Towing Vessel Stability

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

Many thanks
Nigel

 

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

 

Towing Vessel Stability_Specific issues

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

 

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

 

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

 

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

 

Very nice posts, Mori and Nigel, thanks.

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

 

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

 

Nigel

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

Best regards
Mori

 

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.

 

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

 

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

 

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.

 

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.

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