A compressor was used to supply air through a nylon hose to the offshore platform field engineer working at the bottom of the piling. Air quality in the pile was sampled periodically for carbon dioxide, carbon monoxide, hydrogen sulfide, and combustible gases by an universal tester and an explosion meter

Coffin, C. L.

Mull, M. M.

English

NASA Technical Reports Server

PORTABLE LIFE SUPPORT FOR INSTRUMENTATION 
OF AN OFFSHORE PLATFORM 
Michael M. Mull and Clarkson L. Coffin 
Mechanics Research, Inc, 
Los Angeles, Calif. 
17 
INTRODUCTION 
Not all portable life support system design takes place in the laboratory. Occasionally a situation 
arises in an industrial or military operation that requires a unique or unavailable system. The people 
involved must rely on their own ingenuity to develop a new system or to modify an existing one to 
meet their needs. This paper discusses just such a situation. 
Mechanics Research, Inc. (MRI) received a contract from Continental Oil Company to  instrument 
an offshore drilling platform for measurement of structural response of the platform to severe 
storms and hurricanes. A typical platform is shown in figure 17.1. Specifically, this instrumentation 
consisted of the following measurements: 
Structural response of a piling driven into the ocean floor 
Structural response of the jacket members, including typical welded joints 
Figure 17.1 Typical Gulf offshore drilling platform. 
257 
https://ntrs.nasa.gov/search.jsp?R=19720019473 2020-03-17T03:56:04+00:00Z
The hydrodynamic parameters of wave height, direction, velocity, and force 011 a typical 
structural member 
The aerodynamic parameters of wind velocity and direction 
This paper discusses the life support considerations for the field engineer installing the 
instrumentation for the first item listed above. Strain gages were installed in the piling after it had 
been driven through the leg of the jacket into the ocean floor. The life support requirements are 
unique in that instrumenting inside a piling below the mud line is a new activity. In this regard, the 
MRI program constituted a totally new application for portable life support systems. 
Life support requirements for this application included the following elements: 
Sustaining air supply 
Emergency air supply 
Provision for monitoring concentrations of C02, CO, H2S and combustible gases such as 
methane. 
For this program, all system requirements were satisfied by commercially available hardware. 
PLAT FOR M DESCR I PT I ON 
As shown in figure 17.2, a typical offshore 
platform of the type used in the Gulf of Mexico 
consists of three basic structural components: 
jacket, deck, and pdings. 
The ocean depth at the site involved in this 
installation was 176 ft, and the resulting jacket was 
approximately 186 ft high. The pilings are long 
tubular members that are driven through the legs 
of the jacket into the ocean floor to  anchor the 
platform. They are then welded to the top of the 
into the pilings and welded. 
ENVl RONMENT 
The working area for this program was in one of 
the cylindrical pilings. The pilings, which are 
approximately 550 ft  long, taper from an inside 
diameter of 31 in. at the top to 29-1J2 in. at about 
the mud line. As these pilings are driven into the ocean floor, they fill with mud and water. Prior to 
installation of the deck structure, the piling to be instrumented is cleaned out to a depth of 
approximately 70 ft below the mud line. A concrete plug is then poured into the bottom to seal the 
end. The top of the plug is approximately 60 ft below the mud line. 
As shown in figure 17.3, a door cut in the leg at the cellar deck level provided access to  the piling 
work area. Figure 17.4 shows the elevator (powered cage) for transporting the instrumentation 
engineer and his equipment up and down within the piling. Figure 17.5 shows the elevator 
equipment compartment with the electrical equipment and welder in place. Figure 17.6 is a top 
view of the elevator moving within the piling. A view of strain gages in place looking up from the 
elevator is shown in figure 17.7. There were six vertical strain gage locations starting at the mud line 
(ocean floor) and terminating 54 ft below the mud line. Work duty cycles within the piling ranged 
from 3 to 5 hr. 
jacket leg. The deck section legs are then stabbed -JACKET LEG 
Figure 17.2 Assembled p&&om structure. 
258 

The gaseous environment within the piling was comprised of the following constituents: 
Air supplied continuously from the surface 
Carbon dioxide from normal respiration 
Carbon monoxide from internal combustion engines located at the surface 
Acetylene from metal cutting operations 
Gases such as methane and sulphur dioxide from the soil into which the piling is imbedded 
Air composed the major volume of gas present in the piling during the time that people were 
working. However, at the outset, it was not known how the concentrations of the other gases would 
vary during the work period. In this regard, ground gases, such as methane, were of primary 
concern, because the ground structure had been disturbed by the entrance of the Diling. 
Figure 17.5 Elevator equipment compartment. 
260 
Figure 17.6 Elevator moving down in the piling. 
'. 
Figure 17.7 piling with strain gages in place. 
26 1 
LIFE SUPPORT SYSTEM 
Main Air Supply 
The major consideration for life support was supplying clean air to the man working at the bottom 
of the piling. This clean air was necessary both to satisfy his oxygen needs and to continuously 
purge the piling of hazardous gases. This was accomplished by a compressor supplying air to the 
bottom of the piling through a reinforced nylon hose. The compressor used was a two-cylinder 
two-stage vee-type driven by a three-cylinder air-cooled diesel engine. The compressor was rated at 
80 scfm, at a maximum operating pressure of 200 psig. This compressor had a power rating of 35 
horsepower. 
This type of compressor engine combination (fig. 17.8) is commonly used in the gulf by 
commercial divers and is considered to be a diving compressor. However, the actual compressor used 
was a standard commercial unit, not rated as a diving compressor, because of oil lubrication in the 
cylinder area. Rated diving compressors are either soap lubricated or use nonmetallic carbon or 
Teflon-coated piston rings to preclude air contamination. Also, the compression cylinders are 
isolated from the crankcase by a crosshead piston and piston rod seal. However, experience has 
shown that if the piston rings are in good condition, oil fouling of the air supply is not a serious 
problem. 
Figure 17.8 Sustaining air supply compressor-engine combination. 
262 
The air from the compressor was routed to a 50 gallon storage tank where the pressure was 
regulated between 80 and 120 psig. Discharge air was piped from the reservoir through two 
centrifugal separators mounted in series to remove water and condensed oil vapors. From the 
centrifugal separators, the air was routed through an absorption type oil scrubber to remove oil 
vapor. This system was modified after the first day of operation by the removing the oil scrubber, 
because the scrubbing material was becoming saturated with water. The water caused the material 
to pack tightly and thus restrict air flow through the scrubber. The removal of the oil scrubber was 
not a serious omission because the scrubber was fitted with a visual oil-indicating dye. Results of the 
first day of operation indicated that no detectable amount of oil was getting into the air. 
After leaving the separators, the air was transported 300 ft down the pile where it discharged 
through a muffler. The air line, shown in figure 17.9 was a lightweight nylon double braid 
reinforced hose with an inside diameter of 0.5 in., rated for a working pressure of 250 psig. The air 
line size provided sufficient air flow within the pile to carry debris up the pipe and out the door. 
This debris resulted from the grinding that had to be done to remove rust prior to strain gage 
installation. This debris sweeping feature was efficient so when pile grinding was in progress the rust 
and metal powder was processed out through the top. 
Figure 17.9 Main air hose being fed into piling. 
Air Monitoring 
Air quality in the pile was sampled periodically by two devices manufactured by the Mine Safety 
Appliance Company, a universal tester and an explosion meter. 
263 
The universal tester draws a controlled volume of air across a reagent bed contained in a glass
indicating tube. A separate glass indicating tube is used for each gas to be detected. The
concentration of gas present in the air is indicated by either a color change or the length of stain
mark in the reagent bed. If a gas is present in the air sample, the quantity in parts per million or by
volume, is determined by comparing the stain color or length to a calibration sheet. The glass
indicating tubes used are for the detection of carbon dioxide, carbon monoxide and hydrogen
sulfide. This tester required about 5 min for each series tests. On the first round trip to the bottom
of the pile, the air was sampled about every 50 ft down to the 300-ft mark.
The explosion meter detects explosive mixtures of gas and air. The explosive mixture can consist
of one specific gas and air or many gases in a mixture of air. However, this unit only determines the
percent of explosive mixture, and does not identify the constituent gases. The explosion meter
operates on the principle of a particular gas or air sample changing the resistance of a heated
element, which is part of a Wheatstone bridge. If an air sample contains a combustible gas, the gas
will be oxidized by the heated element, and additional heating of the sensitive element will occur,
thus changing its resistance even though there is insufficient gas to support combustion. The device
is enclosed within a screen to prevent ignition of the test area in the event that enough gas is present
to cause an explosion. Readout on the indicator is in percent of an explosive mixture. This device
was used to detect the presence of methane and acetylene, and other explosive gases that may have
been present in the piling. This device proved to be much simpler and faster to use than the
universal tester, and could be used while the elevator was in motion.
Emergency Air Supply
The personnel working in the piling requested that an emergency breathing supply system be used
during the initial trips to the bottom, until confidence could be gained in the life support system.
Several alternative schemes were investigated and the one selected was a scuba diving unit. This unit
consisted of a U.S. Divers single hose two-stage unit attached to a 72 cu ft compressed air tank.
This unit was selbcted because it is extremely quick donning, simple to use, and can be left on
and ready to use without consuming any of the air supply in the standby mode. Storage space was
at a premium in the elevator so the compressed air tank was set on the floor of the basket between
the workers legs. The mouthpiece was placed in a hanger at waist height, where it would be readily
accessible in an emergency.
CONCLUSIONS
The life support system described in this paper proved to be a key item in satisfying the program
goals. The commercial elements that made up the system operated satisfactorily during all work
modes, and provided a high level of confidence for the men working in the piling.
The assemblage of components that made up this system was tailored to the vertical piling
environment. In this regard, the long umbilical (air hose) approach proved satisfactory. On the other
hand, it may be worthwhile to consider the development of a compact portable life support system
that would have universal application for future offshore platform work activities.
264


Portable life support for instrumentation of an offshore platform

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