Appendix 2 - Guidelines for Testing Prior to Backloading
Clasification Society 2024 - Version 9.40
Statutory Documents - IMO Publications and Documents - International Codes - OSV Chemical Code - Code for the Transport and Handling of Hazardous and Noxious Liquid Substances in Bulk on Offshore Support Vessels - Appendix 2 - Guidelines for Testing Prior to Backloading

Appendix 2 - Guidelines for Testing Prior to Backloading

 1 General

1.1 The results of these tests will allow the master, through confirmation with the attached checklist, to establish if the backload is acceptable for carriage on board the vessel. Acceptance is based on the reported analytical data and the measured physical properties, the known nature of the chemical composition and the previous cargo carried in the vessel's tanks. A generic risk assessment should be available on board the vessel and updated when new information and circumstances become apparent. Offshore installation crew should be aware that in certain circumstances the master of the vessel may require advice from the vessel's onshore technical advisers and that a response from onshore may take time to receive.

1.2 Recognizing the relatively complex nature of the cargo, the material intended for backloading should be subject to a series of test to provide an indicative overview of the constituent composition and reactive properties of the material.

1.3 The tests carried out prior to backloading should reflect the conditions in the vessels tanks, that is to say there will be no agitation and no forced ventilation unless specifically required or requested.

1.4 If there is any doubt regarding the result of the test, the test should be repeated and reviewed.

2 Testing prior to backloading

2.1 Flashpoint

The minimum acceptable flashpoint of 60C (Pensky-Martens closed cup method or equivalent) is applicable to wet bulk waste. Sampling should be set up to detect the worst case situation, particularly where there is potential for crude oil or condensate contamination where the oil will rise to the surface of the tank. Base oils typically have flashpoints in the range of 70 to 100C. If the only oil component in a bulk waste is base oil then the flashpoint cannot be lower than that of the base oil itself. If the flashpoint is relatively low (60 to 70C) an explanation should be provided before the analysis form is presented to the vessel's master. Prior to sampling, the material should be left without agitation for at least 30 min and then surface sampled.

2.2 Lower Explosive Limit (LEL)

The LEL gas detector will confirm potential flashpoint issues. The noxious gas test is modified to simulate the unvented vessel's tanks. The sample is placed in a closed container with a sampling port on top and left to equilibrate for 30 min. A tube is then connected from the port to the gas analyser and the sample is analysed. The flashpoint and LEL results should be consistent with each other. LEL gas meters are normally set so that the alarm goes off in the range of 10 to 20% LEL methane equivalent. Any number above 25% would be considered high. Other gases potentially present can have a different LEL range than methane.

2.3 Hydrogen sulphide (H2S)

2.3.1 H2S most commonly arises from the activity of sulphate reducing bacteria (SRB). SRB will become active provided there is a "food" source and low oxygen conditions. This would be typical of stagnant oil-contaminated fluid stored for a long time. H2S is an extremely poisonous gas which is heavier than air. The maximum exposure limit is 10 ppm over an 8 hour period. Offshore sensors and routine offshore analysis methods will detect if H2S is a potential problem in backloads. In the event of a positive test another sample should be collected to confirm the result. If this second result is positive further work may be required to determine the source of the H2S. The sample should be taken from below the surface of the unagitated tank. Most oil will be in the top layer and will give a worst case oil content.

2.3.2 As a precaution, treatment of the material may be required. The SRB organisms thrive in a pH range of 5.5 to 8.0. The lower the pH the greater the breakout of H2S. The backload can be treated on the installation to prevent breakout of H2S in the vessel tanks. Biocides kill the bacteria but do not remove dissolved H2S. H2S scavengers will remove dissolved H2S but do not stop biological activity. Caustic soda will raise the pH and prevent H2S gas breakout. In the event that H2S is detected, tests should be carried out offshore to determine the best treatment prior to backloading. After treatment a final H2S test should be carried out to confirm zero H2S and noted on the analysis form before the hose is connected to the vessel for backloading.

2.4 pH

The pH of seawater is typically 8.3. Oil mud is alkaline and could raise the pH slightly. Cement contaminant is highly alkaline. In general alkaline pH (above 7) protects from corrosion. Highly alkaline materials can be caustic and require care in handling. Cement and sodium silicate can lead to high pH value. Low pH (less than 4) is highly acidic and an explanation should be provided on the analysis form. Acids such as citric acid or acidic chemicals such as hydrochloric acid can lead to low pH. It should be noted that pH less than 9 means that H2S will already have broken out as a gas.

2.5 Retort analysis (solids, water, oil volume %)

This should match the estimated composition (volume %) on the analysis form. It should be noted that it may be difficult to get representative samples if the liquid tends to separate. Some divergence is expected, for example if oil is noted as 5%, the range could be 3 to 10%. If separation is likely a range is preferred, for example 5 to 10%. The solids component can form a residue in the vessel tank and be a potential location for SRB activity and H2S.

2.6 Specific gravity (SG)

The specific gravity of common water-based fluids cover the range of 1.03 (seawater), sodium chloride (1.2), and calcium chloride (1.33). Rarely used brines such as caesium formate can reach an SG of 2.2. Oil mud is typically 1.1 to 1.5, but can exceed 2.0. Mixtures will have intermediate values, most tending towards 1.03 as seawater is the major component. It should be noted that if mixtures separate the top half can have a different density than the bottom half.


Sample description   Sample reference  
Vessel   Date  
Offshore asset   Producer  
Well name & number   Waste company  
Total number of barrels   Waste note number  
Component name Concentration Units MSDS available
    % Volume  
    % Volume  
    % Volume  
    % Volume  
    % Volume  
    % Volume  
    % Volume  
    % Volume  
Test Method Units Results Range of results / Guidance
Salinity (Chloride) Titration mg / l    
Flashpoint Closed cup flashpoint C   Should be >60C to backload
If flashpoint is low (<70C) then explanation should be provided
Gas test (H2S) Gas meter ppm   Should be zero
Indication of bacterial activity
Gas test (LEL) %   <25%, ideally zero.
Meter alarm typically set to 10-20% LEL.
Should be consistent with flashpoint
Gas test (Oxygen) %    
pH pH meter     4-11 is acceptable range for OSV tank coatings.
SHOULD be 9.5-10.5 to keep any H2S in solution
Water Retort % Volume    
Oil content Retort % Volume   Confirm retort report agrees with appendix 10 F,
Section 4 components and waste consignment note
Solids Retort % Volume   Confirm retort report agrees with appendix 10 F,
Section 4 components and waste consignment note
Bulk specific gravity   SG   <2.5
If >2.5 seek further guidance on vessel capability
Date and time of analysis        
Analysis to be conducted by person competent to do so Comments
(Yes / No / Details)
This liquid has been analysed as per GOMO appendix 10 F and it is my opinion that it is safe for carriage in a standard clean OSV bulk tank.  
This liquid has been analysed as per GOMO appendix 10 F and will be loaded into a tank with residues / existing cargo. Compatibility has been risk assessed and found to be safe for carriage.  
H2S Avoidance
Details of mandatory wet bulk waste treatment with biocide (chemical / quantity)  
Details of wet bulk waste treatment in order to produce pH of between 9.5 and 10.5 (chemical / quantity)  
Has waste handling facility been informed of volume and ETA onshore? (Yes / No)  
Does the waste handling facility have the capability to take off the waste at the first port call? (Yes / No)  
Name Signature Date
Operations Representative        

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