Launch Garage

Moderator: Jeff Snyder

Launch Garage

Postby Jeff Snyder » Sat Oct 23, 2010 11:46 am

Hi everybody,

Figured I would freshen up the forum by submitting a thread on a launch garage that I recently had fabricated. It is prototype #1 as I start to look into developing a proper launch garage for the Pro4.

As you all probably know, trying to free-fly a small ROV from the surface to deep depths is problematic. Even 75 feet of tether, subject to currents from the surface to the bottom, can make ROV operations with a small ROV a challenge. It is even more challenging if you are presented with a situation of operating from a dynamically positioning vessel, whose azimuth drives are cycling constantly and present a significant entanglement hazard for the ROV as it descends to work.

Many operators simply go with a clump weight, attached to the tether, so as to decouple the ROV and its working tether from the impacts caused by current acting on the section of tether that runs from the surface to the bottom. Most of the time, this is adequate. But a launch garage provides an alternative for deploying the ROV that protects the ROV on descent and ascent, gives you a "storage basket" for anything you may find on the bottom, and provides the foundation for a more advanced deployment system that could include a subsea tether management system for the Pro4 (ie - a system that actively pays out or retrieves the working tether).

garage-backview.jpg


garage-sideview.jpg


The material is expanded steel mesh that has been bent in a metal brake to create a cage. Reinforcement steel (stainless) has been used to provide a steel frame and integrate into a lifting eye at the top of the cage.

Under the landing platform is an additional small cage with a lip. That is to add ballast weight to get the garage to sink directly down from it's deployment location on the ship. There is also a lip up on the landing platform; that keeps the ROV from sliding out the back during launch/recovery. The primary means of retaining the Pro4 in the garage is a vertical bit of steel rod that is positioned for grabbing with the single-function grabber. This, and thrusting down, keeps the vehicle in place inside the garage.

For this recent deployment on a DP research vessel with the University of Delaware, I decided to deploy the garage with the ship's oceanographic winch off of the stern A-Frame. Due to the location of the azimuth drives and the location of the frame, this was the safest location from which to deploy.

garage-launch.jpg


A shackle, and a swivel attached to the lifting loop of the garage, allow the garage to spin freely in response to the current. I had a small storage area on top of the garage (but beneath the lifting loop) where I could coil the PPT tether for deployment. As long as it is counter-coiled properly, the coil of tether will pay out smoothly upon demand (ie, as you fly away from the garage, the PPT uncoils and pays out from the storage area.

underwater-garage.jpg


As you can see from this underwater shot, the coiled tether stays neatly in the basket during the descent, but it will pay out nicely once the Pro4 must be piloted away from the garage.

The garage makes for a good target, too, so that you can keep track of where you are on the bottom. It shows very clearly in BlueView or Micron imagery.

sand-ripples-garage.jpg


By attaching the down tether (neutral or negative tether) to the garage and then attaching the PPT to the garage, your strain reliefs can work together to keep the splice connection intact but also, importantly, to keep the ROV unaffected by the current acting on the tether that leads from the ship to the bottom.

This is what I would call, jokingly, "passive TMS" because the tether pay-out from the storage basket is one-way. When you return to the garage for recovery, you will have a 20-meter belly (actually 20 x 2) that must be managed by how you recover. The trick is to have the ship positioned so that the prevailing current carries the tether away from the azimuth drives. Or, you can also have the ship clutch in ever so slightly to get about 1/2 knot of headway so that the belly is carried away from the ship as the garage nears the vessel.

garage-recovery.jpg


It's pretty easy to handle and bring up/over the side. All-in-all, without ballast weight, the garage only weighs about 10 or 12 kg.

This is just a first pass at this; the next phase would be to utilize a lifting tether with a powered-winch topside (so that we would not need to use the vessel's winch). The second phase would be to install a powered subsea TMS winch to actively pay-out or recover the PPT. It might seem to be overkill for a small ROV like this, but there are definitely advantages to having this type of configuration, particularly for performing offshore research or offshore subsea inspection work, where the clients and the other contractors are used to seeing work-class ROVs with full launch-and-recovery systems and tether management systems.
Jeff Snyder
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Re: Launch Garage

Postby stinger » Wed Oct 27, 2010 6:40 pm

Thanks for your story Jeff. Another good thing about a garage is you can put a lot of nice things into it and letting the sub being your scout. Suddenly the larger wrov`s don’t look so sexy anymore ;-)
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Re: Launch Garage

Postby rovtom » Wed Oct 27, 2010 10:49 pm

Thats really neat. does the tether deploy with ease? I suppose it's like a packing a parachute, if not done properly it does not deploy well.
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Re: Launch Garage

Postby Jeff Snyder » Fri Oct 29, 2010 10:36 am

Hi Tom,

So, see that U-shaped bit of steel where the tether pays out? I'm taking a grinder to that and getting rid of it. All it would do would create a$$holes behind it and hinder tether payout. If I just counter-coiled the PPT tether in the basket and fed it around the U, then it would pay out very nicely.

Obviously, it's a one-way trip. On recovery, it is necessary to keep that tether away from the business-end of the ship.

This is like a pre-Gen 1 garage. I've seen some other approaches (I really like something that Bjarte Langeland has put together) for the actual construction, and I really want to see if I can get a TMS put together.
Jeff Snyder
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Location: Rhode Island, USA


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