how to ensure h2s safety on offshore rigs

7/28/2019 How to Ensure H2S Safety on Offshore Rigs

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How to ensure H2S safety on offshore rigs

Putting in place OSHA-compliant sour-gas preparedness program demands early,

comprehensive planning

By Angelo Pinheiro, Marathon Oil

INTRODUCTION

Hydrogen sulfide (H2S) is a toxic, flammable and corrosive gas often encountered during

hydrocarbon exploration and production. A byproduct of the decay of protein-containing

substances, H2S forms by bacterial reduction of sulfates in sedimentary rocks. H2S may also

be anticipated in previously uncontaminated reservoirs into which sulfate-reducing bacteria

were introduced through workover and completion fluids containing organic polymers,

including starch, methyl cellulose and polysaccharide base additives.

This article discusses the industrial hygiene and emergency management aspects of H 2S inthe context of US regulations, industry recommended practices, and the authors work

experience of over 20 years in sour-gas drilling.

SIGNIFICANCE OF PROBLEM

H2S is extremely toxic to humans at minute concentrations. At higher concentrations it is

flammable, as well as corrosive to metals. A surface breakout of this gas, if not responded to

and controlled immediately, can result in injuries and/or fatalities, fire and explosion. If an

H2S event were to occur during periods of adverse weather, then personnel evacuation from

the affected rig, plus vessels and installations downwind, could be severely compromised.

Drilling and well control equipment that are not designed for H2S use could suffer a loss of

structural integrity following exposure, which could impede their function and operation

during an emergency.

A blowout involving H2S has the potential to disrupt maritime transportation, fishing, and

manned oil and gas infrastructure downstream of the source. The loss of life and disruption to

business continuity could result in substantial financial and legal liability, compensation

claims, fines and potential prosecution.

PROPERTIES, EFFECTS, INCIDENCE

In the presence of moisture, H2S forms sulfurous and sulfuric acids, which are corrosive to

metals. Corrosivity is enhanced in the presence of carbon dioxide and produced water low in

oxygen content. The effects of H2S corrosion include hydrogen embrittlement, corrosion and

cracking of drillstring components, drilling fluid processing equipment, and drilling safety

equipment, such as blowout preventers. Tensile stress on the suspended drillstring

exacerbates the effects of corrosion and causes sulfide stress cracking.

Physiological Eff ects

The primary route of exposure is by inhalation. In concentrations less than or around 50 parts

per million (ppm), H2S acts as a mucous membrane and respiratory tract irritant; symptomsinclude coughing, shortness of breath, and eye and throat irritation. At concentrations of
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approximately 100 ppm, H2S paralyzes the olfactory nerve and the sense of smell. It is

rapidly absorbed by the lungs and transferred to the blood circulation, where it inhibits the

cytochrome oxidase enzyme system, which regulates the uptake of oxygen at the cellular

level.

As the nervous system and cardiac tissues are particularly vulnerable to oxygen deprivation,symptoms such as headaches, disorientation, cyanosis, pulmonary edema, pulmonary

hemorrhage and cardiac arrhythmia may follow. At 700 ppm to 1000 ppm, H2S acts as a

chemical asphyxiant and produces rapid loss of consciousness, followed by death if the

victim is not immediately rescued and resuscitated.

Occupational Exposure Levels

H2S has an immediately dangerous to life and health (IDLH) concentration of 100 ppm. For

an airborne contaminant, IDLH is defined as the level of the contaminant likely to cause

death or immediate or delayed permanent adverse health effects in the event of failure of the

respiratory protection equipment. Most work areas on an offshore drilling rig are consideredIDLH, as H2S is commonly encountered over 100 ppm and can stack up (increase in

concentration) in low-lying areas or still wind conditions.

H2S has a permissible exposure levelceiling concentration (PEL-C) of 20 ppm with an

acceptable maximum peak above the acceptable ceiling concentration of 50 ppm once duringan eight-hour work shift and which does not exceed 10 minutes (29 CFR 1910.1000). PELs

are stipulated by OSHA and are legally binding.

The threshold limit value (TLV) for H2S is 10 ppm for an eight-hour workday, 40-hour work

week, and the short-term exposure limit (STEL) is 15 ppm (ACGIH, 2009). TLVs and STELs

are recommendations of the American Conference of Government Industrial Hygienists

(ACGIH) and are voluntary standards. As offshore work schedules involve 12-hour shifts

seven days a week, up to four weeks at a time, the TLV should be adjusted as the period of

elimination is decreased. That may be done using several methods. The Brief & Scala method

is shown below:

Adjusted Daily TLV = [TLV x Daily Reduction Factor]

= [TLV x (8/hd) x {(24-hd)/16}] = 5 ppm

Adjusted Weekly TLV = (TLV x Weekly Reduction Factor)

= [TLV x (40/hw) x {(168-hw)/128}] = 3.125 ppm

It is common practice on offshore drilling rigs to set the low and high alarms of H 2S area

monitoring systems at the unadjusted TLV and STEL level of 10 ppm and 15 ppm.


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Figure 1. Toxic hydrogen sulfide is capable of leaking from the drilling mud circulation

system. Mitigation planning should include mud weight control to ensure that hydrostatic

pressure exerted by the mud column overbalances and prevents influx of gas into the mud

column.

Anticipated areas, activiti es

H2S should be anticipated in all areas of the rig where drilling fluid and associated equipment

is present. Those areas include the rig floor, substructure, shale shakers, mud cleaners, mud

pit room, mud pump room and well test equipment. Being heavier than air, H2S will settle inlow-lying and poorly ventilated areas and will dissolve in oil and water present in those areas.

H2S should be anticipated when: a) breaking out, when the run in hole of drill pipe has

been completed and bottom fluids are displaced to surface, b) drill pipe is pulled out of the

well too quickly, resulting in formation fluid entering the wellbore, or swabbing, c)

retrieving core or fluid samples, and d) flowing well testing. During well test operations

involving flaring of formation fluids, H2S and sulfur dioxide should be anticipated around the

rig and on the sea surface. Figure 1 illustrates a drilling mud system, Figure 2, a self-

elevating drilling platform, and Figures 3-7, examples of rig areas and equipment where H2S

may be encountered.

H2S RESPIRATORY PROTECTION PROGRAM

A written respiratory protection program and H2S contingency plan that are developed in the

planning phase for the well will ensure the safety of personnel. Planning should focus on

prevention and accord priority to engineering controls, including:

Mud weight control to ensure that hydrostatic pressure exerted by the mud column

overbalances the formation pressure and prevents influx of gas into the mud column;

Continuous monitoring of drilling returns (mud and cuttings) for H2S;
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Addition of H2S scavengers (zinc carbonate and ironite sponge) in drilling and completion

fluid formulations;

Negative pressure ventilation and air locks in mud-processing rooms;

Local exhaust ventilation over the shale shakers;

Positive pressure ventilation of living quarters, with H2S monitoring at the air intake linked

to HVAC shutdown;

Area monitoring system for H2S with strategically located sensors;

Supplied breathing air system in work areas and at muster/evacuation stations;

Corrosion rings to indicate drill pipe corrosion (they are subject simultaneously to corrosion

and tensile and torsional stresses) and specifying NACE (North American Corrosion

Engineers) standards for well control equipment and tubulars;

Arrangements for emergency flaring when the well cannot be shut; and

Using dynamically positioned semisubmersible rigs (in preference to self-elevating or

anchored semisubmersible rigs), which can be oriented to the wind direction without

disrupting rig operations, such that gases are blown away from living and work areas.

Figure 2. A typical offshore drilling rig has a number of places where H2S can possibly leak

into the environment, causing air contamination. H2S should be expected in all areas of the

rig where drilling fluid and associated equipment is present.

A respiratory protection program complying with 29 CFR 1910.134(c) would include the

following steps:

1. Respirator selection.


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2. Fit testing for tight-fitting respirators.

3. Medical evaluation.

4. Procedures for reasonable foreseeable emergencies.

5. Maintenance arrangements and procedures.

6. Procedures for supplied air respirators (SAR).

7. Training on use of respirators for routine and emergency situations

8. Training in the donning, doffing, care and maintenance of the respirator selected.

9. Procedures for evaluating effectiveness of the respiratory protection program.

The implementation of this standard are described below from an offshore drillingperspective.

Respirator Selection

Respiratory protective equipment for H2S is selected following an assessment of the hazard

and exposure. The hazard assessment takes into consideration factors such as anticipated

concentration; environmental/weather conditions, including seasonal wind directions, rig

layout, the placement of temporary equipment onboard for the well test, personnel exposure

patterns in terms of work activities and duration of those activities; detection and monitoring

arrangements; existing and required respiratory protection equipment; NIOSH and DOTcertification requirements for breathing apparatus and compressed air cylinders; and the

engineering and administrative control measures that will be implemented.

Personnel should be involved in respirator selection, and concerns such as comfort, fit and

adaptability for use with other personal protective equipment should be considered. The

assigned protection factor (APF) should be carefully reviewed in order to determine the

maximum use concentration. The APF of a tight-fitting airline respirator is 1,000, which

might be a limiting factor if the H2S concentration exceeds 10,000 ppm. An airline

respiratory system is usually provided on offshore rigs to permit continuous work while

masked up.

Important considerations in selecting/designing such a system include multistage

compressors (to reduce charging time during an emergency), reserve air capacity, number and

placement of low-pressure air manifolds, air pressure and flow requirements, capacity/size of

the auxiliary air cylinder to permit escape, air quality requirements (CGA specification Grade

D), low-pressure alarms, ability to communicate while masked up (i.e., speech diaphragm),

etc.


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Figure 3. The drilling derrick, the shale shaker area, the mud gas separator, the mud pit room

and the well test area are all examples of rig areas where hydrogen sulfide may beencountered. A leak could occur when drill pipe is pulled out of the well too quickly or

during well test operations involving flaring of formation fluids.

Medical Evaluation

Employees required to use respirators must undergo a medical examination by a physician or

licensed health care provider (PLHCP) in accordance with 29 CFR 1910.134(e)(2)(ii). A

follow-up medical exam may be needed if a positive response was made in the health

questionnaire or if deemed necessary by the PLHCP. The PLHCP should be provided

information on the respirator type and weight, when and for how long it will be used at any

given time, whether it is required for routine and/or emergency use, the expected physical

demand/effort while wearing the respirator, additional protective clothing to be worn (e.g.,

firefighters suit), and temperature and humidity conditions at the workplace. The PLHCP

should be given a copy of the respiratory protection program and 29 CFR 1910.134 rules for

reference.

The physical efforts of the job may be estimated by preparing a physical demand analysis

(PDA) for various jobs, particularly those involving wok while masked up, e.g., rig floor

activities. PDAs that were developed for ergonomic assessments should suffice for this

purpose.


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On an offshore rig, temperature extremes, at a level that would cause heat stress, may be

anticipated in work areas close to the flare, in engine, boiler and machinery rooms, and those

areas receiving and processing mud returns from the well. High-humidity areas include the

shale shakers and mud pit room.

Written recommendation should be sought from the PLHCP on whether the employee ismedically fit to use a respirator and if there are limitations that preclude its use under the

work conditions specified. The medical examination results should be discussed with the

employee.

F it Testing

Each employee should be fittested following medical evaluation and prior to assignment of

requiring the use of a respirator. The fit test procedure should be documented in the

respiratory protection program and conform to 29 CFR 1910.134, Appendix A, for tight

fitting respirators. Fit tests are performed using a variety of facepiece sizes to select the one

that fits best, and are recorded and repeated annually. The fit test is usually conducted

onboard by the H2S safety officer using Bitrex or saccharin solution. H2S respiratorsself-

contained breathing apparatus or air line respiratorsare of the pressure demand type;

therefore, the fit test should be performed in the negative pressure mode to detect leaks.

Training
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Training helps employees to recognize the dangers of H2S and is required to comply with 29

CFR 1910.1200 and 29 CFR 1910.120. API RP 49 recommends Action Levels of 10 ppm

H2S and 2 ppm sulfur dioxide (SO2) as triggers for training. 29 CFR 1910.134 mandates

initial training prior to the assumption of duties requiring the use of a respirator, followed by

annual refresher training.

The training should address the sources; hazards; properties and characteristics of H 2S and

SO2; proper use of detection equipment (personal or portable gas monitors); recognizing and

responding to alarms; location of muster points; symptoms of exposure and biological effects;

rescue techniques and first aid; proper storage, use and maintenance of the respirators;

limitations and capabilities of the respirators; practical donning, operational checks, and

doffing of the respirators; recognizing and responding to respirator malfunction and

emergency situations; and recognizing medical signs that may prevent or limit the user from

wearing a respirator.

The implementation of an H2S training program conforming to ANSI/ASSE Z390.1

(Accepted Practices for H2S Training Programs) is recommended.

Monitoring

The objective of monitoring is to immediately identify the presence of H2S when

predetermined action levels have been exceeded, so that the necessary control measures canbe implemented. A fixed H2S detection system should be provided for continuous monitoring

of areas where H2S is likely to be released. H2S sensors should be located in the mud logging

unit, Bell Nipple/mud return trough, shale shaker, mud pit room, drill floor, and air intake to

the living quarters.

The sensors should be located nearest to the point of release, at floor height (but off the floor)

and protected against steam, water, mud and chemical splashes, and mechanical damage. H2S

sensors work on the principle of diffusion, following Ficks laws, and are susceptible to

poisoning or drifting. It is therefore essential to have written requirements for periodic

calibration and ensure that those are performed as scheduled.

The system should be capable of being function-tested and calibrated at the control panel, and

include a power failure and fault indicator for each channel. The control panel should be

located in the rig control room, which is manned continuously, and include auxiliary alarms

and/or visual annunciators at the central switchboard/engine room and drillers cabin, where

controls for ventilation systems and well control equipment are located.

Personal H2S monitors should be provided to personnel that work in high-noise areas or

where an area alarm might not be easily heard. Visual alarms, such as strobes, should be

provided in those areas. At least two portable, direct-reading instruments should be available

for personnel leading emergency response efforts, including the offshore installation

manager, driller and safety officer. A calibration kit, battery charger and sampling tube/probe

should be available for electronic instruments.

An adequate supply of colorimetric indicator tubes, in various detection ranges, should be

available as backup for the electronic instruments. Indicator tubes are recommended when

drilling wildcat wells or situations where the H2S concentration cannot be predicted withcertainty. Caution should be exercised to ensure that the aspirator is compatible with the


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colorimetric tubes selected and that the tubes are properly stored and consumed or discarded

within their useful shelf lives. Colorimetric tubes have lower accuracy and should not be used

to confirm the absence of H2S.

Respir atory Protective Equipment

Respiratory protective equipment (RPE) for H2S is selected on the assumption that all work

areas outside the living quarters fall under the immediately dangerous to life and health

(IDLH) definition, for the H2S concentrations that may be encountered.

The workplace and user factors to be considered in selecting RPE include: a) the time,

distance and effort required to travel while masked up from work areas to the muster/safe

briefing area; b) the level of physical effort required to perform the job or assist with a rescue

while masked up; c) the physiological and psychological state of being of the user; d) the

maximum use concentration; and e) extenuating circumstances such as fires, spills or

structural damage to escape routes that preclude the safe use of those routes.

The respiratory protection program should include written procedures for selection; use in

routine and emergency situations; cleaning and disinfection; inspection, repairs and

maintenance; and procedures for testing the quantity, quality and flow of breathing air in

airline systems and self-contained breathing apparatus (SCBA).

The RPE for H2S and SO2 on an offshore drilling rig comprises of a work/escape airline

breathing air system and 30-minute SCBA units for emergency response and rescue

personnel. Figure 4 illustrates a typical airline breathing air system.

The airline system comprises of a multistage breathing air compressor that charges one or

more accumulators or cylinder banks (2,400 psi to 2,600 psi). The reserve or stored air

capacity of the cylinder bank should be adequate to support work crews for a period of at

least two hours without recharging. The high pressure from the cylinder bank is regulated

down to 125 psi and distributed to low-pressure breathing air manifolds into which

employees can plug at their work areas.

Manifolds are commonly located on the rig floor, within the drilling derrick (at the monkey

board and stabbing board), crane operator cabs, shale shaker area, mud pit and mud pump

rooms, emergency control switchboard and at the muster stations. The pressure within the

facepiece is maintained at a slight positive pressure to prevent ingress of contaminated air

through the seals.

The other components of the airline system include a quick-connect hose that attaches the

facepiece to the manifold and a pre-charged five- to 15-minute capacity compressed air bottle

for escape purposes, that is worn at the hip. The air compressors should be located at an

elevated level on the rig to avoid the aspiration of H2S. This is achieved by placing the

compressors at the port and starboard side of the uppermost deck of the living quarters.

One of the compressors should be engine-driven so that the system is available in the event of

a power blackout on the rig during an H2S release.

The intakes for the air compressors should be routed away from the exhaust and located in acontamination-free area. High-temperature and carbon monoxide alarms (that alarm at 10


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ppm) should be installed on oil-lubricated compressors, and the inline moisture trap and

sorbent filters should be drained daily and replaced in accordance with a planned

maintenance schedule.

Where longer hose lengths are required to permit movement, pressure-drop calculations are

performed to ensure that the manufacturer-recommended pressure and flow requirements aremet. Low-pressure alarms should be available for each breathing apparatus and high-pressure

cylinder bank of the SAR system.

In harsh-weather environments, the rig lifeboats are commonly outfitted with airline

manifolds into which occupants can plug while awaiting the signal to abandon the rig.

Employees disconnect from the air manifold and breathe off their escape bottle during

platform abandonment. The lifeboat air system augments this air reserve with a 10-minute

supply.

Breathing air quality should be tested periodically to ensure it meets ANSI/CGA Commodity

Specification for Air G-7.1-1989, Grade D, and the tests should be recorded. Thespecification of Grade D breathing air is: oxygen content between 19.5% and 23.5%,

hydrocarbon content below 5 mg per cubic meter, carbon monoxide below 10 ppm, carbon

dioxide below 1,000 ppm, moisture content below dew point of -50F at 1 atm pressure, and a

lack of noticeable odor.

When the respiratory protection program (RPP) is properly implemented, the MUC for airline

respirators, which have an assigned protection factor (APF) of 1,000, equates to an H2S

concentration of 10,000 ppm (MUC = APF x TLV). The MUC for pressure demand self-

contained breathing apparatus (10-minute escape pack or 30-minute unit) is 10 times as

much, i.e., 100,000 ppm, because the APF for SCBA is 10,000. Adjustments for non-standard

work hours should be considered if the exposure is expected to be steady and continuous.

RPE should be stored in areas that are free of dust, sunlight, excessive moisture, chemicals

and extreme temperatures. They should be packed to prevent deformation to the facepiece

and other components and inspected prior to and after each use. Function tests should be

performed by the user prior to taking an escape respirator into a known H2S work area.

SCBA cylinders should be inspected monthly and recharged when the pressure drops below

90%. SCBA cylinders should be tested and maintained to DOT shipping container

specifications 49 CFR 173-178. 30-minute SCBAs should be provided for emergency

response team members and rescuers, and those units should be labeled to identify them assuch. When defects are noted, the respirator should be tagged and removed from service for

repair by trained personnel, using manufacturer-recommended (NIOSH certified) parts and

instructions.


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Table 1: This table provides the physical and chemical properties of H2S. The effects of H2Scorrosion include hydrogen embrittlement, corrosion and cracking of drillstring components,

drilling fluid processing equipment, and drilling safety equipment, such as blowout

preventers.

Program Admin istration

The RPP on offshore rigs is administered by the safety officer designated by the drilling

contractor. Often, the services of an experience H2S specialist is offered as part of the

package for the airline system that is leased for the duration of the well to be drilled. The

program administrator should be qualified by training or experience and conduct reviews of

the program effectiveness.

Other responsibilities may include assigning responsibility and frequency for RPE

inspections (or performing those inspections); reviewing implementation of the RPP;

verifying that employees know how to properly use their RPE; scheduling repairs of RPE

when needed; maintaining records of medical examinations, fit tests, training and inspections;

and making upgrades to the RPP when needed.

Emergency Response and F ir st A id

Despite the numerous precautions taken to prevent H2S incidents, emergencies should be

anticipated due to the extreme toxicity of H2S. A written H2S contingency plan is expected

under OSHA 1910.138 and should be developed to include response procedures at the tactical

level. The plan should dovetail with the companys shore-based incident command system or

plans of the operator.

The key elements of an H2S contingency plan include: a) an onboard emergency response

organization; b) notification and alarm system; c ) arrangements for safe muster and

personnel accounting; d) procedures for well shut-in and confining the H2S leak; e)

procedures for gas-freeing affected areas and confirming they are safe for resumption of

work; f) procedures for declaring the end of the emergency and standing down resources; g)

search and rescue arrangements; h) training and exercising of personnel assigned emergency
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response duties; i) first aid and medical aid arrangements for victims overcome by H 2S; and j)

evacuation or abandonment arrangements in the event the emergency gets out of control.

CONCLUSION

This article discussed the industrial hygiene and emergency response aspects of H 2S from anoffshore drilling perspective. It underscores the importance of planning early for H2S

operations and advocates the use of engineering controls in order of priority. The article also

discussed administrative controls, including key elements of a respiratory protection program

and H2S contingency plan and discussed the respiratory protection equipment that is typically

selected and used in offshore H2S areas

how to ensure h2s safety on offshore rigs

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