Towing Vessel Stability

Would be very grateful if some could explain the following, it was lifted from the Australian Transport Council National Standard for Commercial Vessels

Assuming a towrope angle of 0 degrees to the horizontal, the residual area (AR) between the righting lever curve and above the towrope heeling lever curve up to 40 degrees angle of heel (or up to the angle of down flooding, θf degrees, if this angle is less than 40 degrees) shall not be less than 1.03 metre degrees plus 20 per cent of the total area under the modified righting lever curve from 0 degrees to 40 degrees (or from 0 degrees to θf
degrees

The first part is no problem, its the 20% bit I dont get. Is it 20% of the area between the tow heeling lever curve and righting curve from the angle of first intercept (angle of heel due to towing lever) up to 40 degrees (or angle of flooding)

Thanks in advance

Nigel

 

Does it matter? will it make you a better Towing Master? Will it keep your crew (and you) out of danger?

Sorry Nigel I am (was) but a simple seaman with 17 years experience as mate / second mate on AHTS so I guess your on your own with this - but now I've spoken some mathamatician will dearly love to show how wrong I am (am I bovvered?) but see my first paragraph on my view! You know Nigel I'm kinda glad I'm out of that stuff now! Twas easier in the old days we just towed the bloody things!

 

Downflooding is bad. But for the life of me I cannot understand what they are asking there. I think the 20% is of the new righting area after the CG Lever has been moved back due to the tow. But Im probably wrong.

 

Yea safe all this info is useless. Things like sight reduction, vessel stability table, boxing compasses. All useless. Glad your retired.

 

Nope ,--- your retirement did that.

Common Safy tell the truth,--you were dismissed.

Nigel --you'lle learn nothing in Lancashire, get yerself over to Hull.

 

Thanks Kay9, thats sort of the way I read it, but in that case, they could have made the critera 1.236 which is 1.03 plus 20% of 1.03.
It does not seem to make sense

All the best
Nigel

 

I did go to Hull once or twice, and Immingham, Goole as well. Got to drive a Voight Schneider tug while over there. Three of my present crew are from Hull, a bosun, AB and cook, I'll put this question to them, they may know the answer being from the east coast rather than Manchester

All the best
Nigel

 

What would make sense is if they meant 20% of the area UNDER the heeling arm curve. I sent them an email and hope someone will get back and explain it

Nigel

 

My first job on tugs was with Smit Tak Towage and Salvage (Singapore). One of the first jobs I was on was the salvage of the AHTS "OSA Ghent", which had capsized on the berth at Jurong Marine Base Singapore.
Cause of the capsize:
Vessel had just left a dry dock up in Jurong, and was meant to go to the petroleum anchorage for bunkers. Charterers requested the vessel to first load drill pipe on deck before bunkering as that would save time. The Master agreed. The vessel berthed and loading commenced. The vessel seemed tender while loading, and the Master decided to add more ballast. The only ballast tanks empty at the time were the two aft peak tanks, so these were filled. These are high tanks and did nothing to lower the CoG, and added to an already large stern trim, which was increasing all the time as the drill pipe was loaded. The deck crew reported that when they went to check on the aft moorings, the water on deck was up to their knees.
When the boat rolled, they were lucky she rolled towards the dock, and those boats had big funnels. The only thing that stopped the boat completely rolling was the funnel landing on the dock and holding the boat.

http://www.ddghansa-shipsphotos.de/osaghent100.htm


Later, at an enquiry, the Master admitted that he never checked stability. If he had, that incident would never had taken place

Nigel

 

Nigel I think your right on. I just think the way they are wording the question is Bull, and ment to trip you up. We have the same problem here with the USCG.

 

Here is 2 sites that might help. Allthough I cannot find a direct example of your question.

http://trawler.ca/stabilit.htm

http://www.mcga.gov.uk/c4mca/mcga-g...b_load_line/mcga-gr_gos_loadline-chapter8.htm

 

Nigel,
I've sent a PM to Mori Faplan at the MCGA, who is also a member of these forums, asking him to be so kind to clarify the 20% thing for us.
Cheers.

 

I am clueless about the 20% thing - I always enjoy all of the requirements being put forth and then looking at the end result. I would assume, from all of the regulations, that there would be a requirement to have a functional and useful stability program - yet on most of the AHTS vesels in the GOM all that is required is a stability booklet. This booklette is so cumbersome to use that it is all but ignored by the crew. The one on our vessel is missing three tanks in the stability calculation page, mentions nothing about towing, has no provisions for calculating towing conditions, and only gives provisions for sounding tanks with an innage. When, do you suppose, was the last time an Engineer sounded the sewage tank for a proper measurement?

If they really cared about the stability of a vessel they should require an easy to use program that could be used and checked by any officer aboard the vessel, including the engineering staff.

I also remain curious about the ultimate stability of an AHTS that has a large force astern (such as working anchors in deep water) at maximum power suddenly loosing propulsion. How many AHTS vessels could survive that fate? Does anyone else think that may have had something to do with the BD's tragedy? I think that if a vessel become inherently unstable when getting towed astern then the 20% in question really becomes a bunch of extraneous nonsense.

Just some rambling opinions - thanks for letting me vent.
--Chuck

 

Australian NSCV criterion

Dear All

I will attempt to answer your query the NSCV towing criterion.

The criterion is a modification of the well known US Navy criterion that the residual area between the righting lever (GfZ) and heeling lever curves up to 40 degrees or the point of downflooding should be not less than 40% of the total area under the righting lever curve up to 40 degrees.

The problem with applying 40% across the board is that it penalises vessels that have good stability; i.e., the greater the area under the righting lever curve, the greater the residual area needs to be.

On the other hand, requiring a residual area no less than 40% of the minimum required area of 5.16 mdeg in all circumstances (i.e. 2.06 mdeg) would result in too little a relative proportion of residual area when a vessel has very high stability.

The compromise is to take 50% of the "40%" required residual area from the minimum area of 5.16 mdeg (i.e. 1.03 mdeg) and 50% of the "40%" required residual area from the area below the actual righting lever curve; i.e. 20% of the total area below the righting lever curve up to 40 degrees.

Hence the required area = 1.03 + 0.2 x A(subscript 0-40 deg)

where A(subscript 0-40 deg) = the area under the righting lever curve up to 40 degress.

Reading the criteria in the draft standard, I note the last sentence refers to the 'modified righting lever curve'. The term 'modified' might be causing confusion. This is an error as the righting lever curve is not modified for the determination of the towing criteria (unlike snagged net criteria, lifting criteria, etc). Clause F4 in appendix 4 states that the vertical component of the towrope pull need not be taken into account for the purposes of application of the criteria, so the the righting lever curve need not be 'modified' in any way. I will correct the draft accordingly.

I hope the above answers your query.

Thank you for your interest and feedback.

By the way, just a word of caution. The standards you have access to are drafts that have been altered since being posted for public comment.

The final intact stability criteria standard is awaiting approval by the Australian Ministers of Transport and will be posted on the web as soon as possible after that happens. We were hoping that it would be in the next few weeks, but unfortunately their meeting has been postponed by the recent Federal election in Australia. They are now scheduled to meet at the end of February.

Best regards
Mori

 

Thanks a lot, Mori.
Have you statistically checked this for existent Towing vessels, comparing it against the US Navy criterion?
If so, is it posssible to know results?

Cheers.