1 Regulation 18.15 of Annex I of MARPOL 73/78 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 B/15 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 D/5 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 D/5 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 D/5 inboard from the intersection of the shell with a horizontal line D/5 above the base line (see figure 2).

3 The PA_c 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 PA_s value in the first case and to a PA_c 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 PA_c for double bottom tanks with sloping tank top. Figures 9 and 10 represent the cases where credit is given in calculation of PA_s to part or the whole of a double bottom tank.

w must be at least 2 metres along the entire length of the tank for the tank to be used in the calculation of PA_c

h must be at least 2 metres or , whichever is less, along the entire length of the tank for the tank to be used in the calculation of PA_s

If h_db is at least 2 metres or , whichever is less, along entire tank length,

PA_c = h_db × double bottom tank length x 2

PA_s = B × double bottom tank length

If h_db is less than 2 metres or , whichever is less,

PA_c = h_db × double bottom tank length × 2

PA_s = 0

If h_db is at least 2 metres or , whichever is less, along entire tank length,

PA_c = h × double bottom tank length x 2

PA_s = B × double bottom tank length

If h_db is less than 2 metres or , whichever is less,

PA_c = h × double bottom tank length × 2

PA_s = 0

If W is 2 metres or more,

PA_c = D × tank length × 2*

PA_s = W × tank length × 2*

If W is less than 2 metres,

PA_c = 0

PA_s = W × tank length × 2*

If W is 2 metres or more,

PA_c = D × tank length × 2*

PA_s = b × tank length × 2*

If W is less than 2 metres,

PA_c = 0

PA_s = b × tank length × 2*

PA_c = h × double bottom tank length × 2*

PA_c = h x double bottom tank length x 2*

If h is less than 2 metres or , whichever is less, anywhere along the tank length, but h_db is at least 2 metres or , whichever is less, along the entire tank length within the width of 2b, then:

PA_s = 2b × cargo tank length

PA_s = B × cargo tank length

* To include port and starboard.