarines have been refitted at Devonport Dockyard since the 1970s.
In 1993 it was confirmed that Devonport would be the single UK site that would carry out future deep maintenance, refitting and refuelling of the UK submarine fleet, including the Vanguard class submarines. In addition to increased job security with the prospect of future contracts for submarine refit work, this meant that all the existing submarine support facilities within the dockyard would be upgraded to meet the modern stringent standards for nuclear safety.
The contract for the facility redevelopment programme, which became known as the D154 Project, was agreed in March 1997. The timely success of the project depended upon mobilisation of diverse UK engineering skills. In order to discharge the contract Devonport Royal Dockyard Limited (DRDL) formally created an alliance partnership to bring together a wealth of professional knowledge and experience from six companies. The members of the Devonport Alliance Redevelopment Team (DART) had the following responsibilities:
Devonport Royal Dockyard Ltd was the prime contractor to the Ministry of Defence for the D154 Project.
Halliburton KBR was responsible for the design of buildings and infrastructure.
Rolls Royce designed, developed, safety-justified, procured and erected the fuel handling equipment and nuclear process systems.
Strachan and Henshaw Ltd was responsible for the design, procurement and installation of the TSSBN reactor access house, RAH crane and submarine cradles.
BNFL Engineering Ltd was responsible for the preparation of safety cases across the project and for the completion of specialist nuclear engineering design.
Babtie Group Ltd was the main civil engineering and building services designer.
In February 2002, on the originally agreed date, HMS Vanguard arrived at the breakwater in Plymouth before commencing her refit in the new facilities. At its peak, in August 2001, the D154 project employed 2,865 personnel. It is estimated that, at various stages, more than 100 separate contractors and 40,000 people were involved in turning the facility proposals into timely reality. This article provides an overview of the new facilities, which stand as a visible testimony to the efforts of all involved.
Nuclear engineering and regulation at Devonport Dockyard
During all operations at Devonport the safety of the dockyard workforce and the public outside the dockyard is the primary consideration. The achievement of nuclear safety in all aspects of dockyard operation is the sole responsibility of DRDL, the site licensee.
All activities associated with the handling, storage and movement of radioactive materials take place within a defined Nuclear Licensed Site. At Devonport the Nuclear Licensed Site includes all the shore based docks and berths in the area surrounding 5 Basin.
Nuclear activities at Devonport are regulated by two principal authorities. HM Nuclear Installations Inspectorate administers the 1965 Nuclear Installations Act (and subsequent amendments) on behalf of the Health & Safety Executive. The Ministry of Defence Naval Nuclear Regulatory Panel, the body that regulates submarine nuclear operations during active service, monitors and authorises all reactor plant activities. In addition to the regulation of site activities, environmental issues associated with waste generation and disposal are monitored by the Environment Agency. The Environment Agency, English Nature and the Department of the Environment, Food & Rural Affairs operate very strict disposal and monitoring regimes in the areas in and around the dockyard.
The Licensee must demonstrate that all proposed activities on the site are fully justified and that the nuclear risk to the operators and public is tolerable and reduced to As Low As Reasonably Practicable (ALARP). Statutory licensing conditions are mandated onto DRDL as the owner and operator of the licensed site to ensure that this is achieved. Staged safety case submissions containing the necessary justifications are considered by the regulating bodies and formal consent is required before any site activities progress.
As the new facilities were developed, their performance against a comprehensive range of potential internal and external hazards was analysed. Internally generated hazards such as fires, vehicular impacts and dropped load accidents are examples of risks in an operating dockyard. External hazard examples include high wind, high tide and earthquakes. For each hazard a design basis event was defined and the design performance against this event quantified. For example, the majority of the new facilities are justified against a 1 in 10,000 year return period earthquake. Designing for a seismic event of this magnitude produces very robust structures. In formulating design solutions, consideration was also given to meeting ‘defence in depth’ and ‘common cause failure avoidance’ principles so that, should a primary safeguard fail, secondary and tertiary safeguards are available.
From the outset, the D154 project employed ‘gated’ design development processes. As the designs progressed through concept, preliminary and final design iterations, formal structured peer and operator review activities probed the design for potential weaknesses and unrecognised operating characteristics. Any observations were fed back into the design before progressing to the next phase. Key elements of the design analyses were also checked by independent technical assessment organisations. Although these processes are time-consuming and costly to implement they ensure that the product is technically correct, robustly engineered, reliable and that it will reveal no surprises in operation.
UK nuclear inspectors place high emphasis on the quality of engineering delivery. More than 80% of the regulator guidance contained in their Safety Assessment Principles are engineering-related, the objective being to ensure that the licensee owns and controls the design intent from original definition through build, testing, operation and maintenance to ultimate disposal. Regular inspections are undertaken by the licensee and the regulators to ensure that practices accord with the prescribed methods and specifications.
Refit and refuelling sequence
Refit and refuelling operations are performed most safely and efficiently in a dry dock facility. The submarine is brought into a flooded dock, a caisson is positioned to seal the dock entrance, and the dock is dewatered, lowering the submarine onto supports on the dock floor. Following docking, electrical and cooling water services are connected to support the onboard systems and reactor plant, then the refit activities commence.
Before any refuelling operations take place, the primary circuit may be chemically decontaminated to reduce the background radiation in the reactor environment. During refuelling, access holes are cut in the submarine hull and a reactor access house (RAH) is installed over the submarine. The top of the reactor is removed and a shielded water tank is installed. Reactor components and single fuel elements are then taken out of the reactor through the water tank into shielded containers. Used fuel is transferred from the dock to on site interim storage facilities. When all the fuel has been removed, the reactor components are inspected and serviced, and single new fuel elements are then installed in the reverse sequence. Subsequently, used fuel is shipped by rail to Sellafield in dedicated transport containers.
The concept of low level refuelling
An important principle in developing cost effective facilities is for the new designs to satisfy their functional objectives while avoiding or minimising the introduction of new hazards. At Devonport the concept of low level refuelling was a key influence in the design of all of the facilities. The greater the lift height for both the fuel elements and containers, the greater is the potential energy available to cause damage should they be dropped. Furthermore, raising the centre of gravity produces greater overturning moments in the supporting structures during an earthquake that, in turn, require stronger or more elaborate isolated buildings. Low-level refuelling principles ensure that all lifts and structural heights are the minimum practicable.
Applying low level principles, the dock floors are raised so that the submarines now dock down just below dockside cope level. A RAH structure spans the dock, supported on the dockside cope. The RAH provides a clean refuelling environment, a dedicated nuclear crane and all the support systems necessary for de-fuelling and refuelling the reactor. Under the new arrangements the submarine docks stern first so that the bridge fin does not need to pass under the RAH when the latter moves on rails from the parked position at the head of the dock into the refuelling position. During used fuel removal individual fuel elements are lifted a short distance into shielded containers. The containers are then lifted a short distance into the RAH and thereafter all container handling is performed at ground level using a rail based low level transfer trolley system.
Fuel storage in the low level refuelling facility
A new seismically qualified low level refuelling facility (LLRF) provides interim new and used fuel storage for all classes of submarines. The LLRF was constructed on two sites.
A site on the basin side supports the personnel change areas and plant rooms to support the fuel handling operations. An island site, connected by a link bridge, provides a new fuel storage area and pits for the storage of used fuel. The LLRF facilities are serviced by a dedicated nuclear crane and again lift heights are kept to a minimum. In 5 Basin a ship impact barrier was constructed to protect the facility from potential ship collision hazards.
During construction, 32,500 tonnes of concrete were poured under 15 metres of seawater to form the base of the LLRF. Subsequently, 112 rock anchors with an average length of 35 metres were installed in order to fix the island site to the bedrock.
9 Dock facilities
9 Dock was built between 1896 and 1907 to accommodate the Dreadnought battleships of the day. It has not been used for nuclear submarine work before. To bring it up to the standards of a modern nuclear structure the original dock floor was removed and a new dock floor with integral drainage system was constructed. The Victorian dock walls were strengthened by a series of reinforced concrete counterfort walls constructed on top of the new dock floor. A new dockside cope (edge structure) with integral services subways was constructed on top of the counterforts. The cope is secured by seventy seven, 760 millimetre diameter steel piles anchored in 12 metre sockets in the rock under 9 Dock.
The cope level subways and ducts in the structures provide electrical and pipework services to the submarine. Services are duplicated on both sides of the dock and segregated to avoid common cause faults. Interconnecting pipework links the submarine to plant in the primary circuit decontamination and alternative core removal cooling (PCD/ACRC) system building at the head of the dock. The PCD/ACRC building contains all the process plant necessary to cool the reactor, apply chemical decontamination and inject or remove boronated water reactivity suppressant. The PCD/ACRC building has 92 rooms with equipment and plant connected by over 20 km of pipework and 150 km of electrical cable.
A multi-cellular concrete caisson has been constructed to seal the dock entrance. The water level in the cells can be raised and lowered to allow the caisson to be floated into the dock entrance and then ‘sunk’ firmly in position. Redundancy in the cellular structure makes the caisson capable of withstanding accidental dropped loads from cranes and 5 Basin ship impacts.
Seventy-four docking cradle blocks attach to plinths on the dock floor to support the submarine. The new cradle blocks feature timber faced rubber mounts that hold the submarine upright and reduce the seismic loads generated during an earthquake.
The dock structure, the submarine cradle, three new refit support dockside cranes, new RAH and integral high integrity crane are all seismically qualified.
New refuelling tools and fuel module removal containers (MRCs) have been supplied. In addition to two production MRCs, a third full-scale container was built and drop tested to confirm its adequacy against the site design basis drop hazards.
Finally, in recognition of the importance of operator training, a reactor compartment test and training rig has been constructed in front of the PCD/ACRC building beneath the RAH parked position. This allows the operators to train on simulated reactor plant using the actual RAH environment and equipment.
14 and 15 Dock facilities
Swiftsure and Trafalgar class nuclear defuel and refuelling activities are performed with the submarines dry docked in either the 14 or 15 Dock facilities within a purpose built submarine refit complex (SRC) at the northern end of 5 Basin. The SRC complex was built in the 1970s specifically for the task of maintaining and refitting submarines. As part of the D154 project the two dry docks have been extensively strengthened. This work has progressed in stages in order to support the navy with planned submarine refit and refuelling in parallel with upgrading work.
During redevelopment, the void spaces along the side of both docks have been filled with reinforced concrete tied to the dock walls with retaining dowels. Again, the new dock walls and floor structures have been built within the confines of the existing docks. The floors have been raised, multi-cellular caissons now seal the entrances and new isolating submarine cradles have been installed. On 14 Dock new seismically qualified dockside cranes have been installed.
The installation of new dockside cranes on 15 Dock and the RAH and refuelling equipment upgrades on both docks remain to be completed at a subsequent stage in the improvement programme.
Nuclear transfer route
An important feature in the redevelopment of the 14 and 15 Dock facilities and the creation of new facilities at 9 Dock is the rail-based nuclear transfer route. The existing rail network has been extended to create a ground level rail-based transfer system. This extends from each dock to the LLRF and equipment maintenance storage facilities. Movements within the Nuclear Licensed Site are accomplished using new dedicated locomotives that draw low level transfer trolleys between facilities. The nuclear containers are supported on the trolleys by transportation frames. An electrical drive capability is provided on the trolleys to facilitate manoeuvring within the RAH and storage facilities without the locomotive.
The route taken by the rail network was surveyed and selected to avoid existing site seismic collapse hazards. Where avoidance was not possible, some structures were demolished. The Devonport NTR is probably the only seismically qualified rail system in the UK.
In order to meet the increased power demand of the Vanguard class submarine both at 9 Dock and at selected berths, the site-wide electrical services system has been extensively enhanced. A new dual ring main electrical distribution arrangement has been created with the addition of two new sub stations. Dual power supplies from the national grid are backed up by a multiple diesel generator capability. Under the new arrangements, no single fault anywhere in the distribution system will result in an inability to re-route and re-supply electrical power. During the upgrade over 200 km of electrical cable and 20 km of pipework has been installed into the site service ducts.
Concluding comments In such a short article I can only hope to convey an impression of the rigorous nature of the design development processes and high build quality controls applied to the development of new nuclear facilities. The Government and the Ministry of Defence have made a significant capital investment at Devonport to ensure that the UK nuclear submarine fleet has refit and refuelling support facilities that satisfy modern standards.
On 15 March 2002 His Royal Highness The Duke of Edinburgh opened the Vanguard Class Submarine Facility and named it ‘The Queen’s Golden Jubilee Dock’. At Devonport we think this is an impressive name to attach to an equally impressive facility. We hope you agree.
All illustrations used by permission of DML Media Services.
FORMER TECHNICAL DIRECTOR DEVONPORT FACILITIES REDEVELOPMENT PROGRAMME
Malcolm Smith was Technical Director on the Devonport Facilities redevelopment programme. He joined the project in 1996, seconded into the Devonport Alliance team from Rolls- Royce Marine Power. He has over 18 years experience in the Naval Nuclear Programme working on the design and safety substantiation of nuclear reactor systems and support equipment. He is a Chartered Engineer and a Fellow of the Institution of Mechanical Engineers.