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Mojo Working Miracles at Woolacombe
01 Dec 2005
Conducting marine operations in surf zones can be extremely difficult, particularly when the project is a geotechnical survey off Devon UK's premier surfing beach. Such was the challenge posed to Mojo Maritime Ltd when approached by client Lankelma UK Ltd to carry out an intertidal marine site investigation for an outfall extension project using a 4.5 ton subsea ROV crawler unit combined with a Neptune core penetrometer (CPT) system within the surf zone at the surfer's paradise which is Woolacombe Bay.
The ROV was designed to operate in the North Sea at water depths up to 2,000m and the combined ROV/CPT system had never been used successfully before. In exposed marine environments such as Woolacombe, downtime for conventional geotechnical equipment such as drilling platforms, barges and survey vessels can be 10 to 50% even under the management of experienced marine contractors.
Surveys within surf zones are normally conducted by high cost jack-up platforms.
Throughout the Woolacombe project, Mojo successfully deployed the system in 0.5m to 1.0m seas, operated it in 2.0m to 2.5m seas, and recovered in seas of from 1.5m to 2.0m with a crane mounted on their workboat Portree II. During the first falling tide, Portree II was moved onto a small section of beach next to the rocky foreshore. The craft then lowered its bow door and the crawler ROV was driven down onto the beach.
The vessel was then moved to the edge of the surf zone and the ROV positioned and made operational within the survey area. Portree II was moored on a three point mooring system and held within range of the 150m umbilical, remaining on site throughout the survey.
Precise coordinates and levels were required both for the exploratory locations and the topographic survey. A conventional shore based EDM total station was therefore used with an 8m mast, detail pole and single prism. All 19 CPTs and 55 topographic survey locations were determined to 0.1m horizontal and 0.01m vertical accuracy according to OS National Grid datum.
The survey was completed to client satisfaction with minimal weather downtime. Portree II remained on site in onshore winds of Force 6 with 2.5m breaking swell along the beach. The project enabled Mojo Maritime to demonstrate their ability to deliver innovative solutions to the problems posed by challenging marine works in difficult locations.
MJ Information No: 21308
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