A historical reconstruction of ships fuel consumption and emissions

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006jd007630, 2007 A historical reconstruction of ships fuel consumption and emissions Øyvind Endresen, 1 Eirik Sørgård, 2 Hanna Lee Behrens, 1 Per Olaf Brett, 1 and Ivar S. A. Isaksen 3 Received 7 June 2006; revised 10 February 2007; accepted 29 March 2007; published 16 June [1] Shipping activity has increased considerably over the last century and currently represents a significant contribution to the global emissions of pollutants and greenhouse gases. Despite this, information about the historical development of fuel consumption and emissions is generally limited, with little data published pre-1950 and large deviations reported for estimates covering the last 3 decades. To better understand the historical development in ship emissions and the uncertainties associated with the estimates, we present fuel-based CO 2 and SO 2 emission inventories from 1925 up to 2002 and activity-based estimates from 1970 up to The global CO 2 emissions from ships in 1925 have been estimated to 229 Tg (CO 2 ), growing to about 634 Tg (CO 2 ) in The corresponding SO 2 emissions are about 2.5 Tg (SO 2 ) and 8.5 Tg (SO 2 ), respectively. Our activity-based estimates of fuel consumption from 1970 to 2000, covering all oceangoing civil ships above or equal to 100 gross tonnage (GT), are lower compared to previous activity-based studies. We have applied a more detailed model approach, which includes variation in the demand for sea transport, as well as operational and technological changes of the past. This study concludes that the main reason for the large deviations found in reported inventories is the applied number of days at sea. Moreover, our modeling indicates that the ship size and the degree of utilization of the fleet, combined with the shift to diesel engines, have been the major factors determining yearly fuel consumption. Interestingly, the model results from around 1973 suggest that the fleet growth is not necessarily followed by increased fuel consumption, as technical and operational characteristics have changed. Results from this study indicate that reported sales over the last 3 decades seems not to be significantly underreported as previous simplified activity-based studies have suggested. The results confirm our previously reported modeling estimates for year Previous activity-based studies have not considered ships less than 100 GT (e.g., today some 1.3 million fishing vessels), and we suggest that this fleet could account for an important part of the total fuel consumption (10%). Citation: Endresen, Ø., E. Sørgård, H. L. Behrens, P. O. Brett, and I. S. A. Isaksen (2007), A historical reconstruction of ships fuel consumption and emissions, J. Geophys. Res., 112,, doi: /2006jd Introduction [2] Over the last 100 years the total fuel consumption and emissions by the oceangoing civil world fleet has significantly increased as the fleet expanded by 72,000 motor ships to a total of 88,000, with a corresponding increase in tonnage from 22.4 to 558 million gross tonnage (GT) (Lloyd s Register of Shipping, statistical tables, 1964 (year 1900), and world fleet statistics and statistical tables, 2000). This growth has been driven by increased demand for passenger and cargo transport, with 300 Mt cargo 1 Det Norske Veritas, Høvik, Norway. 2 Centre of Innovation and Entrepreneurship, Bodø, Norway. 3 Department of Geosciences, University of Oslo, Oslo, Norway. Copyright 2007 by the American Geophysical Union /07/2006JD transported in 1920 [Stopford, 1997] and 5,400 Mt in 2000 [Fearnleys, 2002]. There is a significant delay in building up the concentrations of some of the greenhouse gases (e.g., CO 2 ) and in the climate impact. Knowledge on how ship emissions have developed over time is required to quantify climate effects and trends, and to implement effective regulations. Only limited information has been published relatively to the historical development of fuel consumption and emissions by the world fleet, and even for the last 3 decades the estimates being presented differing significantly. [3] Smith et al. [2004] have reported an emission inventory covering the period before The validity of these estimates may be questioned as the annual coal consumption figures appear to be substantially lower than reported in the literature (coal used for shipping purposes in 1915 was taken to be only 80 Kt instead of the 80 Mt that was published by Annin [1920]). Eyring et al. [2005] have 1of17

2 reported simplified activity-based inventories from 1950 up to After 1992, different approaches and assumptions have been applied to estimate global shipping emissions, but significant differences are apparent among these reported emissions inventories. This study has therefore established an historical time series of marine fuel sales to oceangoing ships from 1925 to 2002, and used a fuel-based approach to estimate the emissions of CO 2 and SO 2. [4] It is an ongoing scientific debate regarding the reliability of marine bunker sale statistics to be used for estimates of fuel-based ship emissions [Corbett and Koehler, 2003, 2004; Eyring et al., 2005; Endresen et al., 2003, 2004, 2005]. Activity-based estimates reported by Corbett and Koehler [2003] and Eyring et al. [2005] are significantly higher compared to historical sales data covering the last decades. Moreover, the variation over time between reported sales and estimated consumption does not correspond. It has been argued that underreporting of sales explains the large differences [Corbett and Koehler, 2003; Eyring et al., 2005]. An alternative explanation could be that changes in structure, technology and activity of the expanding world fleet have to be better captured in the activity-based models. It has recently been questioned if the assumed activity level for the fleet is representative [Endresen et al., 2004], and especially for medium and smaller ships (which dominate by number). This study argues that any activity-based approach must take into account variation in the demand for sea transport and operational and technical changes over the years, to better represent the real fuel consumption and corresponding emissions. To better examine these factors this work has established an improved activity-based modeling approach to estimate fuel consumption for the oceangoing civil world fleet larger or equal to 100 GT. This model is applied to the period 1970 to 2000, and limited to the main engines of the nonmilitary fleet. The estimated fuel consumption is then compared with historical marine sales data, to investigate if marine sales data reported over the last 30 years are representative and could be used as proper bases for emissions estimates. [5] In this study we first present a description and discussion of the developed CO 2 and SO 2 emissions inventories for the period 1925 to 2002 (section 2). We describe the activity-based model developed, the input data applied and the modeled fuel consumption for the last 3 decades in section 3. Section 4 compares the modeled fuel consumption for the last 3 decades with marine sales data. In section 5, major findings are summarized and discussed. 2. Fuel-Based Emissions Inventory: [6] This section presents estimates from 1925 up to 2002 of global CO 2 and SO 2 emissions from oceangoing civil ships by combining estimates for marine fuel sales of coal and oils with their respective fuel-based emissions factors Methodology [7] Detailed methodologies for constructing ship emission inventories based on fuel sales have been published by the Atmospheric Emission Inventory Guidebook [European Monitoring Evaluation Program/Core Inventory of Air Emissions (EMEP/CORINAIR), 2002]. The emissions are calculated by means of E g; j ¼ S j e g; j ; where E g,j denotes the emissions of individual exhaust compound g (g =1=CO 2, g =2=SO 2 ) from burning fuel type j ( j =1=diesel, j =2=heavy fuel, j =3=coal), kg emissions/year; S j denotes the total sales of fuel type j, kg fuel/year; and e g,j denotes the emission factor for exhaust compound g in relation to fuel type j, kg emissions/kg fuel. [8] The emission estimates presented in this study are based on bunker sales data (marine oil and coal) obtained from several sources (section 2.2), and average fuel-based emission factors assumed representative for the different time periods (section 2.3) Marine Sales [9] Data exists for sales of fuel to foreign bound ships from 1925 to 1970, but assumptions have to be made for the national sales. Data for the total marine sale exists from 1971 up to 2002, except for the fishing fleet for which separate sale estimates are made. The sources, assumptions and inventories are presented below for two periods (1) and (2) The sales figures given in bold in Table 1 are used to estimate the emissions for the period. Note that no data is available for the World War II period Period I: [10] The international marine sale figures from 1925 up to 1970 are based on coal and oil statistics reported by Darmstadter et al. [1971] and United Nations (UN) [ ]. These data cover sales to foreign bound ships and aircrafts, irrespective of flag [Darmstadter et al., 1971; UN, ]. It is expected that sales to oceangoing military vessels in international operations are included in the presented inventory. However, no data is available for oceangoing military vessels in national services. This sale is small compared to sales to the nonmilitary fleet, according to estimates reported by Endresen et al. [2003], and is not included in this work. As national fuel sales to the nonmilitary fleet are not included, the total oil and coal sale for this period is estimated on the basis of ratios between national sales versus international, which can be established for years when data is available. The calculated average ratio between IEA sales figures of national bunker (category: internal navigation ) and international bunker (category: international marine bunkers ) figures for the period 1971 to 2002 is calculated to be 0.27 (i.e., national sales is on average 27% of the international sales). However, the estimated national sales do not include the fishing fleet as IEA reports this sale under the agriculture category for this period. A separate estimate for the fishing fleet is given in section and indicates that the fishing fleet consumes some 10% of the total sales. Assuming that these ratios also are representative for the period 1925 to 1970, the total sale (Total) is calculated by means of Total Int þ National þ Fishing ¼ Int þ 0:27 Int þ 0:1 Total: ð2þ ð1þ 2of17

3 Table 1. Reported Worldwide Marine Sales of International Marine Bunkers (Oil and Coal) for the Period and Estimated SO 2 and CO 2 Emissions (Equation (1)) a International Bunkers as Coal, b Mt International Bunkers as Oil, Mt International Total Estimated Bunkers as National Estimated Sales Emissions, Tg Year Coal, b Mt UN EWE UN EWE IEA c Sales, Mt as Oil, d Mt SO 2 CO a The sales data are reported by Darmstadter et al. [1971], UN [ ], and IEA [ ]. The sales figures given in bold are used to estimate the emissions for the period. b Coal equivalents, based on UN [ ] for c Only the category international bunkers. d Oil equivalents, using 1/1.416 as conversion factor from coal to oil equivalents [UN, 1998]. [11] Table 1 shows the estimated total figures for the period 1925 up to 1970, as well as the reported international sales (Int) used as input to equation (2) Period II: [12] Data for the total marine fuel sales exists for 1971 up to 2002, but this does not include the fishing fleet. The applied sales figures from 1971 up to 2002 are based on oil statistics reported by International Energy Agency (IEA) [ ]. Our data is based on IEA categories international marine bunkers and internal navigation. The sales to oceangoing military vessels are expected to be included, as IEA defines the international marine bunkers to cover those quantities delivered to seagoing ships of all flags, including warships [IEA, 2001]. The fishing fleet sale is reported in the IEA category agriculture [IEA, 2001]. It is not possible to extract sales to fishing vessels from this overall category and these sales figures are therefore estimated from the 34,379 MW main engine power that was reported to be installed in the 1.3 million decked fishing vessels in 1998 [Food and Agriculture Organization of the United Nations (FAO), 2006a]. This represents nearly twice the power reported by Corbett and Koehler [2003] (18,474 MW) for the fishing fleet larger than 100 GT, and about 13% of the total main engine power of the oceangoing civil world fleet larger than 100 GT. We may assume that the main engine power installed is roughly proportional with the sales to the fishing fleet. The entire fishing fleet consumption in 1998 is then calculated to be of the order 3of17

4 Figure 1. Development of the world fleet of oceangoing civil vessels above or equal 100 GT and transport work, (not including the military fleet). (left) Development of size and tonnage (Lloyd s Register of Shipping, statistical tables, 1964 (year ), and world fleet statistics and statistical tables, 1992 (year ), 1995 (year ) and 2000 (year 1996 and 2000). (right) Development of average size (including also noncargo ships) and transport work (Btm, billion tonne-miles) [Stopford, 1997; Fearnleys, 2002]. Note that no data are available for World War II. 10% of the total sales by assuming less days at sea for the fishing fleet compared to the cargo ships [Endresen et al., 2004] and by taking into account that some of the reported IEA sales include military vessels. This study assumes that the estimated 10% consumption by the entire fishing fleet in 1998 is representative for the period Lloyd s fleet data for the years 1970 and 2000 shows little change in the relative fraction by numbers of fishing vessels, but the relative tonnage fraction was higher and the average vessel size was lower in 1970 than in Global fishing fleet data up to 1970 is not available, but the fish catch statistics indirectly support our assumption. Fish catches in 1938, 1950, 1970 and 2000 within marine waters was 19.3 Mt (reported by FAO, figures given by Aschehougs Konversasjonsleksikon [1956]), 17.3 Mt, 59 Mt and 88 Mt [FAO, 2006b], respectively and this development corresponds well with the total fleet development over this period (Figure 1). On the basis of this assumption, the total sale (Total) is calculated by equation (2). Table 1 shows the estimated total figures for the period 1971 to 2002, as well as the reported international (Int) and national (National) IEA sales data used as input to equation (2) Discussion of the Data Sources [13] We find a good match between international oil fuel sales reported by Darmstadter et al. [1971] and IEA [ ] with reference to 1965 and 1971, respectively. The match between international oil fuel sales reported by UN and IEA is generally good for the period 1971 to 1978, and by UN and EWE from 1953 to 1965 (Table 1). However, the 1977 and 1978 sales reported by UN are lower than IEA data. The IEA data does not include fuel sales to aviation, indicating that for this time period the reported fuel sales to aircraft is likely to be insignificant in both the EWE and the UN data. This is supported by the fact that airplanes do not use coal or heavy fuel oils, and that EWE states that the reporting of aviation fuel sales appear to be incomplete. Thus we assume that the aviation fuel included in UN and EWE data is small or negligible, and that the sources give representative data for sale to international shipping. It is important to recognize that IEA and UN define international bunker differently. UN includes only sales to foreign bound ships, while IEA defines the international marine bunkers to cover those quantities delivered to seagoing ships of all flags, including warships [IEA, 2001]. The good match between international bunker sales reported by UN and IEA indicates that the IEA international bunker category mainly includes sale to ships in international operations Emission Factors for CO 2 and SO 2 [14] We have used a factor of 3.17 tonnes CO 2 per tonne oil consumed [EMEP/CORINAIR, 2002] and 2.58 tonnes CO 2 per tonne coal consumed to model emissions to air. The emission factor for coal is calculated by combining the molecular weight ratio of CO 2 to C (44/12), with tonnes C/tonne fuel [Society of Naval Architects and Marine Engineers (SNAME), 1983]. The sulphur content varies over time, as pointed out by Endresen et al. [2005]. We have assumed a global average of 2.5% sulphur for marine oils up to From 1971 up to 1995, a global average of 2.7% sulphur is assumed for heavy fuel and 0.5% for distillates. Global values reported by Endresen et al. [2005] are applied from 1996 to For year 2002 global weighted sulphur contents (heavy fuel and distillates) are applied [Endresen et al., 2005]. The globally weighted average content for heavy fuels is found to be 5% higher than the average (arithmetic mean) sulphur content commonly used. The likely reason for this is that larger bunker stems are mainly of high-viscosity heavy fuel, which tends to have higher sulphur values compared to lower-viscosity fuels [Endresen et al., 2005]. Smith et al. [2004] reports a sulphur content in coal of 1.1%, and we assume this value to be representative for the entire period. The relation between burned sulphur and generated SO 2 is 2.0 kg SO 2 /kg S (derived from the chemical equation [Lloyd s Register of Shipping, 1995; EMEP/CORINAIR, 2002]) Modeling of Ship CO 2 and SO 2 Emissions [15] Figure 2 shows the calculated CO 2 and SO 2 emissions (equation (2)) using the sale numbers (section 2.2) in combination with the emission factors per tonne oil and coal 4of17

5 Figure 2. Development of CO 2 and SO 2 ship emissions, based on estimated sales of marine fuel (Table 1), (including the fishing and military fleet). Note that no data are available for World War II. consumed (section 2.3). Ship emissions are estimated at 229 Tg (CO 2 ) in 1925, growing to about 634 Tg (CO 2 )in The corresponding SO 2 emissions are 2.5 Tg (SO 2 ) and 8.5 Tg (SO 2 ) Factors Determining Development in Worldwide Fuel Sales [16] This study expects that the demand for sea transport, technical and operational improvements as well as changes in the fleet composition and size will explain most of the development in fuel consumption by the fleet during the last 100 years. The coal sale peaked in 1913 (80 Mt coal is reported by Annin [1920]), and dominated up to around 1920, as the fleet grew and steamers replaced sail ships (Table1)[Fletcher, 1997]. After 1920, the oil sale started to dominate the emissions, as coal gradually was replaced by marine oils, following the shift to diesel engines and oilfired steam boilers (Table 1) [Fletcher, 1997; Corbett, 2004]. Increased focus on fuel economy [Kofoed, 1926], and a shift from coal to oil, combined with depressions in both the world economy and the sea borne trade in the Figure 4. (top) Fleet data for cargo ships versus noncargo ships, (middle) trade volumes of oil and dry bulk versus average haul for oil tanker and dry bulk ships, and (bottom) fleet productivity for cargo ships (tanker and cargo fleet) versus non trading tonnage. Figure 3. Reported IEA [ ] sales of marine oil products (Mt) worldwide (including the IEA categories international marine bunkers and internal navigation, but not sales to the fishing fleet) versus world sea borne trade (Mt cargo), [Stopford, 1997; Fearnleys, 2002]. 1930s [Stopford, 1997] (e.g., some 21% of the fleet was out of service in 1932) partly explain the gradual reduction in sales from around 1925 (Table 1) toward the Second World War. [17] Table 1 illustrates the significant increase in marine sales after the Second World War and up to The main reasons for this significant growth was the demand for sea transportation, as the sea borne trade grew from around 500 Mt in 1949 to 3,233 Mt in 1973 [Stopford, 1997] (i.e., more than a sixfold increase). The growth in sea borne transport was not reflected by a corresponding growth in the fleet by vessel numbers (i.e., a twofold increase) or tonnage (3.5-fold increase), indicating the influence of modern, larger and more efficient cargo ships, with improved cargo handling in ports. For instance, the volume of 5of17

6 Figure 5. (top) Daily main engine fuel consumption, (middle) installed main engine power, and (bottom) operation speed for crude oil tankers with diesel engines built in the periods 1956 to 1979 and 1980 to 2005 [LRF, ]. cargo transported per tonne fuel sold significantly increased in this period (see Figure 3). Note that the transports of passengers, important up to around 1960, have had an influence on the volume transported per tonne fuel sold. We expect that passenger ships, the largest ship type in the fleet up to around 1960 [Aschehougs og Gyldendals store Norske Leksikon, 1999], account for some of the coal consumption for this period and before. For instance, the annual number of European emigrants transported to the US increased from 350,000 around 1890 to around 1.4 million in 1910 [Aschehougs Verdenshistorie, 1982]. The largest of the old passenger liners such as the Olympic and the Titanic burned on average 620 tonnes of coal per day at 21.7 knots (Encyclopaedia Titanic, Daily fuel consumption for Titanic and Olympic, available at org/discus/messages/5919/6509.html, visited 2005). [18] The marine sales decreased from around 1973 to 1983, followed by a nearly steady growth up to 2002 (Table 1 and Figure 3). The main reasons for the decrease were the slowdown in world sea borne trade (Figures 3 and 4), the reduction in sailing distances (Figure 4), improved energy efficiency of the fleet (e.g., phasing out of steam ships) and a reduction in speed (and installed power) within some dominating segments (Figure 5). World economy generates most of the demand for sea transport, through either the import of raw materials for manufacturing industry, or trade in manufactured products [Stopford, 1997]. Figure 3 illustrates that development in bunker sale follow development in seaborne trade with a correlation of r = 0.92 for the period 1975 to The correlation between transport work (measured in tonne miles, Figure 1) and fuel sale for the same period is even better (r = 0.97) (Figure 6, right), indicating that the average length of haul, not surprisingly, is affecting the sales. However, the correlation between these variables is lower (r = 0.88) for the period 1971 to 2000 (Figure 6, left), indicating that other factors also were important, especially between 1971 and For instance, the transport work by the old passenger fleet is not included, as well as effects of changed operational speed and shift to diesel powered ships. The typical operational speed has also varied widely over time, which significantly influences the power requirements. For example Very Large Crude oil Carriers typically operated at 10 knots when freight rates were low in 1986, but this increased to 12 knots when the rates were higher in 1989 [Stopford, 1997]. A reduction in the average operating speed by 2 3 knots below design speed may halve the daily fuel consumption of the cargo fleet [Stopford, 1997; Wijnolst and Wergeland, 1997]. Lloyd s fleet data [Lloyd s Register Fairplay (LRF), ] also indicates a reduction in installed power and operational speeds for diesel powered crude oil carriers built after 1980, followed by a significant reduction in fuel consumption (Figure 5). The annual fuel consumption by the fleet is also strongly affected by the installed propulsion systems (engine, gear, shaft, propeller arrangement), as modern diesel engines have about half the daily fuel consumption compared to the old inefficient steam engines with the same power outtake (Table 2). The shift to modern marine diesel engines has typically occurred in periods with high oil prices. For instance in 1961, there were still over 10,000 steam engine powered ships and 3,536 steam turbine powered ships in operation (36% by number) (Lloyd s Register of Shipping, statistical tables, 1961). By 1970 these numbers had decreased to 4,425 and 3,534 ships respectively (15% by number) (Lloyd s Register of Shipping, statistical tables, 1970). By 1984 only 1,213 ships were steam engine powered and 1,743 turbine powered ships (4% by number) remained in service (Lloyd s Register of Shipping, statistical tables, 1984). [19] The decrease in marine sales from around 1973 is also explained by the decline in both sea borne transport of oil (represented about 50% of the sea borne trade) and the average sailing distances (Figure 4, middle). For instance, 6of17

7 Figure 6. Correlation between reported IEA [ ] sales of marine oil products worldwide (Mt) (including the IEA categories international marine bunkers and internal navigation, but not sales the fishing fleet) and transport work (billion tonne miles (Bmt)) [Stopford, 1997; Fearnleys, 2002]. (left) Period The correlation is (right) Period The correlation is crude oil tankers reached a peak in productivity in 1972 (measured in tonne miles per deadweight (total carrying capacity)). By 1985 this had nearly halved, and a few years later it increased by 40% [Stopford, 1997]. More efficient and specialized ships have also pushed their way into the marked (e.g., the first deep sea cellular container ship in 1965 [Stopford, 1997]). The specialized ships have different operational and technological characteristics, which results in a particular logistic efficiency and energy and emission profiles. For example, passenger ships have on average 2.2 main engines per ship, while the large passenger ships have 5.7 main engines per ship (greater than 100,000 GT) [LRF, 2005]. The civil world fleet have on average 1.3 main engines per ship (above or equal to 100 GT) [LRF, 2005]. Engine capacity is not likely to be fully exploited at all times, resulting in a lower fuel consumption in practice than what might be expected on the basis of the power installed. It is reasonable to expect that these effects are likely to have different impacts on the total fleet within the period and so we have included most of them in the fleet modeling approach outlined in section 3. Table 2. Reported Specific Fuel Consumption (SFC) for Different Engine and Fuel Types Reported SFC Engine Type Ib./S.H.P.h a g/kwh Diesel ships 0.36 b 0.47 c d,e Turbine Oil 0.75 b d,e Coal b 2.4 c,f Steam engine Oil 0.9 b 700 d Coal 1.35 b 1.54 c a Ib-pound = kg; S.H.P., shaft horse power; 1 H.P = kw; h, hour. b Le Mesurier and Humphreys [1935]. c Baker [1915]. d SNAME [1988], average figures. e Cooper [2002]. f At low speeds, while for high speeds 1.2 Ib./S.H.P.hr Uncertainty [20] The uncertainties in our estimated sales figures are significant. Reliable inventories are probably best developed by comparing the different modeling approaches and the different data sets that are available. This extends our knowledge base and improves our understanding of the governing processes. The inventory presented in this study aims to cover sales in oil equivalents to all oceangoing ships worldwide (Table 1). However, the marine sales for the period 1925 to 1970 is likely to be slightly underestimated, as sales to the oceangoing military fleet in national services are not included. Our best estimate today is that the data after 1970 should be within a range of ±15%, while the data before 1970 should be within a range of ±25%. Underreporting by some countries has been [Endresen et al., 2003] and may still be a problem. The national sales reported by IEA from 1971 to 2002, also include sales to smaller ships operating on inland waterways. This fleet is reported to account for 42,000 engine powered ships and 38,000 push-towed vessels in 1992 [Organisation for Economic Cooperation and Development, 1997]. The engine powered ships are small (300 Dwt) and represent around 2% of the cargo capacity of the oceangoing fleet. Thus it is assumed that the sale to this segment is small. [21] The variation in carbon content for marine oil products is small [Energy Information Administration, 1994]. The uncertainty in the average CO 2 emission factor for marine oils is less than ±5%. This is in line with Skjølsvik et al. [2000]. The assumed carbon content in coal for marine purposes is reported to be 70.4% [SNAME, 1983]. We expect the error in the assumed carbon content for coal to be within ±10%. Taking into account uncertainties in sales numbers, it is expect that the uncertainties in the CO 2 emissions after 1970 should be within a range of ±20%, while the estimates before 1970 should be within a range of ±30%. Limited data exists for average sulphur content of marine oil up to This is supported by the fact that the use of residual fuel in marine diesel engines dates to the 1940s. Prior to the 1940s, residual fuels for navigational purposes had been used by steam ships [Cullen, 1997]. We 7of17

8 Table 3. Input Data to Equation (3), Modeling Main Engine Fuel Consumption for the World Fleet of Oceangoing Civil Ships Larger or Equal to 100 GT, Period (Presented per Decade) a Parameter Variable Time at sea, days t Average main engine size, kw p Average engine load ( ) m Number of active ships (10 3 ) N % main engine powered by Diesel n 1, Steam, oil n 2, Steam, coal n 2, Average SFC b at sea for main engine, g/kwh Diesel b 1, Steam, oil b 2, Steam, coal b 2, a Note that yearly input data is used in the modeling. b SFC, specific fuel consumption. judge the error in sulphur content in oil to be within ±20% up to 1990, and within ±10% after It is expected that the total uncertainties in the SO 2 emissions after 1990 should be within a range of ±20%, while the estimates before 1990 should be within a range of ±30%. We realize that the modeling is based on a number of assumptions regarding average sulphur and carbon content (coal) and the results should therefore be interpreted carefully. 3. Activity-Based Fleet Modeling, [22] This section presents an improved activity-based modeling approach that uses high-resolution time series as input data to estimate fuel consumption for the oceangoing civil world fleet larger or equal to 100 GT. Modeling is made for the period 1970 to 2000, and only for the main engines. Historical data available is limited, and do not allow for detailed modeling, such as the baseline approaches reported by Corbett and Koehler [2003], Endresen et al. [2003] and others (e.g., breakdown on ship types and sizes). Simplifications and assumptions are therefore made. However, compared to past activity-based modeling studies [Eyring et al., 2005; Corbett and Koehler, 2003], this study has developed and applied a more detailed approach, which includes the variation in the demand for sea transport, as well as operational and technological changes Modeling Approach [23] The average ship size approach applied in this study assumes an equal size for the total number of ships in the world fleet of oceangoing civil ships larger or equal to 100 GT, and calculates the average size of the ships by dividing total tonnage with the total number of ships. The fuel consumption for an average ship is estimated on the basis of average characteristics of installed main engine power, main engine load, bunker fuel consumed per power unit (kw) (depends on propulsion and fuel type) and days at sea (based on demand for sea transport). The fuel consumption is calculated separately for the diesel and steam ships, as steam ships have a significantly higher fuel consumption (Table 2). The main engine fuel consumption for the period for all oceangoing civil ships above or equal to 100 GT is then estimated by F i; s ¼ b i; s t p m per ship n i; s N number of ships ; ð3þ where F i,s denotes the total fuel consumption of average ships with main engine type i burning fuel type s, kg fuel/ year; b i,s denotes the average specific fuel consumption for an average ship with main engine type i burning fuel type s, kg fuel/kwh; t denotes the average number of operating hours at sea per year for an average ship (see equation (5), below), h/year; p denotes average installed main engine power for an average ship, kw; m denotes the average main engine load for an average ship; n i, s denotes the fraction of the average ships in the fleet with main engine type i burning fuel type s (see equation (4), below); and N denotes the total number of active average ships in the fleet (i.e., not laid up and used as storage). [24] Input data for the different terms in equation (3) is described below and given in Table 3. It should be noted that table figures are only given every tenth year, while the numbers applied in our modeling are given year by year Number of Active Average Ships, Diesel and Steam Powered [25] As outlined above, the number of average ships corresponds to the total number of ships reported in the world fleet of oceangoing civil ships above or equal to 100 GT (Figure 1, left). The total number of ships and GT in the fleet from 1970 to 2000 is based on fleet statistics (Lloyd s Register of Shipping, world fleet statistics and statistical tables, 1992 (year ), 1995 (year ) and 2000 (year 1996 and 2000)). The fraction of the average ships (n i, s ) in the fleet with main engine type i burning fuel type s is calculated on the basis of the following equation: n i; s ¼ D i; s D 1;1 þ D 2;1 þ D 2;2 ; where D i,s denotes the total tonnage in the fleet with engine type i burning fuel type s, GT. [26] The tonnage in the fleet with engine type i burning fuel type s is based on yearly fleet data from Lloyd s Register of Shipping (statistical tables, and ). From 1985 to 1992 the yearly fleet data is only available for the steam powered tank and bulk fleet (Lloyd s Register of Shipping, statistical tables, , and world fleet statistics and statistical tables, 1991, 1992). However, we have assumed that the tanker and bulk fleets are representative for the total steam tonnage, as they were the dominating ships by tonnage. Fleet data is not available for the steam powered segment from 1993 onward, and the changes in the steam tonnage are estimated by interpolation between the 1992 tonnage and the actual tonnage in 2004 [LRF, 2005]. The percentage of the coal fired tonnage is available from Corbett [2004] up to Detailed fleet data from Lloyd s is available for the years 1961, 1962 and 1963 (Lloyd s Register of Shipping, statistical tables, ), but no data is available thereafter according to our information. A linear reduction from 3.3% of the coal fired tonnage in 1963 to zero in 1979 is assumed, on the basis of the development of coal sales (Table 1). It ð4þ 8of17

9 Table 4. Input Data to Equation (5), Modeling Time at Sea for the World Fleet of Oceangoing Civil Ships Larger or Equal to 100 GT, Period (Presented per Decade) a Parameter Variable Average haul, b nm l Average speed, c knots v Total GT (10 6 ) Average ship size, Dwt d Seaborne transport, d Mt q Utilization of Dwt ( ) h Ballast factor ( ) a a Note that yearly input data is used in the modeling. b The average haul length (l) from 1970 to 2000 is based on Fearnleys [2002] and Stopford [1997]. c Based on data collected from Lloyd s fleet database. d Based on Fearnleys [2002] and Stopford [1997]. should be recognized that other engine types are represented in the oceangoing civil world fleet (e.g., gas turbine), but these types are negligible by tonnage and number when compared with steam and diesel. Thus we have not taken them into account in this study. [27] The number of average active ships (N n i,s ) is calculated by subtracting the number of average ships corresponding to the nontrading tonnage. The number of ships out of service is distributed on engine and fuel categories according to the relative tonnage of the fleet with main engine type i burning fuel type s. The development in laid up tonnage and tonnage used for storage from 1970 to 1995 is based on Stopford [1997], and on data from Fearnleys [2002] thereafter. The number of active average sized steam ships (N n 2,s ) will then reflect the total amount of tonnage (in service) for the steam powered fleet (D 2,s ), while number of active average sized diesel ships (N n 1,1 ) will reflect the total amount of tonnage (in service) for the diesel powered fleet (D 1,1 ) Time at Sea [28] The operational profile is derived by combining yearly fleet and trade data. This deviates from recent modeling studies that estimate days at sea on the basis of engine manufactures data for large engines and tracking studies. The development of the yearly total number of days at sea sailing with cargo for the fleet is estimated on the basis of number of voyages required for an average cargo ship (with an average utilization of cargo capacity) to transport the yearly reported worldwide total cargo volumes, combined with average voyage time calculated from reported average length of haul and assumed average operational speed. We have only considered the cargo carrying fleet when estimating days at sea, and assumed that days at sea for cargo and noncargo ships are equal. Noncargo ships normally have less days at sea as indicated by Endresen et al. [2004], and this assumption may bias our fuel consumption estimates toward being too high, compared to the real situation. The development of the yearly average number of days at sea (t) (with cargo and in ballast condition) of the active average cargo ships is calculated by means of t ¼ 1 þ a N l v voyage time q d h ; ð5þ number of voyage where q denotes the total yearly sea borne trade of ships in the fleet, tonnes/year; l denotes of average length of haul (with cargo), nautical miles (nm); d denotes average dead weight tonnage (Dwt) of ships in the fleet, Dwt; v denotes the average operational speed of ships in the fleet, nm/h; h denotes the average utilization of cargo capacity of ships in the fleet, tonnes/dwt; and a denotes the ballast factor, defined as the average number of days in ballast relative to days sailing with cargo. [29] Input data for the different terms in equation (5) is described below and given per decade in Table 4. Average operational speed (v) is derived from data collected for 37,193 cargo and passenger ships in the Lloyd s 2006 fleet database [LRF, ]. The average operation speed for a given year is based on data for all ships the actual year and all older ships in the database. The total yearly reported sea borne trade volumes (q) for 1970 to 1995 are based on Stopford [1997], while data for the period 1996 to 2000 are based on Fearnleys [2002] (Figure 3). The average sailing distances (l) are calculated by dividing the total transport work (tonne miles) by the total sea borne trade volumes. From 1970 to 2000 these data are based on Fearnleys [2002] and Stopford [1997]. [30] Relatively little information is available about the average time spent in ballast. Ideally, a ship should complete all voyages with cargo. However, many trades require return voyages without cargo. For example, a crude oil tanker typically transports a single cargo load between two ports, then returns to its point of origin or another port without cargo. Wijnolst and Wergeland [1997] indicates that it is not likely for the tanker fleet that the average ballast factor will be less than 0.8, and it will hardly ever exceed 1. Tracking data reported for 453 Very Large Crude oil Carriers in 1991 illustrated a ballast factor of Other ship types such as general cargo and container ships, often sail with some cargo and some ballast and have limited time in ballast, but are frequently not fully laded. For about 100 smaller cargo ships operating on a regional basis (most ships around 3,500 Dwt), the ballast factor is reported to be around 0.2 [Wilson EuroCarriers, 2005]. In the modeling, these ships are forced to transport all cargo nearly fully laden, and then spend time in ballast. On an annual basis, it is expected that this simplification, may give representative number of days at sea, as well as cargo volumes transported. Taking into account the fact that larger ships dominate the transported volumes at sea [Stopford, 1997], the average ballast factor (a) is assumed to be 0.7 (i.e., days with ballast is 70% of the number of days with cargo) for all ship types. [31] Wijnolst and Wergeland [1997] reports that utilization of cargo capacity in practice hardly exceeds 0.95, but may become as low as The utilization of cargo capacity of bulk ships and oil tankers larger than 50,000 Dwt in 2001 is reported by Behrens et al. [2003] to be 0.91 and 0.87, respectively. These ships account for almost 55% of the sea borne trade in 2000 [Fearnleys, 2002], and will of course then have a large impact on the average factor. We acknowledge that general cargo and container ships have a lower utilization of the cargo capacity, and that the average density of cargo could make volume the limiting factor. For instance, data reported by Johnsen [2000] for a dry cargo ship with carry capacity of about 5,200 tonnes, indicate that 9of17

10 the ships on average were loaded at 4,000 tonnes. Of this, the average exploited capacity is reported to be 76% (considering different trading routs). Data for container ships serving U.S. trade [PIERS/Journal of Commerce, 2005], indicate a utilization rate of the container capacity of about 70% on average. Important to note is that the utilization rate of the container capacity will typical be higher than the average utilization of the cargo capacity. Clearly the utilization of cargo capacity varies for different ship types, as well as for year considered (depending on the marked). We assume that 0.8 is representative for the average utilization of the cargo capacity (h) for all cargo ships types Specific Fuel Consumption and Engine Load [32] The main engine specific fuel consumption (b i,s )is based on data reported in the literature and engine data reported for individual ships. The main engine specific fuel consumption for an average diesel ship (b 1,1 ) is estimated for different periods (up to 1970; up to 1980;...;upto 2000), combining installed main engine power and the daily fuel consumption reported for 16,465 diesel powered oceangoing civil ships in the Lloyd s 2004 fleet database [LRF, 2005]. A similar calculation is made for the 770 steam turbine powered cargo and passenger ships running on oil [LRF, ]. The daily fuel consumption is normally given at full power (85% MCR: Maximum Continuous Rating). We have therefore assumed in the calculation of specific fuel consumption (Table 3) that utilized power is 85% of installed power. [33] It is important to recognize that no differentiation is made between steam engines and turbine engines. We have defined average steam ships using oil (b 2,1 ) or coal (b 2,2 ). Steam engines have higher fuel consumption compared to turbine engines, as illustrated in Table 2. However, limited data is available on specific fuel consumption for old steamers, and especially for the steam engines. Both steam engines and steam turbines burning coal were phased out of the fleet around We have therefore established typical constant averages specific fuel consumption for these ships. Riksheim [1982] compared the fuel consumption of a diesel powered ship of 142,000 Dwt delivered in 1981, with a coal fired steam turbine ship with the same hull dimension, shaft power and services speed. He reported that the coal fired solution used more than 3 times the fuel by weight, compared to the diesel solution (64 tonnes oil per day versus 198 tonnes coal per day or 140 tonnes of oil equivalents). The data in Table 2 also illustrates that turbines running on coal typically use times more fuel per unit power production, compared to the diesel engines. Thus we assume for all years that turbine engines fuelled by coal consume 800 g/kwh. [34] Table 2 indicates that steam engines running on oil consume 700 g/kwh, while steam engines using coal have slightly higher fuel consumption than turbine engines using coal. Thus, for steam engines using oil and coal we have assumed a fuel consumption of 700 g/kwh and 900 g/kwh, respectively. For average steam ships using oil (b 2,1 ) or coal (b 2,2 ), we have estimated the specific fuel consumption using these specific consumptions with a weighting according to the tonnage. In 1961, the steam engines accounted for 41% of the steam tonnage (Lloyd s Register of Shipping, statistical tables, 1961). This is reduced to 8% in 1970 (Lloyd s Register of Shipping, statistical tables, 1970), 2% in 1984 (Lloyd s Register of Shipping, statistical tables, 1984) and assumed 0% in The estimated weighted specific fuel consumptions (b 2,1 ) and (b 2,2 ) are then applied in the modeling. Table 3 shows the applied averages (only given per decade). [35] We assume an average main engine load of 70% MCR, when including slow cruise, port maneuvering and ballast sailings. This assumption is based on recommendations by Endresen et al. [2003, 2004] and Corbett and Koehler [2004]. The average engine load (m) is assumed for all ships and all years. However, the average main engine load and speed varies a lot for different ship types. For instance, Flodström [1997] reports an average load of 80% MCR based on data from 82 ships. Bulk vessels tend to have slightly lower average values (72% MCR), while tank vessels have higher (84% MCR). The load was ranging from about 60% MCR up to 95% MCR for the 82 different ships. This illustrates large variations in required engine output and average operational speed Installed Main Engine Power [36] From 1978 to 2000 the yearly average main engine power (p) is estimated from Lloyd s fleet database [LRF, 2005]. The average installed power for a given year is calculated using a similar approach as for operational speed. Up to 1978 the change in average power is assumed to follow the development for ships larger that 2,000 Dwt reported by Eyring et al. [2005] (based on data from UK s trade magazine The Motor Ship ). We believe that this better represents the development of average power in the fleet up to 1978, compared to application of detailed fleet data available, where larger merchant ships could be underrepresented because of scrapping after some years (not included in the fleet database). Our calculated average power figures per ship correspond to the data reported for 1990, 1995 and 2000 by Eyring et al. [2005]. However, our estimate for 1980 is about 10% lower, compared to Eyring et al. [2005] Modeled Fuel Consumption [37] Figure 7 shows the modeled fuel consumption by means of equation (3) for the world fleet from 1970 to Our results show that the fuel consumption can be modeled by including the major changes in technology, fleet structure and operational factors. Our modeling differentiates on engine and fuel types, and Figure 7 (as well as Figure 8) illustrates that these effects are important around 1970, and should be included in historical fleet modeling. The increased fuel consumption from 1970 to 1973 (1974) is explained with increased demand for sea transport (i.e., number of days at sea, Figure 9), as well as long sailing distances with cargo (Figure 4, middle). The latter have reduced number of port calls and consequently reduced total time in port (more time at sea). For the tanker fleet, transporting 50% of the sea borne trade around 1970, the average distance was about 6,600 nautical miles (nm) in the mid 1970s, before rapidly falling to a level of 4,600 nm in the mid 1980s (Figure 4, middle). The reasons was that after the oil crisis in 1973, the USA in particular, but also European importers, started to import more from other, Non-OPEC sources, or increased their own production as was the case for United Kingdom and Norway [Wijnolst and Wergeland, 10 of 17

11 Figure 7. Modeled main engine fuel consumption for the oceangoing civil world fleet above or equal to 100 GT separated on main engine and fuel types, (not including the military fleet). The modeled fuel consumption (black) is mostly lower than the estimated worldwide marine bunker sales (red) (including the entire fishing fleet). 1997]. The variation in trade patterns for oil is therefore an important factor determining demand for crude oil transportation, as well as fuel consumption by this fleet segment and the entire fleet. [38] The results show that growth in the fleet is not necessarily followed by increased fuel consumption. For instance, the stagnation and decline from around 1973 to 1983 in fuel consumption is explained with decrease in number of days at sea (Figure 9), combined with reduction of average sailing distances (oil tankers), increasing number of ships laid up (Figure 4) and the rapid shift to diesel powered ships. The stagnation and decline around 1973 in oil transportation (Figure 4), the largest individual commodity trade by sea, is explained with the very high oil prizes resulting in shift back to coal as fuel for power stations [Stopford, 1997]. Around 1985, when the laid up tonnage was reduced and most of the ships in the fleet were diesel powered, the stagnation in fuel consumption can mainly be explained by a slow down in sea borne trade (Figures 3 and 4). After 1990, the fuel consumption has almost followed the development in sea born trade and fleet growth (Figure 4 versus Figure 7). Figure 8. Sensitivity analyses, considering alternative input data. Modeling is made for all oceangoing civil ships above or equal to 100 GT, of 17

12 Figure 9. Estimated average number of days at sea for the oceangoing cargo fleet, 1970 to Discussion [39] Our results indicate that better activity data on a yearly basis over time is required when fleet modeling is used to determine the actual fuel consumption for the entire world fleet. If our method for estimating days at sea is extended to cover the main cargo shipping segments separately, we expect that the uncertainty will be significantly reduced. However, the method fails for service ships (noncargo). Yearly tracking data (e.g., movement data available) for such vessels would then increase the reliability in model results. Lack of actual service speed data for the fleet significantly influences the uncertainty as the specific consume is speed-dependent and because the service speed defines number of days at sea required to carry out the transport demand. It is recommended that reported days at sea are applied with care since the model results are particularly sensitive to this parameter. [40] We expect that in the future the actual service speed will be estimated on the basis of AIS (automatic identification systems) tracking data for individual oceangoing ships. Such data also makes it possible to indirectly estimate the engine power utilization per ship (and for fleet segments) by combining recorded service speed with installed main engine power for each individual ship (available from Lloyd s fleet databases). AIS is primarily an anticollision system, and is designed to be capable of automatically providing position and identification information about the ship to other ships and to coastal authorities [U.S. Coast Guard, 2002]. The International Maritime Organisation (IMO) requires AIS to be fitted aboard all international ships of certain size. Dalsøren et al. [2007] also report that in the future local and regional ship emission inventories (geographical distribution of emissions) will be based on AIS statistics. [41] The model has been applied to periods before 1970 as well. However, the presented approach fails around 1960, when the world fleet still transported large numbers of passengers (see equation (5)). It was not until 1958 that airplanes transported more transatlantic passengers than large passenger ships [Hansen, 2004]. Another problem is the fact that about half the US fleet of 28 million GT was laid up in This was the US reserve fleet and represented some 17% of the total world fleet by tonnage [Aschehougs Konversasjonsleksikon, 1951]. Also, historical shipping data for this period is limited. This illustrates some of the challenges that have to be considered when modeling fuel consumption before Comparison With Other Studies [42] Our new estimate for year 2000 is some 30 Mt higher than previously reported by Endresen et al. [2003] (Table 5). The main reason for the deviation, is that our new modeling estimates also include about 45,000 noncargo ships, not considered by Endresen et al. [2003]. It is interesting that these two models using different approaches and data sets, Figure 10. Distribution of main engine (ME) power (%) within the predefined size categories for the year 2004, cargo and noncargo fleet [LRF, 2005]. 12 of 17