MARPOL Consolidated Edition 2017

Appendix 2
Interim recommendation for a unified interpretation
of regulations 18.12 to 18.15
"Protective location of segregated ballast spaces"
1 Regulation 18.15 of Annex I of MARPOL relating to the measurement of the 2 m minimum width of wing tanks and
the measurement of the minimum vertical depth of double bottom tanks of 2 m or ! in respect of tanks at the ends
of the ship where no identifiable bilge area exists should be interpreted as given hereunder. No difficulty exists in the
measurement of the tanks in the parallel middle body of the ship where the bilge area is clearly identified. The regulation
does not explain how the measurements should be taken.
2 The minimum width of wing tanks should be measured at a height of j above the base line providing a reasonable
level above which the 2 m width of collision protection should apply, under the assumption that in all cases ~ is above
the upper turn of bilge amidships (see figure 1). The minimum height of double bottom tanks should be measured at a
vertical plane measured 0/ 5 inboard from the intersection of the shell with a horizontal line 0 / 5 above the base line (see
figure 2).
3 The PAc value for a wing tank which does not have a minimum width of 2 m throughout its length would be zero; no
credit should be given for that part of the tank in which the minimum width is in excess of 2 m. No credit should be given
in the assessment of PAs to any double bottom tank, part of which does not meet the minimum depth requirements
anywhere within its length. If, however, the projected dimensions of the bottom of the cargo tank above the double
bottom fall entirely within the area of the double bottom tank or space which meets the minimum height requirement and
provided the side bulkheads bounding the cargo tank above are vertical or have a slope of not more than 45° from the
vertical, credit may be given to the part of the double bottom tank defined by the projection of the cargo tank bottom.
For similar cases where the wing tanks above the double bottom are segregated ballast tanks or void spaces, such
credit may also be given. This would not, however, preclude in the above cases credit being given to a PAs value in the
first case and to a PAc value in the second case where the respective vertical or horizontal protection complies with the
minimum distances prescribed in regulation 18.15.
4 Projected dimensions should be used as shown in examples of figures 3 to 8. Figures 7 and 8 represent
measurement of the height for the calculation of PAc for double bottom tanks with sloping tank top. Figures 9 and 10
represent the cases where credit is given in calculation of PAs to part or the whole of a double bottom tank.
Figure 1 - Measurement of minimum width of wing ballast tank at ends of ship
Section view
Centre
tank
Wing
tank
w
w must be at least 2 m along the entire length of the tank for the tank to be used in the calculation of PAc
156 MARPOL CONSOLIDATED EDITION 2017