Tandem Anchoring

[Psy at SEA]

Mooring of Oil Rigs is wrt predominant wind and tidal force, water depth.



So the caternary of the chain is calculated.





Catenary calculator

[Briney Bug]

Our experience mirrors what these fellows have stated. We use a multiple anchor set-up only when we really need to, such as when swing room is limited. All of the anchors used should be of the appropriate design for the sea-bottom, and strong/big enough to hold in the strongest winds that might be encountered.

That said, we did try the tandem anchor configuration once, using a 70 lb Luke and a 50 lb SuperMax in Hurricane Frances (in sand), and it worked for us (no dragging). We rode the backng winds along the eyewall several hours, hoping that the eye would pass over to give us a reprieve, which it never did. In retrospect, we were glad that it didn't, because we weren't sure that the tandem set-up would have re-oriented quickly enough to the sudden shift of 110+ knot wind on the back side of the eyewall.

[Don C L]

Originally Posted by Psy at SEA

Mooring of Oil Rigs is wrt predominant wind and tidal force, water depth.



So the caternary of the chain is calculated.





Catenary calculator

That's what I was looking for. Thanks!

[Steadman Uhlich]

Originally Posted by kas_1611

Not strictly true as floating oil rigs will have 8, 12 or even 16 massive anchors out from all 4 corners. The anchors are so big they require specialised anchor handling vessels to deal with them. And they are cross tensioned with anything up to 8 tonnes of force applied to them.

With that many anchors and that much chain out you are not going to drag and I've ridden out some pretty wild storms on such rigs. They might roll a few degrees port/stbd but they are not going to drag.

Unless it is an ultra deepwater floating rig when all it relies on for position holding is 6 or 8 massive thrusters, satellite fixes, big computers and engines that burn 75 cubic meters of fuel a day.

But Zeehag is right that good technique is vital for any anchor design to hold. Dump it over the side and hope and it doesn't matter how shiny or "next gen" the anchor is

Thanks for adding your comment above to this discussion. I found it interesting to think about those multiple HUGE anchors and how they are deployed and recovered, the special vessel required for that work, and the difference with the other rigs that use dynamic positioning gear instead.

Thanks!