Info What is Saturation Diving?

[Akimbo]

Does anybody have a list, or at least the number, of operational DSVs these days? The definition of what defines a DSV can be a little slippery but I would put a permanently installed sat system and moonpool(s) on the list.

What's a Moonpool? A moonpool is a vertical well through the vessel where the diving bell enters the water. They are located roughly amidships, ideally at the most stable point of hull rotation in all axis (also a slippery definition). Most are square or rectangular and are open from the keel to a deck well above the waterline where the bell is launched.

Most moonpools I have seen have a cursor that sits on top of the bell until it reaches the keel. It is designed to keep the lifting cable centered in the moonpool and guide the bell's umbilical.

Many bell handling systems also have two guide cables connected to a weight that is lowered below the bell to reduce twisting at depth. The cables slide through bushings on the bell. Non-rotating cable is used to lift the bell to reduce twisting, but it doesn't eliminate it. Getting the umbilical twisted around the lifting cable causes excessive chaffing, especially at greater depths.

There are sea conditions where the bell can act like a piston in the moonpool. I have seen bells lifted 3-4' and dropped as the swell rolled by. It is not unusual for air (lots of it) to be injected at the bottom of the moonpool to aerate the water as the bell passes. This effectively reduces the water density, thus the bell's buoyancy, until it clears the keel.

Moonpool is term was that was adopted from the offshore drilling industry. They required the most stable point on the vessel to keep their drill string from being tied in knots. They also need a lot of room to work around it. I have heard several stories on where they got the name and I'm not sure I believe any of them.

Does anyone have any decent photos or illustrations? I always had a hard time getting photos of bell handling systems that didn't just look like a jumble of machinery.

 

[DCBC]

..Does anyone have any decent photos or illustrations? I always had a hard time getting photos of bell handling systems that didn’t just look like a jumble of machinery.

 

[DCBC]

This provides a clear look at a Sat System:

http://www.offshore-europe.co.uk/__novadocuments/27321?v=635011019923400000

 

[kelemvor]

Great post, absolutely fascinating!

 

[Akimbo]

Here is an interesting link that anyone interested in becoming a saturation diver should read: Six Divers Trapped on Sea Bottom

I think it is fair to say that you have to develop a somewhat uncommon mental state to be able to do this kind of work. The discussion includes a lot of insights into systems and lifestyle.

 

[shermhw]

GREAT information, intresting thread for a newbe like myself.

 

[kelemvor]

So, if I understand correctly. The bell is lowered down the moonpool until it reaches a depth equal to that which the system was pressurized to? I'm guessing there's a hatch in the bell that is then opened for exit/re-entry?

 

[Akimbo]

... The bell is lowered down the moonpool until it reaches a depth equal to that which the system was pressurized to?...

More or less, yes. The saturation or holding or storage depth is typically on the shallow end of the working depth range since most tables allow far greater excursions below than above the depth that deck chambers are pressurized to. It isn't unusual for the bell to be pressurized a little deeper on the way down or after the hatch seal breaks to minimize the bell to worksite distance.

Remember that gas expansion from 1,000-850' is far less than 150-0'. This allows much longer no-decompression excursions below the sat depth than surface divers are allowed.

Here is an abbreviated section of the US Navy Table 15-7, Unlimited Duration Downward Excursion Limits, Page 15-26:

Edit: Tables updated

 

[ScubaInChicago]

Forgive me for asking, but is it possible or is there a safety factor in pressurizing a chamber and bringing it to the surface with divers in it with decompression taking place at the surface like a decompression chamber?

 

[DCBC]

Forgive me for asking, but is it possible or is there a safety factor in pressurizing a chamber and bringing it to the surface with divers in it with decompression taking place at the surface like a decompression chamber?

Each Saturation System must comply with various construction standards that are appropriate to where the unit is being used. For a unit to receive an AMSE certification (for example) it must be built and tested to withstand 400% more pressure than it's internal operational pressure.

If you want more information, it's covered under AMSE's Safety Standard for Pressure Vessels for Human Occupancy. This is defined as any enclosure designed to withstand a difference in pressure of 2 psi or greater (Aircraft are in a different category).