The dangers of cargo tank overpressurisation by the introduction of nitrogen whilst undertaking the common practices of drying, purging or padding cargo tanks is well known and documented.
What is less well addressed is the danger of tank overpressurisation when pigging or line blowing, especially when using air as a propellant.
These dangers were first promulgated by IMO in ‘BLG 7/INF.7’ on 19 April 2002, in an ICS submission: ‘Chemical Tankers receiving nitrogen at high flow rates from shore’, where paragraph 13 stated:
This ICS submission resulting in additional questions being added to the standard ISGOTT Ship Shore Safety Check List (SSSCL) 5th Edition published 2006, first at Item 50 of Part ‘A’ Bulk Liquid – General and additionally at Item 11 Part ‘C’ – Bulk Liquid Chemicals, which asked:
If one then delved further into the ISGOTT Guidance at Section 26.4, Item 50 or 11 this stated:
It is obvious from the above excerpts that although the use of air as a propellant is mentioned in both the BLG 7/INF.7 paper and the ISGOTT 5th Edition Guidance, the word ‘air’ is omitted from the ISGOTT 5th Edition SSSCL 50 and 11.
This oversight of the word ‘air’ from the ISGOTT 5th Edition SSSCL Items 50 and 11, has resulted in more than one vessel to answer ‘N/A’ to these questions, when a terminal used air for both pigging and line blowing operations following the discharge of vegetable oils, with the somewhat inevitable result of cargo tank overpressurisation and structural damage resulting in expensive visits to a repair facility.
Has the ISGOTT 6th Edition addressed this anomaly?
The ISGOTT 6th Edition SSSCL address the nitrogen question as Item 60 and 70 respectively on the updated checklist as such:
The new reference 220.127.116.11 closely resembles section 18.104.22.168 in the 5th Edition.
The ISGOTT 6th Edition completely fails to address the use of ‘air’ as a propellant, presumably on the ground that only nitrogen should be used.
However, many terminals, predominately those in Malaysia, Indonesia, Brazil & Argentina etc involved in the production of Vegetable Oils, use air as a propellant for blowing or pigging the shore lines, which may be in the tens, or hundreds of meters in length.
When a terminal requires the line-clearing operations, including pigging, to be conducted using air as a medium, the recommendations below may be considered.
- Follow all ISGOTT and CDI recommendations as if the gas medium to be used is nitrogen.
- Complete the ISGOTT SSSCL Items 60 and 70 as appropriate.
- Conduct such an operation into a tank with as large a vapour space as possible.
- Open the tank main hatch lid.
- Maintain control of the incoming gas under ship’s personnel.
- Always pig a shore line away from the vessel.
The Chemical Distribution Institute (CDI) Publication
The Chemical Distribution Institute (CDI) addressed the practices of the use of nitrogen in the publication ‘CDI Best Practice Recommendations Regarding the use of Nitrogen’ in 2012, a copy of which should be on-board and consulted.