The Ivory field, operated by Centrica, is located in the Norwegian Sea, license PL528 near the Aasta Hansteen development in just over 1400m of water. Prior to the Q4 2014 drilling campaign a suction conductor anchor node (CAN) was installed with SSBL by DOF Subsea, to a tolerance of better than ±10m in absolute position and ±1.0 degree in inclination.
As the contracted construction support vessel "Normand Reach" was equipped with a Dual HiPAP 501 SSBL/LBL acoustic positioning system, cNODE transponders operating with CYMBAL digital ranging and telemetry were chosen to meet the positioning and attitude tolerance of the CAN installation. The equipment for the job was mobilized by Kongsberg Maritime Ltd, Aberdeen, which stocks a large rental pool of cNODE transponders and ROV transducers to meet the needs of subsea construction and survey projects. One cNODE transponder was interfaced to an Ashtead external gyro package providing heading, pitch and roll. For redundancy two additional cNODE transponders equipped with Multi Sensor Modules, including high accuracy inclinometers, were fitted to the top of the CAN.
A dedicated APOS survey operator station is installed in the online room to allow shared use of the HIPAP between DP and survey.
The cNODE transponders were interrogated in "Fast track" mode. Simultaneous update of position and sensor data for all of cNODE transponders was acquired every 3 seconds in 1400m of water. Data from the gyro package and the inclinometers are embedded in the normal navigation pulse, and don't require any extra telemetry messages to be transmitted.
The suction CAN was installed in its final position within tolerance. The position, gyro and inclinometer data was telemetered acoustically continuously from the time the suction can was hung into the water, throughout the lowering and for the entire suction operation. In addition the gyro package displayed the heading pitch and roll on an ROV LED display.
Below from the online room during the operation. This is the control room for the two Kystdesign Supporter Work Class ROV systems and for the survey team.
After final installation the transponders and gyro package were recovered from the CAN and an LBL array of 8 Kongsberg transponders, were installed for the next phase of the project.
The shared use of the vessels HiPAP as a DP and construction survey tool enhances operational efficiencies, by providing high speed positioning and communication of target transponders, when the ROV is on deck and without the need for a dedicated over the side transducer pole.