Towing Vessel Stability

[Nigel1]

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

[safewalrus]

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!

[Kay9]

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.

[Kay9]

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

[Frosty]

Will it keep your crew (and you) out of danger?


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.

[Nigel1]

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.

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

[Nigel1]

Nope ,--- your retirement did that.

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

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

[Nigel1]

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.

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

[Nigel1]

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


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

[Kay9]

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.

[Kay9]

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-guidance-regulation/mcga-dqs-ss_guidance_to_surveyors/dqs-ssb_load_line/mcga-gr_gos_loadline-chapter8.htm

[Guillermo]

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.

[cfvelie]

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

[mflapan]

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

[Guillermo]

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.

[Nigel1]

Many thanks Mori and Guillermo, thats cleared it up for me

Many thanks
Nigel

[Nigel1]

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.


Best regards
Mori


Just a couple more points to clarify. In the calculation for the heeling arm, the horizontal distance from CL to tow wire attachment point is also taken into account.
On my particular vessel, the tow wire is fitted to an "Underhand drum".i.e. the wire leads off from the bottom of the drum.
If the wire were to go out to the vessels beam (worst case), it needs to lead upwards to go over the cargo rail/ crash barrier), and cannot go any further forward than the tow wire stop hook (see attached photo).
Would the position of that tow wire hook stop be the point at which the calculation is made rather than the wire attachment point?

Another point, the rules state to use maximum bollard pull. If I were to use maximum bollard pull on a towing job, it would most likely rip out the towing connection on the object being towed. In most cases, I dont normally use any more than 100t pull (and not the 236t which is available).
Would this not be a better figure to use for the calculation??. I realise than max pull would err on the safe side, but in our
case, its a very big margin

Best regards
Nigel

[mflapan]

Dear Nigel

The leading of the towrope from the underside of the winch drum up to the bulwark rail will create an additional heeling lever when the vessel is at small angles to the horizontal that will add to the effect of the towrope lever. So there will be a double disadvantage in that the line of pull with be further above the vertical centre of lateral resistance plus there will be an additional heeling lever due to the vertical component of load at the bulwark.

However, as soon as the vessel heels sufficiently for the towrope to clear the bulwark, the towrope heeling lever will be as per the NSCV calculation. If, as I suspect, this happens before the angle of heel at which the heeling lever intersects the righting lever curve, then it will have no effect on the residual area calculation and so would not need to be taken into account. However, this needs to be checked.

On the issue of the value of maximum bollard pull to be used in the calculation, any reduction of the value assumed would have to be considered in the context of an equivalent solution (See NSCV Part B Chapter 2 that can be downloaded from the NMSC website at www.nmsc.gov.au). How would equivalent safety be provided if a lesser value were assumed, and what controls would be in place to ensure that this value was never exceeded? These matters would need to be considered in the context of your particular vessel, remembering that operational restrictions alone are generally not a very reliable method of controlling risk. At the very least, you would have to consider what engineering controls are in place such as governors or tension limiting devices on the towing equipment.

These are matters for consideration by yourselves working with the relevant regulatory authority.

I hope this answers your queries.

Best regards
Mori

[Nigel1]

Thank you Mori for the informative reply.
As soon as I return to my vessel, I'll double check our stability book and check what criteria have been used for towing.
As regards controls, one which we do use is only using 2 engines and not 4 when towing in some instances, that will half the max bollard pull.
Another control is if using 4 engines, to reduce the engine load limiter setting, that will prevent extra power being inadvertantly put on the engines

Best regards
Nigel

[Nigel1]

However, as soon as the vessel heels sufficiently for the towrope to clear the bulwark, the towrope heeling lever will be as per the NSCV calculation. If, as I suspect, this happens before the angle of heel at which the heeling lever intersects the righting lever curve, then it will have no effect on the residual area calculation and so would not need to be taken into account. However, this needs to be checked.

On the issue of the value of maximum bollard pull to be used in the calculation, any reduction of the value assumed would have to be considered in the context of an equivalent solution (See NSCV Part B Chapter 2 that can be downloaded from the NMSC website at www.nmsc.gov.au). How would equivalent safety be provided if a lesser value were assumed, and what controls would be in place to ensure that this value was never exceeded? These matters would need to be considered in the context of your particular vessel, remembering that operational restrictions alone are generally not a very reliable method of controlling risk. At the very least, you would have to consider what engineering controls are in place such as governors or tension limiting devices on the towing equipment.

Best regards
Mori


Mori, this has gotten me thinking on the subject.
A towing vessel normally excerts the greatest bollard pull when the tow wire is in line with the vessels centre line. As the wire moves off the centreline and out towards the beam, the wire tension will decrease. In theory, when on the beam the tension should be zero (discounting wire weight, and water resistance on own and towed object). If a tug master is unlucky enough to let the wire get a couple of degrees forward of the beam, then he will be steaming towards the towed object, and then its only tow wire weight to contend with (plus how to get out of an awkward situation.

A ship handling tug on the other hand has a completely different problem to contend with, that being girting/girding, in that vessel they are assisiting could in fact end up towing the tug, and if a conventional tug, and no gog rope has been rigged, they may find themselves being towed and beam on, which some tugs have been capsized by.
Modern tractor and voight schnieder tugs get over this by having the towing point either close to the stern or the bow. A conventional tug should have a gog rope rigged to allow the tugs stern to be heaved back in line with the stern.

Would appreciate your thoughts on this, particularly the first paragraph

Best regards
Nigel

[mflapan]

Dear Nigel

Thanks for your query.

To answer it, I need to go into a philosophical discussion.

You have to ask "What is the objective of these standards?". After some years of working in this field, I have come to the conclusion that the main purpose is to set a minimum capability to forgive human error. The problem is not to ensure that the vessel is fit for its purpose when everything is going well. The problem is to design the vessel to be reasonably forgiving of human error. It is not a matter of if human error occurs. It is a matter of WHEN.

The problem is that we cannot normally afford to make vessels intrinsically safe. We have to strike a balance between safety, utility and economics. Without having access to all the facts, I feel the Bourbon Dolphin incident is probably a perfect example of the conundrum we face. The designers probably never expected that the vessel could be exposed to a heeling lever created by a 300 ton athwartships load. The operator probably never envisaged that happening either. Is it reasonable to design every OSV to be capable of withstanding such extreme loads? Probably not. However, it is reasonable to expect that sufficient information is provided to show what the vessel can safely withstand, which would lead one to the conclusion that the vessel could not safely withstand such loads should they arise. Maybe that could have helped the decision makers to make better decisions.

So, within that context, let us look at the towrope heeling lever loads. You are perfectly correct in saying that in the normal course of events, a conventional screw vessel streaming a long tow at sea would not normally be exposed to the magnitude of transversel heeling lever that is assumed. However, there needs to be a margin for mechanical or human failure, for the effect of waves and winds, and for the Murphy's Law factor when something goes wrong. I have heard converse arguments that the heeling moments could potentially exceed those arising from the athwartships thrust of the bollard pull due to inertial loads applied suddenly. However, we cannot design for every possible scenario. The heeling moment calculation attempts to provide a balance that is plausible but not improbable and matches the risk and solution to the size and type of vessel.

You will note the NSCV standard contains different criteria for azimuthing or tractor towing compared to conventional shaft and propeller arrangements. This is an acknowledgement that the likelihood of full bollard pull applied transversely is going to be less for the latter compared to the former which are designed with this in mind. This probably provides a better approach than the long tow/ship handling differentiation because an azimuthing or tractor tug used for long tows today could be used for ship handling tomorrow.

You will also note that the standard also alters the initial angle of the towline to the horizontal for both ship handling and long towing.

The risk model behind the criteria cannot hope to address all the factors that can vary the nature and magnitude of risk. However, we hope that the standard provides a reasonable measure that achieves its objective.

I hope this answers your query.

Best regards and merry Xmas to all

Mori

[Guillermo]

Very nice posts, Mori and Nigel, thanks.

Merry Xtmas to all forums members from my corner of the world, too.
Cheers.

[Nigel1]

Dear Nigel


The risk model behind the criteria cannot hope to address all the factors that can vary the nature and magnitude of risk. However, we hope that the standard provides a reasonable measure that achieves its objective.

I hope this answers your query.

Best regards and merry Xmas to all

Mori

I believe you have answered the question, I can see that not everything can be taken into account, but as you pointed out, its important that those on the vessel know what she can withstand, and to keep her in a condition well within those limits

I've added three pictures to indicate the conditons typical in the North Sea for a OSV towing a rig

Happy Xmas and all the best for the New Year

Nigel

[mflapan]

Nigel

Great photos. Thank you for the posting. I will respond to the OSV discussion separately.

Best regards
Mori

[Guillermo]

I and a couple of colleagues in my office have been recently traveling around, inspecting a fleet of 63 tugs (harbour and coastal) outside Spain, ranging from 2000 to 5000 HP, and I found to my surprise the stability booklets, even for the more modern units, did not consider any kind of Tow Tripping or Self Tripping like criterions. Nor even the 350 mm minimum metacentric height requirement. Just the general IMO criterions for merchant ships, including wind effect.

I'd like to know what the statutory towing vessels' stability criterions are for the different countries, so I ask for your kind contributions here. I know Australia's, USA's and UK's ones already, but want to know about other countries, particularly europeans. Thanks a lot in advance.

In Spain we have to comply with the 35 mm minimum GM thing (IMO), and with a Tow and Self Tripping criterions very much in the line of the USCG's.(http://www.uscg.mil/hq/g-m/nvic/12_83/n12-83.htm).

Cheers.

[Nigel1]

Just got back home, hence low profile
I checked our stability book, approved by Lloyds, for an Isle of Man Registered vessel. No towing stability criteria mentioned. The only reference to towing was in the Notes to Master, avoid having deck cargo when towing.
The stability criteria listed is only the standard IMO criteria for OSV's
For anchor handling, one condition is included in the book. Very oddly, it makes use of the fact that the vessel has two stern rollers, so the condition inlcudes 600 tonnes on the port roller, and 600 on the stbd roller, so no list.
It also uses the rollers axis for the CoG of the wire tension.
I will try and contact the Isle of man Adminiistration sometime and get their views

[Guillermo]

Thanks, NIgel.
I'll be interested in knowing what the I of M Adm tells you.
Cheers.

[Nigel1]

Thanks, NIgel.
I'll be interested in knowing what the I of M Adm tells you.
Cheers.

I've just called the IoM Registry, and spoke to the Principal Marine Surveyor. They dont have any additonal requirements for stability criteria as applied to either towing or anchor handling operations.
You mentioned the UK's criteria for towing operations, can you let me know what these are, I recall something about a minimum GM of 35 cm. Is this an IMO criteria??

All the best
Nigel

[Guillermo]

If I'm not wrong, IMO applies in the UK, but general requirement for GM is 15 cm (35 only for FV). This is the same as in France, as per my post of 02-24-08

Spain submitted a paper to the 2006 IMO Stability and Load Lines and Fishing Vesels Safety Sub-Committee (SLF 49/5/9), regarding its experience with Spain's statutory Towing Vessels Stability Criteria, but I have not that paper. Any help?

Cheers.

[cfvelie]

This is the specs for one of the newest AHTS being delivered...just wanted to get what the community here feels about the stability, etc. They are planning to install A-frames on the stern of a couple of these.

http://www.seacormarine.com/pdf/Flex_Class_AH_John_Coghill.pdf

--Chuck

[Guillermo]

At first sight it seems to be a narrow boat with big winches. Also the bollard pull seems only feasible if transverse thursters are not working. I would like very much to to have a look at the adopted stability criteria and the loading and working conditions studied.

Cheers.

[cfvelie]

They have a stability program for this vessel - I have no idea about the specifics. I am going to try and go over there at some point this fall to get some time on a DP 2 vessel. I will try to find out something then.

I agree with the narrow observation. I also am not too fond of the shallow hull and thrusters being so close to the surface when at a light draft.
--Chuck

[charmc]

Quick impressions from an "experienced layman":

" Continuous" bollard pull seems optimistic for the rated power output, even given nozzles, and thrusters off.

Based on dimensions, just wondering how stability will be affected significantly by loaded stores? If so, is working under extreme AH conditions to be recommended without the vessel carrying max stores?

HP seems light for an AHTS of that hull length. I note the emphasis on "eco friendly" design. Today that seems to mean sacrificing power for greeness. Does that create an oxymoron? The vessel seems built for less than extreme (medium duty) anchor handling, i.e. not for the depths the new rigs are working at. Is there a standard for determining vessel job assignments? This design has more power than a straight supply vessel, but significantly less than BD and others of similar length. Are there subcatagories within the AHTS type?

I don't know how valid any of these thoughts are. Those of you who have the design and operating experience in this field will know if there is merit in any of these questions/observations.

[Guillermo]

In my opinion 120 Mt bollard pull is about right for the 10.728 HP if applied to the main propellers in an efficient configuration (A rough number is BP =+/- 1% HP, being efficient systems slightly over that).

Narrow beam will not allow to carry the load of the 400 Mt winch much forward over the side in an emergency (Another candidate for a BD like accident?).

Cheers.

[charmc]

Just curious; is that formula for engine rated HP or net HP at the shafts?

[Guillermo]

This is just an statistical relationship between nominal HP and bollard pull for a series of tugs I've had the opportunity to appraise.

[Guillermo]

Interesting video:
http://www.splashvision.com/Video/14164_melbourne-tug-boat-tips-over-towing-ship-and-man-overboard.html

Cheers.

[Nigel1]

Yep, should have fitted that gog rope.
Classic tug boat error

Nigel

[murdomack]

This is the specs for one of the newest AHTS being delivered...just wanted to get what the community here feels about the stability, etc. They are planning to install A-frames on the stern of a couple of these.

http://www.seacormarine.com/pdf/Flex_Class_AH_John_Coghill.pdf

--Chuck

Hi Chuck,

There is something about the specs that interests me. The weights, etc. are quoted in Lt and Mt when I know that US (short) tons are in common use over there. Coupled to this we have volumes in USG and BBLs and pumping rates in GPM which I know from conversion of the Metric figures is US gallons.
My question is; are Long tons the standard used in measuring US shipping weights?

Murdo

[Nigel1]

Hi Chuck, does seem a bit on the narrow side, for comparison, the specs for my new boat can be seen here

http://www.maersksupplyservice.com/vessel.lasso?fleettype=Newbuild(AHTS)&fsc=Newbuild(AHTS)

Nigel