Environmental Report 1999 Index

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Chemical Use and Discharge Offshore, massive structures and large machines play the lead roles in oil and gas production. On a more subtle level, chemistry also plays a part. Typically, an installation will use about 600 tonnes of various chemicals over the course of a year, and both their use and subsequent disposal are strictly controlled both by voluntary standards and Government regulations. The most important chemicals are: production chemicals added to oil and gas processing systems; drilling fluids used to drill and complete wells; chemicals used for cleaning and testing pipeline systems. In 1979, the DTI and UKOOA worked together to establish the Offshore Chemicals Notification Scheme (OCNS), a voluntary scheme for controlling the use of chemicals offshore. The OCNS is administered by the DTI, who receive scientific advice from Government Departments concerned with the fishing industry. The Government has indicated it intends to introduce a mandatory chemicals control scheme and industry will work with regulators for a scheme which seeks to evaluate chemicals on a risk base dependant on their characteristics and use. Under the OCNS, all chemicals intended for use offshore are tested to determine their toxicity, biodegradability and persistence in the marine environment. Then, the regulatory authority assigns each chemical to a 'group', with Group A the most and Group E the least hazardous. For each installation, the weights of chemicals used (rather than discharged) are added together for each group, and the annual sum should not exceed the 'trigger levels' shown in Figure 6. Operators are required to notify the DTI if they expect to exceed the trigger levels. In such cases, a site-specific risk assessment will be carried out, and the DTI may advise on product disposal, press for substitutes or call for further tests or data. The industry has made a considerable effort to develop chemicals which have maximum effectiveness and minimum impact on the marine environment. Recently, a new group, Z, has been introduced to cover chemicals which are either non-toxic or not discharged (for example, oil based muds). Figure 6 Group Production chemicals (tonnes) Drilling chemicals (tonnes) A 40 All proposed usage to be notified B 70 3 C 150 15 D 375 350 E 1,000 4,750 Production Chemicals Chemicals such as corrosion inhibitors and emulsifiers are added at various points in the production process. Inevitably, small amounts of the oil-soluble chemicals dissolve and are exported with the product, while a proportion of the water-soluble chemicals dissolve in the produced water. The quantity of production chemicals used varies from year to year depending on, among other things, the quantity of oil and gas produced. Figure 7 shows the quantities of production chemicals discharged to sea from 126 installations during 1998. Figure 7 Thousand tonnes Year Number of installations E D C B A Unclassified 1996 85 5.43 2.54 1.26 0.31 .33 1.52 1997 106 10.37 2.99 1.21 0.18 0.43 1.65 1998 126 13.71 3.05 1.99 0.66 0.37 0.52 Drilling Chemicals As we explained earlier, drilling mud is pumped into a well to remove the rock cuttings, help prevent collapse of the side walls, and to control the flow of oil and gas - if they are encountered. There are three types of mud, formulated with base fluids of either water, mineral oil or synthetic oil: water-based mud (WBM) oil-based mud (OBM) synthetic-based mud (SBM) Apart from the base fluid, the main components of mud are the inert agents, primarily barite (barium sulphate), that are added to give it weight. These clay-like solids make up the greatest proportion of a drilling mud. The mud carries drill cuttings back to the surface, where they are separated from the fluid. However, even with the most advanced treatment techniques, the cuttings retain a thin coating of mud, and what happens to them next depends on the type of mud being used. Water-based muds consist mostly of water, inert solids such as barite, and non-toxic salts. Consequently, throughout the world, WBM-coated cuttings are discharged into the sea. If a synthetic mud is being used, the cuttings may be discharged to sea under the terms of the phase-out arrangement agreed between the operator and the DTI. After the end of the year 2000, synthetic muds will no longer be discharged, and operators will either use water-based muds or the alternative disposal routes devised for OBM-coated cuttings (these are either returned to shore for disposal or re-injected into the wells). The disposal routes and quantities of cuttings disposed of by UKOOA member companies in 1998 are shown in Figure 8. Figure 8 Base fluid Number of wells Quantity of cuttings (tonnes) Disposal route Quantity of oil/ fluid on cuttings (tonnes) Synthetic 30 13,428 Shore 1,356* Synthetic 120 54,367 Discharged to sea 5,005 Synthetic 3 1,643 Injected into the well 136* Oil 12 1,837 Shore 250* Oil 7 1,357 Injected into the well 196* * not discharged to sea In addition to drilling mud, a number of other substances may be used during the lifetime of a well. Special chemicals may be added to adjust the properties of the mud so that it is suitable for drilling certain geological formations, or in anticipation of certain situations (for example, to prevent the well bore from collapsing). Cement is used to support and stabilise the well, and other chemicals may be used during a 'workover' to maintain the well in good working order. Figure 9 shows the total amounts of such substances discharged to sea from a total of 318 wells during 1998. This includes WBM's. Figure 9 Tonnes Year No of pipelines E D C B A Unclassified 1996 14 0 8 0 41 28 19 1997 36 152 5 0 1 0 7 1998 25 24 3 2 0 12 0 Pipeline Chemicals Seabed pipelines form the North Sea's plumbing system, carrying oil and gas hundreds of miles from offshore installations to processing facilities onshore. But before they can be used, pipelines must be cleaned and tested by filling them with a mixture of seawater and chemicals. For example, a dye is added to so that leaks can be detected before oil or gas is flowed through the line, while other substances are used to prevent corrosion. Under a consent issued by the Government, the treated seawater may be discharged directly into the sea after the cleaning and commissioning process or, where practicable, it can be processed by the onshore facility. In 1998, twenty five pipelines were treated before commissioning, and the seawater discharged to the sea. The quantities of discharged chemicals in each OCNS category are shown in Figure 10. Figure 10 Tonnes Year E D C B A Unclassified 1996 186,241 5,402 5,191 508 405 19,622 1997 187,372 6,232 2,215 352 524 15,883 1998 173,951 6,697 2,569 813, 394 14,706 * Includes revisions to 1997 and 1998 data (see below) Environmental Impacts Of Chemicals The chemicals discharged to sea comprise a wide variety of substances that may have an effect on the environment. However, the potential for environmental impact must be judged against the amounts of chemicals for each OCNS group, and where and under what conditions they were discharged. In fact, almost 90% of discharges on the UKCS in 1998 were Group E chemicals, which have been independently assessed and certified to have little or no environmental impact. The more toxic chemicals (Groups A and B) may have an impact close to the discharge point, but studies have shown that they are quickly diluted and do not represent a major environmental hazard. The changes in some of the chemicals data has been caused by revisions to company records being fed through to the industry database. This has been caused by the greater focus on chemicals usage and discharge arising from the commitment on chemicals described earlier.