Unsolved Engineering Problem: Runaway Anchor Drops

[erobz]

Here is what I get for the chain velocity in the absence of drag, either from the capstan or fluid drag. The velocity is almost linear in time.

I don't know if this is of interest anymore, but there it is.

 

[jrmichler]

The thing is mounted on the bow of ships and got permanent battering of water, salt and every harm seas can offer.
I think it's a requirement that maintenance could be done by excess amount of grease only and field repair is by hammer and wrench

This. But I would change it slightly to say BIG hammer and BIG wrench.

 

[anorlunda]

Here's my thinking. I would use a water brake as a velocity limiter, or as a runaway preventer.

https://en.wikipedia.org/wiki/Water_brake

And this video,
https://en.wikipedia.org/wiki/File:Tech-Talk_Animation_on_How_Water-Brakes_Work.webm

The torque is controlled by the flow of cooling water. I would use a flyball governor to monitor capstan wheel RPM, and use the governor to control flow of water to the brake. The goal is to provide a soft limit to capstan wheel RPM; and to do it mechanically with no aux power.

The harder question is the required rating. I'm not sure how to calculate that. So partially arbitrarily, I'll choose the case of decelerating 30 feet of chain from 25 to 5 m/s velocity within 2 seconds. Here are the calculations using @erobz 's numbers

Initial V	25​	m/s
Final V	5​	m/s
Delta V	20​	m/s
Period	2​	s
Avg Acceleration	10​	m / s^2
len	30​	m
weight	269​	kg/m
total mass	8070​	kg
Initial K.E.	2521875​	Joules
Final K.E.	100875​	Joules
Power	1210500​	joules/sec
MW Power	1.2105​	MW
HP Power	1622​	Hp

I tried to find a 1.5 MW water brake on alibaba.com, but I find nothing similar. Perhaps I'm using the wrong search terms.

@erobz, @jrmichler , I'm not a M.E. Do my calculations appear correct?

The thing is mounted on the bow of ships and got permanent battering of water, salt and every harm seas can offer.

That's true. Ocean sailors must deal with salt and corrosion all the time. We use bronze alloys or for many objects. For example, the windlass I used to raise anchor chain on my sailboat was 100% bronze, lubricated with special water resistant grease.

 

[erobz]

Here's my thinking. I would use a water brake as a velocity limiter, or as a runaway preventer.

https://en.wikipedia.org/wiki/Water_brake

And this video,
https://en.wikipedia.org/wiki/File:Tech-Talk_Animation_on_How_Water-Brakes_Work.webm

The torque is controlled by the flow of cooling water. I would use a flyball governor to monitor capstan wheel RPM, and use the governor to control flow of water to the brake. The goal is to provide a soft limit to capstan wheel RPM; and to do it mechanically with no aux power.

The harder question is the required rating. I'm not sure how to calculate that. So partially arbitrarily, I'll choose the case of decelerating 30 feet of chain from 25 to 5 m/s velocity within 2 seconds. Here are the calculations using @erobz 's numbers

Initial V	25​	m/s
Final V	5​	m/s
Delta V	20​	m/s
Period	2​	s
Avg Acceleration	10​	m / s^2
len	30​	m
weight	269​	kg/m
total mass	8070​	kg
Initial K.E.	2521875​	Joules
Final K.E.	100875​	Joules
Power	1210500​	joules/sec
MW Power	1.2105​	MW
HP Power	1622​	Hp

I tried to find a 1.5 MW water brake on alibaba.com, but I find nothing similar. Perhaps I'm using the wrong search terms.

@erobz, @jrmichler , I'm not a M.E. Do my calculations appear correct?
That's true. Ocean sailors must deal with salt and corrosion all the time. We use bronze alloys or for many objects. For example, the windlass I used to raise anchor chain on my sailboat was 100% bronze, lubricated with special water resistant grease.

I’ve got the illness that shall remain unnamed. It’s starting to wear on me. Last night my fever broke long enough I program that solution. So I’m a little more foggy than usual.

I think you need to consider the mass of the anchor as well? Also the final mass of the chain will be the initial hanging mass + the anchor+ the additional 30 m of chain at 5 m/s. You also have to change the kinetic energy of the mass that is traveling vertically inside the ship.

 

[anorlunda]

I think you need to consider the mass of the anchor as well? Also the final mass of the chain will be the initial hanging mass + the anchor+ the additional 30 m of chain at 5 m/s. You also have to change the kinetic energy of the mass that is traveling vertically inside the ship.

I'm assuming that the runaway begins after the anchor touches bottom. Also assume that as soon as chain speed exceeds ship speed, the chain becomes nearly vertical, and the length of chain in motion remains constant.. But yes, I should include the K.E. of the ascending chain also; that would need about 50% more power.

Sorry to hear you're not well. Get well soon.

 

[erobz]

I'm assuming that the runaway begins after the anchor touches bottom. Also assume that as soon as chain speed exceeds ship speed, the chain becomes nearly vertical, and the length of chain in motion remains constant.. But yes, I should include the K.E. of the ascending chain also; that would need about 50% more power.

Sorry to hear you're not well. Get well soon.

So the anchor is on the bottom, but the chain hangs close to 30 m before it touches water correct?

 

[jrmichler]

Here's how I would attack the problem of a anchoring system for a large ship:

1) Assume a worst case hanging mass. The anchor is just above the bottom of the deepest water in which the ship will be anchored.

2) Assume a maximum allowable speed of drop at that point. This value may be iterated.

3) Design a self contained passive (no electronics, operator adjustments, or electric water pumps) water brake to maintain that velocity at that load. Feel free to calculate the effect of water drag on the anchor and chain, but it's much easier to assume zero drag. That approach is most likely only slightly conservative because I think that water drag will be small compared to gravity force on the anchor and chain at any reasonable velocity.

4) Design a mechanical brake to stop that mass from that velocity in a reasonable distance. The mass and velocity are from above, the distance may be iterated. The brake should have enough braking torque to stop that mass at that speed, and enough thermal capacity to lower the full length of chain under mechanical brake control at the maximum depth at the maximum speed over ground. Design the brake operator for maximum of one or two turns from full off to full on. It should be operable by one person of average strength.

5) Design a hard stop on a spring and shock absorber arrangement to stop the chain in the case of a mechanical brake failure. The first design criteria is the free hanging mass of the chain at the maximum anchoring depth. The second design criteria is the worst case chain velocity, which may include the ship velocity over ground. The anchor will be on the bottom at this point. You may or may not want to consider the case of a powerless ship driven by wind and/or current and the anchor not slipping. The spring and shock absorber arrangement may activate if the brake is applied too hard, so it should be self resetting.

6) Design the system weak link. The existing weak link is the attachment of the chain to the ship. Remember that it's better to lose an anchor and chain than to pull the bow of the ship off.

This system should allow both fast drops and controlled drops, and allows for operator error on the brake. This system will allow manual control of drops with a failed water brake. It will allow anchoring with a mechanical brake that failed in the open (unbraked) position by letting the chain run all the way out, then winching back to the desired length.

 

[Vanadium 50]

Here's a nice very short compendium on magnetic brakes:

I considered that. These are incredible things - they can stop very, very, very heavy objects on a dime. However, I think in principle they suffer from the same problem as mechanical brakes - what do you do with the energy?

 

[256bits]

6) Design the system weak link. The existing weak link is the attachment of the chain to the ship. Remember that it's better to lose an anchor and chain than to pull the bow of the ship off.

A weakest chain link is supposed to be built into the chain near the end.
The chain can be manually emergency separated from the hull - ie weather deteriorates to the anchor drag situation

 

[256bits]

I'm assuming that the runaway begins after the anchor touches bottom. Also assume that as soon as chain speed exceeds ship speed, the chain becomes nearly vertical, and the length of chain in motion remains constant.. But yes, I should include the K.E. of the ascending chain also; that would need about 50% more power.

Ship speed should be zero when dropping anchor.

 

[hutchphd]

I considered that. These are incredible things - they can stop very, very, very heavy objects on a dime. However, I think in principle they suffer from the same problem as mechanical brakes - what do you do with the energy?

Clearly I have no "feel" for this size force. That being said three things mitigate for magnetic brake in this circumstance:

1. Water will not directly affect braking force by lubricating
2. Heat is delivered throughout disk...not just the surface
3. The force increases with speed (to saturation current I guess)

I believe the temperature dependence is not large (Curie temperatures may need consideration for permanent magnets). One worry is that all the new rare Earth magnets seem to oxidase very easilly and deeply. Salt air.
Surely there is an effective way to exract heat when an unlimited cold bath is meters away...directly submerge the entire brake somehow?

 

[anorlunda]

1) Assume a worst case hanging mass. The anchor is just above the bottom of the deepest water in which the ship will be anchored.

2) Assume a maximum allowable speed of drop at that point. This value may be iterated.

I like that. It's a rational way to rate the system based on a knowable worst case.

But what you called a "hard stop" and a "weak link" are contradictory, and the hard stop seems to fill the same purpose of the water brake.

This system should allow both fast drops and controlled drops, and allows for operator error on the brake. This system will allow manual control of drops with a failed water brake. It will allow anchoring with a mechanical brake that failed in the open (unbraked) position by letting the chain run all the way out, then winching back to the desired length.

Music to an engineer's ears, fault tolerance and contingencies.

A weakest chain link is supposed to be built into the chain near the end.

Yes. The last link is called "the bitter end". Often it is not attached to the ship at all. No attachment and a weak link are approximately the same thing.

Surely there is an effective way to exract heat when an unlimited cold bath is meters away...directly submerge the entire brake somehow?

Yes, a water bath for cooling should be doable. I thought about both water brakes and magnetic brakes. Both may be viable.

But when I read that the resistance of the water brake can be controlled by a valve on the water feed, that tipped the scale for me. Then I could control that valve proportional to speed, and adjust the proportionality constant to change the "steepness" of the brake force versus speed curve.

With magnetic brakes, how does one make the braking force adjustable, and the slope of force versus speed adjustable?

Ship speed should be zero when dropping anchor.

No. That is discussed in #17.---

Great discussion everyone, I think we are zeroing in on a consensus solution. Too bad nobody wants to hire the PF community to design ship systems.

 

[hutchphd]

With magnetic brakes, how does one make the braking force adjustable, and the slope of force versus speed adjustable?

My stationary bicycle (only as a last resort) has magnetic drag. I think the adjustment is to move the magnet away. In any system I think it involves lowering the effective magnetic strength (and probably force it is quadratic in B). There are many simple ways to do this...I do not see it as a problem. The slope may be a little trickier but maybe it is as simple as radius of the disk? More thought required.

 

[Vanadium 50]

1. Water will not directly affect braking force by lubricating
2. Heat is delivered throughout disk...not just the surface

1. No, but salt water will interfere with the EM fields.
2. Heat is actually delivered by (and energy to) a dump resistor. The problem is what happens if the dump resistor overloads or otherwise fails.

 

[hutchphd]

1. No, but salt water will interfere with the EM fields.

A static B field? I will need to think about this, but I think it would just ad to the braking slightly

2. Heat is actually delivered by (and energy to) a dump resistor. The problem is what happens if the dump resistor overloads or otherwise fails.

I am not sure we are considering the same system. I am thinking about permanent magnets inducing eddy currents in a rotating disk. The braking is from the intrinsic conductivity (resistivity) of the disk (or drum)
My thought was to have a reversable electric motor drive to raise and lower the anchor. In addition there would be a passive permanent magnet brake. This has the happy characteristic of being (passively) more effective at high speed and probably would need no active components. Sort of like dropping a magnet through a conducting tube. (maybe that is the appropriated geometry but it is not obvious how to make that work at scale). Perhaps separate windlass with a cooled disk and magnets. One would need to look at the numbers of course.

 

[anorlunda]

My stationary bicycle (only as a last resort) has magnetic drag. I think the adjustment is to move the magnet away. In any system I think it involves lowering the effective magnetic strength (and probably force it is quadratic in B). There are many simple ways to do this...I do not see it as a problem. The slope may be a little trickier but maybe it is as simple as radius of the disk? More thought required.

If you use a flyball governor to move the magnets, then you have the adjustment of the slope and intercept of the force versus speed curve.

 

[hutchphd]

If you use a flyball governor to move the magnets, then you have the adjustment of the slope and intercept of the force versus speed curve.

For the bicycle I think the fixed magnetic drag supplies a simulacrum of the air drag, although I simply think of it as a midievel rack so who cares. However I still enjoy the real two wheeled variety on early summer mornings...(not dead yet!)

 

[Rive]

I too thought of more sophisticated solutions, but at the end what I would add is really just an (oversized) elevator-style speed limiter. The kind which not only doing speed limiting, but making a full stop till further intervention. (With optional usage as secondary brake - so it could be initiated manually too, not just by the overspeed.)

 

[Stormer]

Seems pretty simple to solve. Just use a eddycurrent brake. It will brake the chain more the higher speed it gets. So it is self regulating.