32
World Pipelines
/
JANUARY 2015
The job of any gas detector is to sense gas in concentrations
low enough to provide an alarm before a hazardous concentration
has accumulated. However, the type of detection method needs
to be chosen carefully to match the type of gases present and
prevent false alarms.
On any rig, gas detectors are connected to a central control
system, which is responsible for indicating the current gas level and
triggering alarms when pre-defined gas thresholds are exceeded.
Gas detectors are usually located within a hazardous area, with the
control system mounted in the ‘safe area’ potentially hundreds of
metres from the detectors.
The problem with gases is that they often do not behave as we
expect them to; so having a robust gas detection system in place
is vital.
Explosive risk
In order for gas to ignite there must be an ignition source, typically
a spark (or flame or hot surface) and oxygen. For ignition to
happen, the concentration of gas or vapour in the air must be
a level such that the fuel and oxygen can react chemically. The
power of the explosion depends on the fuel and its concentration
in the atmosphere.
Controls and barriers are put in place offshore to prevent a
release finding an ignition source, such as open electrical box or
loose wires that may be live.
Critical points where the release of gases could occur include
all types of storage vessels, including valves and pipes, as well as in
machinery and pumps. Dynamic loading as a result of temperature
and pressure variations, corrosion and material fatigue are
important factors that may lead to leakage from joints and seals
and filling and tapping points also need special attention. There is
also the issue of technical leakage to consider, whereby material is
deliberately released through valves, diaphragms and overflows to
prevent malfunction.
The most notable incidents are those in which dangerous
substances are released. In this form of release, gases form a cloud
around the leakage point and the edge of the cloud becomes a
turbulent boundary zone.
In an effort to increase safety offshore, Dräger is using its ATEX-
approved thermal imaging technology offshore to detect loose
wiring, the fill level of tanks and also detect hotspots in lengths of
pipe that could be due to a leak.
Used by firefighters globally, thermal imaging can provide an
instant and clear picture of different heat signatures not visible
to the naked eye. The detection of even the smallest gas leak can
avert a chain reaction that would otherwise take on a life of its
own and threaten lives.
As well as differences in gas behaviour, there are also
differences between gas detection in confined spaces and in
the open air. In cases like this, the architecture, convective flows
and ventilation determines the spread of the gas. Pools of heavy
vapours are especially dangerous as they can form on the floors of
enclosed spaces such as cellars and tanks.
Dräger’s dedicated safety centre in Aberdeen has specialist
facilities for the offshore sector, including a Confined Space
Entry Training Rig. Located just 20 minutes from the international
harbour, the centre on Tern Place in Bridge of Don, offers a
dedicated Dräger training facility with a range of courses to
prepare its customers prior to going offshore.
Confined spaces pose unique problems, such as the potential
for ‘engulfment’ by materials in and around the space and toxic or
restricted oxygen supply.
Because of restricted freedom of movement, a lack of
adequate lighting and problems with stability, such as when
working on ladders – work in confined spaces requires absolute
attentiveness and, all too often, poses a serious psychological
burden on workers.
The potential for leaking gas pipelines and changing concentrations
of contaminants while working also causes additional stress and
necessitates the use of gas detectors.
The likelihood of an accident occurring in a confined space
is also frequently increased just by working in cramped quarters,
often closer to potential hazards than would normally be the case.
Escape devices such as Dräger’s Oxy K are designed to
give workers immediate protection against potentially deadly
environments, primarily confined spaces. Whether it is smoke,
toxic gases, a lack of oxygen, or where there is a mixture of
hydrocarbons and air, the device offers workers a vital lifeline in
order to escape.
Toxic risk
Gases and vapours produced by offshore activities can, under
many circumstances, have harmful effects on workers exposed to
them by inhalation, being absorbed through the skin, or swallowed.
Wherever toxic or flammable chemicals are being
manufactured, processed, stored or shipped, there will always be a
risk of an accident or substance release. Even the smallest release
of substances can cause harm to people, damage the environment
or even destroy property. Essentially, any gas is potentially
dangerous, whether liquefied, compressed or in a normal state.
In fact, it is hard to believe, but even oxygen, the very elixir
of life, can be a potentially dangerous fire hazard. The normal
concentration of oxygen in fresh air is 20.9%. An atmosphere is
hazardous if the concentration of oxygen drops below 19.5% or
goes above 23.5%.
It behaves very differently to air, compressed air, nitrogen and
other inert gases and is very reactive. If the concentration falls to
Figure 2.
Portable gas detection being used to check the safety
of a confined space.
