Friday, December 23, 2011

About that awesome ferry crash

Yes, well. The BC Ferries Super C class ships were described by the corporation itself as being awesome. I'm reckoning you know that's not what I mean.

The 21 December crash into Berth 1 at Duke Point was awesome. And before anybody tries to correct me, 5 knots headlong into the ramp is not a "hard landing"; it's a crash.

Some initial reading
There has been a fairly busy exchange of information going on at On The Waterfront by Vancouver journalist Christina Montgomery. This initial post is merely an expansion of some of the information there and I would recommend reading through the posts and the comments related to the crash of Coastal Inspiration.

Different ship - same gremlin
BC Ferries reports that an electronic failure occurred preventing the pitch control on the drive propeller to respond to bridge controls. That means that this is the second time a Super C has lost pitch control when approaching a berth. Coastal Renaissance experienced something similar two years ago. The difference there was that the ship was well back from the berth and the problem occurred as the ship was supposed to be slowing to make its approach. When the ship failed to slow in response to bridge command it sent senior engineers scrambling and the problem was rectified before the landing was affected.

The decision that saved a few million bucks
The Super Cs are propelled by a diesel-electric machinery system. In simple terms "prime movers" generate electrical propulsion power which is transferred to electric drive motors. In a decision which caused Flensburger Schiffbau-Gesellschaft, (the builder), some concern, the drive motors run at a constant speed turning shafts at either end which mount variable pitch propellers. Speed is controlled by changing the pitch angle of the propeller blades. In transit mode the "forward" propeller is feathered to reduce drag and the ship is driven by the "aft" motor, shaft and propeller. To reduce speed in transit mode the pitch angle on the drive propeller blades is reduced to give it less bite. While this is effective and efficient, the motor, shaft and propeller continue to turn at full speed - and therein lies a problem. If, for any reason, the pitch on the propeller blades does not come off when the controls are worked, the ship will not slow.

FSG was apparently not thrilled with the decision to use constant speed motors. Nor was ABS, the classification society overseeing much of the construction. A better option would have been to install variable frequency drives with cyclo converters or tap converters to control the electrical output to the drive motors and thus be able to control the speed of the drive shaft. If the propeller blade pitch fails to come off when commanded, the shaft speed could be reduced or even stopped thus slowing the ship.

The problem boiled down to dollars and euros. Variable frequency drives would have come in somewhat more expensive and the maintenance requirements for cyclo or tap converters is relatively high. BC Ferries went for the cheaper, much simpler propulsion system. 

The kicker here is that BC Ferries knew all about this. In December 2007 BCFS was handed a report prepared by the firm of consulting engineers The Glosten Associates Inc., who were contracted to conduct a due diligence review of the Super C construction. They highlighted an area of concern entitled Propulsion System Reliability and it now appears those concerns were well founded. From that report:

There was considerable discussion at the site regarding the use of constant-speed propulsion motors instead of variable-frequency drives. While the constant speed motors simplify the overall system and eliminate the high maintenance cyclo converters, it results in the propulsion machinery running at full RPM all the time. The primary drawback is at the dock; when the nearly-feathered propeller is rotating at full RPM, any failure in pitch control could have severe consequences. This system has been fully reviewed and completely accepted, so while we do not suggest any physical changes, BCF should fully understand the pitch control reliability and failure modes, and consider including this topic in their vessel training program.

So, save money on simpler propulsion. Spend a lot more rebuilding a destroyed terminal berth.

Added: At least one reader here is interpreting the above incorrectly. Seems to think I believe that the forward system is engaged when the ship is in Transit mode (Mode 1). That is not the case. When the ship reverts to Transit mode from Harbour mode, after switching to Normal, the forward drive motor is disengaged, the shaft is braked and the propeller is feathered to reduce drag. My point, (and the issue pointed out by the surveying engineers), was that the drive propeller, which is nearly-feathered as the ship approaches the berth is running at full RPM. If a pitch control failure occurs the ship is suddenly launched forward.

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