Bourbon Dolphin capsizes

More doubts,
In Rayks last posted image on BC with the anchor aboard, the rode chain is over deck and the vessel is pulling it. As the surviving crew member said they were paying out chain (even stating the paying speed) I asume this was been done from the vessel and not from the rig. As chain is not rolled in drums, do the BD had a chain locker and devoted chain capstan? Or is it rolled in drums?

 

In this thread on a Norwegian ship forum you can find photos showing construction, trial and naming of the Bourbon Dolphin:

http://www.skipsfarts-forum.net/read.php?TID=779

 

The BD had rig chain lockers so the chain would have been in the lockers, lowering the VCG. However with most anchors set most of the chain would have been out already.

On the question of height of lightship VCG on this type vessel, I always assume the main deck height for preliminary studies. My experience is that the actual inclined VCG rarely exceeds the main deck height for AHTS vessels.

As far as the comment about seeing the on deck anchor from the PH, you likely could not, however it is normal to have camera setups to cover the winch and aft deck areas. These cameras are on continuous display on the bridge for the master.

740 tons of deck cargo is VERY low for a vessel of this size.

 

Guillermo,

Glad you can run some numbers. Your vessel's B & D is close enough. Which stability software are you using?

VCG of lightship: You'll be surprised how fast the VCG climb with a focsle this size. The deck cargo capacity for the S part of this AHTS is small due to all the towing & other 'goodies' above M.D.

Best & quickest way to get a good approximate VCG is to calculate the IMO max. deck cargo condition = 100% crew (35) + 100% stores + 100% consumables W/ F.S. correction + max deck cargo (Ulstein specs: 740 T @ 1m above M.D.). The W.L. for this condition is usually around 1m below the L.L. Adjust assumed lightship vcg until you meet the IMO criteria we used (IMO A.469(XII))

 

Not sure about the BD but some AHTS if you load max cons, + deck cargo + Bulk + OBM you will be overloaded.

(on anohter AHTS) Have loaded max fuel, converted OBM tanks to Fuel, loaded them with fuel (lower SG than OBM), + max PW, we were requested to load max DW, had to say no as we knew she would be at tropical marks and still with 150 tonne capacity in the DW tanks. and NO deck cargo at all. Stability was well within the criteria required.

 

With all those tanks below M.D. it is easy to overload these type of boats. For the max. deck cargo loading condition the diesel oil, fresh water, potable water, lube, have to be carried in their dedicated tanks. No dry mud, no liquid mud, no ballast. The draft in that condition is usually 1m below L.L.

 

I take it L.L is Loadline, From stability info for a 66m LOA AHTS, 1m below the loadline means you have a deadfreight of 900tonnes, with that particular vessel loaded to the state you say (dedicated tanks only, no ballast etc) gives a deadfreight of 40tonnes.

I agree it is easy to overload these vessels - often the charterer sees that you have only 50% fuel onboard and is pushing you to load more fuel and deck cargo when you are already at your marks.

 

As a person who has never seen this done ,maybe i have a stupid question. as i can understand while the dolphin is pulling the anchor away from the rig ,the rigs hoist or crane pays out chain or cable. has anybody talked about a failure of the crane brakes or the hoist brakes and what that would have done to the dolphin .i would imagine that the pull backwards would have been terific and mabey not able to be overcome by the dolphin that was moving away from the rig slowly . sorry if this doesent make very much sence but i am not a mariner.

 

Hi, Brydee, and welcome to the forum,

If you've read all the earlier posts on this subject, you probably realize that there are still many questions regarding precise details and the sequence of events. Most of what you read here is speculation, although some of it is very well informed speculation, as there are people here with ship design and seagoing experience on anchor handling tugs.

As I understand the process, you're correct, the tug would attach a cable or chain to the rig anchor and, as the tug moves toward the anchoring point, anchor rode, usually chain, is payed out by a winch on the rig. The load on the tug will increase gradually as more chain is paid out. If the rig winch stopped suddenly, it would create a "pull", but it would not result in an immediate stoppage of the tug. The long length of chain is suspended from its two ends at the tug and the rig, forming a catenary, or curve. Any additional force applied to one end will raise the catenary (make the curve flatter) the catenary serves to dampen the force so it does not reach the tug abruptly, the same way the catenary will serve to dampen wind and wave forces on the rig when it is anchored. In theory, if the tug and rig winch had infinite power, the catenary would become a straight line. In practice, the load on the tug increases to the limit of its winch and/or bollard pull. If the load exceeds that, the tug would begin to move backward and the rear deck would begin to submerge. There are procedures and safety equipment to ease or release the load in those circumstances. One procedure is for another tug to hook onto the cable or chain and pull so the total load is split between two tugs.

All of what I just said is based on the cable or chain leading off the proper location at or near the center of the rear deck. Keeping it there is the function of tow pins, and of shiphandling to keep the tug aligned with its stern pointing toward the load.

I hope this answered your question. There are others with much more knowledge than I who can explain the finer points.

 

hi again. I guess what i was asking is if the chain or cable atatched to the rig and the dolpnin was droped sudenly from the rig would it then become a pull downwards instead of backwards .the other question would be that if the second tug had the chain at one point and had the load only to loose it would that cause the dolphin to go side ways or backward. I guess what im saying is that if the dolphin only had the weight of the anchor and a short peace of chain and the cable was droped by the second tug wich now had the load would the dolphin now with the full load be in big trouble. Also if the second tug was not perfectly lined up with the dolphin when it lost the load would that make the cable go all to one side of the dolphin. I hope this is not to crazy question .brydee

 

Brydee,

You may not be a mariner, but those are perceptive questions. Rayk gave a very good picture of what happens as things change. I'd just add that with the full load, BD was in trouble but not in imminent danger so long as the load was over the center of the rear deck. The final problem seems to have developed when the starboard pin was lowered. This allowed the lateral component of the force to pull the chain around quickly, so it led over the side of the tug. That seems to be what pulled her over to a steep angle, allowing other elements to upset stability further (possibilities include equipment sliding to the lower side, water intruding to one or more decks, water entering engine air intakes, loss of engine power, probably more), causing the capsize.

 

Appologies guys been unable to get to a monitor for the past week, so got kinda outof touch with things! Sorry about blowing my top earlier, several years in that game does things to your head - especially as I've lost a few friends in various incidents (not all related to anchor handling, but oil related never the less!)

I have some problem getting my head around this statement about the chain on drums - wire maybe, but no drum could hold that sort of amount of chain! It would be in 'lockers' below the cable holder/gypsy/winch. Pressing the emergency release should in theory release the gypsy and let it 'freewheel' the chain off! But it didn't! why? Looking at the picture of the BD put up by Rayk (?) I would hazard a guess that what happened was that when the towing pins where released the chain rode up the crash barrier and due to the weight sheered the pins off the centre position (they appear very flimsy compared to earlier vessels, [strong yes but strong enough?] we used to have big crash barriers and pins to match - the crash barrier on the BD was to my mind more for a vessel normally used in the supply mode that could run anchors but was not primarily a dedicated Anchor Handler (which would explain why it was originally decided that she would be used for back up rather than the main job). This would mean that the chain would be at right angles to the winch drum/ gypsy, and then at right angles again running over the side of the vessel - not much chance of being able to start freewheeling in that configuration, but once started the weight would keep her running - but you got to get the damn thing started!!! Which would explain why the Rolls Royce (?) engineer stated that his equipment worked properly (how did he know, dive on the ship and have a look?) Plus of course he would wouldn't he?! As the (?) mate stated when the rig came up with the bright idea of releasing the pins the Master and Chief Mate (steersman) where not happy - neither would I have been!!! the one thing that any tugman is scared of (no matter what size the tug) is girting, and releasing those pins would allow such an event to happen! I firmly believe that the suggestion to release those pins was the final 'nail in the coffin' for the BD, how it was worded etc to make a sceptical, experienced, set of officers to actually carry out a manouver that every fibre of their being was screaming out NO! remains to be seen, but that suggestion ended it!! OK so other errors were made and the domino effect certainly came into effect but that's how I see it!! It'll upset a few but that's the way of it!

the Walrus (Mike)

 

Maxsurf has been used in this case.

But as much as to bring the LS condition's VCG 1.3 m over deck height?

Yes, all those high weights, including superstructure, penalize the vessel's stability and its ability to carry deck cargo. Interesting to note in Rayk BD's posted picture, that the upper drum was empty.

Thanks for the info.
I'd like to know what your experience about the inclined VCG for these vessels. Acearch72 states (thanks acearch!):
"My experience is that the actual inclined VCG rarely exceeds the main deck height for AHTS vessels"
Do you agree?

Cheers.

 

Jack, forgive me mate, but I think those two last post of yours on Rayk's contribution are not constructive.

 

Just to remind everybody - my Small experience consists of

1 37 years - 10 Royal Navy; 27 Merchant Navy, including 17 on AHTS most as mate/chief Officer! [6 months fishing! proffesionally]
2 none (there again most Naval architects have NO experience as sailors - some do and THEY are very good, the rest - well according to them the standard person is three foot tall with 17 feet long arms)
3 built my own boat, modified a few

In other words I just might know something this subject - but probably not!

and yours?