For drilling and workover tasks, the integrity and availability of position data for station keeping is of upmost importance. Digital Wideband acoustic transceivers and seabed transponders represent the latest in acoustic positioning performance. Acoustic positioning systems are now more reliable than ever before with increased resilience to noise and multipath combined with greater positioning precision. Inertial navigation complements Wideband digital acoustic signals, with high integrity inertial measurements. It is important to choose an inertial navigation solution that is tightly coupled with the acoustics, with the INS co-located with the acoustic transceiver if the performance and efficiency gains are to be achieved. A tightly coupled INS solution makes use of individual range and bearing observations instead of computed positions and can provide reliable position data even in marginal acoustic conditions. The high update rate of an INS navigation solution (1Hz) more closely matches the GNSS data rate and the vessels dynamic model leading to more robust station keeping when compared to vessels not using INS.
The inertial navigation only needs acoustic position updates every 12-15 seconds. Not only does this save seabed transponder batteries, it “frees –up” the drilling vessel’s acoustic system to perform other tasks. For example, between position updates the acoustic system can be used to collect riser angle measurement data, confirm the acoustic BOP status and collect riser monitoring data including current, attitude and displacements. A tightly coupled INS solution is resilient to acoustic disruptions meaning between only one and three transponders per system are needed rather than a full LBL array. This means fewer ROV trips to the seabed and quicker deployment and calibration time. More time is saved if multi-user transponders are deployed allowing a dual independent system to safely share a seabed array of transponders using “subscription mode” which will allow multiple systems to ‘subscribe’ to transponder channels.