Sinking a Frigate

Photo courtesy of Dave Peake

Two years ago Europe’s first artificial reef was created off the coast of south east Cornwall. Michael Leece OBE from the National Marine Aquarium explains why this innovative disposal solution was employed and how it has served as not only an exciting outreach programme but also an important new home for marine life.

The selection of HMS Scylla, a Leander Class Frigate, as an artificial reef was not random. To Plymouth and the surrounding area, HMS Scylla had a particular appeal – she was the last warship to be built in Devonport Royal Dockyard. This was an important factor which contributed to securing the public funding (circa £1.4 million). Scylla was built in 1968, is 113 metres long with a displacement of 2,500 tonnes and a beam of 13 metres. She had served in many locations including the Cayman Islands and the Persian Gulf and was decommissioned in 1993. HMS Scylla was then bought by the National Marine Aquarium in 2003 to be placed on the seabed as a reef.

Underwater Attraction

The creation of a reef using a frigate is a well-proven system – vessels have been placed on the seabed in Canada, Australia and New Zealand. The opportunity of the Scylla project arose at a time when the National Marine Aquarium (NMA) had decided to pursue more actively ‘man’s interaction with the oceans’; the key criteria of its charitable objectives.

The idea was to allow Scylla time to become colonised by anemones, sea squirts and other marine life, so that fish and other mobile animals would be attracted to the developing reef. Scylla would then become an exciting destination for divers. Like all reefs, Scylla is a fragile environment, and there is a voluntary 'no take zone' around the reef in the hope that fishing will stop in the zone, allowing marine life to develop to its full potential.

Being close to the placement site, Plymouth was selected as the location to carry out preparation works on the vessel. To maximise the economic impact, an early decision was made to have Scylla submerged and ready for the beginning of the 2004 dive season. We at the NMA quickly enlisted the services of consultants with experience in explosive cutting on similar projects. Their organisation was based in Canada and was supplemented by members of the Devonportbased Royal Navy Southern Diving Unit, who also have expertise in handling explosives.

Insurance covering risks for Scylla’s tow from Portsmouth to Plymouth, her conversion period, final tow and placement, and subsequent post-placement events entailed an interesting series of negotiations. UK underwriters had never been approached by a ship owner who had deliberately bought a vessel with the intention of sinking it at a pre determined location and time!

Preparing Her Bodywork

Defra (Department for the Environment, Food and Rural Affairs) is the UK Government licensing authority, and Scylla had to be made environmentally sound according to standards laid down by Defra. Any hazards or harmful materials were removed in order to make the ship fit for the seabed and marine life. Once complete, the ship then had all diving hazards removed. Potential snagging hazards were eliminated, doors welded shut or open to ensure diver safety, and additional diver access holes were opened up throughout the four-deck ship.

Scylla was docked down to remove the sonar domes and to enable the neox stern glands to be drained and the vessel propellers removed. A four metre clearance between the highest point of the reef and the Lowest Astronomical Tide (LAT) has to be maintained so that the reef is not visible from above sea-level or land. In order to maintain LAT clearance the main mast and the funnel were removed.

The bilge area and diesel tanks were steam cleaned and major electrical cables removed. Final removal of hydrocarbons from the bilges and similar areas was achieved through the use of ‘Oclansorb’, a dried peat which absorbs hydrocarbons. The ultimate test was the absence of any oil slick during the vessel’s sinking.

Diver access openings and routes were created. Each space had two openings for access and/or egress. Where spaces were deemed unsafe they were physically blocked off and entrances closed and locked. Divers have been given access to the superstructure, all of decks 1 and 2 and partial access to decks 3 and 4. Diving highlights include the captain's cabin, the bridge, galley, mess decks and living accommodation, operations room and the engine room. The boiler room became a ‘no go’ area due to its confined and congested space. Video recordings were made of all areas and copied to the local police and other rescue services in the event that they were required post placement.

That Sinking Feeling

One of the project’s greatest challenges was the installation of explosives, since the Dockyard and Devonport Naval Base facilities operate under the auspices of a Nuclear Site Licence. Only the explosive charges and their associated firing cords were placed prior to departure from the Dockyard – the detonators were fitted outside of the Dockyard Port of Plymouth. To achieve the required trim and stability during the sinking process a timed sequenced for the detonation of charges was incorporated. The Southern Diving Unit (SDU) of the Royal Navy controlled the deployment of the detonators and their firing. In addition, the SDU team were to be the first to dive Scylla post placement to ensure the effective discharge of all explosives and declare Scylla safe to dive.

A total of 168 explosive charges enabled high-tech cutting devices to cut several holes in strategic positions along the side of the ship. This would allow water on board and bring the ship down in a controlled manner. Figure 1 illustrates the location for some of the holes. A tab cutter is a reference to the final corner cutting of pre-burnt through areas of shell plate above the waterline.

The primary explosive used on the day was LSC (linearshaped charge). LSC is an explosive enclosed in a seamless metal sheath and fabricated in continuous lengths shaped in the form of an inverted ‘V’. When detonated, LSC produces a uniform linear cutting action. On detonation, the focusing of the explosive high pressure wave, as it becomes incident to the side wall, causes the metal sheath of the LSC to collapse – creating the cutting force. This high velocity plasma jet impacts the target with pressures exceeding the target’s yield strength, and literally pushes the target material to either side of the path of the jet. The sheath used was formed from copper and contained a core load of 1,200 grains per foot. This cutting technique produces clean, smooth edges, ideal for diver access.

Simulated Sinking

Our insurers required confirmation of Scylla’s structural integrity and stability during transit to placement at site, and final sinking operations. So we performed an inclining experiment before the vessel left the dock that confirmed that she had adequate stability for the specified conditions of tow to site. Computer modelling tests – excluding dynamic effects – that were required to simulate the sinking of the vessel were carried out at WUMTIA (Wolfson Unit for Marine Technology and Industrial Aeronautics). The openings in the vessel were modelled, in sequence with the time that they were to be detonated.

The important aspects of the simulation were to ensure that the vessel had a positive GM for as long as possible (see figure 3). This would be hindered if the method and sequence of flooding resulted in water on a deck above a compartment that was not fully flooded, creating a free surface situation. The other key factor was to ensure that the vessel had a bow down trim (this means that the ship sinks bow first). However, this could not be too extreme, or there is a risk of breaking the vessel in if there is a very fast speed at impact. During the initial stages of the sinking process the vessel needs to remain upright and initial stern down trim starts to be corrected. The vessel’s GM steadily decreases, as one would expect with rapid flooding (see box). The vessel then, on losing considerable amounts of GM, begins to heel. This heeling creates more water plane, which is required for the vessel to remain stable.

Figure 4 shows that past 24 seconds the vessel has little or no GM and will therefore capsize. The heel remains constant at 10 degrees and is not shown to increase during the sinking. The trim steadily increases throughout the sinking in a bow down aspect. The simulated actual sinking time was recorded as two minutes eight seconds.

Down She Goes

On 27 March 2004, Scylla was successfully placed on the seabed. Through four balls of fire, reaching 20 metres into the air, spectators were treated to a pyrotechnic display. The placement operation went smoothly with Scylla resting on the seabed almost vertically, her bow facing the south west, although not all of her keel is in contact with the seabed. She has now remained stable for over two years. Scylla is located 50º 19.64N 004º 15.2W.

Coverage of the event achieved unprecedented success, with the sinking being broadcast live to over 26 countries worldwide, and an audience of around 60 million witnessing the final placement. Dive activity began almost immediately. Installation of web cameras was carried out during the summer and autumn of 2004 with connection to the surface radio transmission buoy in 2005. The electronics and cameras in the system provided live images as designed. However, the mooring system holding the communication buoy on station has failed on two occasions due to severe weather in shallow water. Images have been restored in 2006.

Hidden Depths

Results to date suggest that the vessel is colonising much quicker than anticipated and no unusual environmental issues have emerged. Figure 2 shows examples of the colonisation to date. Because of the different conditions on the two sides of the reef, different communities will grow up. The starboard side of the wreck is expected to be the richest in marine life. Sea squirts, sea anemones and soft corals such as dead man's fingers will dominate the communities. Curious fish like pollack, bass and possibly scad or mackerel should begin appearing soon. The deepbodied bib will take up residence around the wreck, and it won't be long before the first conger finds a suitable hole to make its lair. Fairly quickly a colony of tiny animals called sea firs will develop and filter feeding animals like barnacles will settle. Blennies and, in time, triggerfish will start to feed on these settlers.

Scylla now provides a venue for leisure diving and a centre for education and research as she becomes a reef. The National Marine Aquarium team carries out regular dives in Whitsand Bay and coordinates many marine based scientific programmes with the support of Plymouth's renowned Marine Sciences Partnership.

The Scylla project demonstrates that it is possible for public sector organisations to fund novel and contentious projects in partnership with professional organisations. An initial Economic Impact Study by the South West Regional Development Agency suggests that the £1.4 million capital expenditure has been recouped within the first year of operation. The NMA’s involvement with Scylla was instrumental in securing £3.6 million of further funding to develop an innovative science and technology exhibition that enabled ‘ExplorOcean,’ illustrating man’s interaction with the oceans, to open in March 2006.

GM

In the simplest terms GM is a distance, the distance between the centre of gravity and the vessel’s metacentre. GM is a measure of stability where positive values = positive stability.

Any freely floating object displaces its own weight of the liquid in which it floats. This weight acts downward through the centre of gravity (G) and is resisted by an upward buoyant force (equal to w), which acts through the centre of buoyancy (B). This is the geometric centre of the submerged volume displaced by the vessel. The point through which all these vertical forces act when heeled is called the metacentre (M).

Further reference

Visit www.national-aquarium.co.uk/scylla

Biography – Michael J Leece OBE CEng FIMarEST FIMechE

Michael Leece has been Chief Executive of the National Marine Aquarium since 1997, and in that time he has overseen further expansion and development of the Aquarium into an internationally recognised leader in the public engagement of science. He was awarded the OBE in November 2003 for services to tourism.