Diving Familiar is at Ion

8/3/2019 Diving Familiar is at Ion

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AN APPRECIATION OF OFFSHORE DIVING

David Sheppard

May 2004

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An Appreciation of Offshore Diving Dave Sheppard

February 2003

TABLE OF CONTENTS

GENERAL HISTORY OF NORTH SEA DIVING .............................................................4

INTRODUCTION ..................................................................................................................7

AIMS ........................................................................................................................................8

TYPES OF DIVING .............................................................................................................10

SURFACE SUPPLIED AIR .......................................................................................... .......11

SURFACE SUPPLIED NITROX ........................................................................................12

SURFACE SUPPLIED DIVE SPREAD .............................................................................13

SATURATION DIVING ......................................................................................................14

BASIC DIVING PHYSICS ..................................................................................................15

PRESSURE ...........................................................................................................................16

PARTIAL PRESSURE 1 ......................................................................................................19

PARTIAL PRESSURE 2 ......................................................................................................20

OXYGEN ..............................................................................................................................23

NITROGEN ..........................................................................................................................24

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DECOMPRESSION .............................................................................................................25

TYPES OF DECOMPRESSION SICKNESS(DCS) ..........................................................27

SYNOPSIS .............................................................................................................................30

MIXED GAS DIVING ..........................................................................................................31

SATURATION ......................................................................................................................33

LEGISLATION AND REGULATION ...............................................................................35

SAFETY ...................................................................................................................... ..........38

DIVING EQUIPMENT ........................................................................................................42

TOOLING/PLANNING/DESIGN .......................................................................................43

DYNAMIC POSITIONING OF VESSELS ..................................................................... ...46

DEEP WATER STATION KEEPING RELIABILITY .....................................................50

DYNAMIC POSITIONING .................................................................................................50

APPENDIX 1 HAZARD IDENTIFICATION CHECKLIST ............................................59

APPENDIX II DIVING HISTORY BIBLIOGRAPHY .....................................................66

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GENERAL HISTORY OF NORTH SEA DIVING

Its a fact that many people involved in the underwater engineering industry are not

completely familiar with diving, diving physics, diving equipment and diving safety.

This is especially true nowadays when diving has become a bit of a rarity on most

projects.

The advent of deepwater fields, where diving will never be an option, the growth in

the use of remotely operated vehicles (ROV) and the design of ROV friendly subsea

hardware has seen a steady decline in the diving industry since the late 1980`s.

Looking back it seems that there was always a threat hanging over the industry, which

was considered extremely hazardous in the early days of North Sea development. It

does seem that, during those heady days, when it was fiscally important for the United

Kingdom to have an oil industry of some kind, that the cost in human life was

bearable, but once the industry infrastructure was in place it was inevitable that the

steely gaze of the legislature would turn on the diver and, under the guise of safety,

restrict mans adventures beneath the waves, at least as far as the North Sea oilindustry was concerned. The common theme was always that, to keep divers safe, it

was better not to put them in the water. Indeed, the removal of man from the water is

written into the subsea handbooks of more than one oil major.

Regardless of todays view, it must be recorded that, without the diver, much of the

North Sea development would have taken significantly longer to progress. Indeed, a

lot of the major development would perhaps only have been possible in the last few

years, given that the technology for useful workclass ROV`s has only been around

since the late 1980`s, early 1990`s. Be that as it may, there are still 300/400 divers

registered for working in the North Sea with the main underwater contractors. At the

height of development, pre 1986, there were over 4000.

Whilst offshore activities began in the North Sea in the early 1960`s, it was not until

the late 60`s and early 70`s that the push for offshore oil and gas really gathered pace.

The oil crisis in 1973/74 partly paved the way for a massive effort but the oil majors

had already put by large sums for North Sea exploration after the discovery of the

huge Groningen onshore gas field in Holland in 1959, and the discoveries offshore

from Great Yarmouth and the Humber seemed to point to the possibility of large scale

deposits under the seabed in the Southern, Central and Northern North Sea.

Diving on the other hand had been marking time. Although the phenomenon of tissues

saturating with gas had been known about since the early 1900`s, there had been no

driving force for men to want to spend time at depth in the oceans. Experiments had

been carried out to try and find answers to the nitrogen narcosis and oxygen toxicity

problems associated with deep air diving. There have been many claims as to who

started the idea of saturation diving and given the different locations around the world

where experiments had been going on, there is probably no one single person who can

lay claim to the idea. It has been recorded though that the first proper working

saturation dives were carried out in August of 1965, in the Smith Mountain Dam in

Virginia, USA. A team of divers spent a total of 4 months clearing the debris chute of

the dam at depths varying between -50m and -80m. This same system, called

Cachalot, owned by Westinghouse Electric, was used in the Mexican Gulf in 1966 tocarry out the first commercial saturation dives at sea. It wasnt long after this that the

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forerunners of todays diving systems were being built around the world in the drive

to find and exploit offshore oil and gas reserves.

Throughout the early days of the North Sea exploration, diving had been going along

much as before, with a lot of the diving carried out using standard gear (the copper

and brass helmet and equipment on the front page) but as the push toward the deeper

waters of the Central and Northern North Sea came, methods were needed to enablemen to work for longer at deeper depths. The limits of safe diving breathing air were

reached and other methods were introduced. Bell bounce diving, where the diver

attempted to complete tasks before going into a saturated state were used. These were

on the edge dives very often and resulted in long periods of decompression, with

men sitting in chambers breathing oxygen from masks (built in breathing systems) for

many hours. The fast compressions and fast de-compressions to the first de-

compression stop were later found to be the worst possible things to do. Bell bounce

diving is no longer carried out.

Mixed gas saturation diving became the accepted method for all dives over 50m

following the 1975 Submarine Pipelines act. Air diving was limited to a maximum of50m. New legislation has followed, culminating in the latest Diving at Work

Regulations in 1997. This legislation was history making in as much as it was put

together by a working party from the industry, alongside members of the HSE Diving

Inspectorate. The key notes of this 1997 legislation are the Accepted Codes of

Practice, issued alongside it, to cover all non-military aspects of men working

underwater, not just Offshore Diving.

The diving industry came a long way in a very short space of time. The depth trail

blazed northwards, from the discovery of the Hewett, Arpet and Leman in `63, `64

and `66 respectively, in the -20m to -50m range, up past Ekofisk, located in `69 in

80m, Forties in 1970, Argyll and Brent in `71, requiring 100m plus interventions and

on up to Magnus in the far North at 180m. Dive depths of 300m are now taken as

technically routine operations, although not carried out very often.

A few experiments are still being carried out around the world, using new gas mixes,

including hydrogen, helium and oxygen (hydreliox) in an effort to further understand

mans limitations at depth but to nothing like the levels of the 1960`s and 70`s, which

in hindsight now appears to have been the golden age for diving. Research and

development of diving equipment has virtually stopped in the commercial sector and

there is very little new blood coming through since government sponsoring of suitable

candidates was withdrawn. In the very near future, the older generation of divers,supervisors and superintendents will leave the industry as they reach retirement age

and this will leave a vast knowledge gap.

It appears that the North Sea diving industry will continue for the life of the existing

fields and the smaller step outs and subsea completions from these fields but that in

the end, it will die out. It will have been a two or three generation industry that

allowed a few lucky men to not only have earned a good living but to have done so

doing something they lived for and enjoyed. Most divers are fiercely independent but

when required, come together as a team in a way that most modern company

leadership schemes would die for.

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It is certainly not only the money that makes men want to live in small cramped

chambers and wander around in the cold dark waters at the bottom of the sea. It has

much to do with a sense of achievement, of being somewhere that not many people go

and perhaps, part of a wish to be out on the edge. It is job that carries a huge amount

of satisfaction with it. To overcome many problems and to succeed in completing

complex tasks in a hostile environment is very often the greatest driving force and

keeps people coming back for more.

This is only a brief history of commercial offshore diving. A list of books containing more information is included

in Appendix II, should you wish to know more.

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INTRODUCTION

Offshore Diving has been an important tool in the development of the oil industry in

the North Sea. It was not until a requirement existed for deeper intervention that

money became available for commercial exploitation of diving techniques and

equipment.

Whilst there is not as much diving work as there used to be, it is still important for

engineers and topside personnel to have an understanding of the different techniques

and methods involved.

An understanding of the basic physics and methodology can help advance safety and

result in cost savings through better planning and design.

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AIMS

This short familiarisation course will attempt to focus on the items listed below and

should give anyone involved in subsea engineering a sufficient depth of detail to

understand the problems involved in diving programmes, with a view to assisting indesign, planning and safety.

To give an overview of the types and limitations of Offshore Diving

Surface Supplied Air & Nitrox Diving

Saturation and Mixed Gas Diving

To examine the basic physics and physiology of diving and the inherent risks

involved

Pressure

Atmospheric Diving

Ambient Pressure Diving

Partial pressures

Oxygen & Nitrogen

Decompression

Decompression Sickness

Mixed Gas/Saturation Diving

To describe and explain the basic equipment and hardware that make up a diving

system

Surface Air Diving System

Surface Decompression Chamber

Saturation Diving System and Complex

To look at the regulations and legislation involved in Offshore Diving

Diving Regulations and Accepted Codes of Practice

Major Legislation

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To understand the risks to divers from outside influences and the methods of

control of these influences

Risk Assessment

Platform Issues

Diving Vessels

Pressured Systems

Controls

To broadly examine the tooling, planning and access issues involved in diving

General Underwater Tooling

Specialised Equipment and Design

Planning

Access and Design

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TYPES OF DIVING

In the North Sea we employ 2 basic methods of diving: -

These are:

a) Surface supplied air or nitrox to -50m

b) Saturation diving covering roughly -12m to -200m in the North Sea but usable

down to -450m+

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SURFACE SUPPLIED AIR

Used for shallow diving tasks

Limited by Regulation to a maximum of -50m

Really only useful in the 0m to -20m range due to decompression penalties

Weather dependent

Useful where space is limited, i.e. platforms. Systems can have a small footprint

Limitations from Dynamically Positioned vessels due to thruster proximity

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SURFACE SUPPLIED NITROX

Oxygen/Nitrogen mixed in various ratios ranging from 25% to 50% Oxygen

Allows longer bottom times with little change to decompression times comparedwith air

Limits depth of usage. Deeper the depth the less oxygen to a maximum of 1.55b

partial pressure of oxygen

Same risks as air diving.

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SURFACE SUPPLIED DIVE SPREAD

A surface spread does not differ much whether situated on a vessel, rig or platform. It

normally consists of two dive baskets on skids, with man riding winches to lower the

basket to the water. One basket is for the standby diver and the other for one or twoworking divers. The winches are independently powered so that, in the event of a

power loss, one winch will still be operable.

There will be a dive control shack, containing the dive control panels,

communications etc. A further container will house the deck decompression chamber

(DDC) and therapeutic gas bottles, in case of a decompression incident.

Compressors will supply the divers panel in the dive control with air, or gas banks

(racks or quads) will be used if Nitrox is being used.

The supply of gas is regulated at the panel and sent down to the diver through an

umbilical. The umbilical will have a main air hose, hot water hose, a pneumo hose

with which the Supervisor monitors the divers depth and an electric cable for

communications, hat camera and light.

The divers helmet is connected to the main gas hose and a further short hose is

connected to the bottle on the divers back via a regulating 1 st stage valve. If the

surface supply should fail, the diver opens a valve on the helmet that closes the route

to the surface hose and opens the route to the bailout bottle.

Surface dive spreads can be made to fit into small areas, which make them ideal forplatform and FPSO locations.

On some locations a modified system can be used which utilises a wet bell. This

enables the divers to stand inside it with their heads in the dry and to change over to

an onboard oxygen supply for decompression.

When installing a system on a platform, rig or FPSO, the Safety case must be changed

to take the system into account. A method must also be available to evacuate the DDC

in case of fire (PFEER regs).

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SATURATION DIVING

Can be used in calm conditions as shallow as -12m

Allows long dive times (6 or 8 hrs)

Divers breathing a mix of oxygen and helium

The normal method of diving if long bottom times required for complex tasks

Requires good planning if more than one diving depth required as divers are

limited to a depth band dependent on storage depth

Limitations in shallow depths from vessels due to thruster proximity

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BASIC DIVING PHYSICS

Just like all animals, human beings need oxygen in order to survive. When we inhale

our lungs extract oxygen from the air which is then used by our bodies for

metabolism. Metabolism being where our bodies convert the food that we eat and theoxygen that we breathe, into energy. There are three by-products of metabolism,

carbon dioxide, water and the bodies waste products. The carbon dioxide is eliminated

whenever we exhale. Therefore we can conclude that the main purpose of breathing is

to supply our bodies with oxygen, and rid our bodies of carbon dioxide. It is the

amount of carbon dioxide in our lungs that triggers the breathing mechanism.

If we believe evolution, it is long while since our ancestors left the water and it is

evolution that has adapted us to breathing air. As stated above, we only need the

oxygen from the air. The nitrogen that makes up 79% of the air is, at surface

pressures, inert. Its purpose, perhaps, to dilute the atmosphere.

Human beings are mammals, and as such our lungs are designed to breathe gas, but

unlike fish our bodies have no gills and so obviously we are unable to extract oxygen

from the surrounding water in order to breathe. If we enter the water and wish to stay

there for any length of time, the first problem we are faced with is finding a means by

which we can get the oxygen we need to stay alive.

If this were the only barrier that we need to overcome in order to explore the sea we

would have discovered and solved many of the oceans mysteries a long time ago. In

theory all that is required in order to remain underwater for long periods is a long tube

between us and the surface, a long snorkel perhaps. Unfortunately there is yet anotherproblem that must first be solved before we can explore the depths safely. This is the

most difficult to overcome. This is the problem ofpressure.

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PRESSURE

Earlier it was suggested that we could use giant snorkels in order to remain

underwater indefinitely. This is not correct of course unless we never intend to

venture beyond the first metre (3ft) or so of water. This is because it's extremelydifficult to inhale through very long tubes. When we exhale, our bodies are ridding us

of unwanted carbon dioxide, and when we inhale our bodies expect pure air. When

our bodies exhale carbon dioxide it will leave our lungs and be passed out into this

tube, but there is absolutely nothing to force it further up the tube. It is physically

impossible to expand our chests against the water even at these shallow depths.

Therefore the next time that we breathe in, the air will be mixed with some of the

carbon dioxide that we have just exhaled. As time goes on it will become harder and

harder to breathe, and the problem worsens the deeper we go, as, the further we

descend, the greater the pressure becomes.

This pressure is a force that acts on you from all directions. On the surface at sea

level, we are exposed to a pressure of approximately 1 bar, or about 14.7 pounds per

square inch (psi). This means that on each square inch of our bodies we have an

equivalent force of about 14.7 pounds pressing on it. The source of this pressure being

due to the earth's gravity, and is a result of the weight of the air in our atmosphere.

Like all other gases, the air that we breathe is composed of molecules of various

different gases and in this case, about 21% of these molecules are oxygen, about 78%

are nitrogen with the remainder being composed of various trace gases. These gas

molecules all have weight, which means that gravity is pulling them towards Earth.

Try to imagine a column of air, one square inch in cross-section (25.4mm), extending

from sea level all the way to the edge of the atmosphere, some 100 miles above. All

the gas molecules contained within that column of air would have a combined weight

of about 1 bar, or 14.7 pounds, hence the pressure of 1 bar or 14.7 psi of pressure on

our bodies at sea level.

This pressure is also defined by the scientific unit of measurement, the "atmosphere"

(abbreviated to "ATM"), which is a unit measurement of pressure being equal to the

pressure exerted by Earth's atmosphere at sea level (1 bar or 14.7 psi).

Like air, water also exerts a pressure due to its weight. However, water is

considerably denser (i.e., heavier for a given volume) than air. If we again visualise

our column, but rather than air this time imagine a column of sea water, again with a

cross sectional area of one square inch (25.4mm), we would find that our column

would only need to be 10m / 33 ft tall to weigh same 14.7 pounds. Therefore, at a

depth of 10m / 33 ft beneath the surface of the sea, the total ambient pressure is about

2 bar / 29.4 psi.1 bar of this pressure is due to the weight of the air in the Earth's

atmosphere, plus a further 1 bar for the weight of the 10m / 33 ft column of seawater.

This is called the ambient pressure, where ambient can be considered to be the total

pressure on our body, including the pressure exerted by the atmosphere. The word

ambient means surrounding. In order to avoid any confusion, divers refer to pressure underwater using the term bars absolute, or absolute pressure. Absolute

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pressure describes the total, or "absolute" pressure created by both the water and the

air above the water.

Remember that long tube that we were going to use to breathe from? In addition to

carbon dioxide build up which we have already looked at, the muscles that our bodies

use to expand and contract our lungs during the breathing cycle are not strong enoughto overcome much pressure. The pressure exerted by just a few feet of seawater is

great enough to prevent these muscles from expanding our lungs against the water

pressure in order to inhale a breath of air from the surface. Our bodies simply are not

designed to do that.

One way to overcome this problem is to protect the diver's body from the ambient

pressure. To do this underwater involves using "Atmospheric Pressure Diving"

technology, or in other words, a submarine. The pressure on the inside of a

submersible vehicle is maintained at 1bar, exactly the same pressure that we

experience at the sea surface. Underwater, the increased ambient pressure acts on the

submersible's hull rather than the diver inside, and so therefore the person inside thesubmersible is protected from the ambient pressure at all times and has no difficulty

breathing.

Another way we could overcome the problems presented by breathing under pressure

is to supply pressurised breathing gas to the diver. If this breathing gas supply can be

delivered at exactly the same pressure as the surrounding ambient pressure then the

diver's lungs do not have to work against the water pressure (i.e., the pressure in the

diver's lungs is balanced with the pressure of the surrounding water). However, when

using this sort of "Ambient Pressure Diving" technology, the diver's body is directly

exposed to the ambient pressure but more importantly, the gas inhaled into the diver's

lungs is pressurised. To understand the physiological effects of this, it's important to

firstly understand what effect this increased pressure has on gases.

All gases are composed of molecules or atoms, whether it be a gas from which we can

breathe or any other gas. As the pressure of this gas increases then the molecules

within the gas, which are in constant motion, become packed more closely together

and therefore occupy a smaller volume, or in other words a given volume will be

occupied by a larger number of gas molecules.

Lets suppose we have a balloon that we inflate at the surface. Now, let's take our

balloon diving. As we descend from the sea surface our balloon will start to shrink in

size, and at a depth of 10m / 33 ft it will only be half the size that it was at the surface.

Descending further, down to 20m / 66 ft it would then be one-third the size it was at

the surface; and descending further, now down to 30m / 99 ft our balloon would be

one quarter the size, and so on.

Now, while we are at 30m / 99 ft we will add some more gas to the balloon so that we

can expand it back to its original size.. Before adding this extra gas the balloon is one

quarter the size that it was on the surface, so therefore in order to re-expand it at depthwe are going to have to fill it with four times as many gas molecules as were required

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at the surface. As we make our ascent back to the surface we will notice that our re-

inflated balloon is now expanding and on return to the surface it will actually be four

times its original size (if it hasn't burst on the way back to the surface).

Now, let's replace our balloon with another expandable air container, our lungs. It

doesn't require much imagination to visualise what would happen to a diver's lungs ifhe took a full breath at depth, and then held it while ascending to the surface. Like our

balloon our lungs would obviously over inflate, the consequences of which could be

catastrophic. This is why the golden rule of diving is "never hold your breath". Divers

who forget this golden rule, maybe due to having a problem that has resulted in them

panicking, run the risk of suffering from ruptured lungs. Humans have no built in

reflex to exhale, only to inhale so we dont sense the damage until it is done. Lungs

are not very strong. They are like wet tissue paper when it comes to over pressure.

This rupture will allow gas bubbles from the lungs to directly enter the blood, a

condition referred to as an embolism, or more correctly, an Arterial Gas Embolism

(AGE), with very serious symptoms as a result.

Our understanding of the relationship between pressure and volume is due to the

research carried out by a seventeenth century Irish scientist by the name of Robert

Boyle. Boyle's research determined that at a constant temperature the volume of gas is

inversely proportional to the absolute pressure.

This is exactly what our balloon is demonstrating in the above example.

The key point to remember from this section is that the greater the increase in

pressure, the more tightly packed i.e., more highly concentrated the gas molecules are.

An appreciation for this will assist in the understanding of the other aspects of diving

physics and physiology.

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PARTIAL PRESSURE 1

To understand the physiological effects of breathing gas mixtures under pressure it

becomes very useful to understand the concept of partial pressure. The partial

pressure of a particular gas constituent in a gas mixture is a representation of theportion of the total pressure of the gas mixture exerted by that constituent. On adding

together the partial pressures of all the different components of a gas mixture, their

total would be equal to the total pressure of the mixture. This concept was first

defined by the English scientist John Dalton, who is also noted for being the first of

the modern scientists to describe the structure of matter as being comprised of atoms.

Daltons law states that "the total pressure exerted by a mixture of gases is equal to

the sum of the pressures of each of the different gases making up the mixture - each

gas acting as if it alone were present and occupied the total volume".

As an example, at sea level we breathe air that contains (in "round" figures, and

ignoring the trace gases) approximately 80% nitrogen and 20% oxygen. As discussed

earlier, the ambient pressure at the sea surface is 1bar, so therefore the total pressure

of the air that we breathe is also 1bar. To arrive at the partial pressure of the nitrogen

within this inspired air we simply multiply the fraction of nitrogen in the air (80%) by

the total pressure of the air (1bar), which gives us a nitrogen partial pressure of 0.8b.

Similarly, multiplying 20% oxygen by the air total pressure of 1bar will result in an

oxygen partial pressure of 0.2b.

Now we need to consider what will happen when we then descend down to a depth, of

say, 30m / 99 ft, where the ambient pressure is 4bar. In order for us to be able to

breathe at all, the inspired air pressure must be the same as the ambient pressure.Therefore, the inspired partial pressure of nitrogen is 80% times 4b, or 3.2b. The

oxygen partial pressure is 20% times 4b, or 0.8b. At 30m / 99 ft the ambient pressure

is four times greater than it is at the surface, and the partial pressures of each of the

gases is also four times greater than at the surface (although the percentages of each

gas are the same in both cases).

As discussed earlier, the gas molecules within the air are more closely packed when

under this increased pressure. At a depth of 30m / 99 ft there are four times as many

gas molecules (in both the nitrogen and oxygen) in a lung-full of air as there were at

the surface.

An easy way to consider partial pressures of gases is remembering that the partial

pressure represents an absolute concentration of that gas, regardless of the depth or

the pressure. If a person were to breathe a gas mixture containing 80% oxygen at the

surface, the oxygen partial pressure would be 0.8b. This is exactly the same partial

pressure of oxygen when breathing air at a depth of 30m / 99 ft. In both cases (80%

oxygen breathed at the surface and air at 30m / 99 ft), the concentration of oxygen

molecules in the lungs (i.e., the total number of oxygen molecules in the lungs on

each inhaled breath) is exactly the same.

In their gaseous forms, both oxygen and nitrogen are what are termed as binary

molecules; that is, they are bound as pairs of atoms. An oxygen gas molecule consistsof two oxygen atoms bound together, and a nitrogen gas molecule consists of two

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nitrogen gas molecules bound together. The chemical notation for oxygen is the letter

"O", so oxygen gas is referred to as "O2"; the subscript "2" indicating two atoms of

oxygen. Similarly, nitrogen gas is referred to as "N2", and carbon dioxide as "CO2.

When discussing partial pressures of gases, the gas notation is usually prefaced by

two "P" capital letters. Thus, "oxygen partial pressure" is written as "PPO2", and

"nitrogen partial pressure" is written as "PPN2".

PARTIAL PRESSURE 2

Anyone who adds sugar to a cup of tea or coffee is experiencing the effects of Henry's

law, and will be familiar with the chemical phenomenon of substances in solution. We

add sugar to our drinks in order to sweeten them. This sweetening process comes

about as a result of the sugar being reduced into particles that are small enough to be

held in solution and distributed evenly within the drink. In this way a solid substance

can be held within the molecules of the liquid.

This also holds true for gases. Consider the effects of shaking a bottle of fizzy drink

and opening it quickly. Yes, most if not all of us have done this at sometime, usually

with dramatic consequences. The reason that the drink generally ends up covering

everything in sight is that gas bubbles form as the gas comes out of solution. This

demonstrates that gas can be contained within liquid, or it can at least until the

conditions are changed.

A liquid, scientifically analysed, is a substance in a state that lies between a solid and

a gas. This means that the distance between the molecules within the liquid is greater

than that of a solid, but less than that of gas, and due to this molecular distance, it's

easy for individual gas molecules to become trapped between the liquid molecules

and electrically associated. In this state the gas is considered to be dissolved in the

liquid, or in solution. Although the gas is dissolved within the liquid it still retains its

gaseous properties, and although surrounded by liquid, the gas exerts pressure inside

the liquid. The pressure that is exerted within the liquid by a particular gas held in

solution is termed the gas tension.

The amount of gas that can be dissolved within a liquid depends on a number of

factors, researched by English chemist William Henry. In conclusion to this research

Henrys law stated that "the amount of any given gas that will dissolve in a liquid at a

given temperature is a function of the partial pressure of that gas in contact with theliquid." Therefore the two factors here that control gas solubility are temperature and

pressure.

Let us suppose that we have a glass of water that does not have any gas dissolved in

it. Therefore the gas tension within this glass of water is zero. On coming into contact

with a gas, the gas molecules will rush to penetrate the water, as the gas molecules are

flowing from high pressure, i.e. the air surrounding our glass of water, into an area of

low pressure, similar to opening the valve on a diving cylinder. The gas that enters the

water exerts a pressure, or gas tension that continues to rise until the pressure within

the water is equal to that of the gas at the surface.

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In accordance with Henry's law each gas that is dissolved within the water exerts a

partial pressure of the total gas tension independent of other gases present. Therefore,

our glass has been subject to the pressure of the air surrounding it, which has a partial

pressure of approximately 0.2 oxygen (20% oxygen at a pressure of 1 bar / 14.7 psi )

and 0.8 nitrogen (80% nitrogen at a pressure of 1 bar / 14.7 psi). If we now increase

the partial pressure of the oxygen in contact with the surface of the water we wouldn't

get any more nitrogen dissolving into the liquid because the nitrogen partial pressurewithin the liquid is in equilibrium with the nitrogen partial pressure on the surface of

the water.

However, the oxygen partial pressure in contact with the water would be greater than

the oxygen partial pressure within the water, and once again the oxygen gas tension

will start to increase. More oxygen molecules will enter the water, only stopping

when once again the partial pressure of the oxygen within the glass of water is in

equilibrium with the partial pressure of the oxygen in contact with the surface of the

water.

The transient difference between the partial pressure of the gases in contact with theliquid and the gas tension within the liquid is called the pressure gradient. When the

pressure gradient is high the rate of absorption of the gas into the liquid is high. As the

number of gas molecules continue to dissolve into the water, the gradient decreases

and the rate at which the molecules are dissolved into the water slows down.

Eventually the gas tension within the liquid will reach equilibrium which is where the

partial pressure of the gases within the liquid equal the partial pressure of the gases

which come into contact with the liquid and no further net exchange of gases will

occur. However, an equal number of molecules will continue to pass in and out of the

water, at which point the water is said to be saturated.

If we were then to place our glass of water in an environment where the air pressure

is greater, for instance in a recompression chamber, the pressure gradient would be

increased. The air pressure surrounding the glass would increase, resulting in an

increase of the partial pressure of the oxygen and nitrogen in contact with the water,

once again setting up a high pressure gradient.

Henry's law can be used to explain how more oxygen and nitrogen would become

dissolved in the water until once again equilibrium exists between the gas tension of

each gas within the liquid and the partial pressure of that gas exerted on the liquid.

Therefore the more pressure exerted by the gas in contact with the water the more gas

will dissolve in the water until once again reaching saturation.

If the pressure within our decompression chamber is reduced then the opposite

occurs. The partial pressure of the gases in contact with the water will be less than the

gas tension within the water. Therefore the pressure gradient is now acting in favour

of the gas dissolved within the water, which is said to be supersaturated until again the

gas within the water has equal pressure with the gas surrounding the water. So long as

the pressure reduction occurs gradually and the pressure gradient is not too high the

dissolved gas will come out of solution without forming any gas bubbles.

However, if the pressure decrease occurs rapidly, creating a high pressure gradient, or

if there are other factors present such as agitation of the water (remember our

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discussion about shaking our bottle of fizzy drink?) the gas may come out of solution

faster than it can diffuse into the surrounding air and bubbles will be formed.

As well as pressure affecting the gas solution in liquids, temperature also has an

effect, but in diving physiology temperature usually has little effect as our bodies

normally regulate our temperature within quite narrow limits. However, when a liquid

becomes warmer molecular motion increases, and as the molecules move faster theywill require more space in which to move, limiting the space which the gas molecules

have to occupy. Therefore the warmer the liquid, the fewer gas molecules it can hold

in solution. When you boil water you will notice that the water releases small bubbles

just before it comes to the boil. Here the accelerated water molecules are displacing

the dissolved air, so the air diffuses into small gas pockets on the irregularities of the

container to form small bubbles that will eventually rise out of the water.

This gas absorption phenomenon has implications for divers in that gas molecules

from the gas that they breathe will dissolve into the blood in proportion to the partial

pressure of the gas in their lungs.

When we breathe air at sea level the dissolved gases contained in our blood and

tissues are in proportion to the partial pressures of the gases in our lungs. As we

descend underwater, breathing compressed gas from a bottle or a surface gas supply

the ambient pressure increases, and therefore the pressure of the gas in our lungs also

increases accordingly. Because the partial pressures of the gases in the lungs are now

greater than the partial pressures of the gases dissolved in our blood and tissues, gas

molecules will begin diffuse from the lungs into our blood and tissues, just as the air

diffused into our glass of water.

Eventually, the pressures of the dissolved gases in the blood and tissues will be equal

to the partial pressures in the breathing gas, or in other words, our bodies will be in a

state of equilibrium, or saturated. As we ascend back to the surface the opposite

effect happens, in that the ambient pressure decreases, thus reducing the partial

pressure of the gases in our lungs. The gas partial pressure in our lungs will now be

less than the partial pressures of the gases dissolved in our blood and tissues and the

dissolved gases will start coming out of solution.

If we ascend too quickly these gases will come out of solution much too quickly,

forming bubbles in our blood and body fluids, just as it did in our boiling water

example above. Therefore we have to adhere to the strict ascent rates laid down in the

company guidelines and decompression schedules. If we ignore these guidelines,bubbles could form in our blood and tissues and result in decompression sickness,

otherwise known as the bends.

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OXYGEN

Oxygen is the only gas we really need to breathe in order to stay alive. If we don't

breathe oxygen, or at least not enough oxygen, we would soon die. However, we can

also breathe too much oxygen, and that will also cause us problems. As we breathe atsea level we inspire approximately 21% oxygen from the surrounding air, or in partial

pressure terms, a partial pressure of oxygen (PPO2 of 0.21 bar). If this PPO2 in our

breathing air drops much below 0.1 bar (i.e., 10% oxygen at sea level), our bodies will

begin to shut down, a condition known as hypoxia, which describes a state of oxygen

deficiency in our bodies. At the other end of the scale breathing a PPO2 greater than

0.21 bar of oxygen is generally fine up to a certain point.

If we maintain our inspired PPO2 above about 0.5 bar for prolonged periods of time

(many hours to days), we will begin to suffer from what is usually referred to as

"pulmonary" or "chronic" oxygen toxicity. The effects of oxygen toxicity include a

burning sensation or irritation in our lungs that can affect our breathing.

However, as the inspired PPO2 increases beyond this 0.5 bar level towards levels of

1.2 to 1.4 bar, a different kind of oxygen toxicity, termed Central Nervous System

(CNS) or acute oxygen toxicity presents itself, and this is a significant problem. A

variety of symptoms such as facial muscular twitching and tunnel vision are attributed

to acute oxygen toxicity (also referred to as the Lorrain Smith effect after one of the

early researchers who investigated it), and a further, far more serious symptom is

severe, uncontrolled convulsions. By themselves these convulsions do not appear to

cause any permanent damage, but the problem a diver could face when experiencing

such convulsions goes without saying. Whilst in itself an unlikely event to occur,when it has occurred it has been known to result in the diver drowning when

underwater, apparently as a result of oxygen-induced convulsions. This is perhaps the

most serious of diving maladies, because it appears unpredictably and without

warning when the increased PPO2 is available.

Unfortunately medical science doesn't seem to have a clear understanding of the exact

biochemical processes which take place under conditions of CNS oxygen toxicity.

However, there are theories suggesting the high oxygen concentration can temporary

overwhelm the bodies defences and interfere chemically with enzymes used by the

tissues for metabolism, resulting in symptoms of facial twitching, muscular trembling,

nausea, convulsions and unconsciousness. Neither is there a clear consensus on whatthe "safe" upper PPO2 limit should be. Convulsions have occurred in divers breathing

an inspired PPO2 level as low as 1.2 bar, but such cases usually involve extenuating

circumstances such as medical conditions which pre-disposed the divers to these

convulsions. Conversely, commercial divers routinely breathe oxygen partial

pressures as high as 1.9 bar in the water, and hyperbaric chamber facilities regularly

expose patients to 2.8 bar or more of oxygen without difficulty. Amid these

ambiguities, there are two trends that do seem to be very consistent.

The first is that strenuous exercise whilst diving can result in high levels of CO2

within our blood, a factor which appears to increase the probability of a convulsion

occurring if this high CO2 level coincides with us simultaneously breathing a highPPO2 of oxygen. A second pre-disposing condition is our immersion in water which

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seems to reduce our body's tolerance to elevated concentrations of inspired oxygen.

This second condition explains why divers in recompression chambers and undersea

habitats are able to breathe far higher concentrations of oxygen without experiencing

CNS oxygen toxicity.

A further factor of note, and one that unfortunately cannot be avoided or necessarily

explained is that there is an extremely large range of variation both betweenindividuals, and within a single individual overtime. When immersed underwater,

most training agencies regard a PPO2 of 1.4 bar as a safe upper limit during periods

of physical exertion, and 1.6 bar during periods of rest, such as during decompression

stops.

NITROGEN

Eighty percent of the gas molecules in the air are nitrogen (N2), and our bodies absorbthis gas as a direct consequence of Henry's Law. While oxygen absorption doesn't

cause us problems, providing of course it remains below oxygen toxicity limits,

nitrogen is physiologically inert and as such is not used by our bodies. Oxygen

absorption doesn't lead to decompression problems because our bodies metabolise this

oxygen. However, nitrogen absorption during deep dives can result in nitrogen

narcosis, a euphoric, anaesthetic effect nicknamed "rapture of the deep" or "the

narks". Divers experience this form of narcosis most commonly with nitrogen,

although other gases can also create this effect and so a more correct term is "inert gas

narcosis" or simply "narcosis", as even though oxygen is not an inert gas, it can still

have narcotic properties.

When diving on ordinary compressed air nitrogen narcosis develops as the nitrogen

partial pressure increases, and becomes most noticeable at depths of about 30m / 99 ft

where the PPN2 has increased to about 3.2 bar. The exact biochemistry behind the

development of narcosis is not fully understood, but almost any gas can cause

anaesthesia when breathed at high partial pressures. Theories suggest that the nitrogen

becomes dissolved in the lipids in neurons (nerve cells), and then interferes with the

transmission of signals between these neurons. Narcosis has many effects, which

seem to differ from person to person and usually from dive to dive. These effects may

cause the diver to feel drowsy, sleepy, and may provide him or her with a false sense

of security. A diver suffering from narcosis may also exercise poor judgement and

become uncoordinated. Hallucinations and giddiness have also been reported, and the

divers recollection of events during the dive may also be affected. The effects of

narcosis can be described as being similar to those that follow a night of inebriation,

followed by a hangover.

Being an inert gas, nitrogen plays no part in our body's process of metabolism, and

what nitrogen we breathe in, we also breathe out again. Although inert, nitrogen is by

no means harmless, as during our dive this gas will saturate our body tissues. If at the

end of our dive we make too fast an ascent, the nitrogen may come out of solution to

quickly, forming bubbles and causing decompression sickness.

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DECOMPRESSION

Decompression Sickness, more commonly known as the "bends", has been well

documented for many years. At the beginning of the twentieth century it was foundthat labourers working in pressurised caissons whilst laying the foundations for the

Brooklyn bridge piers suffered from pain in or near joints when leaving their

pressurised chamber. These caissons used compressed air to hold back water while

working on the river bottom, and the sickness that the labourers experienced on

leaving these caissons became known as "caisson workers disease".

The labourers soon learned that if they bent this joint the pain would subside. They

compared their contortions to the Grecian bend, an awkward but fashionable posture

adopted by women of that period. What these caisson workers were actually suffering

from was decompression sickness due to the fast rate of change of pressure when

leaving their caissons. At this time nothing was known about decompression sickness,their simple solution to the joint pains that they experienced was to bend the joint to

relieve the pain. This is where the term "the bends" comes from.

In 1906 Professor John Scott Haldane M.D. F.R.S, a physiologist already interested in

the effects of gas on the body turned his attention to decompression sickness in Royal

Navy helmet divers, and it was this work which led to the first dive tables being

published in 1907.

Following J.S. Haldane's initial work the U.S. Navy and other organisations spent a

great deal of time and committed substantial resources to conducting experiments in

order to better understand the physiological processes involved with decompression

sickness. They soon learned by developing theory from empirical data obtained from

numerous experiments that by slowing down the rate of ascent back to surface

pressure after exposure to elevated pressure, divers symptoms could be reduced or

eliminated. The U.S. Navy produced a further set of decompression tables which

detailed schedules that describe slow, staged ascent patterns back to the surface after

exposures to various depths for various lengths of time, a process which today is

termed decompression. These tables were eventually released to the general diving

public for use by commercial and recreational divers.

Unfortunately, no matter how conservative these schedules were, they were notperfect. In many cases, people following the schedules would suffer decompression

sickness symptoms anyway. In addition it was also found that a great many dives

were being conducted which followed ascent patterns much less conservative than the

schedules suggested, and which resulted in no symptoms of decompression sickness

at all. Therefore it became clear that there were many other factors contributing to

decompression sickness than simply depth and time. Since then there has been a long

and continuing effort to understand all the actual factors involved, and to produce a

mathematical model that was better able to predict optimal ascent patterns (i.e.,

decompression schedules). As it turns out, this is an extraordinarily difficult

undertaking.

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Since these early tables were produced there has been a great deal of hyperbaric

research undertaken by many expert bodies, and we now know that the blood within

our bodies already contain microbubbles even before we go underwater. The ratios of

the gas partial pressures within these microbubbles compared with the partial

pressures of the gases dissolved in the surrounding blood (plus a wide variety of other

factors) determine whether or not these microbubbles will increase in size, and if so,

by how much their size will increase. If these microbubbles become large enough theymay damage the walls of the blood vessels in which they are located. In turn this will

invoke a complex cascade of biochemical processes termed the "complement system"

that leads to blood clotting both around the microbubbles and at the sites where the

microbubbles have damaged the blood vessels. This clotting will block blood flow to

certain tissues, wreaking all sorts of havoc.

What follows is an outline description of what seems to cause decompression

sickness.

We can assume that Henry's Law describes the nature of how gasses actually dissolve

in our blood reasonably well, as after all, we can prove this theory to ourselves bysimply boiling up some water. After that though, things start to get a bit complicated.

To begin with, the rules that apply to oxygen are different from the rules that apply to

other gas constituents. A lot of the oxygen that dissolves in our blood is immediately

bound by haemoglobin, the important biomolecule that transports oxygen throughout

our bodies. Furthermore, oxygen is constantly being "consumed" by the process of

metabolism, so that the dissolved concentrations are always somewhat lower than the

inspired concentrations. It is generally assumed that oxygen does not need to be

considered in questions about decompression and decompression sickness, at least not

when the inspired PPO2 is within safe CNS oxygen toxicity limits. For the purposes

of this discussion on decompression, we are only considering the gases in the

breathing mixture other than oxygen.

When divers breathe air underwater, it results in increased concentrations of nitrogen

being dissolved in the divers blood and tissues. If we spend sufficient time at depth,

our blood and tissues will have elevated concentrations of dissolved nitrogen in them.

The nitrogen gas molecules are held in our blood by the ambient pressure acting on

our bodies at depth. If we were then to suddenly ascend to the surface, the pressure

that held the nitrogen in solution would be greatly reduced. In this situation, the

nitrogen gas molecules would either form bubbles, or (more likely) cause pre-existing

and harmlessly small microbubbles in our blood to increase in size.(Investigation has

shown that even at atmospheric pressure our blood and tissues contain thesemicrobubbles). Whether these bubbles cause harm directly by blocking blood flow in

capillaries, or by causing clotting via the complement system, it seems almost certain

that it's these bubbles which are ultimately responsible for us experiencing

decompression sickness.

In order to avoid decompression sickness we need to avoid bubble formation and / or

microbubble growth. During a dive we do not become instantaneously saturated by

nitrogen. The process of nitrogen diffusing into the blood and tissues takes an amount

of time. If while diving we stay shallow enough, or only dive to depth for a very short

period of time we can usually ascend directly back to the surface without

experiencing any of the symptoms of decompression sickness. This type of diveprofile is termed a "no-decompression dive". However, should we remain at depth for

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a long enough period of time, enough nitrogen will dissolve into our blood and tissues

to prevent a direct return back to the surface, which can therefore lead to a higher

probability of decompression sickness symptoms developing. When ascending from

such dives, we must spend time at shallow depths to allow the excess dissolved gas to

escape. Thus we must "decompress".

As a point and to try to show the complexities involved, the circulation periods withinthe body vary. Cerebro-spinal fluid can take 24hrs to fully circulate compared with

the 1-2 minutes required for arterial blood.

As we ascend at the end of a dive, the ambient pressure acting on our bodies begins to

decrease. This will result in the pressure of the gas within the lungs (and thus the

partial pressure of nitrogen in the lungs) decreasing. At this point the nitrogen gas

molecules move out of the blood and tissues and return to the lungs to be vented from

our bodies with our next exhaled breath. The depth at which any required

decompression stops are conducted is critical in that they must be shallow enough for

the PPN2 in the lungs to be lower than the dissolved concentration of nitrogen in the

blood. However, the decompression stop depth must also be deep enough to ensurethat the ambient pressure is sufficiently high to prevent significant bubble growth.

Usually decompression is performed in stages, with 3m / 10 ft intervals between each

stage. These stages allow us to incrementally return to the surface while the excess

dissolved nitrogen to escapes from the body without having to set up a high pressure

gradient within our bodies.

It should be noted that even if a diver surfaces from a no-decompression dive without

experiencing any of the symptoms of decompression sickness, it doesn't mean that

bubbles have not formed or are not forming in the blood supply. It simply means that

the bubbles have not increased to a large enough size to display obvious symptoms, as

decompression sickness may still be present even in the absence of symptoms.

TYPES OF DECOMPRESSION SICKNESS(DCS)

DCS results from the effects of these bubbles on organ systems. The bubbles may

disrupt cells and cause loss of function. They may act as emboli and block circulation,

as well as cause mechanical compression and stretching of the blood vessels and

nerves.

Additionally, the blood-bubble interface acts as a foreign surface, activating the earlyphases of blood coagulation and the release of substances from the cells lining the

blood vessels causing vasoconstriction which can worsen the effects of a blocked

vessel.

DCS may be divided into 3 categories:

(1) Type I (mild)

(2) Type II (serious)

(3) Arterial Gas Embolism (AGE).

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Type I DCS

Characterised by

(1) mild pains that begin to resolve within 10 minutes of onset (niggles);

(2) "skin bends" that cause itching or burning sensations of the skin; or skin

rash, which generally is a mottled rash causing marbling of the skin or a

violet coloured rash which is most often seen on the chest and shoulders. On

rare occasions, skin has an orange peel appearance.

It is important that this is not confused with other causes of a rash whilst diving. A

suit squeeze will generally have a different pattern and look more like bruising,

whilst a neoprene contact dermatitis will be in areas where a suit rubs, such as the

neck or cuffs.

(3) Lymphatic involvement is uncommon and usually is signalled by painless pittingoedema which is a swelling of the lower limbs that a thumb when pressed in will

leave an impression. The mildest cases involve the skin or the lymphatics. Some

authorities consider anorexia and excessive fatigue after a dive as manifestations

of Type I DCS.

(4) Pain (the bends) occurs in the majority (70-85%) of patients with DCS. Pain is the

most common symptom of DCS and is often described as a dull, deep, throbbing,

toothache-type pain, usually in a joint or tendon area but also in tissue. The

shoulder is the most commonly affected joint in most divers after a shallower than

40 metre dive, whereas the knees are affected more in deep divers. The pain is

initially mild and slowly becomes more intense. Because of this, many diversattribute early DCS symptoms to overexertion or a pulled muscle.

Upper limbs are affected about 3 times as often as lower limbs. The pain of Type I

DCS may mask neurological signs that are hallmarks of the more serious Type II

DCS.

Type II DCS

Characterised by nervous system involvement, pulmonary lung symptoms and

circulatory problems such as hypovolaemic shock. Pain is reported in only about 30%of cases. Because of the anatomical complexity of the central and peripheral nervous

systems, signs and symptoms are variable and diverse. Symptom onset is usually

immediate but may be delayed as long as 36 hours.

Nervous system

The spinal cord is the most common site for Type II DCS; symptoms mimic spinal

cord trauma. Low back pain may start within a few minutes to hours after the dive and

may progress to paresis, paralysis, paraesthesia, loss of sphincter control, and girdle

pain of the lower trunk.

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DCS can be dynamic and does not follow typical peripheral nerve distribution

patterns. This strange shifting of symptoms confuses the diagnosis, differentiating

DCS from traumatic nerve injuries.

Other common symptoms include headaches or visual disturbances, dizziness, tunnel

vision, and changes in mental status. Labyrinthine or inner ear DCS (the staggers)

causes a combination of nausea, vomiting, vertigo, and nystagmus in addition to

tinnitus and partial deafness.

Lungs

Pulmonary DCS (the chokes) is characterised by (1) inspiratory burning and

substernal discomfort, (2) non-productive coughing that can become paroxysmal like

a coughing fit, and (3) severe respiratory distress. This occurs in about 2% of all DCS

cases and can end in death. Symptoms can start up to 12 hours after a dive and persist

for 12-48 hours.

Circulatory system

Hypovolaemic shock commonly is associated with other symptoms. For reasons not

yet fully understood, fluid shifts from intravascular to extravascular spaces. The

problems of tachycardia (rapid heart beat) and postural hypotension (dizziness when

you suddenly sit or stand up) are treated by oral rehydration, if the patient is conscious

or via an IV if unconscious. The treatment of DCS is less effective if dehydration is

not corrected.

Thrombi or clots may form from activation of the early phases of blood coagulation

and the release of vasoactive substances from cells lining the blood vessels. The

blood-bubble interface may act as a foreign surface causing this effect.

AGE (Arterial Gas Embolisation)

Pulmonary overpressurisation, for example, during a breath holding ascent, can cause

large gas embolisation when rupture into the pulmonary vein allows alveolar gas to

enter systemic or arterial circulation. Gas emboli can lodge in coronary, cerebral, and

other systemic arterioles. These gas bubbles continue to expand as ascending pressure

decreases, thus increasing the severity of clinical signs. Symptoms and signs depend

on where the emboli travel to. Coronary artery embolisation can lead to myocardial

infarction or abnormal rhythms. Cerebral artery emboli can cause stroke or seizures.

Differentiating cerebral AGE from Type II neurological DCS is usually based upon

suddenness of symptoms.

AGE symptoms typically occur within 10-20 minutes after surfacing. Multiple

systems may be involved. Clinical features may occur suddenly or gradually,

beginning with dizziness, headache, and profound anxiousness. More dramatic

symptoms of unresponsiveness, shock, and seizures can occur quickly. Neurological

symptoms vary, and death can result. Central Nervous System(CNS) DCS is clinically

similar to AGE. Since the treatment of either requires recompression, differentiating

between them is not of great importance.

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SYNOPSIS

We have so far seen the following:-

How pressure causes problems to the diver breathing air, in the followingways:-

Narcotic effect of nitrogen at raised partial pressures.

Nitrogen causing decompression sickness.

Oxygen toxicity at raised partial pressures.

Limited time at depth due to decompression requirements

Problems returning to the surface unless decompression stops arecarried out.

We will now look at the ways around these problems that have culminated in the use

of mixed gases and saturation diving.

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MIXED GAS DIVING

The maximum depth limit for diving on air in the North Sea oil & gas fields is -50m.

The PPO2 at this depth is 21% x 6b/100=1.26b, not very far from our 1.55b upper

limit in the safe zone and in the zone where the variable aspect of toxicity could havean affect on a person with a low susceptibility to oxygen.

Even at -50m, narcosis can be likened to having had three or four very large drinks

and unless a person is aware of narcosis and the effects it can have, it could put them

at risk during a dive.

For every minute of time at this depth, the decompression penalty is likely to be

between 3min-6min, depending on which table is used. Commercial tables are likely

to give less penalty than sports diving clubs. However, the trade off risk against

incident for the working time against decompression penalty is unrealistic and only

rarely is surface supplied air diving used at this depth. Most people are of the opinion

that 30m is a more realistic maximum for surface supplied air.

To start trying to sort out our problems with air, we can quite easily overcome the

oxygen toxicity risk by reducing the amount of oxygen in our breathing mix.

By reducing the O2 to 10%, we can go down to -120m before reaching 1.4b partial

pressure.

The problem is, if we replace the removed O2 with N2, we increase the effect of the

narcosis, but at a shallower depth. We also increase the amount of nitrogen available

to flood the tissues and bloodstream.

So, we need to reduce the quantities of both gases. The best way to do this is to use

another gas in place of the nitrogen. One that does not have the same narcotic effects

as N2 and removes any toxicity problems. While we are choosing our new gas, we

could also look for something lighter, that has a smaller molecule, so that we can

breath it more easily at depth.

The most usual gas for this is Helium. We could choose Hydrogen but the explosive

combination of Oxygen and Hydrogen would mean re-designing our diving systems

to make them spark proof, along with many other changes for gas handling etc.

Hydrogen has been used in tri-mixes and work is still ongoing exploring the use ofthese mixes in very deep diving. It does appear though from early results that

hydrogen does produce narcosis but of a more cerebral/hallucinogenic type than that

produced by nitrogen.

Helium

It has two fundamental advantages over nitrogen:-

Does not cause narcosis, even at very high inspired partial pressures.

It has a much smaller molecule and is consequently much less dense,

requiring less effort to breathe at great depths. It is less dense at -100mdepth than nitrogen is at the surface!

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Before we go further into saturation and deep diving, an explanation of some of the

other mixed gas diving that you may come across:-

NitroxCommonly used these days. Because air is not a good decompression

gas because of its nitrogen makeup, we remove some of the nitrogen

and add oxygen. Because, as stated earlier, oxygen does not factor in

decompression dynamics, this gives us a wider leeway in no-

decompression dives, and increases bottom times for little or no

increase in decompression penalties. Obviously this mix can only be

used at depths where CNS oxygen toxicity is not a concern. (also used

by sports divers).

Trimix

Normally Helium, Oxygen and Nitrogen. Not normally used by thecommercial industry but becoming more and more used by technical

sports divers.

Heliox- Helium and Oxygen. Achieves all our requirements:-

Reduces the concentration of oxygen

Eliminates the nitrogen and risk of narcosis

Reduces the overall gas density making it easier to breathe at great depths.

Unfortunately, for all its plus points, Helium is not a good gas for the kind of

decompression we do in surface supplied diving. Because of its small molecule size, it

dissolves in blood and tissue faster than nitrogen, ie, more dissolved helium in less

dive time. This means that the ratio of dive time to decompression time would be

lower, making it a much less commercial proposition than air or nitrox for surface

supplied diving.

Less worktime for more decompression time

Its use also results in a faster loss of body heat because it conducts heatbetter than O2 or N2

Creates communication problems (mickey mouse voice)

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SATURATION

During experiments aimed at isolating gas absorption rates in tissue, it was noted that,

at a certain point, when the tissue was saturated with gas, the time taken for the gas to

transfer became fairly standard. We have seen in earlier text how tissue saturation isreached.

It was then realised that once the body and bloodstream were saturated with gas, no

further decompression penalty was incurred, regardless of the length of time spent at

depth. So, in essence, it is possible to stay at, say 100m for 8hrs, until tissue saturation

is completed. At this point we have 29 hours decompression to complete to allow the

gas to dissipate from our tissues and bloodstream. It would make no difference if we

stayed at that depth for a week or a month, we would still only require the 29 hours of

decompression.

Hence the term Saturation Diving. It took quite some time for the theory and practice

to come together but once there was a commercial driving force and a reason to want

to spend extended times at deep depths, there was no holding back. Once all the

information was available and decompression tables had been built and tested,

saturation diving became a viable concern.

Saturation curves for exposure at depth are not linear but exponential. Consequently

we measure the time taken for a gas/liquid system to saturate as a period. i.e., if we

have a tissue with a 30-minute period, it is a tissue which will be 50% saturated in 30

minutes. This does not mean it will be fully saturated in double the time (60 minutes).

Divers live under pressure in chambers for up to a month at a time, stored at a depth

slightly less than the required working depth.

A diving bell takes them from the saturation complex to the worksite. The bell is

locked onto the system by a clamp. The pressure in the lock on trunking is equalised

to that of the internal pressure of the saturation complex. The divers then transfer

through the trunking into the bell and secure the door of the bell. Personnel in the

complex close the door of the complex and the trunking is de-pressurised, enabling

the bell to be locked off.

The bell is then moved to the moonpool and lowered to the working depth. At theworking depth, when the pressure equalises, the bottom door of the bell can be opened

and the diver can exit to the worksite.

Depending on the work, bell runs can be contain 2 or 3 men. If 2 men go, they will

work in the water for 3 to 4 hours each, one man remaining in the bell while the other

carries out the tasks required in the water. On 3 man dives, 2 go out into the water and

work together with the 3rd carrying our standby diver duties in the bell. Bell runs are

regulated to 8hrs or 6hrs dive duration.

Divers are only allowed 8hrs in water time during a 24hrs working period.

There are now regulations restricting how much intermediate decompression isallowed during the saturation trip, as there was always a tendency to decompress to

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shallower depths to keep options open. This resulted in divers spending days in transit

between depths.

Internal chamber partial pressures for oxygen are normally kept around 400mb. This

is normally increased to 600mb for decompressions. Depth excursions are also now

limited. (The shallower the diving depth, the less excursion above and below the bell

allowed).

The diver is equipped with a bailout bottle on his back for emergency use. At extreme

depths this gives only a few minutes for him to return to the bell.

Breathing gas is reclaimed through helmet valves and return hoses to the surface. The

gas can be then be scrubbed clean and re-used cutting down on expensive helium

waste. (Helium breathing without reclaim would cost approximately 800-1000 per

hour at 100m, at current rates).

Below 150m the gas has to be heated to reduce body temperature loss. The diver

wears a hot water suit and the gas is normally heated by utilising this system. Thewater temperature is regulated from the surface and flow regulated from the bell and

by the diver at the suit.

The saturation system and internal environment are monitored and run by Life

Support Technicians from Saturation control. They are responsible for the divers

while they are in the chambers. The chambers are mini environments and have to be

monitored constantly for gas content, humidity, temperature and bacteria.

Once in the bell, the divers come under the direct control and responsibility of a Dive

Supervisor.

He is the only person that can start a dive. No-one else on the worksite is recognised

by law or legislation in so far as being in charge of the divers. The Dive supervisor

must have a letter of appointment from the company he is working for before he can

run a diving programme. Other responsible persons can insist that he ends a dive but

no-one can insist that he start one. Supervisors are required to have a certificate of

competence, gained by passing examinations set to IMCA/UKOOA standards.

All saturation systems must have a secondary means of escape for the divers. These

days a hyperbaric lifeboat is normally supplied. This is a pressure chamber fitted

inside a lifeboat hull and attached to the saturation dive complex. It contains lifesaving equipment for the divers and is normally manned by the life support technician

crew in the event of abandonment of a vessel. It has a gas supply built in and must be

big enough to take the maximum number of divers held in saturation. Regular

abandonment drills are carried out.

Although saturation has become a very normal and commonplace item in the offshore

industry, it must be stressed that it is very similar to being in space. Whilst a few

millimetres of steel separate the man on the inside of the chamber from a man on the

outside, they are in fact many hours apart, due to the decompression requirements. It

is impossible for the man in the chamber to pop out for a few minutes to get a briefing

for the next job!

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LEGISLATION AND REGULATION

The diving industry in the North Sea is regulated in several ways. There are self-

imposed rules that vary from company to company. There is the International Marine

Contractors Association (IMCA), formerly the Association of Diving Contractors

(AODC). There are guidelines produced by the United Kingdom Offshore OilAssociation (UKOOA) for various standards within diving and there is the

Government, through the Health and Safety Executive and their own Diving

Inspectorate.

The main legislation involved is the Diving at Work Regulations 1997 (SI 1997 No

2776) and allied Accepted Codes of Practice. (ACOP)

All offshore diving is carried out under these regulations and anyone involved in an

offshore diving project would do well to read this legislation and the accepted code of

practice that goes with it.

In the past there have been many grey areas in diving regulations as to responsibility

and accountability. This legislation actually outlines all those who may be

accountable and the net is fairly wide spread.

This quote is from the ACOP, Regulation 4.

Every person who to any extent is responsible for, has control over or is engaged in

a diving project or whose acts or omissions could adversely affect the health and safety of persons engaged in such a project, shall take such measures as it is

reasonable for a person in his position to take to ensure that these Regulations are

complied with.

This short familiarisation course cannot go very deeply into the legislation and

regulations affecting not only diving but the offshore industry in general except to say

that nearly all major legislation affects in some part the preparation, planning,

execution and reporting of diving and subsea programmes.

Included below is a list of the major legislation that may have an effect on an

underwater work campaign. Anyone not sure as to whether a certain part of some

legislation affects the job being planned is advised to check with the contractor

involved or specialist diving consultant for advice.

1 The Health and Safety at Work etc Act 1974.

2 Employers Liability (Compulsory Insurance) Act 1969 requires employers

to take out insurance to cover their liability for accidents and ill health

sustained by their employees

3 Management of Health and Safety at Work Regulations 1992 require

employers to carry out risk assessments, make arrangements to implement

necessary measures, appoint competent people and arrange for appropriate

information and training.

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4 Manual Handling Operations Regulations 1992 cover the moving of objects

by hand or bodily force.

5 Personal Protective Equipment Regulations 1992 require employers to

provide appropriate protective clothing and equipment for their employees.

6 Provision and Use of Work Equipment Regulations 1992 require thatequipment provided for use at work including machinery is safe.

7 The Offshore Installations and Pipelines (First-Aid) Regulations 1989

cover requirements for first-aid offshore.

8 Noise at Work Regulations 1989 require employers to take action to protect

employees from hearing damage. The Regulations now apply offshore.

9 Electricity at Work Regulations 1989 require people in control of electrical

systems to ensure they are safe to use and maintained in a safe condition. The

Regulations now apply offshore.

10 Health and Safety (Training for Employment) Regulations 1990 set out

how certain people being trained for employment should be treated for the

purposes of health and safety law.

11 Offshore Installations (Safety Case) Regulations 1992 require the duty

holder of an offshore installation to submit at various stages in the life cycle of

the installation a safety case for the management of health and safety on the

installation.

12 Chemicals (Hazard Information and Packaging for Supply) Regulations

1994 require suppliers to classify, label and package dangerous chemicals and

provide safety data sheets for them.

13 Construction (Design and Management) Regulations 1994 cover safe

systems of work on construction sites.

14 Control of Substances Hazardous to Health Regulations 1994 require

employers to assess the risks from hazardous substances and take appropriate

precautions.

15 Offshore Installations and Pipelines Works (Management and

Administration)Regulations 1995 require co-operation between everyone

who has a contribution to make to ensuring health and safety on the offshore

installation or in activities involving the installation.

16 Offshore Installations (Prevention of Fire and Explosion, and Emergency

Response) Regulations 1995 provide for the protection of persons from fire

and explosion and for securing effective emergency response.

17 Reporting of Injuries, Diseases and Dangerous Occurrences Regulations

1995 require employers to notify certain occupational injuries, diseases anddangerous events.

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Controls

The hierarchy on a diving site varies depending on the location. If the diving

site is on a fixed platform, the following persons have the responsibility to

ensure the diving is carries out to legislation and regulation;-

Offshore Installation Manager (OIM)He has the overall responsibility for the health, safety and welfare for all

personnel working on his installation or within its 500m zone. He will be

aware of all activities being undertaken and through his managers, will

authorise the diving programme and monitor its progress.

Client/Company Diving Representative

He is the eyes and ears of the OIM and is involved with the dive programme

and dive team first hand. He is normally a diving specialist with firsthand

knowledge of diving techniques and is responsible for ensuring that the diving

programme is carried out to company, legislative and regulatory requirements.

He is responsible to the OIM and the company that is having the diving workcarried out.

Offshore Manager (OM)

Most vessels or diving worksites have a diving company representative

onboard. He used to be called a Diving Superintendent but is now normally

given the title Offshore Manager.

He is there as the diving company worksite and vessel representative and is

normally the contact between the diving company and the contracting

c

Diving Familiar is at Ion

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