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THE QUANTIFICATION OF ENERGY CONSUMPTION AND GASEOUS EMISSIONS ASSOCIATED WITH DECOMMISSIONING OFFSHORE OIL AND GAS INSTALLATIONS a

a

S. A. KERR , J. C. SIDE & R. GAMBLIN

b

a

ICIT, Department of Civil and Offshore Engineering , Heriot-Watt University , Old Academy, Back Road, Stromness, Orkney, KW16 3AW, UK b

Unocal Britain Ltd , Salveson Tower, Blakies Quay, Aberdeen, AB11 6PW, UK Published online: 04 Sep 2007.

To cite this article: S. A. KERR , J. C. SIDE & R. GAMBLIN (1999) THE QUANTIFICATION OF ENERGY CONSUMPTION AND GASEOUS EMISSIONS ASSOCIATED WITH DECOMMISSIONING OFFSHORE OIL AND GAS INSTALLATIONS, Civil Engineering and Environmental Systems, 15:4, 251-273 To link to this article: http://dx.doi.org/10.1080/02630259908970243

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Civil. Eng. and Env. Sysl., Vol. 15, pp. 251 -273

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THE QUANTIFICATION OF ENERGY CONSUMPTION AND GASEOUS EMISSIONS ASSOCIATED WITH DECOMMISSIONING OFFSHORE OIL AND GAS INSTALLATIONS S. A. KERR

* , .I.C. SIDE a and

R. GAMBLIN

" ICIT, Depurtrnent of Civil and Offshore Engineering, Heriot- Watt University, Old Academy, Back Road, Stromizess, Orkney, KW16 3A W , UK; Unocal Britain Ltd, Salveson Tower. Blakies Quay, Aberdeen, ABII 6PW, UK (Received 11 Jlrne 1997; Revised 25 September 1997: In final form 17 March 1998) Following the failure to ~mplementthe proposed deep-sea disposal of the North Sea Brent Spar oil installation the U K oil industry has identified energy consumption and gaseous emission a s a key determinant of environmental impact associated with the abandonment ol'ofTshore facilities. In the absence of a standardised methodology this paper describes the approach adopted and rcsults achieved using the North Sea Heather platform as a case study. The study develops and then applies a set of rules for conducting such analysts. Results show that in terms of cnergy consumption and greenhouse gas emissions there is little to choose between most partial and complete removal options. Thc cnergy cost advantages of recycling are largely offset by increascd transport costs returning materials to shore. The study also highlights the importance of case specific variables, in particular, marine vessel fuel consumption.

Ke.vlvords: Encrgy consumption; gaseous emissions; environmental impact;North Sea oil gas; abandonment; decommissioning; energy analysis

INTRODUCTION Since the failure to implement the proposed deep sea disposal of the North Sea Brent Spar oil storage hcility both industry and govern*Corresponding author.


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ment have been forced to reconsider the environmental criteria used to identify the Best Practicable Environmental Option (BPEO) for the disposal of offshore oil and gas installations. This process has led the oil industry to focus attention on the related issues of energy consumption and gaseous emissions associated with abandonment options. The public outcry surrounding the proposed disposal of the 13rent Spar has subsequently forced the oil and gas industry to engage a wider audience in the decommissioning process and address conflicts between societal values and scientific evidence [I]. In late 1996 Shell U K Exploration and Production Ltd initiated a dialogue process in an attempt to involve wider groups of stakeholders. This process has identified energy consumption as an important abandonment issue [2]. Shcll has conseyucntly selected energy consumption as a key criterion that will be used to select the final disposal route for the Spar [3]. The six contractors currently competing for the job of disposing of the Spar must conduct an energy analysis of their proposed option. The work presented here conducts such an energy analysis for the Heather Platform operated by UNOCAL Britain Ltd. In the absence of a standardised approach to this type of analysis this paper establishes a set of rules which effectively draw a boundary around the decommissioning process in terms of the emissions and energy consumptions included. The essential systems problem is defining correct boundaries and consistently accounting for energy flows. The results of the study for Heather are presented before discussing the wider implications of this type of analysis. The methodology adopted in this paper is also discussed in the context orother published work. It should be stressed from the outset that such analysis is only one feature of' any assessment of environmental impact or BPEO study.

THE HEATHER FACILITY The Heather Field was discovered in Dccember 1973 and brought in to production in October 1978. The field is situated on the edge of the Shetland Basin some 90 nm NE of the Shetland Islands. The field has been developed using a single combined drilling, production and accommodation platform standing in 143m of water. Produced oil and condensate is exported from the field via a 16" pipeline running


DECOMMISSlONlNG OFFSHORE PLATFORMS: EIA

253

32 km to the Ninian Central platfor111and from there on to the Sullom Voe terminal in Shetland. A 6" pipeline imports gas used as platform fuel and as production well lift gas. The platform itself has a maximum height of 236m consisting of a modular topside resting on a deck support frame (DSF) which in turn rests on a steel jacket piled into the sea floor. The topside comprises a number of dedicated units: production, drilling, utility and quartcrs, and two flare booms. The total dry weight of the topside metals, including DSF, is estimated to be 12,300 tonnes. The Jacket comprises a steel tubular space frame with eight legs. Six piles support each of the four corner legs. The estimated weight of the jacket including piles and grout within the pile sleeves to the mudline is 17,000 tonnes. Within the jacket are located 41 well conductors which weigh in the region of4,300 tonnes to the mudline. Thejacket has been itz situ for 20 years and in common with other North Sea installations has accumulated considerable biofouling. Colonies of barnacles, mussels, hydroids and various seaweeds are common to North Sea installations [4]. The total weight of biofouling on Heather is estimated to be up to 2,000 tonnes. Quantities of recoverable materials are given in Figure 1.

DECOMMISSIONING OPTIONS International Maritime Organisation (IMO) regulations require the complete removal of installations, weighing less than 4000 tonnes, in water depths of less than 75 m (or 100 m if emplaced after 1st January 1998). In the case of weight/depth combinations outside the above, the structure may be partially removed subject to 55 m clearance [5]. The regulatory situation may be about to change at the time of writing if the Oslo and Paris Commission adopt more stringent rules for the

Steel

30,000

Aluminium FIGURE 1

Recoverable materials

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S. A. KERR er a/.


North Sea [6]. Standing in 143 m of water it was necessary to consider the full range of partial and complete removal options for Heather. The platform operators have identified twelve abandonment options; these are summarised in Figure 2. The twelve options can be divided into four categories

a

toppling (options A, BI and BII) the jacket structure with various amounts of residual topside; partial removal (options CI and CII) refloating the stripped out DSF and jacket to 55 m; partial removal (options DI, DII, DIII and DIV) lifting the topside, DSF and upper section of the jacket to 55m; complete removal (options EI, EII and EIII) lifting the complete jacket in sections.

The four basic options are further subdivided to give the final 12 according to the extent and destination of the topside removed and the removal methodology and destination of jacket material.

OBJECTIVES OF THE ANALYSIS The primary objective of the study was to estimate the relative performance of each decommissioning option in terms of energy consumption and gaseous crnissions, in particular emissions of greenhouse gases and acidifying emissions. The following measures were adopted: energy (in Joules); greenhouse gases (in tonnes COz equivalents); acidifying emissions (tonnes of sulphur dioxide SO2 and oxides of nitrogen NO,).

C 0 2 equivalents include C 0 2 and other gases with a global warming potential (GWP). The acidifying emissions (NO, and SOz) were not combined in a single unit of acidifying potential as S02/N0, mixes exhibit symbiotic effects; the combined effect not equal to the sum of individual impacts [7]. Energy consumption and gaseous emissions will be referred to collectively as energy costs.


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METHODOLOGY AND RULES OF ANALYSIS When establishing the methodologies, rules and boundaries of the evaluation the analyst is faced with a number of choices. The International Federation of Institutes of Advanced Studies (IFIAS) set out one of the most comprehensive and systematic approaches to energy analysis [8]. Its adoption requires the calculation of all direct and indirect energy costs involved in making available all materials and facilities required by an industrial process and adding to this direct energy consumplion resulting from the operation of the process to calculate the gross energy requirement. For production processes this can be fairly straight forward but nevertheless involves the construction of boundaries as it is not possible lo take a cradle to grave, Life Cycle Analysis (LCA), approach which is absolutely exhaustive including all direct and indirect energy costs associated with a process. The LCA approach outlined by IFIAS examines the energy implications of choices and decisions still to be made. It is inappropriate to adopt this approach with Heather, as the facility is existent. The analysis therefore contains no energy costs associated with the construction of Heather. The analysis is further complicated because decommissioning is clearly not a production process; however, a series of steps may be identified leading to any particular abandonment end point. The adoption of energy analysis in the decommissioning context raises the issue of how to treat wastes and the recycling process. The methodology adopted here is based on the premise that "all activities have positive energy costs associated with them". The task is therefore to identify all component activities and quantify them. Energy costs are calculated by applying unit conversion factors (UCFs) and then summed to give the total energy costs for each option. Energy costs may be eilhcr direct or indirect. Direct energy costs are those that are physically close to the decommissioning proccss and include those associated with the dismantling and transportation of platform materials. Indirect energy costs are spatially or temporally distant from the decommissioning process e.g., replacement costs for materials left at sea. If materials that could otherwise he recycled are left at sea, then in global terms, they will be replaced by material produced from ore. The temporal and spatial incidence of these costs is


DECOMMISSIONING O F F S H O R E PLATFORMS: EIA

257

distant lrom actual decommissioning activities; these costs are however a consequence of making the decision to leave recyclable material at sea. Thc methodology treats recycling like all other activities as cost positive; energy costs arise as materials are transported and then processed at the recycling facility. The mix of component activities and their associated energy costs determines the performance of an abandonment option. Abandonment options with high levels of recycling benefit from a reduction in those indirect costs associated with the replacement of materials left at sea. Having established the underlying principles for the analysis it was necessary to adopt a formal set of rules that would help when deciding if a particular energy costs should or should not be included in the analysis. The following rules wcrc adopted: Rule 1. The indirect cost of capital plant and equipment not altered by the decommissioning process are ignored for the purposes of the energy analysis. The effect of this rule is to exclude the energy costs associated with the construction and maintenance of capital plant and equipment unless it has been specifically constructed or significantly altered by the decommissioning process. For example energy costs associated with the construction and maintenance of vessels is excluded from the analysis. There is, however, a counter argument along the following lines:specialist vessels would not be constructed without an oil industry creating demand for their services and therefore the Heather decommissioning process as part of that industry then is part responsible for the construction and maintenancc of these vessels. However at the case specific level, the vessels used in the decommissioning of Heather would still exist whether the decommissioning proceeds or not. In other words it is assumed that there are no depreciation costs for existing capital plant and equipment in energy terms. Energy costs associated with the fabrication of capital plant and equipment constructed specifically for the decommissioning are included e.g., energy costs associated with the fabrication of required temporary steelwork.

Rule 2. Trivial energy costs have to be disregarded. In order to keep the analysis manageable many trivial items must be disregarded. The

258

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onshore disposal tasks in particular involve many minor energy costs. For example, an allowance was made in the analysis for energy costs associated with sand blasting recovered steel, however costs associated with the extraction and transportation of the sand used were not included. Exhaustive inclusion of such minor items would make the study unmanageable and have little bearing on the results. The assumption is that such costs are collectively trivial.

Rule 3. All renewable energy sources and materials are treated as cost free goods. In principle there is less concern where renewable energy and renewable sources of materials are consumed. Clearly, however, the production of renewable energy and renewable sources of materials also incur direct energy costs, however these are largely covered by Rule 1 . This analysis assumes that all renewable energy and material sources are cost neutral. For this case study this applies to energy costs of aluminium production (see below). Rule 4. All non-renewable materials lost from the global production cycle are replaced by materials of comparable quantity and quality providing that the recycling of these wastes would be possible at this time. This rule requires the inclusion of energy costs to replace materials left at sea. These costs are, however, only included if recycling is technically feasible. Non-recoverable and non-recyclable materials have been omitted from the analysis e.g., sub seabed conductors and glass reinforced plastics. Again this illustrates the difference between the decommissioning analysis of abandonment options and a full LCA. Rule 5. All non-renewable materials that are recycled are treated as substitutes for the basic raw materials that would otherwise be used in their production. For many recycling processes this rule is self evident. However, the recycling of scrap steel, for example, may in some processes require the addition of some pig iron produced from ore; this pig iron has been ignored in the study. One tonne of steel recycled can be considered to substitute directly for one tonne of steel produced from ore. Rule 6. Where the application of Rules 1- 5 involves the cakulation of identical energy costs twice, boundaries must be drawn to ensure that double counting does not occur. This might appear strange at first, but it became clear during the course of the study that there was a possibility


DECOMMISSIONING OFFSHORE PLATFORMS: EIA

259

of double counting; indeed other studies have adoptcd methodologies which double count the benefits of recycling [9]. The energy accounting for temporary steelwork illustrates the possibility of double counting. The accounting procedure is summarised in Figure 3. In this example (Fig. 3 ) the 20 tonnes of steel left at sea are not reconstituted from ore as the costs of constituting the full 100 tonnes of temporary steelwork are included in the first stage of the calculation. T o allow for the reconstitution of the metal left at sea in this case would lead to double counting. In terms of the analysis the approach ensures that the endpoint for all decommissioning option calculations is the same, with all materials accounted for in each option.

VARIABLES, CONSTANTS AND CONVERSION FACTORS Having established the rules of analysis, component activities for each decommissioning option were identified and quantified. Activities and their quantities were divided into constants and variables. Variables are those measures, which are dependent on a particular dccomrnissioning option. Variables, in the main, consist of wcights of materials (brought to shorc, recycled, left in situ or disposed at sea) and hours of vessel, helicopter and generator usage. As many as 80 different variables were identified depending on the decommissioning option. Constants arc those factors, which are the same irrespective of the decommissioning option including: haulage distances- to disposal facilities and recycling centres; rates of fuel consumption- for vessels, helicopters, road haulage and platform power generation. Four-different rates of fuel consumption were included for the 15 different vessel types required depcnding on whethcr vesscls were in port, in transit, working or waiting on weather. The study required a set of Unit Conversion Factors (UCFs) to be eslablished. The UCFs are applied to the appropriate constants and variables to calculate energy costs for specific activities. UCFs calculate energy costs for the recycling and replacement of non-

Outcomes: 80 tonnes of temporary steelwork recycled. 20 tonnes of temporary steelwork lefl at sea

100 tonnes of steel and associated fabrication costs

Outcome: 100 tonnes of temporary steelwork fabricated from ore.

FIGURE 3 The calculation of total energy costs associated with the use of temporary steclwork.

On fulfilment of abandonment oation (+) energy costs of recovering and recycling 80 tonnes of this temporary steelwork. (-) benefits of displacing 80 tonnes of virgin iron in thc smelting process.

Fabrication

(+) energy costs associated w ~ t hthe production of



DECOMMISSIONING O F F S H O R E PLATFORMS: EIA

26 1

renewable materials and for the consumption of fuels. Where possible, published sources were used to select UCFs; where these were unavailable information was sought from manufacturers or calculations were made from first principles. Figure 4 gives a list of UCFs used in the study together with their sources. A total 157 constants and UCFs were used in the study. The calculation of energy costs associated with the production of aluminium from ore highlights the use of the rules of analysis and the need to include C 0 2 equivalents. The production of aluminium is an energy intensive process requiring in the region of 259.6 GJ/t of energy [lo]. Smelting accounts for 176.7 GJ/t of this total. However, 60% of world aluminium is produced using hydro electricity [l 11. In accordance with Rule 3. - all renewable sources of materials andenergy are treated as cos, free - the UCF adopted for the production of aluminium was reduced to 153.6GJIt. This is a difficult matter as in some cases production of aluminium may force other electricity consumers to use more fossil fuels; some hydro schemes exist solely for aluminium production. The use of renewable energy sources means that aluminium s~nelting produces less C 0 2 than might otherwise be expected. The production of aluminium however results in the emission of some other powerful greenhouse gases. The electrolysis of alumina (Na3AIF6) is the only known anthropogenic source of two fully fluorinated carbon compounds CF4 and C2F6; estimated global emissions in 1987 were 28,000 and 3,200 tonnes rcspectively [12, 131. Until recently these compounds were not recognised as significant greenhouse gases. The Intergovernmental Panel on Climate Change (TPCC) [I41 now recognises these gascs at having GWP's of 6,500 (CF4) and 9,200 (C2F6) times that of COz. The production of aluminium may thererore be an important contributor to the enhanced greenhouse effect but not because of C 0 2 production. Ozone depleting en~issionswere not considered in the study.

THE IMPLEMENTATION AND CALCULATION ON SPREAD SHEETS Taking account of the methodologies, rules of analysis, appropriate boundaries, conversion factors and engineering data specific to each


262

S. A. KERR et al.

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Energy Consumption Regarding total energy consumption (Fig. 7) the estimated values for the six decommissioning options considered in this paper are all within 8 % (almost certainly within the confidence of error of the analysis), with the exception of partial removal option C1 and complete removal option EII which involve the disposal of the upper or entire jacket in deep water. Options CI and EII exhibit 13% and 43% higher energy consumption than the lowest energy consuming option (partial removal option DI). Options C1 and EII incur high energy costs associated with transporting large quantities of material and replacement costs of jacket materials left at sea. Marine vessel utilisation accounts for between 24% and 40% of total energy consumption depending on the option. Overall C o t Emissions Estimated overall COz emissions exhibit a similar trend to that of energy consumption (Fig. 8). In this case, the spread of values (other than for deep water disposal option EII) was approximately 10% while the deep water option (EII) would result 46% more C 0 2 emissions than the best performing partial abandonment option

B[1

CI

DI

DIV

El

Ell

FIGURE 7 Variation of energy consumption with dccomrnissioning option.

267

DECOMMLSSIONING OFFSHORE PLATFORMS: EIA

(option DI). Again marine vessel utilisation was a key variable accounting for between 25% and 67% or total emissions dependant on the option.


NO, and SO2 Emissions

A greater variation in both NO, and SO2 emissions is apparent between options than is the case with either energy consumption or C 0 2 emissions (Figs. 9 and 10). The partial removal options are within 25% of each other, while the full removal options exhibited between 90% (EI) and 150% (EII) more NO,, and between 60% and 120% more SO2 than the best performing option (partial removal option BII). Both NO, and SO2 emissions are directly related to marine vessel utilisation accounting for up to 88% of emissions in some options. Of the eight aspects of decommissioning that were identified four were found to be particularly significant: (1) marine vessel utilisation; (2) platform running; (3) platrorm materials recycling; and (4) the replacement of platform materials left at sea. Figures 11 and 12 present the energy consumption results for partial removal option BII and complete removal option EI. Both options exhibit similar total energy requirements (BII, 927 TJ and EI, 940 TJ). Complete removal 140

120 100

80

60 40

20 0

BII

CI

Dl

DIV

El

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FIGURE 8 Variation of overall C 0 2 emissions with decommissioning option.

S. A. KERR et al

268


BRccycling Of Rccovcrcd Platform Masrids

BII

CI

I1 I

DIV

FIGURE 9 Variation of NO, emissions with decommissioning option.

0 1311

CI

Dl

DIV

FIGURE 10 Variation of SO2 emissions with decommissioning option

option EI has the highest level of recycling with all materials returned to shore, the option therefore benefits from zero costs associated with the replacement of materials left at sea. However the benefits of recycling are to a large degree offset by increases in costs associated with vessel utilisation and the recycling process itself.

DECOMMlSSIONlNG OFFSHORE PLATFORMS: EIA Vessels

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Relative energy consun~ptionfor partial rernoval option BII.

Relative cnergy consumption for full rc~novaloption EI.

ACCURACY OF RESULTS

I n a study of this nature it is i~npossibleto be 'correct' in any absolute sensc, boundaries must be drawn which ultimately exclude data from the analysis. Equally, a reliance on often conflicting published data from studies with their own differing terms and objectives introduces an clement of uncertainty regarding the 'accuracy' of ally single measure or result. Various authors note the inherent uncertainty of applying general emission standards in case specific studies [15, 16, 171. Likewise the TPCC recognises ~~ncertainties in its C;WP figures of up to i-35%". Boundary delineation and uncertainties surrounding emission factors and GWP figures mean that the absolute values produced in the study should be treatcd with caution. However, consistency in boundary delineation and the application of consistent UCFs mean


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that the study provides a consistent measure of the relative performance of each decommissioning option. The development of a sensitivity analysis of the results and analyses that can adequately reflect uncertainties surrounding calculated values and factors used are problematic in the extreme. For some factors such as GWP levels of uncertainty may have been estimated, for many others this would require separate studies in order to be able to determine even approximate thresholds. A total of 157 constants and unit conversion factors were used. While beyond the scope of this analysis the estimation of uncertainty in ascribed values used in such studies would seem to be a prerequisite, if such analyses are to be used more routinely.

WIDER DISCUSSION OF RESULTS

After making our initial calculations we sought to compare our work with those of other published studies. While many commentators argue that energy analysis is important, in the context of decommissioning oil and gas installations there is little published work. Brindley and Corcoran[l8] (B&C) were first to consider the development of an appropriate methodology in any depth. There is generally good agreement between B&C and our paper regarding vessel fuel consumption. There are, however, some major differences in the calculation of dismantling energy costs and the UCFs applied. In general this paper adopts UCFs which are more favourable to recycling than B&C. It is, however the methodological differences which gives the greatest variation in results. The assumption that underpins our methodology is that all activities have energy costs, the distribution of these costs determining the performance of a decommissioning option. For each option direct, recycling and replacement costs are summed. Increased levels of recycling lower indirect replacement costs but increase recycling and other direct energy costs. The methodology developed by B&C sums replacement and direct energy costs and then deducts 'savings' for recycling. These savings are the cost of replacement less the cost of recycling. We believe that this



27 1

approach double counts the benefits of recycling, once by reducing replacement costs and then again by deducting recycling savings. Figure 13 (which includes a leave in situ option for Heather for comparative purposes only) illustrates the differences between our approach and B&Cs, using the same input data, conversion factors, weights etc. The B&C methodology suggests that there is a clear advantage to full removal compared to partial removal or toppling. Our methodology suggests that on the basis of energy costs the difference between partial and full removal is at best marginal.

CONCLUSIONS This study has shown that for Heather therc is little difference between partial removal options and the best performing complete removal option in terms of energy consumption. The benefits of recycling are to a large extent offset by increased fuel consumption and recycling costs as materials are brought to shore and processed. Partial removal options do appear to be significantly better at reducing acidifying emissions (NO, and SOz) which are directly related to marine vessel utilisation.

Leave

Topple

Panial

Full

Lravc

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Brindlev.0-

FIGURE 13 Comparison o f energy co~lsumptionaccounting methods.

Full


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S. A . KERR

el al.

The study has illustrated how important it is to consider energy costs at the case specific level. Case specific variables, most notably, marine vessel utilisation, are highly significant in the analysis. If this type of study is to become a regular feature of the BPEO process then industry and its regulators must adopt a standardjsed set of rules for these analyses or it will not be possible to make meaningful comparisons between different abandonment options and different studies. We believe that the methodology presented here is the most rigorous and appropriate developed to date. Input data is inherently uncertain and will change with time: new research and new industrial processes will change UCFs; GWP figures will change with advances in atmospheric chemistry; new regulatory regimes and developments in engineering technology will determine which future decommissioning options are available to operators. These changes can be easily accommodated and their impacts examined if a standardised approach to the analysis is adopted. It is important to consider these results in the wider context of the industry, decommissioning and their impact on the environment. It is worth putting the data presented above in some sort or perspective. The difference between the worst and best performing options, in tFrms of energy consumption is 3.8 x lo5 GJ (41 % of the best performing option BII). The oil industry is not in a position to avoid decommissioning; an option muFt be selected. If one option is selected over another then it is the difference in energy cost between the two that has been either incurred or saved. UK oil and gas production over the past ten years has been in the order of 1.5 x 10' tonnes.yrP (oil equivalent). Assuming an energy content for crude oil of 42.3GJ.t-' 1191, then the annual energy production by the UK oil and gas sector is in the order of 6.3 x ~ O ~ G J . ~The ~ - 'difference . between the best and worst performing decommissioning options for Heather is therefore equivalent to 0.006% of the UK oil and gas sector's annual energy output. It may be tempting for the oil industry to over cmphasise the importance of edergy studies, as unlike other forms of environmental analysis, they generate 'hard' numbers (tonnes of emissions, joules of energy etc.) rather than softer impact points on an environmental assessment matrix. The oil industry would, however, be wise to adopt a cautious approach as it may prove politically difficult for companies,

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which are ultimately in the business of selling energy, to argue for one abandonment route over another on the grounds of energy efficiency alone.


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