MAC   SH Fact Sheet

Hot Work on Hollow or Enclosed Structures in Shipyard Employment

Failure to test the atmosphere prior to hot work on hollow structures
could result in an explosion or injury.  

Hot work on a hollow or enclosed structure on a vessel or shore side
can present a hazard to both the worker and a vessel or a facility
asset. OSHA has regulations which detail safe practices when working on
any hollow or enclosed structure. However, these regulations only apply
to shipyard work and to vessel repair outside of shipyards. The OSHA
longshoring standards found in 29 CFR address the hazards of this type
of work shore side within a longshoring facility. 

What is a hollow or enclosed space? 

A hollow or enclosed space is a structure which is not vented to the
atmosphere. It is normally hollow but may be filled with some material
such as foam, or a preservative to prevent corrosion. Some common
examples are skegs, masts, booms, rudders support stanchions, pipe
stanchions, hatches or railings. Shore side some hollow structures may
include crane legs and supports, drums, mooring bits and tanks. This is
a very limited list and these structures can come in a variety of sizes
and shapes both shipboard and also shore side. It becomes dependent on
the person doing the hot work to recognize if they will be doing hot
work on a hollow structure.   

Figure 1. A hollow area under a mooring bit

What hazards may you encounter when performing hot work on these
structures?                      

The main hazard that may be found in a hollow structure is an explosive
atmosphere. Performing hot work on a hollow structure with and internal
explosive atmosphere will present the obvious danger of an explosion.
The hot work will present an ignition source which if combined with an
explosive atmosphere will result in ignition. 

Another hazard which you may encounter is that a hollow structure may be
filled with a material which could either catch fire or release toxic
vapors. One common material is insulating foam which can catch fire and
gives off noxious gasses. 

Where do explosive atmospheres come from?

Flammable or explosive atmospheres in hollow structures can come from
many sources. One method is if the internal metal is painted and then
sealed up. The paint may continue to release solvents from the paint
which can cause the atmosphere to become flammable. Another way is that
frequently internals are coated with an oil based preservative to
prevent corrosion. While the preservative is intentionally introduced,
repairs can present an unforeseen hazard. This is especially true with
rudders and skegs. 

Additionally, the rusting process of metal can also present an explosive
atmosphere. Water vapor combined with steel causes oxidation or rusting
and in the process can release explosive hydrogen gas. 

The process of welding can release carbon monoxide gas into the hollow
structure. Carbon monoxide in high enough concentrations can become an
explosive gas. 

Another source of an explosive atmosphere can be due to a leak from an
adjacent space, cargo tank or even a spill which may have caused some
flammable liquid or gas to enter the internal space of the hollow
structure. 

You may also have a mixture of the processes above causing an explosive
atmosphere. 

In the case of a hollow structure which is filled with foam or a similar
material, atmospheric testing may not indicate an explosive atmosphere,
but that does not mean that a hazard may not exist. Special precautions
must be used to prevent fires from starting when welding against this
material. 

Figure 2. Hollow Skegs

How do you test the internal atmosphere? 

The safest way to determine if you have an explosive environment is to
have a qualified individual test the atmosphere in the space. In order
to do this you must be able to sample the internal atmosphere. If the
hollow structure has a method to do this such as a vent plug then all
you have to do is remove the plug and test the atmosphere. If no plug is
available, the most common method to test the atmosphere is to drill a
hole into the structure. Since you are drilling into an unknown
atmosphere care must be taken not to overheat the drill bit. This can be
accomplished by using a lubricant to keep the bit cool.

 Testing the internal atmosphere: 

 Once an access hole is established the internal atmosphere can be
tested. It is important to have a qualified individual use a calibrated
meter to test the atmosphere. Due to several issues such as response
factors and unknown gasses, it is important that a qualified individual
test the atmosphere and understand the results to establish safe
conditions. 

Figure 3. A vent plug indicating leaking product

What do you do if an explosive atmosphere is found? 

If, after testing, an explosive atmosphere is found then there are
several ways to eliminate this hazard. The easiest method is to drill
another vent hole and introduce compressed air and ventilate the space
until testing determines that all of the explosive gas has been
eliminated. 

However, in some cases such as when an oil based preservative has been
introduced or a flammable liquid has entered the space, another method
of eliminating the hazard may be needed. This is normally done by
purging the space with an inert gas such as carbon dioxide or nitrogen.
This practice should only be done by an experienced professional as
failure to do this properly may not eliminate the hazard. 

In the case of foam or similar material it may be necessary to purge the
area with an inert gas, use special welding methods or to cold cut the
void and remove the foam. The use of a professional may be required in
this situation also. 

Once the hazard has been eliminated then hot work can safely proceed.
Depending on the length of the hot work it is a good practice to
occasionally test the atmosphere to ensure that safe conditions remain. 

To Obtain Professional Assistance for Hollow Structure Testing: 

www.MarineChemist.org

  HYPERLINK "http://www.aiha.org"  www.aiha.org 

http://www.osha.gov/SLTC/etools/shipyard/index.html

Local Safety Suppliers or Consultants

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This is one in a series of informational fact sheets highlighting OSHA
programs, policies or standards.  It does not impose any new compliance
requirements.  For a comprehensive list of compliance requirements of
OSHA standards or regulations, refer to Title 29 of the Code of Federal
Regulations.  This information will be made available to sensory
impaired individuals upon request.  The voice phone is (202) 693-1999;
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