fastener locking methods....submerged in seawater 1

[endover]

I'm trying to make this short so if you need more info, please ask away.

We are putting a large unmanned buoy (200,000lb) in seawater and anchoring it to the seafloor. (Yes...I know that's stupidly difficult to do but it pays the bills). The only way to moor it correctly is to have the mooring line enter from the bottomside of the buoy through an intricate steel gimbaling system which allows for full range of wave motion.

Here's the question...

When bolting the joints(and other various parts) on the gimbal system, we cannot find a proven method for preventing the standard hex cap screws from possibly backing out(due to wave motion vibration) AND providing the galvanic isolation.

Goops like Loc-Tite don't make waterproof compounds and mechanical fasteners like Nord-Locks don't help with galvanic isolation so we're looking to combine the solutions. The structure is A588 steel and we are using Grade 5 hardware from McMaster.

The gimbal must be serviceable and replacable by divers under water since the bushings are wearing out yearly so the solution has to be reversably by divers with gloves on.

Does anyone have experience with Nord Locks, Vibra-Tite or anything else that may work for us?

bla...bla...bla...again....sorry 'bout the long question.

 

[MikeHalloran]

You couldn't use a less intricate gimbaling system, like, say, anchor chain and shackles?



Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

Or braided mooring ropes?

Mike Halloran
Pembroke Pines, FL, USA

 

[endover]

In an attempt to NOT write a novel, I left out the part where I tell you that the gimbal is designed the way it is becuase there are also fiber optics and copper wire running up through the mooring line and into the buoy. The gimbal system has been designed to allow the buoy full motion ('cause you ain't stopping that thing from moving) yet protect the fiber and copper in the midst of massive sea states. The gimbal system has been up and running for 2 years now with minimal fastener issues however, we are doing a redesign to make it better and would like to improve on that area as well. As anyone on this marine forum could tell you....the sea will eventually move the mountains so whatever you build...will eventually die so make it replacable.

 

[MikeHalloran]

I can understand how a Nord-Lock could survive in the sea if primed and painted after assembly, but underwater assembly has to expose some fresh bare metal.

Have you looked at "Stage 8"s?

http://www.stage8.com/howitworks.htm

Mike Halloran
Pembroke Pines, FL, USA

 

[endover]

I started looking at the Stage 8's today, but I fear it's too many small parts for a diver to be able to handle with his gloves on.

It's not out of the picture though....

 

[IRstuff]

Maybe just a simple collar to minimize the exposure of the hexheads? Sort of like a cup with a hole in it for the fastener that's wide enough to get the socket over the hexhead?

TTFN

 

[MikeHalloran]

What's the nominal size of the cap screws you're trying to secure?



Mike Halloran
Pembroke Pines, FL, USA

 

[endover]

Everything from 3/8" up to 1". All of varying lengths.

 

[MikeHalloran]

I'm pretty sure a diver can handle the clip and tab of Stage 8s in those sizes, even with gloves. Buy a few extras.

If you're contemplating serious changes, you might look to reducing the number of fasteners and increasing their size. I'm imagining that your divers might prefer to work with a few large castellated nuts, locked with cotter pins.

Better yet, ask your divers to participate in the development effort. They'd probably like a few days in a warm dry office.







Mike Halloran
Pembroke Pines, FL, USA

 

[endover]

We'll be doing all of the above as we try and find the best option(s). The divers are usually much more capable than we (the design engineers) give them credit for. They're a pretty amazing bunch. Our particular dive team is overseas right now so it will be a while before we can talk with them.

Thanks.

 

[zeusfaber]

I like Mike's idea. I've done a few in-water castellated nut and cotterpin changes on bobbing objects and never found it that difficult.

As a design solution, it has the big advantage that the spares are cheap, so you needn't feel guilty about losing the odd pin as you're trying to fiddle it into the hole.

Being able to assemble onto a set of fixed studs rather than having to line up two sets of holes while buoy, gimbal and diver are all dancing randomly around one another is an added bonus. I get the impression you aren't planning to do this job in a nice sheltered non-tidal lock?

A.

 

[endover]

Nope....100 miles out at sea!

 

[endover]

...and it moves with the waves while the divers work on it....brutal!

 

[zeusfaber]

Just re-read the original post and converted the buoy's weight into units I can think in. Brutal is one word for it.

A.

 

[MikeHalloran]

Given the dynamics of the environment, you might want to build in means for attachment of a tool for temporarily holding the buoy and the anchor rode in position relative to each other while the divers fuss with the replaceable parts of the gimbal.

Sort of like a roller chain vise on a larger scale...



Mike Halloran
Pembroke Pines, FL, USA

 

[Ussuri]

Is it possible to split the two functions? Have your buoy anchored in the conventional way and have your cables and stuff in an umbilical free hanging in a catenary supported with buoyancy?