Much work with Dynamic Positioning is carried out onboard Dive Support Vessels.  But what kind of diving is done & how does it work?

There are 3 types of diving offshore, typically defined by the depth at which the dive takes place:

1. Air Diving
2. Saturation Diving
3. Hard Suit Diving

In this article we will take a look at each of these operations.

Surface Supplied Diving

Run off the side of a dive vessel by lowering the diver into the water on a platform, surface dives generally run to a maximum depth of 50m.  After this depth, the risk of Oxygen Toxicity is greatly increased & a mix of breathing gases is required.

This style of diving is known as Surface Supplied, as the divers air, power & communications are supplied by an umbilical with runs between the surface vessel & the divers helmet.  This umbilical is handled by the tenders onboard.  The diver will also wear SCUBA type breathing bottles, which act as a backup.  This way there will be a “bail out” in place if the umbilical gets trapped or damaged & needs to be cut loose.

Because surface supplied diving equipment  includes communication capability with the surface, this adds to the efficiency of the working diver. Breathing gas is supplied from the surface, either from a specialized diving compressor, high-pressure cylinders, or both. In commercial and military surface-supplied diving, a backup source of breathing gas is always present in case the primary supply fails.

The types of jobs performed on a surface dive are the same as would be done on a saturation dive.  Some examples are:

• Underwater burning & welding
• Survey work
• Equipment installation
• Operation & monitoring of subsea machinery

When returning to surface, much of the time the diver will go straight into a decompression chamber.  In deep recreational dives the diver will make calculated stops during the ascent to properly decompress.  In a working environment the divers can use the chamber to decompress thoroughly & in a controlled environment.

Remember they will have to get back in the water again next shift!

Saturation Diving

Saturation diving is a diving technique that allows divers to remain at great depth for long periods of time.

“Saturation” refers to the fact that the diver’s tissues have absorbed the maximum partial pressure of gas possible for that depth due to the diver being exposed to breathing gas at that pressure for prolonged periods. This is significant because once the tissues become saturated, the time to ascend from depth, to decompress safely, will not increase with further exposure.

Sat divers work at depths in excess of 50m for weeks at a time.  The deepest ever sat dive achieved in 1988 by a team of professional divers of the Comex S.A. industrial deep-sea diving company performing pipe line connection exercises at a depth of 534 meters  of sea water in the Mediterranean Sea.

An interesting item about the divers equipment is that while working in the water, warm water is pumped into the suit to keep them warm.

Breathing Gas Mixtures

On dry land the air we breath consists of approximately 21% oxygen & 78% nitrogen, with 1% of other trace gases.  However it can only be used in diving to a depth of around 40m, after which Nitrogen Narcosis can occur, which produces a state similar to alcohol intoxication.  This can obviously become very dangerous in a working environment.

Gas mixtures such as Heliox (Hydrogen & Oxygen) & Trimix (Oxygen, Nitrogen & Helium) are mixed onboard the ship by the LST’s & used by the divers for breathing.  These mixtures are prepared to avoid the risks of nitrogen narcosis & oxygen toxicity.

The gases need to be blended at specific ratio’s, depending on the working depth of the divers.

The Saturation System

While not on shift, the divers live in a controlled habitat onboard the vessel.

The “Saturation System” typically comprises a living chamber, transfer chamber and submersible decompression chamber, which is commonly referred to in commercial and military diving as the diving bell. The system can be permanently placed on a ship or ocean platform, but is more commonly capable of being moved from one vessel to another by crane.

The entire system is managed from a control room, where depth, chamber atmosphere and other system parameters are monitored and controlled. The diving bell is the elevator or lift that transfers divers from the system to the work site.  The bell will generally have one diver inside that tends the working divers tether & stands by to assist if required.

The divers will work a 12 hour shift at depth, dividing the time in the water.  Once back on board the time off will vary with the number of dive teams in sat, but it will be at least 12 hours.

At the completion of work or a mission, the saturation diving team is decompressed gradually back to atmospheric pressure by the slow venting of system pressure, at an average of 15 meters per day.  In an emergency this rate could be increased.

Hard Suit Diving

A third type of diving takes place from within a hard, or Atmospheric, suit.

Consisting of a hard & bulky suit fitted with small thrusters & control arms, the diver inside is kept at a steady pressure of 1 atmosphere; the same as we all experience on the surface.  This allows the team to lower the diver down to the working depth where the job is performed, then back up to surface with no special breathing gas or decompression time required.

Divers can work at depths of more that 700m with no ill effects such as decompression sickness or nitrogen narcosis; more than twice that of a saturation diver.

Oceanworks International makes the ADS line of Hardsuits

The Future of Diving

Offshore diving takes many forms & is an important tool is getting jobs done offshore.

With the advent of increasingly sophisticated ROV’s, the need to push the limits of depth on the human body is decreasing, however the ROV will never supersede the diver in all tasks.  The spacial awareness & dexterity of the human diver will never be matched by a machine, although the design of tools & equipment for ease of use by ROV’s is an element taken into consideration more & more.

But for those working in any capacity onboard a DSV, it looks like there will be work for years to come.