Over the years, there have been many incidents of collisions and oil spills at tanker loading buoys due to adverse wind and sea conditions. Fugro GEOS metocean monitoring system was selected as the ideal solution for measuring waves, winds and current profiles in real-time, to avoid such breaches of safety and reduce the risk to the environment.
The monitoring system works by gathering data on key metocean parameters and feeding these back in real-time to the onshore control room and to pilots of tankers and tugs via portable onboard units. This information assists pilots when approaching the buoy and helps them to make critical operational decisions, such as whether to continue loading or disconnect from the buoy. In addition, if there was an oil spill, on-site data are already available to plan an environmental response.
The CALM buoy is moored south of Pointe-Noire in 35m of water and is used to export crude oil from the Nkossa field. Tankers can lie alongside it for up to 36 hours to load the oil. During this time tanker pilots need to monitor the wind and currents constantly. Initially winds have the greatest effect on the tankers but, as their drafts increase, currents start to dominate. Either may cause the tankers to rotate about the mooring.
Fugro GEOS' monitoring system was incorporated within the buoy and designed for operation in hazardous areas with low power and maintenance requirements. The system itself uses wind speed and direction sensors, compass, motion sensor, wave radars and Acoustic Doppler Current Profilers (ADCPs).
Two sets of wave and current profilers are used in the system, so that a wave sensor is always on the weather-side of the buoy. In addition, the buoy can rotate about its six mooring chains, requiring two ADCPs to ensure that one always has a clear beam pattern without reflection from the chains. Specially written software, controlled by an onboard computer, carries out sophisticated quality control procedures so that the best data are picked and relayed by telemetry.
Fugro GEOS’ Project Engineer Mark Palmer expanded on the software’s task: "Not only does it select the most appropriate wave or current sensor, it also compensates both wave sensors’ data for the buoy’s motion. It does this using the heave, pitch and roll sensor located towards the centre of the buoy, making separate calculations for each wave sensor. And because of the buoy’s rotation, the anemometer direction is also compensated using a separate compass installed on the buoy.
The monitoring system at the Djeno CALM buoy has been in use for several months and already the feedback has been extremely positive. Ongoing maintenance for the metocean system has been awarded to Fugro GEOS’ sister company, Fugro Topnav, who also supplied in-country logistical support for the installation of the system.