Catenary - would you buy krypton piano wire?

[zoidberg]

Abrasion and failure of mooring chains is a major and costly problem, for many vessels break free even without the encouragement of storms. Many boat owners are complacent, relying on mooring contractors' diligence ( and insurance ), for the problem is literally 'out of sight'.

Here are some pics taken while researching a PBO article 'How Safe Is Your Mooring', from local harbourmasters' collections of horrors....

mooring trot

courtesy VCox!

Contractors' inspection practices vary widely. There are no standards. One local mother-and-daughter team dive on each of their charges, measure, and produce an annual certificate with recommendations. Another, nearby, replaces riser chains/shackles only when something breaks.

Of course, the water itself is not homogenous, with differing concentrations of suspended grit solids. Many estuaries are contaminated chemically, with the upper reaches of the Fal, for example, contaminated with metals/metalloids pumped out of the nearby tin mines. Some work has been done to bring better understanding - 'Towards Zero Failures In Swinging Moorings' ( Jim Izzard 2010 ) - and more use is being made of Polysteel rope for risers, but we still need shackles.

It emerged that 'fast corrosion' occurs in the threads of galvanised shackles, where over-tightening seems to rupture the 'galv' coating, leading to localised acceleration of the problem. It is suggested this contributes to some shackle pins failing early, then the shackle/chain connection parting.

Frequent diligent inspection seems to be the only available answer.

 

[vyv_cox]

Abrasion and failure of mooring chains is a major and costly problem, for many vessels break free even without the encouragement of storms. Many boat owners are complacent, relying on mooring contractors' diligence ( and insurance ), for the problem is literally 'out of sight'.

Here are some pics taken while researching a PBO article 'How Safe Is Your Mooring', from local harbourmasters' collections of horrors....

mooring trot

harbour ferry

Contractors' inspection practices vary widely. There are no standards. One local mother-and-daughter team dive on each of their charges, measure, and produce an annual certificate with recommendations. Another, nearby, replaces riser chains/shackles only when something breaks.

Of course, the water itself is not homogenous, with differing concentrations of suspended grit solids. Many estuaries are contaminated chemically, with the upper reaches of the Fal, for example, contaminated with metals/metalloids pumped out of the nearby tin mines. Some work has been done to bring better understanding - 'Towards Zero Failures In Swinging Moorings' ( Jim Izzard 2010 ) - and more use is being made of Polysteel rope for risers, but we still need shackles.

It emerged that 'fast corrosion' occurs in the threads of galvanised shackles, where over-tightening seems to rupture the 'galv' coating, leading to localised acceleration of the problem. It is suggested this contributes to some shackle pins failing early, then the shackle/chain connection parting.

Frequent diligent inspection seems to be the only available answer.

Ahem!! Your photo labelled 'harbour ferry' was taken by me on my foredeck while anchored in Pollensa, Mallorca. The full version is shown on my website at http://coxeng.co.uk/metallurgy/corrosion/ It was a light mooring chain that has suffered corrosive wear due to dragging across the sandy seabed.

 

[Neeves]

Interesting thread Zoidberg.

We are insured now with Pants who demand an invoice for a mooring service annually. Fortuitously our mooring contractor also sends us an annual reminder, altruistic though he may be - he also wants the business. Interesting he tells me that I am one of the very few owners who goes on board his barge when our mooring is serviced. We get on well, he teaches me splicing tricks and tells me the local marine gossip Other owners pay less attention to servicing and every year we have a few yachts on the beach with swivels and chains, as in your image, as thin as paper.

Failure is invariably the swivel at the lower end of the riser - which is possibly not surprising as it 'travels' the furthest.

Oddly our contractor does not use galvanised fittings, nor gal chain, at all - as he points out the gal soon wears off. This is reflected in the tests I conducted where the worst offenders had lost about half their gal within 70 days. But he peens and welds all the fittings (so when the mooring is serviced and components are coming to the end of their life - he cuts them off with an oxy torch).

Jonathan

 

[Neeves]

I was going to edit my post with a comment on that final picture - and I'm now further confused.

The query I was going to raise was that the 'wear' seems very even - no preferential wear at the crown - which I might have thought would occur - even if a 'light' mooring. You say its 'corrosive' wear from 'dragging' across a 'sandy' seabed - which in my ignorance I might have called 'abrasive' wear - though my limited experience of abrasion of chain on seabeds - suggests prioritising the long of the link for abrasive wear.

So any further clarification.

I understand the idea that as the chain corrodes the products of corrosion, rust, are continually removed, exposing new steel. Are you saying abrasion of the raw steel is not a major factor and that its the corrosion that is the primary cause of wasting and the abrasion simply removed the corroded product.

And is there a depth at which the rate of rusting is reduced (and abrasion becomes more dominant).

I'm guessing for the anchor rode - corrosion (rust), once the gal is removed, is the primary wastage process, with that rust being removed, by abrasion, every time the chain is used. And if its used frequently - you will not actually see the rust nor know it is actually wasting - unless you measure.

If 10m is too deep for rapid Fe oxidation (which is what my mooring contractor believes) - then I need to bring any abraded chain to the surface fairly frequently to accelerate the oxidation process.

Jonathan

 

[vyv_cox]

Interesting thread Zoidberg.

We are insured now with Pants who demand an invoice for a mooring service annually. Fortuitously our mooring contractor also sends us an annual reminder, altruistic though he may be - he also wants the business. Interesting he tells me that I am one of the very few owners who goes on board his barge when our mooring is serviced. We get on well, he teaches me splicing tricks and tells me the local marine gossip Other owners pay less attention to servicing and every year we have a few yachts on the beach with swivels and chains, as in your image, as thin as paper.

Failure is invariably the swivel at the lower end of the riser - which is possibly not surprising as it 'travels' the furthest.

Oddly our contractor does not use galvanised fittings, nor gal chain, at all - as he points out the gal soon wears off. This is reflected in the tests I conducted where the worst offenders had lost about half their gal within 70 days. But he peens and welds all the fittings (so when the mooring is serviced and components are coming to the end of their life - he cuts them off with an oxy torch).

Jonathan

The only mooring failure that occurred to me was also a failure of the lower swivel. This was on the very tidal Menai Strait, where the swivel has to work hard, turning 180 degrees twice on every tide, plus whatever the wind can achieve.

Like you, our mooring was ungalvanised, except for the pick-up chain. We had two heavy anchors, up and down stream, heavy ground chain with the riser attached to the centre. All connections were welded rebar, no shackles except the one attaching the pick-up chain to the boat. The rebar was bent into a 'paper-clip' shape, with a long weld between adjacent legs.

There are often mentions of 'black iron' shackles and other fittings but I have never been able to find just what these are. Not a metallurgical term as far as I know.

 

[vyv_cox]

I was going to edit my post with a comment on that final picture - and I'm now further confused.

The query I was going to raise was that the 'wear' seems very even - no preferential wear at the crown - which I might have thought would occur - even if a 'light' mooring. You say its 'corrosive' wear from 'dragging' across a 'sandy' seabed - which in my ignorance I might have called 'abrasive' wear - though my limited experience of abrasion of chain on seabeds - suggests prioritising the long of the link for abrasive wear.

So any further clarification.

I understand the idea that as the chain corrodes the products of corrosion, rust, are continually removed, exposing new steel. Are you saying abrasion of the raw steel is not a major factor and that its the corrosion that is the primary cause of wasting and the abrasion simply removed the corroded product.

And is there a depth at which the rate of rusting is reduced (and abrasion becomes more dominant).

I'm guessing for the anchor rode - corrosion (rust), once the gal is removed, is the primary wastage process, with that rust being removed, by abrasion, every time the chain is used. And if its used frequently - you will not actually see the rust nor know it is actually wasting - unless you measure.

If 10m is too deep for rapid Fe oxidation (which is what my mooring contractor believes) - then I need to bring any abraded chain to the surface fairly frequently to accelerate the oxidation process.

Jonathan

The mooring was in shallow water, 2-3 metres. I recovered the chain that I photographed from the seabed using mask and snorkel. There were many moorings in the area, each a simple concrete block or other weight with a riser about 1.5 times water depth. Around every weight there was a circle of clear sand where the chain dragged, most of the remainder being fairly weedy.

Corrosive wear is as you describe, the nascent (worn) surface corroding far more rapidly than one that has been corroding for some time, due to the slight protection that the corroded surface provides. This quite a well-known phenomenon, the subject of many learned papers that compare wear in various media with wear in air or vacuum.

 

[lw395]

Most moorings around here are done using very heavy chain.
The risers swirl around in the mud and wear away.
Most of the chain is bought secondhand, so it's assumed not to be any special grade.
All plain steel.
The professionals all weld their shackles.
I have used old anodes on the swivels and lighter hardware on the buoys, seems to help IMHO.

We had some moorings which were not used for a few years, the chain was very wasted. It still put a couple of tons on the crane dial before breaking.

 

[thinwater]

The mooring block is 10m deep, all the chain is on the seabed, its the 'snubber' and is a combination of 2" mining chain, 5m and a sweep chain of a further 5m of 3/4" chain. Neither chain really lend themselves to joining to 6mm chain, easily. The pendant 1.5", spliced to a swivel on the 3/4" chain, floats, and does not touch the seabed with a bridle spliced into the 'top' at sea level.

Lashing might not last - the swivel lasts about 3 years if left unattended.

Jonathan

I'm far more interested in link-to-link wear than sea bed wear. This would make the pendant a good location. It occurs to me that you already have sea bed wear data and that it is the link-to-link wear that matters more.

 

[thinwater]

There are often mentions of 'black iron' shackles and other fittings but I have never been able to find just what these are. Not a metallurgical term as far as I know.

In the US, black iron is a synonym for malleable iron, a form of annealed cast iron. It is considerably cheaper (and weaker) than forged, but if bought massively oversized for a mooring, should last well.

 

[lw395]

....

There are often mentions of 'black iron' shackles and other fittings but I have never been able to find just what these are. Not a metallurgical term as far as I know.

'Black Iron' seems to mean all things to all people.
Any ferrous metal that isn't bright or plated it seems.
It's a bit like 'wrought iron' which these days is likely to mean mild steel which has met with a blacksmith, or even imitation thereof.
We buy 'black chain'. It's any untested chain which has probably had some chemical surface treatment to preserve it until it's actually in use.
Or even more loosely, steel chain, self coloured.

 

[lw395]

I'm far more interested in link-to-link wear than sea bed wear. This would make the pendant a good location. It occurs to me that you already have sea bed wear data and that it is the link-to-link wear that matters more.

I don't know how much such data will be worth, I am always amazed at how much the wear varies between adjacent moorings and even a few feet up or down the same riser.

 

[Neeves]

Its relative wear - so it does not matter if the environment is more or less abrasive, corrosive as long as the chains sweep randomly in a small area.

Having looked at Vyv's image it does not seem to matter how wear occurs, whether its link to link or simply across the seabed - the end result is the same. Here our swivels do tend to wear, a subjective comment, at about twice the rate of the same components being dragged across the sand. The wear has the same 'contaminant' silica sand - one is accelerated by more, much, pressure (but on our moorings the wear is only increased by a factor of 2) and on Vyv's sample - they are very similar.

The link to link wear this seems a bit overrated - as the wear on the rest of the chain is bad in itself. When I see chain that should be condemned - I always notice the bulk of the chain, not the crowns - and the bulk usually has a solid rust coating.

No test is going to be very quantitative - it will simply say this is better, or less bad, than that - like salt spray test (which is well used even if it is subjective)

I have done some corrosion/abrasion testing - but effectively all the same sized chain. I'll have a slightly different focus as I'll have some 3/8th" and 6mm, some G80 and G100 and some G30, G40 and G43. Some of the chain will be the same as used in the first tests.

I'm not keen on diving as to evaluate needs a dive to retrieve, assess on the surface (cannot do so at 10m) then dive and re-attach. Allowing to sweep across the seabed means I can retrieve, examine and return in a few minutes. But I might speak to our friendly mooring contractor and see if he has a spare mooring I can borrow on his floating barge, and then use his barge as the weight as his barge is in considerably more shallow water.


We have 'black' chain here - it has a very thin coating of black paint (I can only think its applied in a slowly rotating drum), it is then lightly oiled, I believe diesel is commonly used. The coating is not expected to last long - its really just to protect it till its put into service.

I'll be sorting out chain today, but I will not be committed for a few days. Options and comments, welcome.

Jonathan

 

[Neeves]

As you all anticipated I thought I might add to the previous post, my thought processes are slow.

There are 2 questions I think.

1 How does the wear of, say, 6mm galvanised HT chain compare to, say, 10mm G30 galvanised chain. This is really a test of gal abrasion/corrosion rates, different gal processes on different steels and different chain sizes.

2 How does the wear of, say, 6mm raw HT chain compare to, say, 10mm raw G30 chain. This is simply looking at different steels and different sizes.

You cannot combine the 2 questions into one test (too many variables, or too many for me) - as you don't know when the gal has worn off and thus whether you are measuring change in the amount of gal, the amount of gal plus steel and then (once all the gal has gone, unlikely) steel wear rates.

I need to add raw steel samples to my test to compare raw steels.

I have some raw HT (G80 and G100) steel chain 6mm, a number of different samples (so lots of choice). I need to go and buy a metre of raw G30 in 10mm, maybe 12mm as the latter will show up bigger differences more quickly.

I might end up with too many samples to test together - I might need to whittle them down a bit.

Jonathan

 

[thinwater]

^^ I like question 2.

 

[RupertW]

Whereas test 1 is my dilemma as I have the galvanised G30 10mm now and want to replace it with strong but much lighter chain

 

[Neeves]

Rupert,

Question 2 is of interest to some as they wish to, or think they wish to, allow all or most of the gal to wear and they then wonder how quickly the underlying steel will wear. The thinking is that the crown will wear most quickly, so the gal will go from there before other parts of the chain - the crown will follow the gal and will thin out.

I have an open mind - or there are so many conflicting mechanisms - I simply don't know.

I need to buy some 3/8th" gal and ungal G30 chain, its one of the qualities/sizes I don't have but I'll have the tests underway this week.

Our chain has been in good use for 2 years and we have no signs of gal thinning, that's stepping down from 8mm to 6mm (you cannot go smaller - there is a 5mm - but I don't know that there gypsies

Jonathan

 

[vyv_cox]

Having done quite a lot of wear testing many years ago, I can say that the whole process is difficult to predict. However, there is a direct relationship between hardness and wear resistance, so it seems very probable to me that H&T steel will suffer less wear than annealed. In abrasive wear the particles cut or plough through the metal, thus increased hardness is clearly going to benefit the resistance to these mechanisms.

 

[lw395]

Is the wear mechanism mostly metal-metal, or are we really looking at metals forming chemical compounds with whatever's in the water, followed by these compounds wearing away?

I would guess that anchor chains might have a very different life to mooring chains, due to spending a lot of their time out of the water?

But there is a huge market for anchor chain which only spends a few hundred hours a year in the water.

 

[[163233]]

Digressing slightly back to the original topic, if the catenary disappears under any significant strain, is there actually any need to put out more scope with rode over chain?

 

[GHA]

Digressing slightly back to the original topic, if the catenary disappears under any significant strain, is there actually any need to put out more scope with rode over chain?

Over about 8:1 scope rocna reckon not -
http://kb.rocna.com/kb/Scope_vs_catenary

Interesting questions though, could loads of scope make matters worse by allowing the boat to speed up more when it tacks about in the big gusts? Would a riding sail & some buckets off the bow actually help more by keeping the velocity down as the energy the boat has is proportion to speed squared?

Even answerable?

Anyone have any guesses how much worse riding about the anchor is on a skinny chain compared to more normal size?