The single most important structural factor in reducing pollution in the event of damage is the one that's almost never talked about: tank size and arrangement. IMO has developed a method to evaluate a tanker's propensity to spill cargo in the event of hull damage. It is based on a hypothetical collision and a hypothetical grounding. The collision involves a wedge penetrating into the hull about B/5 where B is the ship's beam. The grounding involves damage from the bottom up to about B/15. These parameters were based on a study of past casualties. The collision/grounding is assumed to be equally likely to occur anywhere along the ship's side/bottom. The overall result of this analysis is the ship's Effective Oil Spill (EOS) number, which is the percentage of the ship's cargo which will be spilled on average given the IMO collision/grounding scenario. The IMO system is far from perfect but it is a reasonable starting point for evaluating a design's resistance to spillage.
Table 8.1 shows the IMO Effective Oil Spill numbers for four different pre-MARPOL VLCC's and ULCC's all built in the mid 70's, a typical single hull MARPOL VLCC built in 1986, a 2001 built double hull VLCC, and the Hellepont 440,000 ton double hullULCC. From age 25 on, the pre-MARPOL ships must operate under either IMO REG 13G4 (30% of the side or bottom tanks non-cargo) or IMO REG 13G7, usually known as Hydrostatically Balanced Loading (HBL). For these ships, Table 8.1 shows the EOS numbers for each of these regimes.
|Design||Percent Cargo Spilled||Number of|
|Hellespont Embassy, 1976||2.1||1.8||2.2|||34 five across|
|Empress des Mer, 1976||2.6||2.2||2.4||35 three across|
|Shell L-Class, 1974||3.6||3.1||3.7||22 three across|
|Ludwig VLCC, 1974||3.4||2.9||3.3||22 three across|
|Typical Marpol Single Hull, 1986||4.3||13 three across|
|SHI 1321, Double Hull VLCC, 2001||2.4||2.1*||17 three across|
|DSME 5184 Double Hull ULCC, 2002||1.9||1.6*||21 three across|
Table 8.1 makes a number of points including:
But for present purposes, the interesting feature of Table 8.1 is the wide range in expected spillage under 13G7 for the pre-Marpol ships. The Hellespont Embassy spills 65% less than the L-class in the same casualty scenario. The other two pre-Marpol ships are in between with the Empress des Mer much closer to the Embassy and the Ludwig V's much closer to the L-class.
This is a product of small tank size. The Embassy and the Empress have a lot more tanks than the other two designs. The Empress, which was built to the last pre-Marpol restriction on tank size, has 13 pairs of wing tanks; the Shell L-class has 8. And in the case of the five across Embassy, the tanks are much more intelligently arranged.
An interesting feature of double hulls is that they finesse the IMO tank size limits. The IMO tank size limits apply only to cargo tanks that touch the side shell plate. Double hull wing tanks are thus exempted and the tanks in modern double hulls are enormous. The standard newbuild VLCC with a cargo cubic of 350,000 m3 has only 15 cargo tanks plus two small slop tanks. Even the L-class, which we've just finished castigating, with a cargo cubic of 385,000 m3 has 22 cargo tanks. The Marpol VLCC's were even worse. Most of these ships were built with 13 cargo tanks. If one were determined to increase spillage in a casualty via structural regulation, it's hard to imagine a more effective set of requirements than the Marpol single hull rules.
The main reason why the double hull ULCC is superior to the double hull VLCC is that is has 14 wing tanks while the standard double hull VLCC has 10 wing tanks. All it took was two extra wing tank bulkheads to reduce expected spillage by more than 30%.
Table 8.2 shows a little more detail on the Embassy operating under hydrostatic balance versus new double hull comparison.
|2002 ULCC||2001 VLCC||Hellespont Embassy (13G7)|
This is a little unfair to the double hull because in a grounding a portion of the bottom loss will be captured in the top of the double sides. IMO arbitrarily and without any support says cut the bottom loss number for double hull by 50%. Adopting this rule, the double hull V's EOS number becomes 2.1%, the asterisked number in Table 8.1. But even if we do that, the double hull VLCC still has a 17% higher Effective Oil Spill number than the Embassy under 13G7. Paradoxically, double sides are more effective in containing grounding damage than they are in reducing side damage. The way to go after side damage is sub-division. And the double bottom does almost nothing in grounding that could not be done far more efficiently by HBL.
Sadly Class is implicated in the growth of double hull tank size. The biggest tanks in a modern double hull VLCC are about 50 meters long. Tanks this long have a natural sloshing period which is close to the ship's natural pitch period, 13 to 15 seconds. When a tank's sloshing period matches the ship's pitch period, immense waves can build up in the tank, crashing from end to end. This is known as sloshing resonance.
In the mid-70's no one would be crazy enough to design a tank to operate anywhere near sloshing resonance. Thus, even if an owner cared nothing about pollution -- and few did -- you either built smaller tanks or you had to use real (complete) swash bulkheads. The difference in cost between a complete swash bulkhead and a oil-tight bulkhead is not all that great, so the incentive to go with massive tanks was not strong. But over the years, Class has allowed the swash bulkhead to atrophy and then disappear. The argument is that we can operate these tanks at resonance because we can predict the forces and beef up the structure to handle them.
This argument is a sad joke on a number of levels. Nobody has any way of accurately estimating sloshing forces at resonance, certainly not Class. The best of the current lot of Class tools is probably LRFLUIDS. When Daewoo applied LRFLUIDS to the case of the Hellespont double hull ULCC's center tank, the program indicated that at resonance the tie beams would impose an important dampening on the sloshing. ABS's empirical gouge said the same thing. The state of the art in sloshing analysis is the Hamburg Ship Research Institute's program which implements a full two phase Navier-Stokes but only 2-D. Despite being 2-D, this is an extremely computationally intensive program. One run simulating a little over one minute in real time took a cluster of 8 Dual-Pentium PC's over two days to compute. The results showed that at resonance the tie beams will have almost no effect on sloshing. The basic wave form is a kind of U that sneaks under the tie-beams as it moves from one end of the tank to the other, not a sort of semi-harmonic wave as Class claims. (Any housewife who has had water slosh out of a basin could have told us the same thing.) The Hamburg results are far closer to reality but the people at Hamburg will be the first to tell you that they cannot accurately predict the loads imposed on the structure. But when this wave crashes into a bulkhead, it climbs over 15 m into the top of the tank, dwarfing the wave heights in the 2005 tsunami.
The only reasonable thing to do is to stay away from resonance. And that means real swash bulkheads, not overgrown webs. And once you have real swash bulkheads, the move to full oil-tight bulkheads and far better sub-division is obvious. The result is a big reduction in spillage and far more flexibility in cargo parcels, ballast exchange, and tank inspection.
The tragedy is that this big improvement would cost very little. Shorter tanks mean:
In the case of the massive double hull tanks, we have just the opposite effect: immense forces in the critical lower hopper area near the transverse bulkheads, and in way of the centerline buttress, and at the stringer corners and web toes, despite the Rules' staying away from any really difficult loading condition. The bottom bracket of the centerline buttress on the DSME ULCC has a 57 mm web and a 60 mm faceplate, both High Tensile Steel. The center tank lower web toes have 50 mm faceplates. That's way too much stress in one place.
At a minimum, the IMO Regulation 24 tank size limits should be applied to all double hull wing tanks and the IMO Regulation 13G(7) prohibition against operating tanks near sloshing resonance should be applied to all tankers.