2006 Sonar Survey of the wreck of the Royal Oak, Scapa Flow

Earlier this year Salvage & Marine Operations IPT/DSC of the Ministry of Defence (MoD) had a requirement to survey the wreck of the Sovereign Class vessel HMS Royal Oak, the first battleship to be lost in the Second World War whilst moored in Scapa Flow, Orkney. ADUS undertook the survey in May 2006, using a local vessel Flamborough Light, after a successful tendering process.

The Royal Oak was built in Devonport in 1916 and saw active service at the Battle of Jutland in 1917. In 1922 she was fitted with ‘blisters’ or anti torpedo bulges along the sides, which were also designed to give additional buoyancy and inhibit rolling. She had a major refit in 1936 at Devonport and, at the outbreak of war in 1939 was sent to the main fleet anchorage in Scapa Flow, Orkney. Here she acted principally as an anti aircraft platform to help protect Kirkwall and local military installations. As she was not considered a front-line ship, the compliment included a high proportion of boy sailors.

On the night of the 13th October 1939, in a daring mission, U boat commander Gunther Prien managed to manoeuvre his submarine U47 into Scapa Flow through the narrow channel at Kirk Sound that had been thought to be effectively blocked by sunken ships designed to deter the enemy. Shortly after midnight U47 approached the unsuspecting Royal Oak laying at anchor in Scapa Bay. Most of the fleet was out to sea, and Royal Oak was the only capital ship present.

The Royal Oak was thought to be well protected against submarine attack, but after being struck by four very well placed torpedoes from U47 the ship listed to starboard and sank within 15 minutes, becoming the first battleship to be lost in the Second War. Gunther Prien returned to Germany to receive a hero’s welcome, but 833 men of a crew of 1, 234 aboard the Royal Oak had died.

Salvage & Marine Operations required the survey in order to provide highly detailed sonar images of the current state of the Royal Oak, which are used as a practical aid in their ongoing operations to remove the quantities of oil from the hull that still remain after 67 years. The sonar images created by ADUS are of such accuracy that even small changes in the hull over time, i.e. through deterioration, can now be monitored closely year on year. This approach will not only help reduce the impact of a catastrophic failure of the hull should it occur but, better still, allow a much greater understanding of how and when the wreck might break up.

The survey undertaken by ADUS used a multibeam sonar system manufactured by Reson Offshore Ltd. to produce images of the wreck of the Royal Oak. This system is deployed from a survey boat and is mounted on the end of a framework arrangement (referred to as ISHAP) specially developed by ADUS in order to get the sonar head as close to the wreck as possible when in the water. The sonar system builds a three dimensional model of the wreck and the seafloor by collecting many millions of accurately positioned ‘spot heights’ derived from the sonar ‘pings’ sent and returned to the sonar head as the survey boat travels back and forth over the wreck. The many millions of ‘XYZ’ points generated from the survey are edited in computer software to generate three-dimensional images and fly around movies of the wreck and the surrounding seabed.

In order for the multibeam system to produce such detailed images the sonar head itself, attached to the survey boat, has to be very accurately positioned. This is achieved by using the satellite positioning system we know as GPS, but with an additional correctional basestation set up on the nearby shore. In addition the motion (i.e. roll pitch yaw and heave) of the survey boat has to be effectively compensated for in the data collected by using very sensitive motion reference systems onboard.