PIAS Manual  2023
Program for the Integral Approach of Shipdesign
Outflow: probabilistic computation of oil outflow, with the MARPOL simplified method
The MARPOL regulations set a limit on the amount of oil outflow in case of damage. With this module this can be calculated, and tested against the criteria.

Background of the probabilistic oil outflow calculations

MARPOL contains two sets of probabilistic outflow regulations:

  • Probabilistic oil outflow for oil tankers >5000 ton dwt. This is applicable from construction date January 1, 2007.
  • Probabilistic oil outflow of fuel oil, for vessels with a total fuel capacity >600 m3. This applies to vessels with contract date on or after August 1, 2007, keel laying February 1, 2008 or delivery August 1, 2010. Also applies to ‘major conversions’.

This PIAS module performs a simplified calculation, as prescribed in detail in MARPOL. The alternative would be an exact calculation, as e.g. available in Probdam for the computation of prbabilistic damage stability. Such a calculation is explicitly accepted according to reg. 23.10 for oil tankers. For fuel oil tanks this method is not referred to, however, because the calculations for cargo oil and fuel oil are identical, besides for some detail, it could very well be applied to fuel oil tanks too. By the way, the explanatory notes — explanatory notes on matters related to the accidental oil outflow performance under regulation 23 of the revised MARPOL Annex I, 15 October 2004, MEPC.122(52) — make mention of the fact that in non-rectangular cases the simplified method gives a higher outflow than a more exact approach. If desired, SARC will implement a calculation on the basis of the numerical integration method, if:

  • Such a method is considered to be necessary. Suppose that with the simplified approach the outflow criterion can easily be matched, then there will be no raison d'etre for a more accurate approach.
  • With a certifying organisation can be agreed that this numerical integration calculation will indeed be accepted.

Introduction to this module

The requirements contain rules for determining the probability of an average outflow on the basis of many tank parameters (like distances to shell and bottom, volume, tank boundaries). The ship complies with the rule when that average outflow is smaller than a certain maximum.

This module automatically determines all these tank parameters, but, as usual, not all rules are equally objective. Take, for example, the determination of the distance y, which is the `minimum horizontal distance between compartment and side shell'. Such a definition raises the question what the ‘side shell’ exactly is. Does it run on into the bottom, or into the bilge? And what in case of a rounded gunwhale? In order to have some certainty about this definition, one has added, at the rules for fuel oil tanks anyway: In way of the turn of the bilge, y need not be considered..... This does not solve the problem, however; because where exactly is that way of the turn of the bilge, and what about the fore and aft parts, where there is not a real bilge, but where everything is just curved?

Because the determination of certain dimensions is therefore sometimes subjective, these can also be given manually as the occasion rises. In any case, it is strongly recommended to check the penetration and tank dimensions thoroughly. Finally, two more remarks:

  • In the program and at the output the same symbols are used as in the regulations (although without the typografic refinement of subscripts). In general we are not in favour of including cryptic codes in the input or output, but in this case their meaning has been laid down correctly in the text of the regulations.
  • This module calculates the average oil outflow. The position of the tanks in relation to the outer shell of the ship (like, for example, described for fuel oil tanks in paragraphs 6, 7, 8 and 11.8 of reg. 13A) is not determined, and the consequences of that location (for example, the question whether there have to be made outflow calculations at all) have to be verified by the user himself.

Main menu of this module

This module is activated by chosing from PIAS' main menu, via option Other, the module Outflow. After specifying the PIAS file name this module's main menu appears:

Settings for the oil outflow computations

Type of outflow calculation

At this option one can choose between calculations for fuel oil or cargo oil. Each tank must be assigned the correct content destination type in Layout, see Oil outflow parameters.

Calculation method

In the future this option will be the place to toggle between a simplified calculation and a calculation on the basis of numerical integration.

Ship and compartments are symmetrical

If hullform and compartment are completely symmetrical, then it is sufficient to perform the calculation for one side only (we chose SB). In case of asymmetry the calculation is performed to both SB and PS, and the result is averaged. So, this choice is not governed by the setting of the side of calculation as given at the general project configurations ( Calculate intact stability etc. with a heeling to).

Light ship draft

For the determination of the calculation draft. The drafts as entered here are not integrated with the corresponding data from Probdam.

Load line draft

Please refer to the remark just above, at Light ship draft

Which oil density to apply

Here two choices can be made:

  • Apply the density as specified per tank in module Design density. This method can only be applied if none of the tanks is designated a ‘variable density’.
  • With a generic density, as specified in the next line, for all tanks.

Generic oil density

If the second option is selected at the previous line, then at this line the uniform density for all tanks can be given.

Which tank permeability to apply

Here two choices can be made:

  • Apply the permeability as specified per sub-compartment in module Layout.
  • With a generic permeability, as specified in the next line, for all tanks.

Generic permeability of all tanks

If the second option is selected at the previous line, then at this line the uniform permeability for all tanks can be given.

With fixed minimum height for determination of y

As indicated in the introduction, the penetration of side damage, y, needs only to be determined from the side shell (and, with fuel oil tanks, not below h=min(B/10,3)). As an aid for the question where the side shell ends, at this option a certain minimum height for the determination of y can be specified (by the way, the h=min(B/10,3) will always be applied in case of fuel oil tanks).

Fixed minimum height for determination of y

If the previous line is set to ‘Yes’, then at this line the minimum height (in meters from baseline) can be given.

There are 2 continuous longitudinal bulkheads in the cargo tanks

The answer to this question is relevant for the determination of factor C3, see reg. 23.6 of the cargo oil rules.

Specify damage boundaries and geometrical parameters

Menu with damage boundaries and other geometrical parameters

As motivated in the introduction, it can be desirable to give certain dimensions or distances manually. That can be done in this menu, where the different columns have the following meaning:

  • Auto: with ‘Yes’ these compartment parameters are computed by the program, with ‘No’ entered by the
  • Xa: aft boundary of damage.
  • Xf: forward boundary of damage.
  • Zl: lower boundary of side damage.
  • Zu: upper boundary of side damage.
  • y: penetration of side damage.
  • Yp: portside boundary of bottom damage.
  • Ys: starboard boundary of bottom damage.
  • z: penetration of bottom damage.
  • Yb, Hw and A: Only applicatible to fuel oil tanks, see MARPOL reg. 13A.11.3

Execute the oil outflow calculations

With this option the calculation is executed and printed. An output example is included just below.

Output example