Earlier this year, Lwandle Technologies provided Murray & Roberts with real-time wave measurements to assist them with the towing and installation of concrete caissons for the extension of the Robben Island breakwater (Figure 1).
Figure 1 – Vessel towing concrete caisson to Robben Island
A GPS-based wave buoy from Datawell in the Netherlands was deployed in Table Bay when required over a two month period. Information on wave height, period and direction was transmitted in real-time via a radio telemetry link to a portable computer in Murray & Robert’s satellite office in Cape Town harbour. Datawell’s w@ves21 software was used to display the incoming data (Figure 2).
Figure 2 – Display of real-time data from wave buoy
The GPS buoy itself weights only 17kg, making it the ideal instrument for quick and easy deployment and recovery from a small vessel (Figure 3). It relies on the principle of measuring waves with a single GPS receiver, a technique which is well established in oceanography as evidenced by several publications and users in the field. The buoy can measure waves from–50m to +50m and with periods ranging from 1.6 to 100 seconds. The accuracy is 1-2cm, which is more than sufficient for this type of application.
Figure 3 – Wave buoy on deck of vessel just prior to deployment
Clive Smith of Murray & Roberts commented as follows:
"Stability tests for the two caisson types, (i.e. overall beam and length of caissons similar, only freeboard differed) required Murray and Roberts to ensure that towing conditions relating to swell heights and periods were critically monitored before a decision was made to move the caissons from Cape Town harbour to Robben Island.
Total distance of open sea passage equaled 7 nautical miles, and a total of 4 hours towing time with two small tugs. Conditions along the route vary considerably, with the worst case swell conditions expected off Whale Rock where deep sea swells experienced a shoaling effect near the Island. However, this was not always the situation as it had been observed on occasion that varied wave conditions occurred in different areas.
The cost of installing moored multiple waveriders along the tow route would have been prohibitive in costs and also, as the route crossed busy shipping lanes, a problem existed in the protection of these buoys. The use of the GPS buoy allowed us to have absolute freedom of choice in terms of wave data capture in unlimited positions along the tow route, and, at any time. No large vessel was required to move the buoy, simply deployment from a ski boat. Real-time information was relayed directly to a Site office for quick analysis and critical decision making. We found that the use of this equipment was user friendly, accurate and a worthwhile tool in the successful tow of the caissons and hence the Project"
Based on the success of this project, a number of other clients have expressed interest in the buoy and Lwandle is currently engaged in discussions with a number of companies for rental / sale of the buoy. Lwandle Technologies is a relatively new Black Economic Empowerment (BEE) company providing a range of meteorological and oceanographic (metocean) services to, inter alia, the oil and gas industry, ports & harbours and marine& coastal engineers. A number of Lwandle’s members graduated from the oceanography department at UCT.
For further information, please contact Craig Matthysen at (021) 794-0313 or info@lwandle.co.za.
