mooring weight

dear all,
 this is a quick quiery with regard to how much weight a bottom mounted mooring should have.  we are planning to deploy an Aquadopp mounted on a Thetis frame (supplied by Nortek) in about 12m of water.  The area we are going to deploy in is sand bottom and is very active in terms of waves, the Portuguese west coast, with wave heights of 2-3m not uncommon.  however I unfortunately don't have an estimate of the current magnitudes.
  So, I was wondering if anyone has any suggestions as to the amount of weight we should be thinking about for the deployment, it will be in the water for 15 days.  At the moment we are looking at lead weight with a sum of 60 kilos.  too much, too little?  the mooring will be dived in (and out) so making it overweight is not a major concern.  any advice would be gratefully received.

thanks in advance

brad morris

Hello Brad

The problem is that proper mooring depends on many factors. Is there anyone in the community with experience on this?

- Gustav

Hello Brad,

I have some advice - based on our experience of seabed instrument frames and moorings in UK coastal waters - hope it helps.

We recently had a fairly successfully deployment of a Nortek 1 MHz AWAC in 9m (LW) to 15m (HW) water depth for 3 months on the East Coast of England. Sandy seabed, waves between 1 and 3m occasionally 4m, mean currents of 0.6 with maxima of 1.2m/sec. The frame was serviced monthly.

A very turbulent and dynamic site. (Frame moved only once when towed away by a beam trawler). The frame was also fitted with a lightweight acoustic release with two pop-up floats, and a tide gauge. Mooring system was the usual "L" type to a lit marker buoy with 250kg sinker weight some 60m way.

We used an open multi-purpose frame made of best marine quality 19mm st/st bar - 1.4m x 1.4m x 1.4m overall size - truncated-pyramid style, with the AWAC mounted on an upper plate at seabed + 1m to avoid burial by sand waves(photo attached).

We fixed 6 x 12kg lead weights to the lower rails of the frame. total weight approx. 150kg - but we have never actually weighed it. They usually always stay put however.

It sounds like you may have close to the correct weight - but add some more just in case - say 20 to 40kg?

Regards

Steve
\n\n

<!--EDIT

Hi,
At Nortek we have done a number of short deployments with the Thetis frame, The frame works really well, but it is better to be safe than sorry regarding weight. The frame is a tripod and typically has the battery pod on one of the legs. Things to remember are:
If you can, put the battery inside the leg not outside,
Have equal weight in each side/leg, It has been known for the frame to tip if the weight is too little/uneven.
Make sure the weight is correct underwater, I have seen some users use concrete blocks which virtually float." />'>
If you are using an AWAC with Surface tracking it is important the frame doesnt tilt too far (<5 degrees), this will reduce the quality of the vertical beam, if possible check the frame with a diver/camera.
Best regards
Paul

Hello Brad

Note that Thetis is now IXSURVEY SAS and Oceano Instruments is now IXSURVEY LTD. I have recently been deploying Aquadopp Profilers using our 30° tripods in a river with a nasty tidal regime. 1m water depth at low water and 5.5m at Spring High Water. Less that two hours from LW to HW and very fast current with very high sediment load. Hard sandy bottom and long period sand waves moving up river at Springs and back down at Neaps when the river flow is more dominant. I had 5kg lead disks fixed under each foot. After 8 week deployment the tops of the tripods are now buried 1.5m below the bed. I would like to exchange information with you.