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|Cargo Ship Bunker Tanks: Designing to Mitigate Oil Spillage|
Keith Michel, President, Herbert Engineering Corp., San Francisco, CA
Thomas S. Winslow, PE, Consultant, Oakland, CA
Collision of Alexia and Enif - Gulf of Mexico, 1995
Recent collision and grounding accidents have increased public and industry awareness of the risks of oil spills from bunker tanks. This paper summarizes historical spill data for freighters, and provides case histories for representative collision, allision, and grounding casualties. Arranging double hull protection around the bunker tanks is one means for mitigating the risk of spillage. The location and size of the fuel oil tanks also influence the likelihood and expected volume of oil spills. The relative effectiveness of these alternatives are explored using probabilistic oil outflow analysis techniques.
In the wake of the Exxon Valdez spill and the subsequent passage of the Oil Pollution Act of 1990 (OPA’90), there has been a dramatic reduction in the spillage of oil from tankers in U.S. waters. During the 1990s, the annual spill volume from tankers has been less than one-tenth of the spill volume experienced during the 1980s. At the same time, recent fuel oil spills from freighters involved in collisions, allisions, and groundings (Enif, 1995; Kure, 1997; and New Carissa, 1999) have raised awareness of the risk of oil spills from bunker tanks.
The assessment of risk involves the evaluation of the frequency of accidents together with the consequences of such events. A formal assessment of consequences of oil spills should incorporate many factors including the impact on habitat, costs incurred, and consideration of injuries and loss of life. Such an assessment is beyond the scope of this study, and therefore the quantity of oil spilled is used as a surrogate for consequence.
The United States Coast Guard maintains a database of petroleum spills occurring within the navigable waters of the US. This database includes information on the amount of spillage, the type of vessel involved, and causality. The frequency and volume of spillage from bunker tanks on freighters is estimated through analysis of these historical data. The authors also examined six accidents involving breaching of fuel oil tanks. These case histories provide insight into the types of accidents that occur, and the severity of hull damage encountered.
The use of historical statistics for assessing spill performance does have limitations. Because oil spills from collisions and groundings are low probability events, there is insufficient data to compare the effectiveness of the different bunker tank arrangements currently in use. Furthermore, new concepts cannot be evaluated on the basis of historical data alone. However, probabilistic analysis utilizing historical statistics on damage extents provides a means for calculating the relative effectiveness of designs in mitigating the likelihood and volume of oil spills.
In this study, the probabilistic oil outflow calculation methodology developed by IMO to assess alternative tanker designs is applied to various bunker tank configurations on tankers, containerships, and bulk carriers. The intent is to provide the designer with a better understanding of the influence that the arrangement and location of bunker tanks has on oil outflow from collision and grounding accidents.
There are currently no requirements for protectively locating bunker tanks within cargo vessels. Regulations 13F of MARPOL 73/78 requires all new tankers above 5,000 DWT to have a double hull, a mid-deck, or an alternative arrangement approved by the International Maritime Organization (IMO). OPA 90 mandates double hull construction for all new tank vessels calling U.S. waters. However, both regulations apply only to cargo oil tanks, and any fuel oil tanks located within the cargo tank length. The cargo tank length extends from the aft-most cargo tank boundary to the collision bulkhead. As tankers typically have their bunker tanks arranged in the engine room, these tanks can be located adjacent to the shell.
Restrictions on the storage of fuel oil in double bottom spaces was first proposed at IMO by Finland in the 1980s and more recently by Norway. Norway is also considering an indexing system intended to encourage environmentally friendly operations by differentiating fees for ships. The risk of oil discharge from bunker tanks will be factored into this environmental index. In the wake of a spill from the wood-chip carrier New Carissa, the US Congress has also initiated debate on the need for enhanced regulations related to fuel oil carried on commercial freighters.
HISTORICAL SPILL DATA
The USCG database includes reported oil spills of all sizes occurring in U.S. navigable waters. Figure 1 shows the annual oil spill volume from vessels since 1973. The data is broken down into three categories: tankers, tank barges, and “other vessels”. The significant reduction in spill volumes since 1990 is readily apparent. The performance of tankers is most impressive, with annual spill volume less than one-tenth of the pre-1990 level. Spill volume from tank barges average about one-third of the pre-1990 level, and now represents the single largest source of oil spillage. The reductions in oil spillage realized by tankers and tank barges has not carried over to other vessels, and in the 1990’s other vessels have become responsible for an increasing percentage of the total oil spillage.
Figure 1 –Spill Volume from Vessels in US Waters
(for period 1973-1997)
A more detailed breakdown of spills by source indicates that, during the period from 1992 to 1997, vessels have been responsible for about 54% of the oil spilled into US waters. (see Figure 2). The other vessel category shown in Figure 1 includes freighters, freight barges, tow and tugboats, fishing boats, unclassified vessels, and all other vessels except tankers and tank barges. In Figure 2, freighters are separated from other vessels. The category of freighters includes commercial cargo vessels such as bulk carriers, containerships, ro-ros, and general cargo ships. During the 1992-1997 period, freighters were responsible for about 4% of the total oil spillage.
Figure 2 - Spill Volume in US Waters by Source
(for period 1992-1997)
Between 1992 and 1997, freighters experienced a total of eight oil spill events 1000 gallons or larger originating from allisions, collisions, or groundings (see Figure 3). The combined spill volume from these eight accidents is about 900 cubic meters (237,000 gallons). This represents 42% of the spillage from freighters and only about 3% of the spillage from vessels during this six-year period.
Figure 3 – Spills from Freighters in Collision and Grounding Accidents (for period 1992-1997)
The current level of public concern and political reaction may seem surprising in light of the relatively low spill rate for freighters in collision and grounding accidents (1.33 spills per year with an average spill size of about 112 cubic meters or 30,000 gallons) as compared to tank barges and other vessels. However, a number of recent spills have occurred in environmentally sensitive regions such as the coasts of Alaska and Oregon, and Humbolt Bay in California. Spills such as the Kure (about 4,500 gallons spilled in Humbolt Bay) have demonstrated that even relatively small spills can lead to significant environmental impact and substantial cleanup costs.
REPRESENTATIVE CASE HISTORIES
Case histories for six accidents involving damage to bunker tanks are summarized below. These include two high energy collisions (the President Washington - Hanjin Hong Kong and the Alexia - Enif), two allisions (the Julie N and the Kure), and two groundings (the Kuroshima and the New Carissa).
Figure 4 – President Washington
In May 1994, at the entrance to Pusan Harbor, the containership Hanjin Hong Kong struck the containership President Washington on the port side near amidships. The bow of the Hanjin Hong Kong penetrated the side shell of the President Washington in way of an empty bunker tank and an adjacent ballast tank, extending about 2.5 meters beyond the longitudinal bulkhead. Two adjacent cargo holds were flooded. There was only minor oil pollution to the harbor from the residual HFO in the damaged bunker tank.
The tank arrangement on the President Washington is representative of most containerships where the significant percentage of bunker oil is stored in wing tanks outboard of the cargo holds. However, it is likely that the extent of damage from the penetration of the Hanjin Hong Kong bow would have exceeded any practical double hull protection of the bunker tank in this high energy collision.
In July 1995 the 230-meter bulk carrier Alexia collided with the 157-meter bulk carrier Enif in the Gulf of Mexico near the entrance to the Mississippi River (see cover photo). The Alexia’s bow imbedded in the port side of the Enif, just aft of amidships. It extended into No. 3 Hold, approximately half way through her beam. As a result of the collision three bunker tanks and one diesel oil tank on the Enif spilled approximately 360 m3 (95,000 gallons) of mixed diesel and IFO 180. There was only bow structural damage to the Alexia with no oil spillage. The ships were successfully separated and lightered without additional spillage. After the third day only sheens were reported around the Enif, and visible evidence of the spill disappeared a few days later.
The port bunker tank and centerline diesel oil tank on the Enif were damaged from direct contact with the Alexia bow. The starboard bunker tank was damaged from resultant shifting of the cargo of coiled steel plate. The impact and penetration of the Alexia bow caused the hatch covers to collapse into the hold below causing collateral damage to a double bottom bunker tank at the forward end of the hold.
The force of this collision and the extent of bow penetration was so substantial that double hull protection afforded by the outboard port bunker tank failed to protect the diesel oil tank. Similarly, there are no practical design options that would have prevented the collateral damage to the port side and double bottom bunker tanks.
Figure 5 –Julie N
In September 1996 the product tanker Julie N struck the south side of the Million Dollar Bridge in Portland, Maine as the ship transited the draw span. Pilot error was the cause of the accident. The contact with the bridge buttress resulted in an oil spill of 353 m3 (93,200 gallons) of heavy bunker fuel and 327 m3 (86,400 gallons) of No. 2 home heating fuel, cargo oil. The oil spill covered 13.7 miles of shoreline and led to a massive clean-up response. Total costs reportedly approached $50 million.
The damage to the Julie N occurred below the waterline, on the port side of the bow just aft of the collision bulkhead. The side shell ripped open – the hole measuring approximately 10 meters in length by 4 meters in depth. A HFO bunker tank located immediately aft of the collision bulkhead was breached, and the bulkhead at the forward boundary of the port cargo tank was ruptured.
The transverse penetration into the bunker tank from the contact with the bridge buttress was limited, and there is a good possibility that double hull protection would have prevented this oil spill.
Figure 6 – Pier at Humbolt Bay
In November 1997 the 195-meter bulk carrier Kure contacted the pier while shifting berth at the Louisiana Pacific Dock in Humbolt Bay. Damage to the hull consisted of a 350mm hole about 3 meters above the waterline in way of a forward bunker tank. About 17.2 m3 (4537 gallons) of IFO 180 was discharged into the bay before the hole could be plugged. The local wetlands and shoreline were heavily impacted by the oil spill.
Double hull protection would certainly have prevented the spill from this minor and very localized puncture through the hull.
Figure 8 – Kuroshima
In November 1997, the 116-meter refrigerator ship Kuroshima went hard aground at Summer Bay near Dutch Harbor, Alaska. The grounding resulted in the breeching of two double bottom bunker tanks and about 174 m3 (46,000 gallons) of heavy fuel oil spilled. An additional 288 m3 (76,000 gallons) of HFO was pumped from the ship to holding tanks ashore to prevent further spillage and to lighten the ship. The salvage effort took three months to free the ship, and a costly oil cleanup and recovery operation ensued. This oil spill would likely have been averted if the bunker tanks were located outside of the double bottom spaces.
Figure 7 – New Carissa
In early February 1999 the wood chip bulk carrier New Carissa drifted aground off the central Oregon coast. Initially, though hard aground on a sand bottom, there was no known oil spill. As storm seas pounded the ship against the bottom, oil began to leak from the ship, and pollute the nearby coastline. Bunker fuel was located in three centerline double bottom tanks below Cargo Holds No. 2 to No. 4, and an additional double bottom tank on the portside below Cargo Hold No.5. Diesel oil was stored in the starboard double bottom tank across from the No. 5 DB. At the time of the grounding the ship had approximately 60% bunkers on board, consisting of about 1,363 m3 (360,000gallons) of HFO and 114 m3 (30,000 gallons) of diesel oil. It is difficult to know how much HFO escaped from the grounded vessel, and how much burned-off during the salvage operation. Estimates of HFO spillage range from 189 m3 (50,000 gallons) to 265 m3 (70,000 gallons). To date, salvage and oil spill clean-up costs exceed $20 million.
The bunker tank arrangement on the New Carissa was typical of many bulk carriers where bunker oil is predominantly stored in double bottom tanks below the cargo holds. However, it is uncertain whether alternative bunker tank arrangements would have averted this spill. The structural failure and breaking open of the vessel would probably have opened up any tanks in the midships region of the vessel.
STANDARD design PRACTICEs
Design considerations lead to different bunker tank arrangements for different ship types.
Table 1 – Typical Bunker Capacities
Table 1 summarized typical capacities for HFO and DO tanks for various sizes of tankers, containerships, and bulk carriers. The high-powered post-Panamax containerships have the largest HFO storage requirements, with total HFO capacity for recent newbuildings exceeding 7,600 m3 (2 million gallons). The HFO is usually distributed in a number of wing tanks, such that the capacity of any one tank does not generally exceed 1000 m3 (264,000 gallons). In comparison, VLCC’s have tanks as large as 3,400 m3 (898,000 gallons), as the HFO is typically allocated to one or two pairs of ER wing tanks.
outflow analysis of alternative bunker tank arrangements
To assess the relative effectiveness of alternative arrangements for protectively locating bunker tanks, probabilistic oil outflow calculations were carried out for tankers, containerships, and bulk carriers.
Outflow Calculation Methodology
The IMO guidelines for approval of alternative tanker designs  and the draft regulation for evaluating accidental outflow for new tankers  contain a probabilistic-based procedure for assessing oil outflow performance. Probability density functions describing the location, extent and penetration of side and bottom damage are applied to a vessel's compartmentation, generating the probability of occurrence and collection of damaged compartments associated with each possible damage incident. All oil is assumed to outflow from tanks penetrated in collisions, whereas outflow from bottom damage is based on pressure balance calculations. Outflow parameters are developed by combining results from all damage cases:
References , , and  provide further background on the calculation procedures and assumptions. Although originally developed for the purposes of evaluating alternative tanker designs, the calculation methodology provides a rational means for comparing outflow performance of alternative bunker tank arrangements.
The methodology described in the IMO draft regulation on accidental outflow was applied in these calculations, with the following adjustments to accommodate the analysis of bunker tanks.
Figure 9 – Probability Distribution Functions
for Longitudinal Position of Side Damage
Presentation of Outflow Results
The IMO methodology assumes the vessel has been in a collision or grounding accident of sufficient energy to penetrate the outer hull. If the probability of zero outflow (Po) equals 0.80, then 80% of such collisions and groundings only penetrate into spaces which do not carry oil, and therefore 80% of the cases have zero outflow. Since the mean outflow is the weighted average for all accidents (whether spillage occurs or not), the average spill size is the mean outflow divided by (1- Po).
If the probability of zero outflow is 0.80, it then follows that 20% or 20 out of 100 accidents will be spill events. The combined outflow from these 20 spills is the mean outflow multiplied by 100.
The calculation results for the probability of zero outflow and mean outflow are presented for each tank configuration, as well as spill frequency (number of spills per 100 accidents) and total outflow per 100 accidents. The reader may find it easier to refer to the spill frequency tables when comparing designs.
Outflow Analysis for Tankers
Figure 10 illustrates the five bunker tank arrangements evaluated for tankers. Calculations were run for a 280,000 tons deadweight VLCC, with a 5 long x 3 wide cargo tank configuration. A heavy fuel oil capacity of 7,500 m3 (including service and settling tanks) is assumed for all configurations.
Figure 10 – Tanker Bunker Tank Configurations
Table 2 – VLCC Projected Spill Frequency
(with Cargo Tanks empty)
Table 3 – VLCC Outflow Parameters
(with Cargo Tanks empty)
Table 4 – VLCC Projected Spill Frequency
(with Cargo Tanks 98% Full)
Table 5 – VLCC Outflow Parameters
(with Cargo Tanks 98% Full)
Table 2 and Table 3 contain the projected outflow assuming the cargo oil tanks are empty at the time of the casualty. Table 4 and Table 5 correspond to the full load condition, with all cargo tanks 98% full.
As previously discussed, the bunker tanks are evaluated at three filling levels (10%, 54% and 98% full), and results combined in order to simulate fuel oil consumption during the voyage. For configuration T2, fuel is first drawn from the forward tanks. For configurations T3 and T4, fuel is first drawn from the outer tanks.
Findings related to outflow from tankers:
Authors’ observations and comments:
Outflow Analysis for Containerships
Figure 11 illustrates the three bunker tank arrangements evaluated for containerships. Calculations were run for a post-Panamax vessel of approximately 75,000 tons deadweight. A heavy fuel oil capacity of 7,600 m3 (including service and settling tanks) is assumed for all configurations.
Figure 11 – Containership Bunker Tank Configurations
Table 6 and Table 7 contain the projected outflow for the three containership configurations. The outflow analysis assumes the fuel oil is comsumed proportionately from all tanks (i.e. in the 10% arrival condition, each tank is assumed 10% full).
Table 6 – Containership Projected Spill Frequency
Table 7 – Containership Outflow Parameters
Findings related to outflow from containerships:
Authors’ observations and comments:
Outflow Analysis for Bulk Carriers
Figure 12 illustrates the five bunker tank arrangements evaluated for bulk carriers. Calculations were run for a Panamax vessel of approximately 70,000 tons deadweight, with a heavy fuel oil capacity of 2,200 m3 (including service and settling tanks).
Figure 12 –Bulk Carrier Bunker Tank Configurations
Table 8 and Table 9 contain the projected outflow for the five bulk carrier configurations. The outflow analysis for bulk carriers assumes the fuel oil is consumed proportionately from all tanks.
Table 8 – Bulk Carrier Projected Spill Frequency
Table 9 – Bulk Carrier Outflow Parameters
Findings related to outflow from bulk carriers:
Authors’ observations and comments:
CONCLUSIONS AND RECOMMENDATIONS
Figure 13 illustrates the steps leading to oil spillage. Generally speaking, the higher up the tree corrective measures are implemented, the more effective they will be. Evidence of this is the dramatic improvement in tanker spill performance in the 1990’s. We can surmise that this reduction in spillage is primarily a result of improved operational and management procedures, as these improvements were realized prior to implementation of regulations such as the double hull provision of OPA 90 and escort tug requirements.
Thus, the preferred approach is to eliminate incidents (1st order effects) and eliminate accidents (2nd order effects). However, third order effects such as protectively locating tanks to prevent spillage or mitigate outflow in the event of an accident does offer a last line of defense.
Figure 13 - Spill Event Tree
With regard to the issue of protectively locating bunker tanks, the following general conclusions and recommendations are offered:
1IMO, ” Interim Guidelines for Approval of Alternative Methods of Design and Construction of Oil Tankers under Regulation 13F(5) of Annex I of MARPOL 73/78,” Resolution MEPC.66(37), Adopted September 14, 1995.
2 IMO, BLG3/WP3, “Report of the working group at BLG 3 on Revision of MARPOL Regulations I/22 to 24 in the Light of the Probabilistic Methodology for Oil Outflow Analysis,” including a draft of proposed MARPOL Regulation 19, “Accidental Outflow Performance”, July, 1998.
3 Sirkar et al, “A Framework for Assessing the Environmental Performance of Tankers in Accidental Groundings and Collisions”, SNAME TRANSACTIONS, 1997.
4 Michel, “Oil Outflow Analysis of Double Hull Tankers (Volumes 1 and 2),” Prepared under Contract DTCG23-95-D-HMT001, by Herbert Engineering Corp., for the U.S. Coast Guard, January 1997.
5 Michel, Moore, & Tagg, “A Simplified Methodology for Evaluating Alternative Tanker Configurations,” Journal of Marine Science and Technology, Volume 1 Number 4, SNAJ, September 1996.
6 Motor Ship, “Innovative design for British trio”, December, 1997.
May 14, 1999 SNAME Joint California Sections Meeting