Appendix III - Agreed Interpretations of Certain of the Provisions of the Revised Specifications
Clasification Society 2024 - Version 9.40
Statutory Documents - IMO Publications and Documents - Specifications and Manuals - Crude Oil Washing Systems - Revised Specifications for the design, operation and control of crude oil washing systemsResolution A.446(XI) Amended by Resolution A.497(XII) Amended by Resolution A.897(21) - Appendix III - Agreed Interpretations of Certain of the Provisions of the Revised Specifications

Appendix III - Agreed Interpretations of Certain of the Provisions of the Revised Specifications

  4.1.1 This paragraph requires pipes and valves to be of steel or other equivalent material. As classification societies permit grey cast iron for cargo piping with a maximum permissible working oil pressure of 16 kg/cm2, grey cast iron may be permitted in the supply system for crude oil washing systems when complying with nationally approved standards.

  4.1.3 The requirements of this paragraph allow alternative arrangements. One such alternative is that where the system is served only by centrifugal pumps so designed that the pressure derived cannot exceed that for which the piping is designed, a temperature sensing device located in the pump casing is required to stop the pump in the case of overheating.

  4.1.6 On new oil tankers the steam heater referred to shall be located outside the machinery spaces. However, on an existing oil tanker with an existing steam heater located in the machinery spaces, no more additional isolation will be required other than that which isolates the crude oil washing system from the machinery spaces.

  4.2.4 This paragraph requires each machine to be capable of being isolated by means of stop valves in the supply line. Where more than one submerged machine is connected to the same supply line a single isolating stop valve in the supply line may be acceptable provided the rotation of the submerged machines can be verified in accordance with paragraph 4.2.13(a) or (c) of the revised Specifications for the Design, Operation and Control of Crude Oil Washing Systems.

  4.2.8 With regard to the application of this paragraph a slop tank is considered as a cargo tank.

  4.2.9 Guidelines for the assessment of shadow diagrams

  • (a) Shadow diagrams (to be prepared in accordance with 4.2.9) must be on drawings the scale of which must be at least:

    • (i) 1:100 for tankers of less than 100,000 tons deadweight

    • (ii) 1:200 for tankers of less than 100,000 tons deadweight and above.

  • (b) The drawings must provide at least a plan view, a profile view and an end elevation for each tank, or for tanks considered to be similar.

  • (c) Sufficient detailed drawings of the vessel must be provided to check that all large primary structural members have been included.

  • (d) The term “large primary structural members” is to be construed as those components of a tank structure which contribute significant strength to the ship, such as web frames and girders. It is intended that smaller components such as those that contribute to plate stiffening be excluded. In general the following lists, in conjunction with the diagram showing structural components of cargo tanks, may be used to amplify this interpretation. (See diagram on page 27.)

  • (e) Shadows cast upon the underside of decks, web frames, centre and side girders can be ignored.

  • (f) Calculations must be provided either on the drawing or separately to show how the percentages required by 4.2.8 have been arrived at. The calculations should be itemized so that it is possible to relate each item with a particular shadow area.

  • (g) Where a curved surface is presented to jets it is not necessary to provide exact geometric projections to determine the resultant shadow. A reasonable estimate is acceptable.

  • (h) For the purpose of determining the bottom area of wing tanks, the breadth of the tank is to be taken as the horizontal distance measured across the top of the bottom longitudinal frames to the inside of the shell plating, midway between the tank bulkheads.

  • (i) A swash bulkhead may be taken as a tank boundary. However, in this event the bulkhead must be assumed to have no openings in it.

Include Disregard
(i) web frames (i) longitudinals
(ii) girders (ii) brackets
(iii) stringers (iii) stiffeners
(iv) webs (iv) ladders
(v) main bracket (v) pipe work
(vi) transverses (vi) corrugations on corrugated bulkheads
(vii) crossties in transverse web frames, unless it can be verified by tank inspection that their presence does not affect the cleanliness of the tank. However, for the purpose of making an initial assessment, where there are no more than two crossties and each is less than 1/15 of the total depth of the tank they may be ignored. (vii) face plates

Figure 1

  4.2.10 “Water rinse” means the water washing process carried out in connection with tank cleaning after crude oil washing and is not intended to be construed as limiting the amount of water needed in the process.


  • (a) The “oil monitoring system” referred to in this paragraph means any approved system, including laboratory tests, which verifies that the oil content of the effluent does not exceed the stated level. If laboratory tests are to be conducted, standards contained in resolution A.393(X) should be used as guidance. Oil tankers engaging in a trade where discharge of cargo takes place in one port State and cargo loading in another port State create a special problem with respect to verification. Two alternatives available to confirm the tanker's capability are:

  • (i) The tanker could be required to conduct the entire COW operations at the discharge port, taking inspectors to sea if necessary to observe water washing, handling of departure ballast and discharge of arrival ballast;

  • (ii) Co-ordination between the flag State Administrations and port States to obtain the required documentation.

    However the test is performed, it should be decided on a case-by-case basis taking into account the service of the tanker and the availability of surveyors.

  • (b) The expression “totally discharged to the loading port harbour” which is used in this paragraph shall be so construed as to mean the total quantity of arrival ballast except that quantity which is to be retained on board as specified in section 15 of the Crude Oil Washing Operations and Equipment Manual.

  4.4.3 During bottom washing the stripping capacity shall be at least 1.25 times the total throughput of all the machines that may be simultaneously in use according to paragraph 4.4.3. This does not mean that all the machines in a tank have to be operated simultaneously during bottom washing but the bottom washing may be carried out in steps according to detailed procedures laid down in the Operations and Equipment Manual. The stripping capacity should be at least 1.25 times the throughput of all machines that are in operation simultaneously during any stage of the bottom washing.

  4.4.5 In crude oil tankers having individual cargo pumps in each tank, each pump having an individual piping system, dispensation from the required special small diameter line may be given in cases where the combined amount of oil left in the tank after stripping and the volume of oil in the piping system from the manifold to the tank is less than 0·00085 times the volume of the cargo tank. If a deepwell cargo pump system is provided with an evacuating system for retained oil, the above consideration should also apply.

  5.2(b) This paragraph requires that officers who assume overall charge of a crude oil washing operation must have participated in a crude oil washing operation on the ship for which they are required to undertake the responsibility for cargo discharge, or on a similar ship. However, for new ships, for ships changing for the first time to the carriage of crude oil, for ships new to a particular owner, or for ships which are changing registry in which it may not be possible to acquire the particular experience, the Administration may accept as an alternative:

  • (i) a person such as a shore-based senior officer appointed by the company (additional to the ship's complement) who is experienced in the operation of crude oil washing and is present to advise the ship's personnel; or

  • (ii) a senior member of the crew such as the master, chief officer or cargo control officer who has participated in at least four crude oil washing operations and is on board the ship;

provided that an Operations and Equipment Manual, in a language readily understood by the ship's officers, is available on the ship.

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