In this article, we examine the layout, equipment and initial trials programme for RFA Stirling Castle, the first of the vessels recently purchased by the RN for use as a mothership.
Early in 2023 the former walk-to-work and subsea support vessel, MV Island Crown was purchased from Norwegian owners (Island Offshore) for £40M, primarily as a platform for operating autonomous mine hunting systems. The acquisition of this vessel is positive news and demonstrates visible progress as the Naval Service embraces new ways to deliver capabilities. The generation process has been rapid and she will begin operations within 6 months of concluding the purchase. (It took the RN 9 months for the much smaller XV Patrick Blackett bought off the shelf from the Netherlands to achieve the necessary certifications to go to sea.)
She arrived in Devonport on 29th January still under the Norwegian flag and the MoD purchased her shortly after midday on 30th January. Very little conversion work was done during her time in the dockyard and the focus was on completion of paperwork relevant to the transfer to the MoD as well crew familiarisation and safety aspects. The ship was formally re-named Stirling Castle and registered in London. New nameplates were fitted and the vessel was assigned a pennant number of M01, although this has yet to be marked on the ship’s side. Taking a vessel built and operated entirely to commercial standards and then working out how she will be run using RN and RFA procedures inevitably takes some negotiation and compromises.
By RFA standards she is a relatively compact vessel, 96.8 metres long (including the crane overhang) with a displacement of 5,800 tonnes. She was refitted by the Norwegian owners shortly before sale to the MoD and was handed over in excellent condition, having also been well-maintained in service since she was constructed in 2013. The large tower and compensated gangway used to provide access to and from offshore installations that had been fitted on the port quarter was removed sometime before the ship was handed to the RFA but the long-reach crane has been retained.
In order to save time and expense, Stirling Castle will not be painted grey until the next refit in about 4 years’ time. She has not yet received a military communications fit which enables the handling of secret-grade material although equipment may be embarked later as the RN develops its concept of operations. Force protection weapons have not been fitted yet but mounts for 50cal Heavy machine guns may be added in the future.
The main working deck that runs nearly half the length of the ship will be used to store, prepare and deploy boats and UUVs on mine warfare and hydrographic survey tasks. The boats, accompanying support equipment and C3 systems are mostly designed for transportation in TEU containers and much of the kit will be embarked this way. At least for now, it is envisaged the crane will be used to launch and recover boats over the side. A door in the stern transom and a removable panel in the starboard bulwark also offer alternative routes that craft might be deployed if suitable Launch and Recovery Systems (LARS) were added.
A pontoon could be secured alongside to facilitate personnel transfer on and off the boats. (Despite being fully capable of autonomous or remotely piloted operation, they often have crew on board for safety purposes in busy waters). The Dynamic Positioning (DP) system allows the ship to remain stationary despite wind, tide or currents. The ship can be manoeuvred so that the boats are protected in the vessel’s lee for launch and recovery. Like any small craft, operations will inevitably be subject to safe weather operating limits.
The working deck is well served with adjacent workshops and store rooms for the RN teams to maintain kit. Compared to her sister ships, Island Crown was built with an additional accommodation block aft of the main superstructure. This section will be occupied by the embarked RN personnel numbering up to 43 sailors. There is also a conference room, offices and recreational spaces.
The lightweight aluminium flight deck is not rated for naval helicopter operations and is unlikely to be used except maybe in CASEVAC emergency. Its only role might be as a launch point for small uncrewed air systems. As the flight deck generates considerable windage on the vessel and is slightly surplus to requirements it could be removed in future.
The spacious bridge has a 360º view around the ship and is typical of modern highly automated commercial vessels. When using the DP system, the operator is seated at the rear of the bridge looking aft over the working deck. The RFA currently has very few personnel currently qualified on DP systems and training up more operators will be a priority.
Stirling Castle has diesel-electric propulsion designed for simplicity, redundancy and manoeuvrability. Four diesel generators provide power for the ship’s hotel load as well as the two azimuth thrusters at the stern that can rotate 360º. There are also twin bow thrusters and a retractable azimuth thruster at the bow. In DP mode, the forward and aft thrusters may be set to gently push against each other so that a decrease in power on one will provide an instant response to a change in conditions such as gusting wind.
The two DGs and the stern thruster on each side of the ship are completely independent of the other to provide backup in case of failure. With just 5 marine engineers on board, the machinery spaces are usually unmanned except during routine maintenance. The engine exhaust uptakes that run through the accommodation areas are well shielded the ship is very quiet when underway.
The centre section of the vessel under the working deck is occupied by cargo tanks for semi-liquid products such as minerals used in oil drilling or cement, a legacy of her former employment in support of offshore energy projects. These tanks are not relevant to her new role but remain in place. In-service support will be provided by Cammell Laird as part of their cluster contract with the RFA. CL will conduct maintenance in their shipyard or, if needed, send engineering teams to repair the ship wherever she may be deployed.
Living spaces on board are in line with the usual high standards of RFA vessels. Most of the crew and embarked personnel have comfortable two-berth cabins with en-suite toilet and shower. Many cabins and living spaces have large scuttles and there is plenty of natural light. Officers and crew dine together in a cafeteria and there are several recreation spaces. In common with HMS Protector, Stirling Castle has a sauna, a feature of many Scandinavian-built ships.
Stirling Castle has two crews that rotate every 2 months, (reducing to 36 days once she is operational) in a change from the manning model used by the majority of the RFA where everyone serves at sea for around 4 months, followed by 3 months off before being assigned to another vessel. The ‘one ship – two crews’ model has several advantages as that crew will be on that ship for around two years. This ensures familiarity with the platform and helps build a greater sense of ownership. It also allows a small measure of flexibility for sailors as they can negotiate the timing of when they are relieved by their opposite number on the other crew.
In late June, the RN’s Maritime Autonomous Systems Trials Team (MASTT) and Defence Equipment & Support (DE&S) teams embarked on Stirling Castle to oversee a week of initial trials operating autonomous boats from the new vessel. RNMB (Royal Navy Motor Boats) Apollo, Hydra and Hazard were deployed successfully from the ship in the sheltered waters of Portland Harbour. Stirling Castle has now arrived in Devonport to undergo a specially tailored FOST package to certify the ship and crew are safe before beginning more demanding trials with the MCM systems.
Stirling Castle will ultimately be based in Faslane, co-located with the First Mine Countermeasures Squadron (MCM1) which already operates autonomous boats under Project Wilton. For the foreseeable future, she will predominantly operate around the south coast, from Portsmouth, Portland or Devonport. Thales UK at Turnchapel in Plymouth is working on development of the Anglo-French/UK Maritime Mine Countermeasures (MMCM) programme and French work on the system is being undertaken not far away at Brest in Brittany. Besides MMCM, Block 1 of the RN’s Mine Hunting Capability (MHC) programme includes the Combined Influence Sweep system developed by AEUK based in Dorset as well as a suite of UUVs. Stirling Castle will play an important part in the evaluation of these new systems as well as increasing the range at which they can be deployed.
This vessel adds another capability to the RFA, a service that should be noted as offering careers with plenty of varied and interesting opportunities supporting the RN. RFA Stirling Castle (and RFA Proteus) will provide the navy with useful operating experience of very modern commercial vessels designed for lean manning and efficient operation that may help inform the design of future auxiliaries and warships built around smaller crews. She may also be just the first of the ‘Castle class’ as the RN wants eventually to acquire up to three more motherships to host globally deployable autonomous systems.