Bourbon Dolphin capsizes

8910' chain for a rig-vessel distance of 8827'? That sounds almost physically impossible. Has to be checked.

 

As per a quick check, length and sag of cable would have been of around 2885 m (9,466 ft) and 454 m (1,491 ft) respectively, considering the following:

Both ends are at the same height and there is no hydrostatic vertical push on cable (which they are not, but let's asume this for the easiness of calculations).
Distance Rig-Vessel = 2690 m (8,827 ft)
Tension at winch = 330 MT
Propeller Pull = 194 MT
Estimated angle with horizontal of dropping cable at the stern = 54 deg.
Cable weight for unit length = 160 kg/m (107.48 lb/ft)

For the estimated cable length of 2700 m (8,860 ft), distance Vessel-Rig should have been of 2537 m (8,325 ft) with a sag of 403 m (1,321 ft), the rest of data remaining the same as before.

(If I'm not wrong)

Cheers.

 

Using his mooring program we weren't able to get the reported force of 330 tons at all - there just wasn't enough bollard pull available to get that amount of force in his program. It is made more for determining the force at the fairlead of the rig. I was assuming that with a system being suspended at both ends the force at each end should be the same...might be missing something. --Chuck

 

Probably I'm totally wrong, as I'm checking this with a very simple catenary calculator and asuming several simplifications, but to get a cable length of 8910 ft for a rig-vessel distance of 8827 ft, tension in cable needs to be 907 MT!

 

Working a little bit more accurately on it (I do not have the adequate program, so I have to tediously iterize):

With a horizontal tension of 195 MT (+/- bollard pull), the maximum catenary admisible for a 2700 m long, 160 kg/m chain, hanging from two points with a difference of heights of 25 m, would have a sag of approximately 521 m under lower support (vessel), for a vessel-rig distance of 2320 m. No way of making the catenary more flat with that bollard pull.

Supposing no frictional resistance, that brings a tension to the winch of around 290 MT. Could it come up to 330 MT if we consider friction and dynamic loads? 40 MT is quite a bit!

 

Simple, clutch in the washing machine, that should do it! Lets not be too critical here guys, your talking 'whatifs' not definates! Remember that whilst 100% may be perfect there's not much chance of achieving it so why cloud the issue with talk of it? Remain at the practical level and it becomes believeable, wander into the theroretical and you will be dismissed as 'dreamers' and the good work you can do towards Safety at Sea will be lost!

 

Whilst I wouldn't normally 'bite', to the coward that just took away some points, you probably don't believe me but you know I don't really care! I say as I see and if you don't like it thats your perogitive; but at least be brave enough to admit it, my little coward!

Hey just think I have something not to believe in,you have nothing but a cowardly manner!

Sorry Guys rant over, just can't stand cowards is all!

 

As I understand the bridge witness's evidence he said that the rig let out 912 Mtrs of 88mm chain and this was linked to a 900 Mtr length on the BD. He quoted that just before the capsize there was 1800 Mtrs out, which I presume includes the rig chain

Murdo

 

Murdo:
If chain length is 1800 m, asuming an uniform weight of 160 kg/m, for a 195 MT horizontal pull I get a rig-vessel distance of around 1660 m and a 288 m sag below vessel end.

In this case the maximum expected tension at winch (no dynamic loads, no friction) would be of around 242 MT. So as Chuck says, quite far away from the reported 330 MT.


Mike:
I agree with you. We should identify ourselves if we give someone negative points, as well as explain our reasons for that. The attitude of "throwing the stone and hiding the hand" is not correct at all, in my opinion.

Cheers.

 

I believe your calculations much more than what I was getting (I believe that they had 3" and 3 1/2" wire instead of chain plugged into their program).

Does anybody know how they calculated the force at the winch on the BD - hydraulic pressure? I wonder how accurate this is. The winch on my vessel doesn't have any way to measure except for the motor amperage - and then we need to guess on how many layers are left on the winch to come up with the KIPS of force - this doesn't work on payout at all.
--Chuck

 

I'm ashamed to have to plead ignorance on the whole point thing (I didn't even know we were being graded)...somebody trying to take away your Birthday Walrus?
--Chuck

 

Some interesting info on winches here:
http://www.unols.org/publications/winch_wire_handbook__3rd_ed/10_single_drum_winches.pdf

Cable tension uses to be measured by means of an in line tensiometer based in strain-gauged sheave axle-pins. I don't know what system BD's winch mounted.

Cheers.

 

Another hearing and a preliminary report:

http://www.shetlandtoday.co.uk/Shetlandtimes/content_details.asp?ContentID=24076

http://www.shetland-news.co.uk/news_10_2007/bourbon dolphin report inconclusive.htm


Kind regards

Johannpeter

 

Thanks, Johannpeter,

Enlightening information. I remember in an economics statistics class lecture on the laws of probability, many years ago, the instructor said that it was possible, in theory, for all residents of London to attempt to cross Tower Bridge at the same hour of the same day. That would require millions of individuals all making a similar decision at the same point in time. Extremely improbable, but still possible.

The consultant's report quoted in the first article seems to theorize that a series of conditions with negative effects on BD's stability all occured at the same time. 3 or 4 of them impacting at once might have allowed her to recover, but the combination of all resulted in the disaster.

Some (I hope no one in authority) might call it an act of God and forget it. The proper response, IMHO, however, will be for rig and vessel owners to place safety at a higher priority, and demand that captains be rewarded for doing the same, and not cutting corners to "get the job done or else".

I'm not criticizing captains here; the opposite, in fact. The crews of these offshore support vessels do a dangerous job with a great deal of skill. The subject of pressure from rig owners to get the job done as quickly as possible, however, regardless of circumstances, has been raised in this thread several times by veteran AHTS captains and mates. That attitude and pressure can cost lives.

 

The earlier comments on the trend to add more gear on or above the main deck so the vessel can "do more" seems even more on target in view of the report cited by Johannpeter:

"Apparently the Bourbon Dolphin was 250 tonnes heavier than originally calculated. The shipyard only realised how great the increase was when the ship was launched.

This came to light when deputy director of administration Harald Moller said they knew during the building phase that the ship would be heavier than in design, but they did not know how much the ship deviated from the original design until the ship was launched and they could see how deep she lay in the water.

Mr Moller said: 'It was the weight of the equipment which had changed, not the weight of steel.' " (Shetland Today: The Shetland Times, 26 October 2007)