- Email: [email protected]
Contamination control: Cleaning sludge in the North Sea
36
Application
Filtration+Separation December 2006
Contamination control:
Cleaning sludge in the North Sea S
arah McGillivray, operations chemist at CETCO Oilfield Services looks at a new technique for cleaning contaminated sludge in a floating oil production vessel that cleans the liquids and solids together without the need for prior separation.
Naturally Occurring Radioactive Material (NORM), also known as Low Specific Activity (LSA) scale, is a weakly radioactive insoluble precipitate that exists in oil reservoirs and often contaminates oil production equipment during extraction processes. LSA scale has been contaminating equipment throughout the history of the oil industry. However, with greater knowledge of pollutants and more stringent environmental regulation, NORM has, over time, become a significant issue for the industry. LSA scale does not exist in isolation – it is usually in an emulsion of wastewater, mud and oil. The oil industry refers to it is as contaminated sludge, which, given the constituents, is an apt description. The cleaning of contaminated tanks or equipment, removal and handling of LSA scale requires specialist radiological protection supervision. Items of LSA scale contaminated equipment can be brought to shore and cleaned at specially licensed facilities, but what do you do with six million litres of sludge, which takes up a significant storage capacity of a FPSO (floating, storage, production and off-loading vessel)?
The discharge of LSA scale to the marine environment in the North Sea is monitored and controlled by SEPA (Scottish Environment Protection Agency), and the discharge of oil contaminated water or solids is monitored and governed in the UK by the DTI (Department of Trade and Industry).
filtration, chemical, centripetal and cyclonic technologies. However in some instances these methods are found to fail, leaving operators with long term challenges and growing volumes of ‘untreatable’ fluids in their tanks. The condition of these fluids can degenerate over time as more biomass forms promoting further corrosion.
Operators cannot simply dump water over-theside of their production facilities, so methods of cleaning need to be applied to remove oil, NORM and other contaminants before any discharge can be made to the sea.
The FPSO tank contained approximately 5000 m3 of liquid and 1000 m3 of solids, that had formed a cake-like layer at the bottom of the tank. The liquid was sampled at various depths and displayed an emulsion like consistency of oil in water with biomass and iron sulphides present. Inlet concentrations of oil in water were as high as 20,000 mg/l (2% m/v) during the first stage of operation which aimed to remove the free liquid above the solid sludge layer.
The traditional approach is to first separate solids and liquids and then treat them individually. This process is often ineffective, however, and is both costly and slow. Now there is a new technique, which has been pioneered by CETCO Oilfield Services, which cleans the liquids and solids together without having to separate them beforehand. This is the technique that was successfully applied offshore in the North Sea, in the presence of a DTI inspector, to clean the LSA and biomass contaminated sludge and associated oily water from the six million litres of sludge in CETCO’s client’s FPSO storage tanks.
A new approach The sludge in the FPSO wasn’t just taking up valuable space, it was also causing corrosion, bio-fouling and the generation of toxic gases such as hydrogen sulphide. The actual fluid was a mixture of produced water, crude oil, sea water ballast, deck drainage water and all manner of associated solids including the LSA scale. Together, this mixture forms a fairly stable emulsion that prevents bulk separation of the liquid phases. Figure 1. The CETCO Weirbox.
Traditional methods of treating such fluids have included separation tanks,
To help CETCO prove the system, a temporary dispensation was granted by the DTI which allowed the cleaned fluids to be discharged up to a maximum of 500 mg/l, compared with the normally acceptable level of 30 mg/l. The treatment process utilised a compartmentalised Weirbox and was designed to allow for LSA solids and sand to be cleaned of hydrocarbons without the need for solids separation and the creation of another waste stream. The objective was for the effluent water to be discharged to the sea directly after the treatment process, without the need for further water polishing. Prior to commencement of treatment, a kill dose of biocide was applied to the cargo tank as a large quantity of biomass was evident and oil was clearly congealed in this scum layer. The use of the biocide resulted in greatly increased phase separation within the tank and reduced inlet concentrations to the Weirbox. This was an important step in the successful treatment of the fluids.
Application
Filtration+Separation December 2006
PAH
µg/l
PAH
µg/l
Acenaphthylene
144.32
Acenaphthylene
0.97
Acenaphthene
95.97
Acenaphthene
0.32
Fluorene
200.28
Fluorene
0.85
Anthracene
119.9
Anthracene
2.69
Fluoranthene
24.92
Fluoranthene
<0.01
Pyrene
46.38
Pyrene
0.31
Benz(a)anthracene
91.84
Benz(a)anthracene
3.26
Chrysene
<0.01
Chrysene
<0.01
Benzo(b)fluoranthene
48.07
Benzo(b)fluoranthene
0.69
Benzo(k)fluoranthene
35.61
Benzo(k)fluoranthene
0.26
Benzo(a)pyrene
4.57
Benzo(a)pyrene
2.2
Benzo(g,h,I)perylene
<0.01
Benzo(g,h,I)perylene
<0.01
Indeno(1,2,3,cd)pyrene
<0.01
Indeno(1,2,3,cd)pyrene
<0.01
Dibenz(a,h)anthracene
<0.01
Dibenz(a,h)anthracene
<0.01
Naphthalene
269.84
Naphthalene
0.83
Phenanthrene
265.81
Phenanthrene
5.63
Total PAH
1347.51
Total PAH
18.01
OIW
mg/l
OIW
mg/l
Total hydrocarbons
1627
Total hydrocarbons
37
Aliphatic content
1417
Aliphatic content
17
Aromatic content
210
Aromatic content
20
Alkyl Phenols
µg/l
Alkyl Phenols
µg/l
Total C1-C3 alkyl phenols
<0.50
Total C1-C3 alkyl phenols
<0.50
Other C1-C3 alkyl phenols
Other C1-C3 alkyl phenols
2-methylphenol
<0.02
2-methylphenol
<0.02
3-methylphenol
<0.02
3-methylphenol
<0.02
4-methylphenol
<0.02
4-methylphenol
<0.02
2, 5-dimethyphenol
<0.02
2, 5-dimethyphenol
<0.02
3,4- and 3, 5-dimethyphenol
<0.02
3,4- and 3, 5-dimethyphenol
<0.02
2, 4-dimethyphenol
<0.02
2, 4-dimethyphenol
<0.02
4-ethylphenol
<0.02
4-ethylphenol
<0.02
2-n-propylphenol
<0.02
2-n-propylphenol
<0.02
2,3,5-trimethylphenol
<0.02
2,3,5-trimethylphenol
<0.02
4-n-propylphenol
<0.02
4-n-propylphenol
<0.02
2,3,6-trimethylphenol
<0.02
2,3,6-trimethylphenol
<0.02
Total C4-C5 alkyl phenols
<0.50
Total C4-C5 alkyl phenols
<0.50
Other C4-C5 alkyl phenols
Other C4-C5 alkyl phenols
4-tert-butylphenol
<0.02
4-tert-butylphenol
<0.02
2-tert-butylphenol
<0.02
2-tert-butylphenol
<0.02
4-n-butylphenol
<0.02
4-n-butylphenol
<0.02
2-tert-butyl-4-methylphenol
<0.02
2-tert-butyl-4-methylphenol
<0.02
4-tert-butyl-2-methylphenol
<0.02
4-tert-butyl-2-methylphenol
<0.02
4-n-pentylphenol
<0.02
4-n-pentylphenol
<0.02
Total C6-C9 alkyl phenols
<0.50
Total C6-C9 alkyl phenols
<0.50
Other C6-C9 alkyl phenols
Other C6-C9 alkyl phenols 2,6-diisopropylphenol
<0.02
2,6-diisopropylphenol
<0.02
4-n-heptylphenol
<0.02
4-n-heptylphenol
<0.02
4-n-octylphenol
<0.02
4-n-octylphenol
<0.02
2,6-di-sec-butylphenol
<0.02
2,6-di-sec-butylphenol
<0.02
2,4-di-tert-butylphenol
<0.02
2,4-di-tert-butylphenol
<0.02
4-tert-octylphenol
<0.02
4-tert-octylphenol
<0.02
2,6-di-tert-butylphenol
<0.02
2,6-di-tert-butylphenol
<0.02
4-n-nonylphenol
<0.02
4-n-nonylphenol
<0.02
2,6-di-tert-butyl-4-methylphenol
<0.02
2,6-di-tert-butyl-4-methylphenol
<0.02
Figure 4: Inlet sample.
Figure 5: Outlet sample.
37
38
Application
Filtration+Separation December 2006
The equipment The centre piece of the process is a compartmentalised Weirbox which allows for simple chemical addition, gas venting and recirculation of the contaminated fluids until they met the discharge requirements in place. The Weirbox, holds a maximum of 19,000 Figure 2: The Weirbox consists of four compartments sharing a common gas vent. l of fluid, but here a maximum of 12,700 l The bulk fluids were categorised into three was treated per batch to prevent the oil weir distinct groups: filling with untreated fluids. A small oil weir is located across the length of the Weirbox and • Free Oil – A thin layer likely to be allows any coalesced oil to be removed from contaminated with some solids and the system. Transparent sight glasses allow biomass. for the operator to define between water and • Contaminated Water – In the case of the solvent layers. Ball valves are used to fill the FPSO work, the vast majority of the target compartments one at a time and allow discharge fluid volume which is heavily contaminated of each compartment separately. with biomass, Iron Sulphides and dispersed The Weirbox consists of four compartments oil. sharing a common gas vent as seen in figure 2. • Sludge – A comparatively smaller volume The sight glasses in each compartment allow of extremely contaminated solids, scale and interface monitoring where immiscible fluids are biomass. contained or oil and water separation is required. Each compartment can be individually sampled The objective was to treat the free oil and for analysis and can be isolated and recirculated contaminated water phases to dramatically onto itself, or onto any other compartment, reduce their hydrocarbon content and where allowing great flexibility for complex chemical possible recover the valuable oil. treatment applications. Chemicals can be directly injected into any of the compartments Initially, a simple batch chemical treatment individually or into the recycle line shown in the process utilising patented CETCO process diagram. Inert gas can also be sparged through equipment was used. The process utilised a each compartment to aid the removal of gases carefully chosen organic solvent to extract such as H2S if required. the hydrocarbon components of the fluids and a chemical to kill any bacterial content in the water. After undergoing the solvent process, each batch of fluid was checked to meet DTI requirements and passed through a macerator to ensure compliance with SEPA regulations of discharged solid size of less than 1 mm, before being discharged overboard.
In the case of the FPSO treatment, an organic solvent was pumped into compartment one and a bacterial treatment chemical dosed to compartment two. All fluids entered the Weirbox through compartment one, which allowed the entire batch volume to contact the solvent layer stripping the hydrocarbons from the contaminated waste water. Once full, the contents of the Weirbox were recirculated upon itself to aid in the extraction process. This was achieved by pumping the contents of compartment four into compartment one continuously. All the compartment valves were open allowing the entire fluid to mix thoroughly. After the fluid had been circulating for a predetermined time, it was left to settle and tested to determine if it could be discharged to sea.
Figure 3: When the Weirbox was setup on the FPSO, there were various hose connections for clean fluid to be discharged to the marine environment and waste fluid to waste cargo tanks.
When the concentration of the batch was confirmed as less than the prescribed 500 ppm, each compartment was pumped through the macerator and overboard one at a time. The density difference between the treated water and the solvent allowed for discrimination
between the two phases. The solvent layer floats on the water layer and during treatment some under carry of solvent was experienced within the Weirbox, resulting in a spread of solvent between each compartment. The sight glasses allowed for visual inspection of the interface and subsequent separation of the two phases. The treated water was pumped overboard and the reclaimed oil pumped into a separate cargo tank for re-injection into the crude oil export line. The solvent layer was typically renewed after an average of four batches. Batch concentrations determined when the solvent required to be changed out. Flowmeters were installed in all discharge lines to monitor volume of treated fluid discharged to sea and reclaimed oil and solvent to cargo tanks.
Discharge sampling The oil in water concentration was measured onsite using the DTI IR absorption procedure carried out on the FPSO’s calibrated Infracal. A uniform concentration was achieved within the Weirbox by recirculation and constant mixing of the fluids. The Weirbox design allows for each individual compartment to be analysed separately so CETCO could confirm that the concentration in the fourth compartment was identical to the concentration in every compartment. Each batch was then sampled from compartment prior to discharge overboard to confirm the concentration levels were below the set 500 ppm limit. The tests of the processed fluid showed that the new treatment method was effective and, by the end of the project, 5000 m3 of the fluid had been processed. Laboratory analysis to determine oil in water content confirmed that all fluids discharged to the marine environment during the batch processes were well below the 500 ppm limit set by the DTI for the project. After successfully completed treatment of the contaminated waste water, the same operational philosophy was applied to the solids phase.
Ongoing application During the initial stages of the treatment process the effluent fluid concentration was relatively low. This was thought to be due to the addition of the biocide which aided in the separation of the three distinct phases. Detailed analysis was performed on a sample of the inlet and outlet to the Weirbox. This was performed using gas chromatography by a third party laboratory. (See figures 4 and 5). The analysis results revealed that the organic solvent extraction process reduced the overall total hydrocarbon content without increasing the aromatic content within the discharged water. The analysis results indicated a total hydrocarbon removal efficiency of 98%.
•
Application
Filtration+Separation December 2006
Contamination control:
Cleaning sludge in the North Sea S
arah McGillivray, operations chemist at CETCO Oilfield Services looks at a new technique for cleaning contaminated sludge in a floating oil production vessel that cleans the liquids and solids together without the need for prior separation.
Naturally Occurring Radioactive Material (NORM), also known as Low Specific Activity (LSA) scale, is a weakly radioactive insoluble precipitate that exists in oil reservoirs and often contaminates oil production equipment during extraction processes. LSA scale has been contaminating equipment throughout the history of the oil industry. However, with greater knowledge of pollutants and more stringent environmental regulation, NORM has, over time, become a significant issue for the industry. LSA scale does not exist in isolation – it is usually in an emulsion of wastewater, mud and oil. The oil industry refers to it is as contaminated sludge, which, given the constituents, is an apt description. The cleaning of contaminated tanks or equipment, removal and handling of LSA scale requires specialist radiological protection supervision. Items of LSA scale contaminated equipment can be brought to shore and cleaned at specially licensed facilities, but what do you do with six million litres of sludge, which takes up a significant storage capacity of a FPSO (floating, storage, production and off-loading vessel)?
The discharge of LSA scale to the marine environment in the North Sea is monitored and controlled by SEPA (Scottish Environment Protection Agency), and the discharge of oil contaminated water or solids is monitored and governed in the UK by the DTI (Department of Trade and Industry).
filtration, chemical, centripetal and cyclonic technologies. However in some instances these methods are found to fail, leaving operators with long term challenges and growing volumes of ‘untreatable’ fluids in their tanks. The condition of these fluids can degenerate over time as more biomass forms promoting further corrosion.
Operators cannot simply dump water over-theside of their production facilities, so methods of cleaning need to be applied to remove oil, NORM and other contaminants before any discharge can be made to the sea.
The FPSO tank contained approximately 5000 m3 of liquid and 1000 m3 of solids, that had formed a cake-like layer at the bottom of the tank. The liquid was sampled at various depths and displayed an emulsion like consistency of oil in water with biomass and iron sulphides present. Inlet concentrations of oil in water were as high as 20,000 mg/l (2% m/v) during the first stage of operation which aimed to remove the free liquid above the solid sludge layer.
The traditional approach is to first separate solids and liquids and then treat them individually. This process is often ineffective, however, and is both costly and slow. Now there is a new technique, which has been pioneered by CETCO Oilfield Services, which cleans the liquids and solids together without having to separate them beforehand. This is the technique that was successfully applied offshore in the North Sea, in the presence of a DTI inspector, to clean the LSA and biomass contaminated sludge and associated oily water from the six million litres of sludge in CETCO’s client’s FPSO storage tanks.
A new approach The sludge in the FPSO wasn’t just taking up valuable space, it was also causing corrosion, bio-fouling and the generation of toxic gases such as hydrogen sulphide. The actual fluid was a mixture of produced water, crude oil, sea water ballast, deck drainage water and all manner of associated solids including the LSA scale. Together, this mixture forms a fairly stable emulsion that prevents bulk separation of the liquid phases. Figure 1. The CETCO Weirbox.
Traditional methods of treating such fluids have included separation tanks,
To help CETCO prove the system, a temporary dispensation was granted by the DTI which allowed the cleaned fluids to be discharged up to a maximum of 500 mg/l, compared with the normally acceptable level of 30 mg/l. The treatment process utilised a compartmentalised Weirbox and was designed to allow for LSA solids and sand to be cleaned of hydrocarbons without the need for solids separation and the creation of another waste stream. The objective was for the effluent water to be discharged to the sea directly after the treatment process, without the need for further water polishing. Prior to commencement of treatment, a kill dose of biocide was applied to the cargo tank as a large quantity of biomass was evident and oil was clearly congealed in this scum layer. The use of the biocide resulted in greatly increased phase separation within the tank and reduced inlet concentrations to the Weirbox. This was an important step in the successful treatment of the fluids.
Application
Filtration+Separation December 2006
PAH
µg/l
PAH
µg/l
Acenaphthylene
144.32
Acenaphthylene
0.97
Acenaphthene
95.97
Acenaphthene
0.32
Fluorene
200.28
Fluorene
0.85
Anthracene
119.9
Anthracene
2.69
Fluoranthene
24.92
Fluoranthene
<0.01
Pyrene
46.38
Pyrene
0.31
Benz(a)anthracene
91.84
Benz(a)anthracene
3.26
Chrysene
<0.01
Chrysene
<0.01
Benzo(b)fluoranthene
48.07
Benzo(b)fluoranthene
0.69
Benzo(k)fluoranthene
35.61
Benzo(k)fluoranthene
0.26
Benzo(a)pyrene
4.57
Benzo(a)pyrene
2.2
Benzo(g,h,I)perylene
<0.01
Benzo(g,h,I)perylene
<0.01
Indeno(1,2,3,cd)pyrene
<0.01
Indeno(1,2,3,cd)pyrene
<0.01
Dibenz(a,h)anthracene
<0.01
Dibenz(a,h)anthracene
<0.01
Naphthalene
269.84
Naphthalene
0.83
Phenanthrene
265.81
Phenanthrene
5.63
Total PAH
1347.51
Total PAH
18.01
OIW
mg/l
OIW
mg/l
Total hydrocarbons
1627
Total hydrocarbons
37
Aliphatic content
1417
Aliphatic content
17
Aromatic content
210
Aromatic content
20
Alkyl Phenols
µg/l
Alkyl Phenols
µg/l
Total C1-C3 alkyl phenols
<0.50
Total C1-C3 alkyl phenols
<0.50
Other C1-C3 alkyl phenols
Other C1-C3 alkyl phenols
2-methylphenol
<0.02
2-methylphenol
<0.02
3-methylphenol
<0.02
3-methylphenol
<0.02
4-methylphenol
<0.02
4-methylphenol
<0.02
2, 5-dimethyphenol
<0.02
2, 5-dimethyphenol
<0.02
3,4- and 3, 5-dimethyphenol
<0.02
3,4- and 3, 5-dimethyphenol
<0.02
2, 4-dimethyphenol
<0.02
2, 4-dimethyphenol
<0.02
4-ethylphenol
<0.02
4-ethylphenol
<0.02
2-n-propylphenol
<0.02
2-n-propylphenol
<0.02
2,3,5-trimethylphenol
<0.02
2,3,5-trimethylphenol
<0.02
4-n-propylphenol
<0.02
4-n-propylphenol
<0.02
2,3,6-trimethylphenol
<0.02
2,3,6-trimethylphenol
<0.02
Total C4-C5 alkyl phenols
<0.50
Total C4-C5 alkyl phenols
<0.50
Other C4-C5 alkyl phenols
Other C4-C5 alkyl phenols
4-tert-butylphenol
<0.02
4-tert-butylphenol
<0.02
2-tert-butylphenol
<0.02
2-tert-butylphenol
<0.02
4-n-butylphenol
<0.02
4-n-butylphenol
<0.02
2-tert-butyl-4-methylphenol
<0.02
2-tert-butyl-4-methylphenol
<0.02
4-tert-butyl-2-methylphenol
<0.02
4-tert-butyl-2-methylphenol
<0.02
4-n-pentylphenol
<0.02
4-n-pentylphenol
<0.02
Total C6-C9 alkyl phenols
<0.50
Total C6-C9 alkyl phenols
<0.50
Other C6-C9 alkyl phenols
Other C6-C9 alkyl phenols 2,6-diisopropylphenol
<0.02
2,6-diisopropylphenol
<0.02
4-n-heptylphenol
<0.02
4-n-heptylphenol
<0.02
4-n-octylphenol
<0.02
4-n-octylphenol
<0.02
2,6-di-sec-butylphenol
<0.02
2,6-di-sec-butylphenol
<0.02
2,4-di-tert-butylphenol
<0.02
2,4-di-tert-butylphenol
<0.02
4-tert-octylphenol
<0.02
4-tert-octylphenol
<0.02
2,6-di-tert-butylphenol
<0.02
2,6-di-tert-butylphenol
<0.02
4-n-nonylphenol
<0.02
4-n-nonylphenol
<0.02
2,6-di-tert-butyl-4-methylphenol
<0.02
2,6-di-tert-butyl-4-methylphenol
<0.02
Figure 4: Inlet sample.
Figure 5: Outlet sample.
37
38
Application
Filtration+Separation December 2006
The equipment The centre piece of the process is a compartmentalised Weirbox which allows for simple chemical addition, gas venting and recirculation of the contaminated fluids until they met the discharge requirements in place. The Weirbox, holds a maximum of 19,000 Figure 2: The Weirbox consists of four compartments sharing a common gas vent. l of fluid, but here a maximum of 12,700 l The bulk fluids were categorised into three was treated per batch to prevent the oil weir distinct groups: filling with untreated fluids. A small oil weir is located across the length of the Weirbox and • Free Oil – A thin layer likely to be allows any coalesced oil to be removed from contaminated with some solids and the system. Transparent sight glasses allow biomass. for the operator to define between water and • Contaminated Water – In the case of the solvent layers. Ball valves are used to fill the FPSO work, the vast majority of the target compartments one at a time and allow discharge fluid volume which is heavily contaminated of each compartment separately. with biomass, Iron Sulphides and dispersed The Weirbox consists of four compartments oil. sharing a common gas vent as seen in figure 2. • Sludge – A comparatively smaller volume The sight glasses in each compartment allow of extremely contaminated solids, scale and interface monitoring where immiscible fluids are biomass. contained or oil and water separation is required. Each compartment can be individually sampled The objective was to treat the free oil and for analysis and can be isolated and recirculated contaminated water phases to dramatically onto itself, or onto any other compartment, reduce their hydrocarbon content and where allowing great flexibility for complex chemical possible recover the valuable oil. treatment applications. Chemicals can be directly injected into any of the compartments Initially, a simple batch chemical treatment individually or into the recycle line shown in the process utilising patented CETCO process diagram. Inert gas can also be sparged through equipment was used. The process utilised a each compartment to aid the removal of gases carefully chosen organic solvent to extract such as H2S if required. the hydrocarbon components of the fluids and a chemical to kill any bacterial content in the water. After undergoing the solvent process, each batch of fluid was checked to meet DTI requirements and passed through a macerator to ensure compliance with SEPA regulations of discharged solid size of less than 1 mm, before being discharged overboard.
In the case of the FPSO treatment, an organic solvent was pumped into compartment one and a bacterial treatment chemical dosed to compartment two. All fluids entered the Weirbox through compartment one, which allowed the entire batch volume to contact the solvent layer stripping the hydrocarbons from the contaminated waste water. Once full, the contents of the Weirbox were recirculated upon itself to aid in the extraction process. This was achieved by pumping the contents of compartment four into compartment one continuously. All the compartment valves were open allowing the entire fluid to mix thoroughly. After the fluid had been circulating for a predetermined time, it was left to settle and tested to determine if it could be discharged to sea.
Figure 3: When the Weirbox was setup on the FPSO, there were various hose connections for clean fluid to be discharged to the marine environment and waste fluid to waste cargo tanks.
When the concentration of the batch was confirmed as less than the prescribed 500 ppm, each compartment was pumped through the macerator and overboard one at a time. The density difference between the treated water and the solvent allowed for discrimination
between the two phases. The solvent layer floats on the water layer and during treatment some under carry of solvent was experienced within the Weirbox, resulting in a spread of solvent between each compartment. The sight glasses allowed for visual inspection of the interface and subsequent separation of the two phases. The treated water was pumped overboard and the reclaimed oil pumped into a separate cargo tank for re-injection into the crude oil export line. The solvent layer was typically renewed after an average of four batches. Batch concentrations determined when the solvent required to be changed out. Flowmeters were installed in all discharge lines to monitor volume of treated fluid discharged to sea and reclaimed oil and solvent to cargo tanks.
Discharge sampling The oil in water concentration was measured onsite using the DTI IR absorption procedure carried out on the FPSO’s calibrated Infracal. A uniform concentration was achieved within the Weirbox by recirculation and constant mixing of the fluids. The Weirbox design allows for each individual compartment to be analysed separately so CETCO could confirm that the concentration in the fourth compartment was identical to the concentration in every compartment. Each batch was then sampled from compartment prior to discharge overboard to confirm the concentration levels were below the set 500 ppm limit. The tests of the processed fluid showed that the new treatment method was effective and, by the end of the project, 5000 m3 of the fluid had been processed. Laboratory analysis to determine oil in water content confirmed that all fluids discharged to the marine environment during the batch processes were well below the 500 ppm limit set by the DTI for the project. After successfully completed treatment of the contaminated waste water, the same operational philosophy was applied to the solids phase.
Ongoing application During the initial stages of the treatment process the effluent fluid concentration was relatively low. This was thought to be due to the addition of the biocide which aided in the separation of the three distinct phases. Detailed analysis was performed on a sample of the inlet and outlet to the Weirbox. This was performed using gas chromatography by a third party laboratory. (See figures 4 and 5). The analysis results revealed that the organic solvent extraction process reduced the overall total hydrocarbon content without increasing the aromatic content within the discharged water. The analysis results indicated a total hydrocarbon removal efficiency of 98%.
•