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INTERTANKO
A Guide to Crude Oil Washing
and Cargo Heating Criteria
May 2004
G)
I NTERTANKO
International Association of Independent Tanker Owners
Acknowledgements INTERTANKO is grateful to the author of 'A Guide to Crude Oil Washing and Cargo Heating Criteria', Timothy Gunner, and the INTERTANKO Safety Technical and Environmental Committee (ISTEC) for their valuable contribution to this publication.
,
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© INTERTANKO 2004 Whilst every effort has been made to ensure that the information contained in this publication is correct, neither the authors nor INTERTANKO can accept any responsibility for any errors or omissions or any consequences resulting therefrom. No reliance should be placed on the information contained in this publication without independent verification.
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A Guide to Crude Oil Washing and Cargo Heating Criteria May 2004
TABLE OF CONTENTS 1
Introduction
1
2
The COW Manual 2.1 Section 9 of the COW Manual
2
3
3
What is crude oil?
6
4
The physical behaviour of crude oil 4.1 The physical behaviour of aromatic crude oils 4.2 The physical behaviour of paraffinic crude oils
6
7
9
5
The Cloud Point temperature determination of a crude oil 5.1 Methods for determining the Cloud Point temperature onboard a tanker 5.1.1 Bondi Test method 5.1.2 Primary Cloud Point test method 5.1.3 Density method 5.1.4 The Cloud Point temperature model 5.2 The predicted extent of precipitated sludge before COW
10
11
11
13
16
17
18
6
The practical implications of the Cloud Point determination 6.1 MARPOL regulations and commercial implications 6.2 The crude oil washing of sludge and some practical implications
19
19
21
7
COW equipment and procedures 7.1 COW machines and tank cleaning nozzles 7.2 COW pipeline systems 7.3 Stripping systems 7.4 Crude oil washing concurrent with cargo discharge 7.5 "Closed cycle" crude oil washing
23
23
25
26
27
28
8
Crude oil washinp of sludges 8.1 The theory and background 8.2 Practical circumstances affecting the crude oil washing of sludges 8.3 An alternative, practical programme for the crude oil washing of sludge
29
29
31
34
9
Closing remarks
36
1 Introduction Regulations laid down in the 1978 Protocol to the 1973 Marine Pollution Convention (MARPOL 73/78) require the cargo tanks of crude oil tankers to be cleaned using a procedure called crude oil washing (COW). With the COW procedure the crude oil cargo itself is used as the cleaning medium. During the 196,6s it was discovered that crude oil, when applied to the cargo still remaining on tank floors and c1in~ing to the tank structures, using tank cleaning machines, effectively dissolves and dilutes these residues and mixes it in with the rest of the cargo which is being discharged ashore by the cargo pumps. Prior to the advent of COW, cargo tanks were washed with sea water on their ballast voyage to the next loading port. The mixture of oil and cleaning water resulting from this type of cleaning operation could settle out in the tanker's slop tanks with the decanted water being discharged overboard into the ocean. Consequently, this operation resulted in inevitable operational discharges of oil-water mixture into the sea. However, the use of crude oil to COW the tanks means that the solvent action of the crude oil makes the process far more environmentally friendly than when water is used. Additionally, after undertaking COW, th volume of cargo residues left in the tanks is greatly reduced removing the subsequent risk of opera ional discharges at sea. Modern ankers are designed with segregated ballast tanks (SST), and there are only a few stipulated occasio~s on which seawater comes into contact with the oil cargo system during the course of normal tbnker operations. The requirement for new crude oil tankers to be built with double hulls,
introduc~d
in the 1990s, has further improved the efficiency of COW operations because more of
the structural support members are placed outside the cargo tank and on these types of ships, the amount
bf crude oil residues left in the cargo tank following discharge is much reduced. Overall, the
COW prbcedure and ship design changes have greatly reduced the need for operational discharges from tankers.
RegUlati~n
13S of Annex I of MARPOL 73/78 requires that the COW installation and arrangements
onboardla tanker should comply with the provisions of the "Specifications for the Design, Operation and Conkol of Crude Oil Washing Systems"adopted by the International Maritime Organization (IMO)
in 1978'fThe COW regime requires that before departure on a ballast voyage, after the complete discharg of cargo, sufficient tanks shall have been crude oil washed to preclude the ballasting of a cargo ta~k without it having been crude oil washed. On SST ships approximately 25 per cent of the crude oi carrier's cargo tanks need to be washed, in the prescribed manner, on every voyage for sludge c ntrol purposes provided that no tank need be crude oil washed for sludge control purposes more th n once in every four months. For tankers with insufficient SST capacity, the number of tanks to be crude oil washed has to be increased above this minimum level in order to render sufficient cargo
ta~ks "clean" enough (as defined
by the regulations) to take onboard enough water ballast to
achieve he tanker's required sailing ballast draught for the voyage.
Page 1
INTERTANKO - A Guide to Crude Oil Washing and Cargo Heating Criteria - May 200
In addition to the regulatory controls governing the use of COW, cornmercial or charter party requirements may require the tanker operator to carry out a greater or lesser degree of COW than the specified rninimum in order to rnaximise the discharge of the crude oil cargo. Notwithstanding these commercial pressures for the extent of COW to be undertaken, at no time should a tanker undertake less than the minimum levels specified in paragraph 6 of Section 1 of the mandatory onboard COW Manual.
Although the MAR POL 73/78 COW regime has proved to be eminently successful in minimising tanker operational discharges and improving cargo outturns during the last two decades of the 20th century, the tanker industry has also been learning more about the behaviour of crude oil cargoes over the period. A number of research projects' have led to a better understanding of the COW process and how it could be further improved. As a result of this work and at the initiative and suggestion of INTERTANKO, in 1999 the IMO adopted amended COW requirements that are laid down in the revised "Specifications for the Design, Operation and Control of Crude Oil Washing
Systems". These revised Specifications can be found in the 2000 Edition of the "Crude Oil Washing Systems" publication, issued by the IMO. From an operational perspective the changes provide a more realistic and accurate way of determining the suitability of a crude oil for use in crude oil washing.
2 The COW Manual The 2000 Edition of the "Crude Oil Washing Systems" publication contains the following: -Revised "Specifications for the Design, Operation and Control of Crude Oil Washing Systems" (IMO Resolution A.446(XI), as amended by Resolutions A.497(XII) and A.897(21)); -Standard format for the "Crude Oil Washing Operations and EqUipment Manuals" (IMO Resolution MEPC.3(XII), as amended by Resolution MEPC.81(43)); -Examples of "Crude Oil Washing Operations and Equipment Manuals";
-"Guidelines for In-port Inspection of Crude Oil Washing Procedures". It is a MARPOL requirement that every crude oil carrier has onboard a reference manual, known as the "Crude Oil Washing Operations and Equipmen: Manual", in order to assist the responsible tanker officer in carrying out the prescribed COW operation correctly. Each tanker's COW Manual is customised to meet the specific requirements of the particular ship. The COW Manual is comprised of the following 17 sections: 1. The text of the revised specifications. This section contains the complete text of the revised "Specifications for the Design, Operation
and Control of Crude Oil Washing Systems" and the agreed interpretations of certain of the provisions of the revised Specifications.
1
T.J. Gunner, "Pollution Control for Crude Oil Tankers and the Accuracy in the Measurementof theirCargoes" - University of Wales .1993
INTERTANKO - ,A, Guide io Crude Oil \jVElsh\\lQ
