Snatch loads on mooring in high seas

I am designing a mooring system for a buoy that must survive very high seas (Hs>12m) in water depths of 30m. In the present design, the buoy is connected to a subsurface float then to a long mooring made up of a nylon line connected to sequentially heavier chain that lies on the seabed. The idea is that as a really big wave comes by, the mooring gives compliance by (1) pulling the subsurface float down, and (2) picking the chain up off the seabed. But based on simulations in OrcaFlex, there are still snatch loads: the bridle between the mooring and buoy goes slack then abruptly taut. Any suggestions on how to further mitigate snatch loads?

 

The most common systems I have seen are half chain , half nylon 3x to 4x water depth total.

A chain end weight also helps .

FF

 

I'm sure OrcaFlex wouldn't approve but around here we would shackle an old tyre into the system ,leaving a loop ,allowing the tyre to flex and take the snatch out.

 

Thanks all! Happily this particular mooring isn't long term but going to be a challenge when I have to design a version that can last a long time in all weather with no maintenance. Perhaps not possible to come up with stretchy components (tire, nylon section, snubber) that can take a long-term beating.

 

Are you saying the mooring alone must survive 12m seas in 30m depth or must survive with some sort of vessel moored to it?

 

Sorry, I re-read the post and now understand you are mooring a buoy of some sort. When I was in USCG in 1960s we were responsible for all aids to navigation in the US and I got a pretty good idea of the buoy moorings at the time.
Nothing we used was designed for the 12m sea conditions in the shallow water you mention. The usual setup was a large concrete 'sinker' about 2m cube with heavy stud link chain, then lighter chain. This for a buoy of at least 3 or 4 tons.

 

With Hs > 12m in 30m TWD, that would be near breaking wave conditions depending on bottom slope and wave period. I also don't think a classic buoy mooring would work. What is the buoy type (can, nun, lighted, etc) and what is the function (navigation, sensor, etc)?

Edit; and the watch circle needed?

 

Some time ago I read about Offshore Fish Farm Moorings on the internet.

Offshore fish farms use a similar mooring system as you describe. A robust, all weather , Stand alone mooring and buoy, trailing a shock absorbing mooring Pendant that is connected to the fish farm.

Much useful information was on the site concerning design and failure analysis. Unfortunately Ive lost the link.

Try Googling Gulf of Mexico, Offshore fish farm, Moorings.

 

I found this, -http://www.reefix.com/mcintoshP3.htm
It gave me the idea of putting a weight on the tendon between the spar buoy and the surface float that the mooring attaches to. The weight helps keep the tendon from going taut.

 

You are right, the wave conditions need re-examining. Thanks for the warning about wave breaking.
It is a vertical spar buoy of about 10m draft for a project-specific function (i.e. not a standard buoy you'd tend to come across, like a navigation buoy.)

 

I looked once again for the web site describing offshore fish farm moorings...no luck.

I do remember that a typical failure was caused when the fish farm pendant became slack in "no wind" slack current conditions, then tangled around the main heavy chain mooring, chaffed and parted.

Somehow you must keep the delicate spar buoy pendant separated from the permanent mooring buoy.