Chain dragging trials

[Deleted member 36384]

Are you sure about the bentonite clays being used as lubricants and not to "gel" the liquid mud to support the weighting materials, such as barite or calcium carbonate?

Just asking, because I'm offshore at the moment.

Yes - its one of the fundamental functions of a clay based drilling mud: lubricate, cool, carry cuttings, suspend cuttings, filter cake, maintain overbalance, protect formations from damage, transmit hydraulic horsepower to the bit. Other lubricants can be added which are massively superior but bentonite clay lubrication is suitable for most applications which don't required anything more complex. Brine based drilling fluids (non solid containing fluids) produce a noticeable increase in pipe drag weight (friction) when a bentonite mud is swapped out for them. I have spent my career working with friction factors in wells. Ask the mud engineer, he should be able to tell you this, or the drilling engineer.

 

[Halo]

Anyone on a fore and aft trot mooring?. I was for several years and got involved with the annual "servicing". The heavy ground chain was hauled over a work boat and checked for wear and the risers were done at the same time with the rig being dropped back down as we progressed up and down the trot. The anchors at each end were not enough to hold the whole line of boats so a lot of grip must have come from the ground chain when the river weas in spate!

 

[KellysEye]

>Why bother with an anchor then? It would be much easier to just bung chain out. A lot easier to just throw it in the locker too without having to faff around with however many kilos of steel on the bow roller.

How many times do I have to repeat myself, I have never advocated not having an anchor I have said that I view it as a backup when the wind picks up and a gale is forecast while you make a decision to put out more chain or another anchor or both. Sorry to everybody else for the repetition.

 

[Neil_Y]

I must admit I haven't read all the replies but after the first page I was thinking the point of how the chain/anchor works has been missed.

The anchor works when it has a horizontal pull along the sea floor, as the load on the chain rises first the anchor chain is progressively lifted off the floor. The weight of the chain acts to resist the boat going backwards. With a short scope of chain there is less chain, less weight and the point at which the anchor starts to be lifted comes earlier or with less force. It's not about dragging the chain along the bottom it's about lifting chain and weight of chain. More chain means more is lifted before it looses the horizontal pull on the anchor and hence more force is needed. The test in the yard would be to have the chain along the floor going up to the deck of the yacht and see if you could lift it all off the floor with a horizontal pull.

 

[Heckler]

I must admit I haven't read all the replies but after the first page I was thinking the point of how the chain/anchor works has been missed.

The anchor works when it has a horizontal pull along the sea floor, as the load on the chain rises first the anchor chain is progressively lifted off the floor. The weight of the chain acts to resist the boat going backwards. With a short scope of chain there is less chain, less weight and the point at which the anchor starts to be lifted comes earlier or with less force. It's not about dragging the chain along the bottom it's about lifting chain and weight of chain. More chain means more is lifted before it looses the horizontal pull on the anchor and hence more force is needed. The test in the yard would be to have the chain along the floor going up to the deck of the yacht and see if you could lift it all off the floor with a horizontal pull.

You got it!
Stu

 

[vyv_cox]

. The test in the yard would be to have the chain along the floor going up to the deck of the yacht and see if you could lift it all off the floor with a horizontal pull.

We already know the answer to that one. Calculated by many, and real world observation also by quite a few, shows that dependent upon the boat, chain, etc it happens at around Force 6. Dependent upon whose figures you believe you can then convert this knowledge into force.

Possibly one of the bigger advantages provided by new generation anchors is their ability to keep holding when the pull is not horizontal on the seabed. Even with my Delta, which I would not classify as new generation, I have seen it holding well with the chain to all intents and purposes straight, entirely off the seabed, and maybe 5 or ten degrees to the horizontal.

 

[doug748]

I liked Mr John Barry's contribution here:

http://www.ybw.com/forums/showthread.php?364388-Anchor-Scope/page3

It helps a mug like me to understand how a chain lifted off the bottom, which appears to be bar staight, can in fact offer a 0 degree angulation at the anchor, ie an horizontal pull.