[1] A reliable and up-to-date ship emission inventory is essential for atmospheric scientists quantifying their impact and policy makers implementing regulations and incentives. However, significant differences are apparent among the various global ship emission inventories. The amounts emitted from ships to the atmosphere are largely determined by the bunker fuel consumption. Corbett and Koehler [2003] have a model estimate that is ∼100 megatons (Mt) higher (∼289 Mt), compared with a recent estimate by Endresen et al. [2003]. The main reasons for the discrepancy between the model estimates are identified in this study, and the effect of the differences in input data are discussed and quantified. This study also substantiates our previous suggested estimates that correspond with updated total bunker sale statistics. [2] The global bunker fuel sales and the figures reported by Endresen et al. [2003] are all significantly lower than those reported by Corbett and Koehler [2003]. The world marine bunker sale statistics for 2000 indicate a world bunker fuel consumption of around 180–190 Mt (see Appendix A). Of this, ships in international trade are responsible for 140–150 Mt. The Energy Information Administration (EIA) total bunker figures correspond well with the International Energy Agency (IEA) sales, except for a few important countries (EIA, International petroleum information, in “Crude oil imports and exports, with most countries and world total 2000,” available at http://www.eia.doe.gov/emeu/international/petroleu.html#IntlTrade; IEA, International bunker statistics year 2000 and 2001, available at http://data.iea.org) (see Appendix A). Endresen et al. [2003] modeled the year 2000 fuel consumption by the main engines as 144 Mt for the total cargo world fleet (∼45,000 vessels), separating on vessel type and size categories. We also presented a year 1996 total bunker estimate for the whole world fleet of 170–200 Mt (∼105,000 vessels), separating national and international fuel. Our estimate is supported by unpublished model results provided by R. Meech (personal communication, 2004). Corbett and Koehler [2003] modeled the year 2001 bunker consumption as 289 Mt, covering the cargo (203 Mt), the noncargo (45 Mt), and the military fleet (41 Mt). They have also made uncertainty analyses, illustrating some 190 Mt as a lower bound and ∼350 Mt as an upper bound. [3] To clarify the main reasons for this significant difference between the fleet modeling results, we have calculated the fuel consumption by means of the model reported by Endresen et al. [2003] and the input data given by Corbett and Koehler [2003]. The fuel consumption by the main engines for cargo ships (including passenger vessels) above 100 gross tonnage (GT) is then found to amount to 216 Mt. This corresponds to the amount reported by Corbett and Koehler [2003] for the same segment (203 Mt) and indicates that the models give corresponding results with common input data. However, their estimate for the cargo and passenger fleet includes 52,222 vessels, compared to ∼46,000 vessels greater than 100 GT reported by Lloyd's Register of Shipping (LR) [2000]. A fuel consumption of 203 Mt is 50 Mt higher than estimated by Endresen et al. [2003] with alternative operational profiles depending on ship type and size. The difference corresponds to the modeled fuel consumption for all oil tankers and general cargo ships [Endresen et al., 2003]. The main source for the observed deviation between the estimates is the assumed average number of operational hours while cruising at sea for medium and smaller vessels (which dominate by number) and the average engine load. Endresen et al. [2003] separated the activity profile on vessel size categories. Ships less than 5000 deadweight tons (Dwt) were assumed to have 4000 operational hours per year, while the larger ships above 100,000 Dwt were assumed to have 6000 operational hours per year. Ships between 5000 Dwt and 100,000 Dwt were assumed to have 5000 operational hours per year. The average utilization of main engines, while in operation, was assumed to be 70% for all ships, independent of size. Corbett and Koehler [2003] assume that the cargo ships are in operation 74% of the year (6500 hours) with 80% engine utilization. The tracking studies presented below do not support their assumed profiles, especially for small and medium-sized vessels (dominating by number). [4] The world cargo fleet (oceangoing) is dominated by a number of vessels less than 6000 Dwt (assuming 3000 GT is 6000 Dwt). For instance, ∼13,000 general cargo vessels are in this size range [LR, 2000]. Corbett and Koehler [2003] claim that the general cargo fleet dominated by small vessels stands for 22% of the energy demand (main engines) by the world fleet total and 31% of the world cargo and passenger fleet. This is in conflict with modeling results presented by Endresen el al. [2003, Figure 4] that clearly illustrated that medium and large vessels dominate the fuel consumption inventory. This deviation contributes to the higher total fuel consumption estimates and may be explained with their assumption of 6500 hours in use for the main engines for small vessels, while Endresen et al. [2003] assume 4000 hours. [5] Statistics Norway holds detailed information for the ∼400 smaller vessels between 100 and 3000 GT in coastal trade in Norway [Statistics Norway, 2000a, 2000b] (available at http://www.ssb.no/emner/10/12/40/nos_sjofart/arkiv/nos_c582/nos_c582.pdf and http://www.ssb.no/emner/10/12/40/nos_sjofart/nos_c633/nos_c633.pdf, respectively). The largest cargo ships (500–3000 GT) are typically in service (including port time) for 300 days, decreasing to around 240 days for the smallest categories (100–199 GT). This illustrates that service time decreases significantly with decreasing vessel size (or trade), mainly caused by the higher number of days laid up. This is also illustrated by Institute of Shipping Economics and Logistics (ISL) [2001], which shows that 206 of the 299 laid-up vessels were in the lowest size category (300–4999 Dwt). The annual time used in ports normally increases with decreasing vessel size. Operational data for ships in short sea, coastal, and regional trade indicates that ∼60% of the service time is in port [Norway, 1994]. We then only have 100–120 days at sea. Note that this estimate is based on Norwegian statistics for cargo vessels of 100–3000 GT and may not be representative for the world fleet. However, Norway has a very long coast, and the operation pattern and vessel size may be comparable with regional international traffic (e.g., North Sea trade). [6] From the individual operational profiles in the ship movement database Seasearcher (Lloyd's Maritime Information Services, http://www.seasearcher.com/), the annual number of port calls varies from, typically, 20 for very large cargo vessels up to 100 or higher for small and medium vessels, depending on trade. [7] Data submitted to the International Maritime Organisation (IMO) [United Kingdom, 2002] for time in port indicate great variability between ship types with an average of 1.17 days in port for all vessels (Table 3). Whall et al. [2002] reported that time in port varies between 0.46 and 3.8 days (median values), depending on vessel type. An average of these median values is calculated to be 1.27 days. Assuming 100 port visits and 1–1.3 days on average per port visit as representative for the small vessels (100–3000 GT) gives us ∼100–130 days in port annually. With 240–300 service days [Statistics Norway, 2000a, 2000b] and 100–130 days in port, the number of days at sea for the smaller vessels is probably in the range of 100–200 days (2400–4800 hours). We acknowledge great variability and uncertainty in the data. However, the Corbett and Koehler [2003] assumption of 6500 hours in use for the main engines for small vessels seems significantly too high. [8] Automated Mutual-Assistance Vessel Rescue System (AMVER) holds detailed voyage information (daily reports during a voyage) (E. Carroll, U.S. Coast Guard, personal communication, 2002) for ∼7100 medium and large cargo vessels mainly larger than 3000 GT in international trade. The AMVER data show that the main bulk of reporting frequencies is found to be in the interval of 100–250 days at sea, for cargo vessels, decreasing at lower and higher size intervals, neglecting the category of 1–50 days (Table 1). The AMVER data illustrate that the reporting frequency variability is reduced and the average time at sea is increased with ship size. The activity profiles presented below (and the AMVER data) indicate that on average, vessels spend around 220 days at sea with variation upward to ∼290 days depending on size. The standard error of ∼70 days illustrates vessels that report less than 150 days, report infrequently during a voyage, or do not report for the voyages shorter than 24 hours. We have assumed that all these vessels underreport, and the average profiles are then calculated by removing all vessels that report less than 150 days (Table 1). The average number presented in Table 2 illustrates some 60 days of variation between vessel types and up to 100 days more time at sea for large vessels compared to small-sized vessels. Clearly, size and type dependencies have to be considered when performing fleet modeling (see also Table 3). The data in Tables 1 and 2 indicate that an average of 6500 hours (270 days) at cruising speed as assumed by Corbett and Koehler [2003] is a high estimate. The AMVER fleet average profile at sea for different vessel sizes corresponds well with the proposed operational profiles suggested by Endresen et al. [2003] (see above). However, vessel type dependencies have to be included. The AMVER average reporting frequencies for very large crude oil carriers (VLCCs; ∼150,000 GT) are in the same range (Table 2) as the average 248 days per year reported for 453 VLCCs tracked in 1991 [Wijnolst and Wergeland, 1997]. Uncertainty and limitations related to the AMVER data are discussed by Endresen el al. [2003]. [9] A study that followed 1580 cargo vessels during a period of 16 months supports the above findings and illustrates significant variations in operation profile related to different vessel types/sizes and type of trade [United Kingdom, 2002]. They reported a mean turnaround and total sea time by vessel type, as shown in Table 3. They also reported mean turnaround times by different main bulk carrier sizes, ranging from 38 hours (minibulker) to 120 hours (Panamax). The ship turnaround time is the duration of the vessel's stay in port and is calculated from the time of arrival to the time of departure. Table 3 illustrates large variation in the voyage cycles. On average, 29% of the in-service time is related to the noncruising mode (assuming two turnarounds per voyage), where the main engine normally is not in use. This distribution is supported by a tracking study reporting the annual time fraction in port to be 37% for vessels operating in worldwide trade [Liberia, 1996]. However, this will largely depend on vessel type/size and trade. For instance, the annual time fraction in port is reported to be 60% for 605 Norwegian vessels operating in European waters [Norway, 1994]. In addition, detailed activity data for several ships operating in short sea areas (less than 10,000 Dwt) show that time in port constitutes 60–70% of the year [SAFESHIP, 2004]. [10] It is difficult to identify reliable and detailed statistics for nonservice time. Several sources report tonnage laid up, but little information on incidents and ship waiting, storage, and repair time is available. On the basis of the operational pattern for 453 VLCCs, the average number of days not in service was found to be 76 days per year [Wijnolst and Wergeland, 1997]; 32.4 days were reported for long- and short-term storage, 4.9 days laid up, 7.3 days waiting, 6.8 days related to incidents, and 24.7 days for repair. The waiting time is important to consider for cargo vessels. For example, Fremantle Ports [2003] (available at http://www.freport.wa.gov.au/about/report/docs/Yearend30June3020011.pdf) report that 20% of the bulk vessels waited outside the port for more than 2 days. [11] The average engine load, which is the ratio of actual power output to rated output based on maximum continuous rating (MCR: engine output power available for long periods without stop), will significantly influence the modeled fuel consumption. In open sea, the ships will usually run the engines on 80–85% MCR [LR, 1995; Environmental Protection Agency (EPA), 2000] (the latter is available at http://www.epa.gov/otaq/models/nonrdmdl/c-marine/r00002.pdf). However, periods with slow cruise (typically 10–40% MCR), port manoeuvring (typically 10–20% MCR), and ballast cruise reduce the average engine load (depending on trade and vessel type). Normally, ship speed and fuel consumption increase with increasing engine load. Operational data illustrate that ballast voyages at the same speed use significantly less bunker (typically, a reduction of 20% for large vessels), compared to normal cargo voyages [Wijnolst and Wergeland, 1997; Frontline Management AS, Vika Oslo, Norway, Technical and operational information for Frontline vessels, available at http://www.frontline.bm/fleetlist/index.php3; Clarkson Research Studies, 1998]. In addition, depending on the market situation, vessels operating in the spot market may slow steam in ballast condition. Bulk carriers, tankers, and general cargo vessels cruise at 14–16 knots, while reefers, RO-ROs, and container ships cruise at 20–22 knots [EPA, 2000; Cooperative Programme for Monitoring and Evaluation of the Long Range Transmission of Air Pollution in Europe/Core Inventory Air Emissions (EMEP/CORINAIR), 2002]. However, there is a wide range of cruising speeds within ship types [EPA, 2000; Lloyd's Register, 2002]. Speed variations were reported for 453 very large crude oil carriers tracked in 1991 [Wijnolst and Wergeland, 1997]. The reported average speed was 11 knots, but the variations were substantial, ranging from 10 to 13.6 knots. Assuming 10–12 knots as slow cruising speed, and the percent power required by the cube of the ratio of 12 knots to actual cruise speed, only 14% of MCR for container ships and 40% of MCR for bulk carriers are used. For maneuvering, the actual power applied will be lower. This affects the assumed annual average power rating for the main engines and will reduce it below 85% MCR, depending on type of trade (length of voyages, restricted waters, etc.), bunker prices, and freight rates. Also taking into account that cargo vessels sail in ballast close to 50% of the time [Wijnolst and Wergeland, 1997], our best average estimate is 70% MCR. This corresponds to the average weight load factor of the test cycles defined for main propulsion in the IMO technical code, when applied for verification of compliance with the NOx emission limits in accordance with regulation 13 of Annex VI [International Maritime Organisation (IMO), 1998]. [12] Corbett and Koehler [2003] report a fuel consumption for the noncargo fleet that is 100% higher than the estimate provided by Endresen et al. [2003]. We find a large discrepancy between the simplified activity profiles assumed by Corbett and Koehler and activity data reported for offshore vessels, large and medium fishing vessels, and tugs [Statistics Norway, 1996, 2000a, 2000b; SAFESHIP, 2004; Eidesvik, Technical and operational data for supply vessels, provided by K. Sandaker, Norway, 2004]. For the fishing fleet of ∼23,000 vessels, Corbett and Koehler [2003] assumed 6500 hours (270 days) in operation. However, Statistics Norway [1996] reports 162 days (20–25-m length) to 235 days (25–45-m length) days in operation (at sea) for Norwegian fishing vessels on the basis of activity data. These segments of the world fishing fleet by more than 80% of the vessels in the fleet. Statistics Norway for tugs operating in Norwegian waters, days with at sea and days with port are some in the world fleet [LR, 2000]. The AMVER noncargo vessel voyage data (Table illustrate that the number of days at sea is found to be less than days for of the This clearly illustrates that on average, days at sea is a very high estimate. vessels have engine and engine power for service For example, tugs and vessels have power that is only on a the energy demand in the fleet may be the actual bunker fuel consumption will be lower of several very large vessels and 1995; and a database of all the and ships, by available at and report that more than of the large carriers are An estimate of Mt for the fleet as by Corbett and Koehler [2003] seems to be the activity for the national (e.g., and the that vessels are in mode for military 2000] (available at the in the of which are is also a Endresen et al. [2003] that the fleet consumption is of the of Mt. This estimate is based on size for the fleet and assumed operational The significant deviation compared with Corbett and Koehler [2003] for this fleet segment some of the discrepancy in the estimate for fuel consumption. estimates are and be given in The main operation a depending on vessel For example, offshore and service vessels several days at sea in the mode only of the available power (or Data for the two offshore supply vessels and show that the ships are of annual time in 2 of engines of with load on In mode (at cruising of the year, these ships normally use 2 of engines, with 85% load on engine Technical and operational data for supply vessels, provided by K. Sandaker, Norway, 2004). For supply vessels, detailed activity data show that are only at sea at cruising speed for of the year [SAFESHIP, 2004]. fleet activity varies a depending on vessel type, and size and to be and the of models is to be We claim that a fleet model based on engine without engine and fuel type and with activity data from trade will the fuel consumption. A detailed study of the main input data used by Corbett and Koehler [2003] to model bunker oil consumption shows that 6500 hours in average operation for all ship types and size categories will in of the bunker consumption. Our study shows that available operational data indicate a on ship type and with the average number of operational hours decreasing from 6000 hours for large vessels to 4000 hours for small vessels. In addition, we that the assumed engine power rating of 80% MCR for the days assumed to be at sea is very especially for noncargo vessels. The main for the significant differences in estimated fuel consumption be explained by engine load factor and operational profiles We that our estimates for fuel and operational profiles [Endresen et al., are We find that our estimates are supported by detailed operational data and updated bunker inventories. However, vessel type and size to be and the for fleet segments is to be For of modeling results, it is important to have a common of world fleet of the between the fleet segments may be explained by differences in fleet We to that the model of Corbett and Koehler [2003] results in an that may to an of the impact of on the as well as fleet We to acknowledge Carroll, and of AMVER and of for traffic and operational data. We also to acknowledge the and at for the fuel to ships in international of the of the to as marine The 1996 on countries to estimate emission from international bunker fuel and to these from national The world fleet of vessels [Endresen et al., will use international and marine depending on the type of trade. It is difficult to between national consumption and consumption related to international trade to IMO This is discussed in several studies [Endresen et al., et al., 2000; 2000; 2002]. The Lloyd's fleet statistics includes all vessels greater than 100 GT [LR, independent of and The marine is not to or international for vessels, but to bunker consumption by vessels in international between The International Energy Agency (IEA) marine to those to ships of all by ships in in and coastal waters is not included. and coastal (including small and coastal vessels not their bunker marine is in their coastal, and fishing be in their category to are as For the year 2001, reports Mt, Mt, and Mt marine and for the marine and International bunker statistics year 2000 and 2001, available at a fraction of the bunker fuel within the category is related to fishing vessels. The world marine bunker sale in 2001 is then Mt to IEA, bunker fuel to fishing vessels. The Norwegian fishing some vessels (which includes vessels of less than reported a bunker fuel consumption of Mt in [Statistics Norway, 1996]. Norway has some fishing vessels larger than 100 by and by GT of the world fishing fleet [LR, 2000]. reported by the Statistics Norway [1996] indicates that the large fishing vessels Mt of fuel annually. Assuming that the Norwegian fleet is representative of the world fleet in of activity and we find that the fishing vessels in the world around 10 Mt of fuel. it is that the fishing fleet the difference of ∼100 Mt between the world bunker fuel statistics and the modeled consumption reported by Corbett and Koehler [2003]. The Energy Information Administration (EIA) reports bunker fuel sales to be Mt for the year 2000 (EIA, International petroleum information, in “Crude oil imports and exports, with most countries and world total 2000,” available at this corresponds very well with the data be as by et al. [2000]. and gives the fuel consumption of international marine consumption by while not international and also includes fuel in The data be to give a higher estimate compared with those of the However, Figure illustrates a for most of the countries reported by and The countries with by or small amounts of fuel and have a impact on the total inventories. Table illustrates some differences between the and data. The largest deviation is for and the The total consumption for the is the same for and but varies for the main fuel The consumption is reported to be some 16 Mt by while not report sales at This deviation most of the difference between the total and Figure clearly illustrates that these bunker statistics mainly and that only a few countries the error in the total bunker inventory. However, be made in of common and figures reported by The higher marine consumption reported by especially in (Table indicates that national consumption is for some countries to of The and total figures are Mt, with Mt of fuel and Mt of marine fuel. Our best estimate is 180–190 Mt, the fleet of fishing vessels. The national consumption some of the total The estimate of 289 Mt reported by Corbett and Koehler [2003] is significantly higher than and worldwide sales for marine It is not that the difference of some 100 Mt be explained by in the sales
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