Technology gives new view of HMS Royal Oak
30 Oct 06
The DLO's Salvage and Marine Operations (S&MO) Integrated Project Team (IPT) has completed a cutting-edge survey of HMS Royal Oak, the Second World War wreck in Scapa Flow, Orkney.
Sonar image of HMS Royal Oak.
The survey, conducted with St Andrew's University school of history, employed a Reson 8125 Seabat Sonar head which uses the latest visualisation technology and techniques to gain images of the wreck as never seen before. The survey was so successful that it will now set the standard for all future wreck surveys and pave the way for advancing survey technology.
The new images will help the IPT in its ongoing task of removing oil from the Royal Oak. To date all double bottom tanks have been pumped off and last year a pilot phase was conducted in removing the oil from the inner wing tanks.
Following this, a full scale removal is to take place in the summer. Prior to starting the operation, it is important to have as much information as possible to inform the risk assessment. The new images will help in determining whether or not the wreck has moved or settled further.
HMS Royal Oak was sunk by a German U-boat in Scapa Flow on the night of Friday 13 October 1939. Three torpedoes sank the ship within 13 minutes and of the 1,400 crew, 833 people lost their lives. The oil onboard when it sank has been slowly leaking from cracks and holes in the upturned hull ever since. Because of the wreck’s status as a war grave, where diving is normally forbidden, work on the wreck has to be done sensitively and with minimal intrusion.
The Cutting Gear
[ Source: MoD ]
In the early 1990s oil leakage increased and action was required in order to reduce the impact of a catastrophic failure of the hull and the resulting oil spill. The consequences of a large oil release would be very damaging to the local Orkney environment. After the MOD experimented with a few different techniques to contain the oil, the decision was made to remove the problem rather than contain it.
Hence, it was decided to use a process called hot-tapping, which involves fitting valves in strategic places on the hull, allowing oil to be extracted from within the wreck without releasing any oil into the environment. Using divers, a hose is then attached to the valve and the oil is pumped to the surface into a barge. The task of hot-tapping was passed to the S&MO due to its proven expertise in underwater intervention.
It all sounds quite straightforward, but there are complications that the team has to take into account. There are a number of explosives still onboard, which are very sensitive and the team have to be very careful not to disturb them. The quantity of the oil remaining is based upon assumed leakage rates over the years and the assumption that the ship was fully bunkered prior to the sinking. Also, a large quantity of oil was released when the torpedoes struck which resulted in many of the sailors perishing in the burning and suffocating oil.
The exact configuration of the internal tanks is based on research and drawings of other ships in her class, as all the ship’s drawings were destroyed during WW2. The Double Bottom Tanks immediately inside the hull contained about 50 per cent of the oil and the hot-tapping process has extracted the majority of this, but there are Inner Tanks inside the wreck that could hold anything from 900 -1,600 tons of oil. In summer 2005, the S&MO team began to tackle these.
HMS Royal Oak in 1938
The Inner Tank boundary construction is thinner than the outer hull and it had been predicted that this boundary may have wasted over the years, allowing the oil contained within the inner tanks to migrate into the ‘hot-tapped’ outer tanks. This could result in the majority of the remaining oil being removed over time using the ‘hot-tap’ valves. However, S&MO’s yearly visits to the wreck proved this to be untrue because operations on the wreck in recent years have resulted in very little oil being removed, indicating that the tank boundaries where relatively sound, so relying on oil migration alone would be a lengthy process. Therefore the project team, alongside Briggs Marine Contractors Limited, developed and trialed a cutting tool capable of penetrating the inner tank boundary from the existing ‘hot-taps’. It was predicted that this would enable the oil to flow from the inner tanks and be collected from the outer, ‘hot-tapped’ tanks.
After careful consideration and theoretical study, a small number of areas were identified as potential targets for trialing the newly developed cutting tool. During the 2005 operations the tool was trialed and one single penetration resulted in over 180 tonnes of previously inaccessible oil being recovered. This was a great result for the project team and this year S&MO will try to remove more.
Operations during the first two weeks of September will focus on reducing the amount of oil contained within the wreck by penetrating two further inner tanks. The results from this will help determine further penetrations and give a better understanding of the wreck. S&MO will also be pumping out the oil from the two main leakage areas, reducing the sheen, which builds up slowly over the year.
The wreck is monitored and surveyed regularly to monitor any movement or deterioration. S&MO have to take time extracting the oil in order to keep any disturbance to a minimum. However, they need to get the oil out sooner rather than later because eventually the wreck will deteriorate and as it does so, any intervention will pose greater risk to the divers . Each time S&MO visit Scapa Flow they learn a little more about the wreck and the process of extracting oil from it. The team needs to ensure the environmental risk remains as low as possible without weakening the structure.
S&MO will be carrying on with this work for the foreseeable future. New technology is emerging all the time, which can help them better understand what is happening to the wreck. A multi-beam sonar survey was carried out during the Spring which has produced some of the best images yet of Royal Oak. The wreck is very stable and sturdy considering her age, and the survey will help S&MO continue to reduce the risks from the wreck to the environment.