TOTAL Exploration & Production Angola has recently equipped the GIRASSOL FPSO (Block 17, Offshore Angola) with a Riser Tower Monitoring System.
The aim of such a system is to provide real time positions of the towers and activate an alarm in case of unusual movement. The three 1300-meter-high riser towers, located 200m away from the barge are mounted with MPT319/DT transponders at the top, 50m below the surface.
The KONGSBERG HiPAP 350P acoustic system was chosen to provide the top of the risers' positions relative to the FPSO. The transceiver deployed on a pole alongside the hull is interfaced to the APOS computer through a subsea junction box and a Fibre-Optic topside converter.
The custom built junction box is used to convert the fibre optic signals from surface to the HiPAP 350P transceiver and to convert the surface 220V AC power to 48V DC. Also, due to the limited depth of the top of the risers, this pole is equipped with a special 30 degree angled flange to provide full tracking from surface to 120 degrees down.
This monitoring acoustic solution was mobilised and installed by Fugro Survey BV, which also designed a dedicated application based on Fugro Starfix (SEIS module) to recover positioning data and to compute, log and display it for long term analysis. An initial situation based on the first months of logging will be analyzed and the average positions and standard deviations will be defined as the nominal 3D-positions in order to highlight excessive movements that should set off the alarms. The system is also designed to take into consideration the FPSO capacity using 3 states - full, half empty and empty - that have a direct effect on the relative riser tower elevation.
Any deviation out of the nominal values will indicate damage or malfunction on the buoyancy tanks and thus provide further controls that may not be noticed during the regular inspection surveys.