Safety, practical and legal aspects of handling cargoes with high H2S content
Igor Sterzhantov@2011
Safety, practical and legal aspects of handling cargoes
with high H2S content
By Igor Sterzhantov©2011
www.lawandsea.net
1. Introduction - Safety Aspects
By their nature all crude oils and petroleum products are mixtures of a wide range of
hydrocarbon compounds with the tendency to produce gas, which tendency also called
volatility. One of characteristics of the volatility of crude oils and petroleum products is their
ability to continuously vaporise, or, in other words, they liberate gas into the atmosphere
continuously while being stored in tank space.
Clear realization of liquid’s ability to continuously vaporise is of vital importance in
understanding of the principles of handling of H2S cargoes. Vapour pressure or, simplified,
rate of vaporisation of gases liberated from the crudes and petroleum products depends on
its temperature, constituents and the ratio of gas to liquid by volume in tank1.
Gas Hydrogen Sulphide (H2S) is a product of vaporisation of the constituents mentioned
above. It is a very toxic, corrosive and flammable gas. It has a very low odour threshold and
a distinctive odour of rotten eggs. H2S is colourless, is heavier than air, has a relative vapour
density of 1.189, and is soluble in water2. Danger of H2S gas is well known and recognized
within shipping industry. Exposure to 700ppm in air rapidly induces unconsciousness (few
minutes) and death, when more than 700ppm – immediately fatal (ISGOTT, para 2.3.6.1).
Presence of H2S is a common occurrence in crude oil, naphthas and fuel oil cargoes, it is
imperative therefore that handling of such cargoes require particular attention and special
safety measures during loading, carriage and discharging. Due to negative environmental
effect of H2S gas vapours its presence in cargo tanks atmosphere is limited to certain levels
in many countries and that also places additional legal constraints for all parties involved, i.e.
cargo sellers and buyers, charterers and shipowners.
ISGOTT, at sections 2.3.6.1 - 2.3.6.7 describes exposure limits, typical effects of exposure to
H2S and enumerates safety procedures for handling cargoes and bunkers with H2S. Notable
that sec. 11.1.9. – Loading Cargoes Containing Hydrogen Sulphide (H2S) provides practical
guidance on operational measures to minimise “the risks associated with loading cargoes
containing H2S” (emphasis mine), while no specific guidance considered necessary for
discharging operation.
Naturally, during loading operation H2S vapours displaced by liquid cargo entering the tank
space through cargo piping system and escape into the atmosphere via ship’s venting system
if no vapour return line connected to ship’s manifolds. Therefore safe conduct of this
operation often requires enhanced risk assessment and such additional precautions as use of
1
International Safety Guide for Oil Tankers and Terminals, 5th edt.,ICS, OCIMF, IAPH (ISGOTT) section
1.1.1
2
ISGOTT section 2.3.6
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Safety, practical and legal aspects of handling cargoes with high H2S content
Igor Sterzhantov@2011
personal H2S gas monitoring equipment, EEBD and SCABA (see ISGOTT sec 2.3.6.4) to
protect personnel working on deck of tanker from poisoning.
As a part of risk assessment it should be taken into account that if there is no wind to
disperse the vapours or if the wind direction takes cargo vapours towards the
accommodation the loading operation must be suspended until conditions change3.
Risk of H2S poisoning is much less during discharging because no vapour emission goes out
from ship’s tanks, i.e. while volumes of cargo diminish in the course of discharging,
simultaneously inert gas produced by ship’s IG plant replaces empty volumes and keeps
constant positive pressure in tanks and lines.
2. Sources of H2S Vapours - Practical Aspects
Environmental laws in ports of loading and discharging force the vessel to arrive in port
with H2S content below certain levels. Before considering the ways of handling this issue it
is pertinent to underline that origin of H2S vapours in cargo tanks may be of two sources:
a) vapours from remains of previous, H2S containing cargo, i.e. from ROB;
b) vapours from cargo which is being loaded is of high H2S content.
In the first case, to reduce H2S content up to required level it would be necessary upon
completion of discharging operation to carry out purging of tanks with inert gas to dilute or
displace H2S vapours from tanks. Some standard additional clauses in charterparties address
this issue by way of obliging the owners’ to warrant that H2S shall have been reduced to
below the certain levels prior to arrival to load port4. Generally speaking, purging operation
does not offer any difficulty but requires time, which depends on H2S content, outside
temperature and characteristics of IG plant. It is also necessary to underline in this context,
that any significant ROB of previous cargo in tanks will considerably complicate this task
due to constant emission of H2S gas from remains of last cargo5.
In the second case, presence of H2S gas in vapours of cargo being loaded give rise to a
number of difficult legal issues. First of all, the shippers and terminal are usually reluctant to
disclose, as required by ISGOTT, true H2S content in cargo to the master at the preliminary
stage of loading operation. This reluctance has a natural explanation that the terminal likes to
avoid to be exposed to risks related to safety of H2S cargo handling and, moreover, any such
disclosure will also alert the charterers and/or cargo owners and may provoke a concern
over the cargo quality and probably over its tradability as well. Subsequently, when the vessel
is under way to the discharging port and presence of hydro sulphur has been asserted by
crew, the owners may find themselves in difficult position when they have to comply with
the laws and regulations of arrival port with respect of H2S content. Unlike the situation
3
ISGOTT section 11.1.9.
4
SAC4. BP H2S AND MERCAPTANS CLAUSE (REVISED AUGUST 2007 AND OCTOBER 2009):
(A) OWNERS UNDERTAKE THAT PRIOR TO ARRIVAL AT THE LOAD PORT THE HYDROGEN SULPHIDE (H2S)
CONTENT IN THE VESSEL'S TANK ATMOSPHERE SHALL HAVE BEEN REDUCED TO BELOW THE LOWER OF:
(I) THE THRESHOLD LIMIT VALUE ("TLV") AS DESCRIBED IN ISGOTT AS AMENDED FROM TIME TO TIME –
(II) ANY TLV APPLICABLE BY VIRTUE OF LOCAL OR NATIONAL LAW, RULE, REGULATION AND/OR PORT
REQUIREMENT.
5
See above about dependence of vapour pressure upon the ratio of gas to liquid by volume in tank.
ISGOTT section 1.1.1
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Safety, practical and legal aspects of handling cargoes with high H2S content
Igor Sterzhantov@2011
with empty tanks containing some H2S vapours which can be managed by purging with inert
gas, when tanks are full with cargo such method will not give positive result. Due to volatile
nature of crude oil and other petroleum products, they continuously vaporise gases, H2S
including, from the surface. The rate of vaporisation or vapour pressure, as mentioned
above, depends mainly on temperature and the ratio of gas to liquid by volume in tank.
Obviously, introduction of inert gas in small empty volumes of loaded tanks will neither
displace nor dilute H2S, but rather ventilate it out of tanks space. Temporarily some
reduction of H2S content can be achieved, but this content will rise again quickly as soon as
introduction of inert gas will be stopped.
Apart from zero probability to reach any desirable reduction of H2S, such operation is
obviously in direct contradiction with that interpretation of safety measures for handling
H2S cargoes stated in ISGOTT. Their general idea is to retain or seclude H2S vapours
liberated by cargo from contact with the crew and terminal staff as much as possible or, if
such retention is not practically possible, to ensure safety of personnel by monitoring
atmosphere and using personal protective equipment such as breathing apparatus.
Dispersion of H2S gases by ventilation from ships tanks leads to dangerous exposure of
personnel to poisonous vapours.
Read more about purging in 5.Annex.Purging below.
3. Recommended Procedures
There are several actions which may be recommended to the master as below:
The first step is to request, better in writing and well in advance of loading operation,
Material Safety Data Sheet for the cargo to be loaded. IMO Res. 150(77) requires from the
supplier to provide vessel, before commencement of cargo or bunker operation, with
Material Safety Data Sheet (MSDS) for the product which is going to be loaded.
The second step is to examine MSDS, which shall be submitted to the ship’s staff as a part
of exchange of information during pre-loading safety conference. Master must insist on H2S
content to be specifically mentioned and reflected in Ship/Shore Safety Checklist in the
course of compulsory safety routine. Unfortunately MSDS not always available before the
loading starts, and when available, it seldom, for the reasons mentioned above, states correct
data about H2S content in cargo. In such cases, without knowledge and forward notice as to
dangerous nature of cargo, ship’s crew may potentially be exposed to serious risk of
poisoning.
The third step is to take independent measurements of H2S content in tanks as soon
possible and carry out risk assessment. If or when presence of H2S in cargo vapours has
been established then it is necessary to ensure crew safety during loading.
Read the rest of this article at:
http://www.lawandsea.net/maritime/master_h2s_3.html
http://www.lawandsea.net/maritime/master_h2s_4.html
http://www.lawandsea.net/maritime/master_h2s_5.html
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