Originally Posted by Unusual Attitude
The stipulation within the last tender issued that AUV's would be used for this search raised eyebrows amonst many involved in the subsea search & salvage industry.
An AUV is designed to fly over ideally a flat sandy bottom flying a pre programmed route using waypoints, ideal for a lengthy cable / pipeline / route survey. The terrain involved in the AF447 is about as far from ideal as you could get for the use of AUV's. The second factor being the battery life of an AUV and the time taken to submerge to 4000m, basically by the time it gets down there there is very little time left to actually do any searching before it then needs to start the journey back to the surface.
I'm curious to know what alternative you propose and what its search rate and percent coverage would be in mountainous terrain. All other systems I can think of are tethered and have issues of their own.
While any search system works better on a flat bottom, AUVs have characteristics that make them well suited to this work:
1. I covered the search rate parameters in post
http://www.pprune.org/5692187-post1029.html
It works out to about 40 square kilometers/day for double coverage by one vehicle. [BEA: 100sq-km/day/AUV, maybe for single coverage.] A claim of much time lost in vertical transit is simply incorrect. REMUS-6000 descends and ascends at an average rate of about 60m/min, so it can make a one-way excursion to 3500m in 1 hour. With a 20-22 hour mission time, 2 hours lost in descent/ascent, and a 2-4 hour turn-around at the surface, that yields 18-20 bottom hours out of every 20-26 hours. Even with the worst of those values, the vehicle is on the bottom 70% of the time.
2. When surveying the bottom, the AUV transits at 4 knots. All the towed systems I know of are limited to 1-2 knots in deep water, to prevent the cable drag from lifting the vehicle off the bottom. [BEA: Orion system is listed at 2kts. BEA also lists 100sq-km/day/towed sonar, probably using lower frequency, lower resolution, but longer range to offset the slower tow speed.] Perhaps if the entire cable is faired, then the speed could increase, but that would significantly increase the size and complexity of the cable handling system.
3. AUVs spend no time in turns (other than the few minutes it takes to cover the offset distance between survey lines). [BEA: 15min for AUV.] Towed systems follow the ship track with 30-60min delay in deep water, and it can take 6-12 hours to align with the next track. There are perhaps operating skills that could shorten that time, but it won't be minutes. [BEA: Orion system turn time listed as 3hr.] Turn time is to the towed system, as vertical transit/servicing is to the AUV.
4. AUVs can follow the bottom more closely in rugged terrain than towed systems, at least more closely than I have seen towed operators willing to attempt. Contact of a towed system with the bottom in rough terrain carries the risk of fetching the vehicle under a ledge and breaking the cable if the winch operator and bridge watch don't react in time. For an AUV, collision with a cliff is a survivable event. While no one plans a mission to include a collision, it does happen and it is recoverable, usually with the mission continuing as planned. Aside from a collision, an AUV can follow terrain at about +/-30deg (depends on the AUV). Certainly there are steeper parts in the AF447 search area, but a lot of it is within +/-30deg, especially with "terrain aware" route planning.
5. Multiple AUVs can be operated from a single ship, further increasing search rate for a nominal increase in day rate (cost).
6. Navigation of track lines is generally more precise with an AUV, because there is no influence from ship motion. This improves coverage (fewer holidays) and becomes especially important if the debris field of AF447 is ever found, when the AUVs can be used to photo mosaic the field.
After the wide area search and initial photo work are complete, then tethered systems will be needed to finish any detailed survey or recovery.
Certainly, towed/tethered systems have advantages too. The most obvious being that the sensor data is available in real time, and similarly there is much higher bandwidth for control of the vehicle.
Enough said. I'm ready to be educated about the system(s) you have in mind.