1.1 Under resolution MEPC.51(32), the Marine Environment
Protection Committee (MEPC) adopted, at its thirty-second session,
amendments to Annex I of the MARPOL
73/78 Convention. The key issues of these amendments were the then
new MARPOL Annex I regulations 13F and 13G, which address the prevention
of oil pollution in the event of collision or stranding. MARPOL Annex
I regulation 13G, which covered
the treatment of existing tankers, will not be discussed in this paper.
MARPOL Annex I regulation 13F addressed
oil tanker newbuildings and contained the double-hull requirements
applicable to oil tanker newbuildings, for which the building contract
is placed on or after 6 July 1993.
1.2 Paragraph (4) of MARPOL Annex I regulation 13F addressed the so
called "mid-deck design", which means that the protective double-bottom
ballast tanks may be dispensed with, if a horizontal partition ("mid-deck")
is fitted in such a way that the internal cargo pressure plus vapour
pressure is less than the external sea water pressure. This is called
the hydrostatic balance principle.
1.3 By means of the IMO comparative study on oil
tanker design (OTD)(1)
footnote it
was demonstrated that the oil outflow performance of mid-deck tankers
is at least equivalent to that of double-hull tankers, but it was
recognized that within this overall conclusion each design gives better
or worse oil outflow performance under certain conditions.
1.4 It was therefore recognized early by the MEPC
that there is a compelling need for IMO to establish internationally
agreed guidelines for the assessment of the oil outflow performance
of alternative tanker designs in relation to basic double-hull designs.
This intent was expressed in paragraph (5) of MARPOL Annex I regulation 13F as follows:
-
"(5) Other methods of design and construction
of oil tankers may also be accepted as alternatives to the requirements
prescribed in paragraph (3)footnote, provided
that such methods ensure at least the same level of protection against
oil pollution in the event of collision or stranding and are approved
in principle by the Marine Environment Protection Committee based
on guidelines developed by the Organizationfootnote."
1.5 Interim guidelines were adopted in September
1995. They were included as per Appendix 7 to MARPOL Annex I as "Interim
Guidelines". The word "interim" expresses the intent to update the
Interim Guidelines when experience had been gained during a three
to four years application period. The Interim Guidelines were superseded
by the Revised Interim Guidelines, which were adopted by resolution MEPC.110(49) in 2003.
1.6 The calculation methodology prescribed in
the Revised Interim Guidelines involves direct application of the
provided probability density functions (PDFs) to the design. As there
are five probability density functions (pdfs) for side and bottom
damage this is a calculation-intensive approach.
1.7 Following this development, the MEPC considered
it necessary to reconsider and revise the existing MARPOL Annex I
regulations 22 through 24, which covered a similar issue, i.e. minimizing
oil pollution from oil tankers due to side and bottom damages, in
a more traditional (deterministic) manner. It was recognized that
the existing deterministic regulations did not properly account for
variations in subdivision in general, and longitudinal subdivision
in particular. Therefore, the accidental oil outflow performance regulation 23 was developed for
the revised MARPOL Annex I. The envisaged goal was to provide a performance
based accidental oil outflow regulation that effectively handles variations
in subdivision. This regulation is made consistent with the Revised
Interim Guidelines to avoid the possibility of contradictions in acceptability
of oil pollution prevention regulations due to their difference in
nature.
1.8 While it was felt that the rigorous approach
prescribed by the Revised Interim Guidelines was suitable for the
evaluation of alternative tanker designs and possible unique tank-configurations,
a less complex regulation was considered necessary for application
to all tankers. Thus, a "simplified" method based on the same background
was developed. These explanatory notes describe the assumptions and
philosophy underlying this simplified approach for assessing oil outflow,
provide background on the development of the performance index, and
contain examples demonstrating application of this regulation.
1.9 This simplified method based on minimum clearances
between the cargo tanks and the hull is suitable for tank arrangements.
For certain designs such as those characterized by the occurrence
of steps/recesses in decks and for sloping bulkheads and/or a pronounced
hull curvature, more rigorous calculations may be appropriate.
1.10 Combination carriers are ships designed and
built for carrying both dry and liquid cargo (i.e. bulk cargo and
oil cargo). Traditionally these ships are built without any centreline
bulkhead. The new probabilistic method is suitable also for the combination
carriers, but due to the nature of the design they may not be able
to comply with the outflow performance index (mean outflow parameter)
of a standard oil tanker. For combination carriers, separate mean
oil outflow parameter may be applied provided it is demonstrated by
calculations that the increased structural strength of the hull is
providing for improved environmental protection compared to a standard
double hull oil tanker of the same size. The calculations are to be
to the satisfaction of the Administration.