Chapter 6 took a great deal of trouble to stop choosing damages. This chapter goes straight back to choosing them, and chooses much worse ones. What decides how bad they are is not the ship at all. It is what she is carrying.
7.1 The cargo decides the standard
Everything so far has started from the ship. The chemical and gas codes start from the other end. They look at the product, judge how much harm an escape of it would do, and work backwards to the hull that may carry it. A type 1 ship carries products of very severe hazard and takes the most severe standard of damage with her tanks furthest inboard; a type 2 products of appreciably severe hazard; a type 3 products of sufficiently severe hazard requiring a moderate degree of containment. Carry several products and the most stringent type governs.
The gas code does the same with types 1G, 2G, 2PG and 3G. The 2PG is the interesting one: a ship of 150 metres or less that would otherwise need to be a 2G may be built to the lesser standard if the cargo is carried in independent pressure tanks. The code will trade structure for subdivision, but only on that explicit basis.
7.2 The damage is a box of prescribed size
This is where the codes part company with Chapters 5 and 6. There, the flooded volume was a compartment, because that is where the water stopped. Here the damage is a box of stated dimensions, placed wherever it does the most harm, and every watertight division inside it is assumed to have been breached.
| side damage | bottom damage within 0.3 L of the bow | bottom damage elsewhere | |
|---|---|---|---|
| longitudinal extent | 9.327 m | 9.327 m | 5.000 m |
| transverse extent | 4.840 m | 4.033 m | 4.033 m |
| vertical extent | upwards without limit | 1.613 m | 1.613 m |
One third of L to the power two thirds comes to 9.327 m for this hull, B/5 to 4.840, B/6 to 4.033 and B/15 to 1.613. Every one is smaller than the absolute cap the code sets beside it except one: abaft the forward 0.3 L the IBC Code caps the bottom damage at 5 m, so the after bottom box is 5.000 m long and not 9.327 m. Every other governing figure comes out of her own length and breadth.
One provision is easy to overlook and matters a great deal: if a damage of less than the maximum extent would produce a more severe condition, that lesser damage must be considered too. The box is a maximum, not a specification.
7.3 Where the box may be placed
| type | length | damage is assumed |
|---|---|---|
| type 1 | any length | anywhere in her length |
| type 2 | over 150 m | anywhere in her length |
| type 2 | 150 m or less | anywhere except involving either bulkhead bounding an aft machinery space |
| type 3 | over 225 m | anywhere in her length |
| type 3 | 125 m to 225 m | anywhere except involving either bulkhead bounding an aft machinery space |
| type 3 | under 125 m | anywhere except involving the aft machinery space itself |
MV Ninja is 148 metres long. The lines in red apply to her, and the point worth noticing is that at this length a type 2 and a type 3 must survive exactly the same damage. What separates them is not the damage but where the tanks may sit. Only a type 1 must take damage on the machinery bulkheads as well.
7.4 The rule that the cargo is lost
Several flooding assumptions are stricter than anything met so far. Cross flooding that needs a valve opened cannot be counted on. The buoyancy of any superstructure directly above the side damage is disregarded. But the assumption with no counterpart in a dry cargo ship is this: wherever the damage penetrates a tank containing liquid, the contents are assumed completely lost and replaced by sea water up to the final plane of equilibrium.
Worked example 7.1: suppose No.3 hold were a tank carrying 5000 t of a product of relative density 1.05, the ship in her summer condition, 30456 t at 9.600 m with KG 8.09 m, and the hold the rectangular box of the print chapter’s Table 35.4 (plan area 576.0 m2, floor 2.20 m above the keel). As an ordinary dry cargo hold at permeability 0.60 the damage takes her to 10.421 m with a residual GM of 2.124 m. Under the code’s rule the 5000 t leave first, her displacement falls to 25456 t and, because the cargo sat low, her KG rises from 8.09 to 8.435 m. Sea water then enters to the waterline and she finishes at 9.313 m, which is 0.287 m shallower than she started intact, with a residual GM of 1.571 m, 0.553 m less than in the dry cargo case.
She came up, and she got worse. The weight she lost was the lowest weight in the ship. A tanker that spills her cargo is not lightened in any useful sense, and that is the whole argument for the code.
7.5 The survival criteria
Two features set these apart. They must be met at every stage of the flooding, not only at the end. And the area under the residual curve must not be less than 0.0175 metre radians: a ship can satisfy a maximum lever and a range while having almost nothing between them, and the area criterion asks that she have work in her, not merely that she stand up.
| criterion | the code requires |
|---|---|
| the waterline, at any stage of flooding | below the lower edge of any opening through which progressive or down flooding may take place |
| angle of heel from unsymmetrical flooding, at any stage | IBC: not more than 25 degrees, which may be increased to 30 if no deck immersion occurs; IGC: not more than 30 degrees |
| range of the residual curve | not less than 20 degrees beyond equilibrium |
| maximum residual righting lever | not less than 0.10 m within that range |
| area under the residual curve | not less than 0.0175 metre radians |
| unprotected openings | not immersed within that range |
| emergency power | capable of operating |
7.6 Sliding the damage along her
Take MV Ninja loaded and apply the code’s side damage: a box 9.33 m long penetrating 4.84 m inboard, from the bottom shell upwards without limit. Place it with its after end at every quarter metre, flood whatever it opens, and test.
| positions tested | opening one compartment | straddling a bulkhead | |
|---|---|---|---|
| dry cargo, permeability 0.60 | 555 | 308 of 308 survive | 65 of 247 survive |
| treated as tanks, permeability 0.95 | 555 | 136 of 308 survive | 28 of 247 survive |
Every one of the 308 placings that opens only one compartment is survived. Of the 247 that straddle a bulkhead, only 65 are: the 28 on the after peak bulkhead and the 37 on the bulkhead between No.4 and No.3 holds, amidships, the pair that trims her least. Treated as tanks at 0.95, only 136 of the single placings survive, No.5, No.2 and No.1 holds each putting an end of the ship under on their own, and of the straddling placings only the 28 on the after peak. The lesson is the one Chapter 6 reached from a quite different direction: on a ship of this length and freeboard it is the trim after a two compartment flooding, not the transverse stability, that decides survival. (The verdict here is on the three tests the wall sided estimate can reach: the deck above water at both perpendiculars, a lever of 0.10 m and an area of 0.0175 m rad before the deck edge; the range needs the damaged cross curves.)
Why this ship is the wrong shape for the argument
MV Ninja’s holds run the full breadth between the hopper and topside tanks, so a penetration of B/5 opens a space symmetrical about the centreline and she floods upright. On a chemical tanker with wing tanks and a centreline bulkhead the same box opens the tanks on one side only, and the heel is the criterion that bites first. That is why the code sets an explicit limit of 25 degrees where Chapter 5 set 15 and Chapter 6 set none at all.
7.7 Bottom damage, and her double bottom
The bottom box is shorter, 5.000 m over the after 0.7 L of her length under the IBC Code, and it reaches 1.613 m vertically from the bottom shell at the centreline. Appendix A does not give the height of her inner bottom. The centres of her No.2, No.3 and No.4 double bottom tanks are 1.12 m above the keel, so if those tanks are of uniform depth the inner bottom is at about 2.24 m, and on that reading the assumed grounding stops about 0.59 m short of it and opens only the double bottom tanks in its path. Had the inner bottom been below 1.613 m the box would have gone through it into the cargo space, and no amount of stability would help: the damage would simply be inside the tank. That is why the codes require the cargo tanks of the higher types to sit above the vertical extent of bottom damage.
7.8 Where the tanks may sit, and what it costs
The codes do not stop at survival. For the more hazardous products the cargo must be held away from the shell so that minor contact damage cannot reach it at all. The protective distances are the damage extents turned into an envelope.
| hold | capacity m3 | breadth m | clearance to the shell m | 760 mm met | as a type 2 | as a type 1 |
|---|---|---|---|---|---|---|
| No.1 | 5854 | 22.2 | 1.00 | yes | 5453 | 3301 |
| No.2 | 6900 | 23.8 | 0.20 | NO | 6459 | 4094 |
| No.3 | 6549 | 23.8 | 0.20 | NO | 6131 | 3885 |
| No.4 | 6900 | 23.8 | 0.20 | NO | 6459 | 4094 |
| No.5 | 6032 | 22.3 | 0.95 | yes | 5621 | 3414 |
| TOTAL | 32235 | 30123 | 18788 |
Three of her five holds sit closer than 760 millimetres to the shell already. Rebuilt to type 2 distances she would keep 93.4 per cent of her cargo volume; to type 1 distances, with a 4.84 m void each side, only 58.3 per cent. (The breadths are those of the illustrative box holds of the print chapter’s Table 35.4; the bottom distance costs nothing because the floor at 2.20 m already sits above 1.613 m.) Four tenths of her cargo space given over to empty steel. That is the price of the most hazardous products, and it is why type 1 chemical tankers are small, specialised and expensive ships.
7.9 The IGC Code, and where it differs
A gas carrier sits in the same framework. Two numerical differences are worth carrying away. The vertical extent of bottom damage in the gas code is B/15 or 2 metres, the lesser, against the chemical code’s B/15 or 6 metres. And the longitudinal extent of bottom damage is the same over the whole length in the gas code, where the chemical code shortens it abaft 0.3 L from the bow.
The vertical difference changes nothing for a hull this size: B/15 is 1.613 m, less than 2 and less than 6 alike. The longitudinal difference does apply to her: 9.327 m under the gas code against 5.000 m under the chemical code abaft the forward 0.3 L, so a gas carrier of her length must consider two adjacent double bottom tanks flooded by a grounding aft as well as forward. On a ship broader than 30 m the vertical difference begins to matter, and it is why a gas carrier’s double bottom can be shallower than a chemical tanker’s for the same protective standard.
Chapter 7 in seven lines
- The product decides the type; the type decides the standard of damage and where the tanks may sit.
- The damage is a box placed where it does most harm, not a compartment. Every bulkhead inside it is assumed breached.
- On a ship of 148 m only one of the codes’ absolute caps takes effect, the IBC’s 5 m on the after bottom damage. Every other governing figure comes from her own L and B.
- At 148 m a type 2 and a type 3 must survive identical damage. Only the tank location separates them.
- Wherever the damage opens a liquid tank the cargo is assumed lost. That lightens the ship and raises her centre of gravity at the same time.
- The criteria apply at every stage of flooding and include an area of 0.0175 metre radians, which no other instrument in this volume requires.
- Applied to MV Ninja, every single compartment damage is survived and most damages that straddle a bulkhead are not, the trim putting an end of the ship under.
Test yourself
Questions
- Explain how a chemical tanker comes to be a type 1, type 2 or type 3 ship, and state what happens where a ship is intended to carry several products of different types.
- State the assumed extent of side damage under the IBC Code, and calculate each dimension for a ship of 148.00 m length and 24.20 m breadth.
- State the assumed extent of bottom damage under the IBC Code for the two regions of the ship, and explain why the code treats the forward third differently.
- Explain the essential difference between the way a compartment is chosen for flooding under SOLAS chapter II-1 and the way it is chosen under the IBC Code.
- A ship of 148 m has transverse bulkheads spaced 22 m apart. Explain what happens under the code when the assumed side damage is placed over one of them, and what this implies for bulkhead spacing on a chemical tanker.
- State the standard of damage for a type 2 ship of 150 m or less and for a type 3 ship of 125 m to 225 m. Comment on the result for a ship of 148 m.
- State the survival criteria of IBC 2.9, and identify the one criterion that has no counterpart in the SOLAS deterministic or probabilistic requirements.
- Explain the requirement that the contents of a damaged liquid cargo tank are assumed to be lost and replaced by sea water, and explain why this can leave a ship shallower and less stable at the same time.
- State the protective distances required for the location of cargo tanks in type 1, type 2 and type 3 ships, and explain what they are derived from.
- State two respects in which the damage assumptions of the IGC Code differ numerically from those of the IBC Code, and explain for what size of ship each difference would begin to matter.
Looking ahead
The last three chapters have all asked the same question from different angles: what must she survive, and what does the hull have to be in order to survive it? Chapter 8 turns the question round and asks how deep she may be loaded in the first place. The answer is a freeboard, assigned from a tabular value corrected for block coefficient, depth, superstructure and sheer. The connection is closer than it looks: a type A ship is allowed a smaller freeboard than a type B ship of the same size, and a type B ship may buy back part of that difference, as a B minus 60 or a B minus 100, by demonstrating a damage stability standard. Freeboard and damage stability turn out to be the same argument seen from opposite ends.