Annals of Occupational Hygiene Advance Access originally published online on December 21, 2005
Annals of Occupational Hygiene 2006 50(2):123-129; doi:10.1093/annhyg/mei065
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© 2005 British Occupational Hygiene Society Published by Oxford University Press
Original Article |
Benzene Exposure on a Crude Oil Production Vessel
Section for Occupational Medicine, Department of Public Health and Primary Health Care, University of Bergen, Kalfarveien 31, N-5018 Bergen, Norway
* Author to whom correspondence should be addressed. Tel: +47 55 58 61 65; fax: +47 55 58 61 05; e-mail: Jorunn.Kirkeleit{at}isf.uib.no
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Objectives: The aim was to describe the personal exposure to benzene on a typical crude oil production vessel and to identify factors influencing the exposure level.
Methods: The study population included process operators, deck workers, mechanics and contractors on a production vessel in the Norwegian sector of the North Sea. The personal exposure to benzene during ordinary activity, during a short shutdown and during tank work was monitored using organic vapour passive dosimeter badges (3MTM3500). Information on the tasks performed on the day of sampling was recorded. Exposure was assessed by grouping the measurements according to job category, mode of operation and the tasks performed on the sampling day. Univariate analysis of variance was used to test the differences between the groups.
Results: Forty-two workers participated in the exposure assessment, comprising a total of 139 measurements. The arithmetic and geometric mean of benzene exposure for all measurements was 0.43 and 0.02 p.p.m., respectively. Twenty-five measurements (18%) were below the limit of detection (0.001 p.p.m.), while ten samples (7%) exceeded the occupational exposure limit of 0.6 p.p.m. The geometric mean exposure was 0.004 p.p.m. (95% CI 0.003–0.006) during ordinary activity, 0.01 p.p.m. (95% CI 0.005–0.02) during shutdown and 0.28 p.p.m. (95% CI 0.16–0.49) during tank work. Workers performing annual cleaning and maintenance of tanks containing crude oil or residues of crude oil had higher levels of exposure than workers performing other tasks, including work near open hydrocarbon-transport systems (all P < 0.001). However, because of the mandatory use of respirators, the actual personal benzene exposure was lower. The job categories explained only 5% of the variance in exposure, whereas grouping by mode of operation explained 54% of the variance and grouping by task 68%.
Conclusion: The results show that, although benzene exposure during ordinary and high activity seems to be low in the processing area on the production vessel, cleaning of tanks and performing maintenance work in a cleaned tank have a potential for high exposure.
Keywords: benzene exposure • upstream petroleum industry • production vessel • tank work
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Crude oil from undersea reservoirs is a complex mixture of hydrocarbons, sulphur and nitrogen. After arriving at an offshore production facility, the effluent is piped through a closed system of separators and treaters where it is separated into gas, oil, water and solid waste (sand and sediment). Oil and gas are transported to an onshore terminal, but can also be reinjected into the well. The water produced by this process is either reinjected into the well or purified and disposed overboard. During ordinary operation most of the processes are performed in a closed system, but whenever the system is opened there is potential for exposure to aromatic hydrocarbons such as benzene (C6H6). A causal relationship between exposure to benzene and the development of leukaemia has been established (Hayes et al., 1997
; Rinsky et al., 1987, 2002), and an increased risk of leukaemia associated with cumulative benzene exposures and benzene exposure intensities has been reported in the downstream petroleum industry (Glass et al., 2003). IARC has classified benzene as a confirmed human carcinogen Group A1 (IARC, 1987). Furthermore, significantly lower white blood cell and platelet counts have been reported for workers exposed to benzene in the working environment, even at concentrations <1 p.p.m. (Lan et al., 2004).
Published data on benzene exposure in the petroleum industry are limited, and most measurements have been performed in the downstream refining and marketing industry. A retrospective exposure assessment of benzene in the Australian petroleum industry suggested an estimated exposure of 0.02 p.p.m. benzene for the job group ‘upstream operator offshore’ based on six measurements from one oil company (Glass et al., 2000; Health Watch, 2001). In the conventional oil and gas sector of the upstream petroleum industry in Canada, 198 personal long-term samples from 1985 to 1996 had an arithmetic mean of 0.064 p.p.m. and a geometric mean of 0.011 p.p.m. The range was reported to be <0.001–2.431 p.p.m. The corresponding values for the personal short-term samples (n = 21) were 0.399 and 0.114 p.p.m. (range 0.005–3.844 p.p.m.) versus 0.207 and 0.007 p.p.m. (range <0.001–2.486) for the area long-term samples (n = 23) (Verma et al., 2000). Health and Safety Executive in UK (HSE, 1999) has measured the personal exposure to benzene (n = 241) at 11 offshore oil and gas facilities, including 3 production vessels. Of the total number of measurements, 91% was <0.005 p.p.m., while 6% was between 0.1 and 1 p.p.m. At the production vessels 59 measurements (84%) were below the level of detection (0.02 p.p.m.), while only 1 measurement had a benzene value >0.1 p.p.m. Other studies show that exposure can be high during maintenance of tanks, separators and vessels containing crude oil (Durand et al., 1995). Runion (1988) summarized common operational exposure data from the petroleum industry in the US (n = 124) and reported that personal short-term benzene exposure levels during crude storage tank gauging range from <0.032 to 160 mg m–3, corresponding to <0.01–50 p.p.m. benzene.
As several existing oil fields are at the tail-end of the production phase and the oil fields being discovered are getting smaller, the demand for crude oil production vessels is increasing. Production vessels, also called floating production, storage and offloading systems, have several advantages compared with fixed oil platforms. They can eliminate the need to lay expensive long-distance pipelines from the oil well to an onshore terminal and can be used economically on smaller oil fields. Once the field is depleted, the production vessel can move to a new location. However, as pointed out by Gardner (2003), the introduction of production vessels has given rise to increased or new exposure opportunities. In contrast to fixed oil platforms, which often deliver the crude oil directly to onshore terminals by pipeline, the production vessels store the crude oil in cargo tanks before it is offloaded and transported onshore. Workers cleaning and maintaining the cargo tanks potentially have high benzene exposure.
The objectives of this study were to describe the exposure to benzene in the processing area on a typical crude oil production vessel and to identify the factors influencing the exposure.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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The production vessel
The study population included employees on a production vessel in the Norwegian sector of the North Sea. The production company contacted us because they wanted to assess the employees' exposure to benzene and other hydrocarbons. The production vessel is tied to an unstaffed wellhead platform, which is a transfer point for crude oil from the oil wells on the seabed. From the wellhead the oil is transported to the production vessel through pipelines and risers. The vessel started production in 1998 and is 214.7 m in length, 38.2 m wide, and contains nine cargo tanks with a storing capacity of 470 000 barrels of crude oil. Further, it has the capacity to process 57 000 barrels of crude oil per day and 53 million standard cubic feet of gas per day for fuel or for reinjection into the well. At the time of air sampling, the water produced was purified and disposed overboard.
Composition of the crude oil
The compositions of crude oil and the fraction of benzene differ between the oil fields depending on several factors such as geological conditions in the reservoirs and the production period of the oil field. According to a crude oil assay of the present oil field from 2004, the benzene content was 0.52% by weight. Crude oil assays from different regions on the Norwegian continental shelf (n = 14) reported a mean and median value of 0.28% benzene by weight, with a range of <0.01–0.66% (Statoil, 2005). The same content of benzene has been reported for crude oil samples from Canada (Verma et al., 2000), while crude oil assays from Central Asia, Far East and West Africa ranged from 0.20 to 1.73% benzene by weight (Statoil, 2005).
Job categories
The workers included in the study were divided into four job categories according to department. Process operators survey the production process via computers in a central control room but also have practical tasks such as sampling and analysing oil and the water produced, fault-finding and repairing. The deck workers are in the marine department and are responsible for maintaining the vessel itself, such as the deck, tanks and hull. The mechanics repair, replace, adjust and align components of various types of machinery and equipment such as compressors, turbines and pumps. The contract workers are employed by contractors and perform jobs for a limited period of time, such as surface treatment, isolation and tank maintenance. Administrative personnel, catering personnel and workers from the department of electricity and instruments were not included in the study.
Modes of operation
The activity on a floating production, storage and offloading system is normally divided into two main modes of operation. During ordinary activity, the production phase, most of the processes are performed in a closed system. However, periodically the production is completely or partly shut down for cleaning and maintaining the processing system. Shutdowns are characterized by high manual work activity and may last from days to several weeks. Production ceased for 2 days during the shutdown assessed in this study.
Work in the crude oil tanks is done during the shutdowns but also periodically after tanks are offloaded during ordinary activity, where the tanks have to be cleaned and entered for maintenance. Tank work is considered as a third mode of operation in this study because of the potentially high exposure.
Tasks
All tasks performed during the sampling period were recorded using a personal log, filled in daily by each worker. The tasks were divided into four main groups: manual cleaning of tanks containing crude oil or residues of crude oil (Task 1), maintenance work in a cleaned tank (Task 2), work near open hydrocarbon-transport systems (Task 3) and other tasks (Task 4).
Work near open hydrocarbon-transport systems (Task 3) includes sampling and analysing crude oil and the water produced; changing, opening and closing blind flanges and valves; preparatory work before entering the tank; and safety measures during tank work. Other tasks (Task 4) include work not expected to cause hydrocarbon exposure, such as maintaining compressors and lift operations. Task 1 was assumed to have the highest potential for exposure, Task 2 the second highest, Task 3 the third highest and Task 4 the lowest. Workers who performed more than one of these tasks during one sampling period were assigned to the task assumed to have the highest potential of exposure.
Occupational exposure limit
The recommended occupational exposure limit for benzene in Norway is 1 p.p.m. averaged over an 8 h workday. Norwegian offshore workers work 12 h shifts 7 days a week for 2 weeks with 4 weeks of leave between the tours. In the guideline to the Activities Regulations, the Norwegian Petroleum Directorate recommends a safety factor of 0.6 to correct the standard for a 12 h shift. Thus, the occupational exposure limit for benzene is 0.6 p.p.m. over a 12 h workday.
Sampling strategy
Benzene exposure during ordinary activity and a brief shutdown
The personal exposure to benzene during ordinary activity and during a brief shutdown was assessed in March 2004. Three of six work shifts were covered in a 3 week period. For each sampling period, all eligible process operators, deck workers and mechanics entering the processing area on the first day of sampling were invited to participate. All the workers agreed to participate. Contract workers were included during the shutdown. The selected workers carried the dosemeter badges on three consecutive days for ordinary activity and two consecutive days during the brief shutdown.
Benzene exposure during tank work
Benzene exposure was sampled during tank work in two different types of tanks: drain water seal tank and cargo tank. The drain water seal tank, assessed in March 2004, receives excess water and hydrocarbons from the deck. The oil-contaminated water is pumped to the slop tank, where it is separated into the different phases. The cargo tanks, assessed in July 2004 and April 2005, are larger and contain only crude oil. The workers used half-mask respirators with combination filter, containing both a dust and an organic gas filter, while cleaning and maintaining both types of tank. Chemical protective clothing was mandatory during tank cleaning. During maintenance work in the tank the use of chemical protective clothing and ordinary working clothes varied.
Air sampling and analytical method
Personal exposure measurement. Personal exposure to benzene was monitored by organic vapour passive dosimeter badges (3MTM 3500) attached to the worker's collar. We aimed at a full-shift sampling time of 12 h. However, for tank work, where the potential for cutting and/or contaminating the badge is significant, the sampling time was shorter. The exposure measurements were used without further adjustment for unsampled time as we regarded them as representative of the full shift exposure. The exception is measurements sampled for two workers cleaning the cargo tank, where two sequential samples were added up for each worker. After sampling, the badges were stored in a freezer (–4°C) until they were transported to X-lab AS in Bergen, Norway, for analysis. The benzene was desorbed in C2S and analysed quantitatively and qualitatively by gas chromatography with mass spectrometry (NIOSH, 2003). The level of detection was 0.001 p.p.m.
Real-time monitoring. The direct reading device MiniRae 2000 (Rae Systems, Sunnyvale, CA, USA) was used for real-time monitoring of hydrocarbon exposure. The MiniRae 2000 is a photoionizing detector equipped with a 10.6 eV lamp. Air was drawn through the detector at 400 ml min–1, and one measurement was logged every second. The instrument was calibrated with a mixture containing 100 p.p.m. isobutylene and measured the total exposure to volatile organic compounds (in p.p.m.) in isobutylene equivalents. In this study this measure is defined as total hydrocarbons. The measurements were used to examine the variability of exposure to hydrocarbons during selected tasks.
Statistical analysis
The benzene exposure was assessed by grouping the measurements according to job category, mode of operation and tasks performed on the sampling day. Exposure levels are given as arithmetic mean, geometric mean, 95% confidence interval (95% CI) of the arithmetic and geometric mean, range (minimum and maximum) and geometric standard deviation. Since the distribution of the data was skewed, log-transformed data were used in the analysis. Univariate analysis of variance (ANOVA) was used to test the difference between different job categories, modes of operation and task groups. Concentrations below the level of detection were replaced with values equal to the level of detection divided by 2 (Hornung and Reed, 1990). Tests were considered significant at the level of 0.05. The data were analysed using SPSS version 12.0.1 for Windows.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Forty-two workers participated in the exposure assessment, comprising a total of 139 measurements. The mean duration of sampling time was 592 min (SD 182). The arithmetic and geometric mean personal benzene exposure for all measurements (n = 139) were 0.43 and 0.02 p.p.m., respectively (Table 1).
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Twenty-five measurements (18%) were below the level of detection, and ten measurements (7%) exceeded the occupational exposure limit of 0.6 p.p.m. benzene. These 10 samples were taken during cleaning of tanks containing residues of crude oil (n = 8) and during maintenance work in a cleaned cargo tank (n = 2). Most of the measurements (85%) were below half the occupational exposure limit, while 71% of the measurements were below one-tenth of the occupational exposure limit. Table 1 presents descriptive statistics and the results of univariate ANOVA.
Exposure differed significantly between the job categories (ANOVA, P = 0.016), mainly caused by the low exposure for the mechanics versus the contract employees (Scheffe's post-hoc test of difference between groups, P = 0.021) (Table 1). The differences were similar when comparing the job categories after excluding the tank work (data not shown). The job categories explained 5% of the variance in exposure.
The mode of operation explained much more of the variation in exposure (R2adj = 0.54, ANOVA, P < 0.001). The main difference was found between tank work and ordinary activity (Scheffe's post-hoc test, P < 0.001) and between tank work and shutdown (Scheffe's post-hoc test, P < 0.001).
Grouping the data according to the task performed explained the most variance in exposure (R2adj = 0.68, ANOVA, P < 0.001). All tasks differed significantly from one another (Scheffe's post-hoc test, all P < 0.001), with cleaning of tanks having the markedly highest exposure (Table 1).
Cleaning of tanks included two measurements during cleaning of a cargo tank and six measurements during cleaning of a drain water seal tank. Eight buckets of asphalt were removed during 100 min of work cleaning the cargo tank. The personal benzene exposures for these two measurements were 1.17 and 1.14 p.p.m., respectively. The arithmetic mean benzene exposure while cleaning the drain water seal tank was 7.84 p.p.m. Because the dosimeters had to be replaced owing to the risk of contamination, the mean sampling time for this task was short (218 min, range 43–531 min). The reported exposure values are, therefore, not representative for a full shift. Nevertheless, after the exposure values were adjusted to a 12 h full shift by assuming zero exposure during the residual time of the shift the exposure for tank cleaning was still high, with an arithmetic mean of 1.66 p.p.m.
The highest benzene exposure during ordinary activity was 0.22 p.p.m. weighted over a full shift. This exposure was related to a process operator who turned blind flanges during preparatory work before cleaning the drain water seal tank during the last 2 h of his shift. The exposure to total hydrocarbons during part of this work was monitored by a direct reading device showing short periods of high exposure (Fig. 1). Sampling of the water produced and crude oil was also associated with high short-term variability in exposure (Fig. 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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The overall exposure to benzene on the production vessel studied was low. Nevertheless, exposure differed considerably, with some measurements being very high. The task performed was a major determinant of exposure; grouping the measurements according to the task performed explained 68% of the variation.
The highest exposure was found during cleaning of tanks, with a geometric mean of 4.42 p.p.m. Because of the mandatory use of half-mask respirators during cleaning the actual personal exposure was probably lower, although little is known about the effectiveness of such protective equipment during tank work. The use of respiratory protection is imperative for this type of work. The measurements taken during maintenance of cleaned tanks had a geometric mean of 0.15 p.p.m., 
13 times higher than the exposure during work near open hydrocarbon-transport systems and 74 times higher than the exposure measured during other tasks. Opening hydrocarbon-transport systems are brief tasks, as illustrated by Figures 1 and 2. The results indicate that such short-term exposure contributes significantly to total exposure, as also pointed out by Gardner (2003). The job categories explained only 5.3% of the variance in the exposure level, indicating that job category is a poor determinant of exposure. The reason is that the workers from the different departments often work in teams on a range of activities that are associated with large variation in the day-to-day exposure to benzene.
As expected, cleaning of tank was associated with the highest benzene exposure, ranging from 1.14 to 16.75 p.p.m. Exposure was highest while cleaning the drain water seal tank. The personal benzene exposure while cleaning crude oil vessels has been reported to be comparatively high, ranging from non-detectable to 5.8 p.p.m. when averaged over the duration of the whole task (Durand et al., 1995). In a retrospective exposure assessment for benzene in the petroleum industry in Australia, Glass et al. (2000) estimated an exposure level of 2.01 p.p.m. benzene during cleaning of crude tanks when using crude (n = 13) and slops storage data (n = 46). Cleaning and maintaining tanks, separators, etc. are episodic activities. Many tank workers are employed by small companies specializing in cleaning or maintaining oil tanks or separators onshore and offshore. Tank work is physically demanding, and workload is one factor shown to increase the uptake and to modify the distribution and biotransformation of organic solvents (Lof and Johanson, 1998; Csanady and Filser, 2001). Since the health effects of such high episodic benzene exposure are not known, these tasks should be considered independently during assessment of exposure rather than just being included in the workers' cumulative benzene exposure.
The exposure during ordinary activity was low, and our results are in accordance with the base estimate of 0.02 p.p.m. benzene for the ‘upstream operator offshore’ in the petroleum industry in Australia (Glass et al., 2000) and the exposure levels reported from the UK offshore industry (HSE, 1999). However, since exposure during cleaning and maintaining tanks is omitted both in the Australian and UK study, the actual exposure is probably underestimated for some work categories in the upstream area. In the present study the benzene exposure during the brief shutdown was twice as high as the exposure during ordinary activity. The maintenance work during shutdown included opening the hydrocarbon-transport system for visual inspection, but did not include entering separators, tanks or vessels containing crude oil residues, which is done during ordinary shutdowns. The exposure level might, therefore, be fairly representative for high activity rather than for a real shutdown, which probably would have shown even higher levels of exposure. The workers on the present production vessel were predominantly handling crude oil, and they were only exposed to gas condensate when opening vessels in relation to reinjection of gas into the well. As all hydrocarbon-leading vessels were isolated and purged with an inert gas before being opened, even this operation gave minimal exposure to gas condensate.
Passive dosimeter badges were chosen because they are easy to administrate and do not need pumps that could disturb the workers. However, the producer of the badges recommends sampling time <12 h, as was the full shift time in the present study. Theoretically some analyte could have been lost owing to desorption, and the exposure level might have been higher then presently reported. However, considering the low exposure levels and a mean sampling time of 9.9 h (range 43–931 min) we do not think that the sampling time has affected the measurements significantly.
The lack of published exposure data from the upstream petroleum industry might partly be owing to the expected low exposure on the offshore installations. It could also be owing to the practical limitations for exposure assessment, such as the need for helicopter transport, the unpredictability in planning of tasks and the priorities for personnel owing to the limited number of beds offshore. When benzene is monitored in the working environment on offshore installations it is to comply with the regulations under the Working Environment Act or to ensure that control measures are effective. More effort should be made to strengthen the co-operation between the petroleum industry and researchers to optimize the assessment of exposure to hazardous chemicals and the publication of the results in scientific journals.
The exposure in this study was assessed on one production vessel from one company that itself took the initiative to be included in the project. The results might not be representative for other vessels or other types of installations in the upstream petroleum industry. Nevertheless, we have no reason to believe that the working environment differs much from other crude oil production vessels, although the exposure level will be influenced by the benzene content of the crude oil produced. Further, the tasks described in this study also take place on crude oil tankers and onshore installations where crude oil is stored or handled.
In conclusion, the results show that, although the benzene exposure during ordinary and high activity seems to be low on the production vessel, cleaning of tanks and maintenance work in a cleaned tank have a potential for high exposure. Proper control measures should be taken during tank work.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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This study is part of a project financed by the Norwegian Research Council. Pertra A.S., the operator of the oil field during the study period, financed the laboratory analysis. We thank the management and the employees of PGS Production and the production vessel Petrojarl Varg for their hospitality, co-operation and flexibility throughout the study.
Received July 14, 2005; in final form October 8, 2005
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