Sea Technology

JUN 2013

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

Issue link:

Contents of this Issue


Page 11 of 75

Mulberry A were destroyed, not by the storm directly but by drifting ships that had run out of fuel or broken down and were driven against them by the winds. After the storm subsided, the Americans at Mulberry A dispensed with the Whale units and landed stores over the beaches sheltered by a reinforced Phoenix breakwater. At Mulberry B, the Whale was enhanced, partly using items salvaged from Mulberry A, and both the Gooseberry and Phoenix breakwaters were reinforced, allowing the Mulberry harbors to remain in operation until November, by which time the great port of Antwerp had been captured and took over the role of supplying the Allied army ashore. After the Mulberry harbors were shut down, useful items were salvaged, while the wind and sea gradually reduced the other items to rubble. However, substantial elements still remain, many permanently submerged, and form a lasting monument to the audacity of the planners and engineers who built the harbors during the mid1940s. It is also reported that many of the kite anchors remain since they grip the seafoor so well that, when being 12 st / June 2013 "Although exactly who thought of the idea has slipped into the mists of history, the idea was simple in its scale: if the Allies needed a port but could not capture one, they would have to build one and take it with them." recovered, cables have snapped before the anchors were dragged out, leaving them embedded and buried in the seafoor. Mulberry B Multibeam Survey Although the U.S. had surveyed in 2001 much of the remains of their Mulberry A with multibeam sonar (RESON SeaBat 8125), the British Mulberry B had never been systematically explored with multibeam equipment, although the French Hydrographic Offce (SHOM) had conducted a survey in 1993 using single beam and side scan as a precursor to the 50th anniversary celebrations planned for June 1994. With the SHOM survey being complete, it was not expected that anything new would be discovered, although the new survey, using modern multibeam sonar, would provide much better images of the wreckage and, hence, provide a baseline for any future studies regarding the rate of deterioration. The plan was to berth the survey boat in Port-en-Bessin in France about 5 miles west of Mulberry B. Port-enBessin is a tidally constrained port with the lock gates only opening for four hours around each high water, so the survey boat, Xplorer, hired from Falmouth Divers (now Marine & Towage Services of Brixham, England), was working roughly 14-hour days, leaving on the early tide then returning on the later one. Xplorer is a 12-meter twinhulled workboat with a 1.2-meter draft and capable of 25 knots but also suitably maneuverable at slow speeds. The survey was to cover fully the wrecks of the Bombardons, as well as all the Phoenix caissons and any remaining blockships. Within the harbor, the magnetometer was to be used to locate any remaining kite anchors, while the scanning laser was to be used to record the above-water remains. Xplorer arrived at Port-en-Bessin on September 24, allowing MMT-NetSurvey and UKHO to start mobilization the next day. The RESON Seabat 7125 was mounted on a pole located on the starboard side and hinged so it could be raised when not needed. Raising the pole allowed Xplorer to transit rapidly to and from Mulberry B and, hence, maximized time on station before returning to catch the lockgate opening window. The 7125 was operated at 400 kilohertz, 1 degree along-track transmit beam width and 0.5 degree across-track receive beam width.

Articles in this issue

Links on this page

Archives of this issue

view archives of Sea Technology - JUN 2013