International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 1

Number 4

December 2007

421

Safety and Environmental Concern Analysis for

LNG Carriers

I. D. Er

Istanbul Technical University, Maritime Faculty, Istanbul, Turkey

ABSTRACT: The main attempt of this study is to overview and to discuss occupational safety and

environmental conciseness for the transportation of LNG with gas carriers. LNG is transported by a fleet of

157 LNG tankers of varying sizes from 18,500 m3 to 140,000 m3. This study investigates the technological

development and innovation in LNG transportation while considering safety and environmental standards and

regulations for LNG shipping. It is also originally contributing the process safety for decision making process

for the MET institutions while planning the further needs of LNG industry during the planning of their related

curricula. The further research activities could also be concentrated on quantitative risk evaluations of LNG

equipment, based on risk maintenance and reliability concepts.

1 INTRODUCTION

1.1 LNG Definition

LNG is the common acronym for “Liquefied Natural

Gas”. Natural gas is a mixture of gases that is

produced with or without oil in gas and/or oil fields

and consists primarily of methane. Liquefaction of

methane is achieved by cooling the gas to below

minus 160°C under normal atmospheric pressure.

Natural gas in liquid form is some 600-620 times

less in volume than its gaseous equivalent. The

actual percentage of methane in the natural gas

depends on the characteristics of the oil field where

it is produced.

1.2 World Energy Demand and Supply

World energy production relative with the demand

and supply intention has a great significance on the

determination of usage and transportation. Therefore

Figure 1 illustrates the world energy production in

2005 to give an idea on the change of energy

supply-demand and how the LNG takes a significant

portion in the world total energy production (BP,

2006). When the world energy production in 1990’s

analyzed, similar distribution among the different

kinds of energy production still remains with only

slight differences and increase in: oil 40%, gas 23%,

coal 28%, nuclear 7% and hydroelectric 2% (U.S.

Energy Information Administration, 2005).

It would seem that production share changes in 15

years have been quite insignificant. However,

considering the very large figures involved in the

total world produced energy, even a variation of 1 %

percent is noticeable. The above data clearly indicate

that there is a clear trend of increase of the “clean”

sources of energy (gas and water) and a decrease of

the “dirty” sources (oil and coal), while the nuclear

remains almost constant (U.S. Energy Information

Administration, 2005). This trend appears more

evident comparing the rate of increase of the world

energy consumption with the rate of increase of the

various types of fuels for the 15 years period

between 1991 and 2005 (BP, 2006).

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WORLD ENERGY PRODUCTION ON 2005

37%

23%

28%

Oil

Natural Gas

Coal

Nuclear Energy

Hydro-electricity

Fig. 1. Energy source production rates (BP, 2006)

1.3 LNG Supply and Demand

Up to 1970, almost all natural gas was transported

from the producing countries to the consuming

countries via pipelines. Pipelines are still used to

transport the majority of natural gas today,

particularly in Europe and United States. Figure 2

respectively illustrates LNG Trade by Exporting

Country (BP, 2006). This figure is also useful to give

an idea of the dimensions, the trends, the routes of

gas and LNG in the world.

Fig. 2. LNG trade depending upon the demand of exporting

countries

Fig. 3. World LNG trade expansion

Figure 3 indicates how the transportation of LNG

has increased for the last ten years, since 1996 and

continuing up to the end of 2006 comparing with the

existing pipelines and the maritime transportation

(BP, 2006).

2 WORLD LNG SHIPPING CAPACITY

According to LNG Shipping Solutions, number of

LNG tankers were in operation worldwide as of End

2006 shown in Figure 4,: 16 ships with a capacity of

less than 50,000 cubic meters, 21 in the 50,000 to

120,000 cubic meters range, and 120 larger than

120,000 cubic meters (BP, 2006). Fifty-five ships are

under construction, of which 46 are designed to

carry at least 138,000 cubic meters of LNG

(equivalent to 2.9 Bcf of natural gas). Much larger

ships with 250,000 cubic meters of capacity

(equivalent to 5.3 Bcf of natural gas) are under

consideration, but may not be compatible with all

existing LNG terminals. The addition of new ships

to the fleet will raise total fleet capacity 44 percent

from 17.4 million cubic meters of liquid (equivalent

to 366 Bcf of natural gas) in October 2003 to 25.1

million cubic meters of liquid (equivalent to 527 Bcf

of natural gas) in 2006.

Shipping accounts for 10 to 30 percent of the

delivered value of LNG (depending on the distance

from the reserves to the market), compared with less

than 10 percent for oil, because of the relatively high

cost of manufacturing LNG tankers. Tankers

currently cost $150 to $160 million for a 138,000-

cubic-meter ship, more than double the price of a

very large crude oil tanker which carries 4 to 5 times

as much energy. One reason for this high cost is that

LNG ships require expensive, insulated cryogenic

containment for the cargo.

Fig. 4. LNG fleet expansion statistics

Only thirteen shipyards in the world currently

build LNG tankers: five in Japan; four in Korea,

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three in Europe and one in China. However, India,

and Poland are planning to develop LNG tanker

construction capabilities in their shipyards (U.S.

Energy Information Administration, 2005).

3 SAFETY AND ENVIRONMENTAL

PROTECTION FOR LNG OPERATIONS

3.1 LNG Hazardous and Mitigation

The potential hazards of most concern to operators

of LNG facilities and surrounding communities flow

from the basic properties of natural gas. LNG

hazardous and its consequences to assure safety and

to prevent environmental pollution are subdivided

into four categories. These are the primary

containment; secondary containment, safeguard

systems, and separation distance provide multiple

layers of protection (American Bureau of Shipping,

2000). These measures provide protection against

most hazards associated with LNG.

Primary containment is the first and most

important requirement for containing the LNG

product. This first layer of protection involves the

use of appropriate materials for LNG facilities as

well as proper engineering design of LNG containers

onshore, offshore, and on LNG ships. LNG storage

containers are specially designed, constructed,

installed, and tested to minimize the potential for

failure. The containers are designed to: safely

contain the liquid at cryogenic temperatures, permit

the safe filling and removal of LNG, permit boil-off

gas to be safely removed, prevent the ingress of air,

reduce the rate of heat input consistent with

operational requirements, and prevent frost heave,

withstand the damage leading to loss of containment

arising from credible factors, operate safely between

the design maximum and minimum pressures,

withstand the number of filling and emptying cycles

and the number of cool-down and warming

operations that are planned during the design life

(ABSG Consulting Inc., 2004).

Secondary containment ensures that if leaks or

spills occur at the LNG facility, the LNG can be

fully contained and isolated. In many installations, a

second tank is used to surround the LNG container

and serves as the secondary containment. Secondary

containment systems are designed to exceed

the volume of the LNG container for onshore

installations; dikes surround the LNG container to

capture the product in case of a spill. Secondary

containment should be designed to minimize the

possibility of accidental spills and leaks endangering

structures, equipment, adjoining property, or

adjacent waterways. NFPA 59A requires that LNG

containers be provided with a natural barrier, dike

impounding wall, or combination to contain a leak

or spill of LNG (National Federal Protection

Agency, 2001). Additionally, a drainage system can

be used to remove the LNG to a holding area where

the LNG can vaporize safely. NFPA 59A provides

guidance on the location and sitting of LNG

containers from adjacent property lines, equipment,

and other facilities at terminals. EN 1473 is

performance based in its approach to sitting and

location. The outcomes of a risk assessment can be

used to justify the distance and locations specified

(EN 1473, 1996). On offshore facilities, trenches are

used to channel LNG flow to a safe location where

the LNG can vaporize under controlled conditions.

Safeguards’ goal is to minimize the frequency

and size of LNG releases both onshore and offshore

and prevent harm from potential associated hazards,

such as fire. For this level of safety protection, LNG

operations use technologies such as high level

alarms and multiple backup safety systems, which

include Emergency Shutdown (ESD) systems.

Fire and gas detection and fire fighting systems

all combine to limit effects if there is a release.

The LNG facility or ship operator then takes action

by establishing necessary operating procedures,

training, emergency response systems, and regular

maintenance to protect people, property, and the

environment from any release (SIGTTO, 2003).

There are many safeguards required by

regulations. These can be summarized in terms of

detection, emergency shutdown and fire protection.

The ability to detect a leak of LNG or natural gas is

important for emergency response actions to begin.

Hydrocarbon gas detectors can be used to detect a

natural gas leak if properly located. Hydrocarbon

detectors need to be located higher than suspected

leak points and placed where natural gas can be

expected. Hydrocarbon detectors are generally

located over vaporizers, in metering stations, and

in cargo tanks where natural gas is stored and

processed. Hydrocarbon detectors will not detect

a LNG spill because vapors are insufficient.

Temperature detection is used to sense a spill of

LNG. The set point for the alarm is set low enough

that ambient freezing conditions do not cause a fault

trip. Temperature detection is located where spills

can occur. In some instances, the temperature

detection is used to activate a high expansion foam

system that helps control vaporization.

ESD Systems are required to shut off operations

in the event certain specified fault conditions or

equipment failures occur. They should be designed

to prevent or limit significantly the amount of LNG

and natural gas that could be released. The ESD

systems should be designed such that a spill or leak

does not add to or sustain an emergency condition.

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The ESD systems should fail to a safe condition.

NFPA 59A is pseudo–performance based when it

comes to fire protection. Because of the wide range

in size, design, and location of LNG facilities, NFPA

59A does not identify specific details of fire

protection. The extent of fire protection should be

determined by an evaluation based on sound fire

protection engineering principles, analysis of local

conditions, hazards within the facility, and exposure

to or from other property. The evaluation should, as

a minimum, consider (Mizner & Eyer, 1983):

− The type, quantity, and location of equipment

necessary for the detection and control of fires,

leaks, and spills of LNG, flammable refrigerants,

or flammable gases.

− The type, quantity, and location of equipment

necessary for the detection and control of

potential non-process and electrical fires.

− The methods necessary for protection of the

equipment and structures from the effects of fire

exposure.

− Fire protection water systems.

− Fire extinguishing and other fire control

equipment.

All LNG terminals are required to be provided

with a fire water system. The amount of water will

be determined by the number of fire protection

systems and demand for these systems. The duration

of water supply is typically not a problem because

LNG terminals are generally located next to water.

Fire protection systems for LNG facilities consist

of water spray, high expansion foam, dry chemical,

or a combination of these. Water spray is used to

control radiant heat exposure on equipment and

structures. LNG pool fires are neither controlled nor

extinguished by water. In fact, the application of

water on the LNG surface will increase the

vaporization rate, and thus there is the potential

to increase burning rate with negative consequences

on fire control. The use of water spray systems

should be carefully considered in the design.

High expansion foam can be used to control

the vaporization rate on the surface of an LNG spill.

The foam works by warming the LNG vapors and

reducing the fire thermal radiation back to the LNG

pool, thereby reducing the LNG burning rate.

High expansion foam is generally provided for

impounding areas or where a LNG pool can form.

Dry chemical extinguishing systems are used to

extinguish an LNG fire. The dry chemical should be

applied such that the surface is not agitated, which

will allow additional vaporization. Dry chemical

systems have been installed at unloading area, LNG

pumps, boil-off compressors, and LNG vaporizers.

Separation (Safety Excursion Zones) LNG facility

designs are required to maintain separation distances

to separate land-based facilities from communities

and other public areas. National regulations have

always required that LNG facilities be sited at a safe

distance from adjacent industries, communities, and

other public areas. Also, safety zones are established

around LNG ships while underway in territorial

waters of a nation and while moored. The safe

distances or exclusion zones are based on LNG

vapor dispersion data, thermal radiation contours,

and other considerations as specified in regulations

(Paik et al. 2001). Most hazard analyses for

LNG terminals and shipping depend on computer

models to approximate the effects of hypothetical

accidents. Regardless of the cause, the formation of

a methane/ air mixture and its movement depends on

the quantity of the spill, whether on land or water,

atmospheric stability, wind direction and velocity,

and temperature of the atmosphere and water.

3.2 Emergency Procedures for Low Temperature

Effects and Pressure

Spillage of LNG may lead to the following dangers:

Fire starting from the vapor, which is generated

during the spillage, brittle fractures of the ship

structures, which are in contact with the spilled

LNG, accidental contact of LNG or its vapor with

ship personnel. However, should a spillage be

detected on a LNG ship the measures to be taken are

the following: Stopping the flow, avoiding contacts

with liquid and vapor, extinguish all possible

sources of ignition, flooding the area where the

spillage happened with a large amount of water in

order to disperse the spilled LNG and to prevent the

risk of brittle fracture.

As liquefied gas cargoes are often shipped at low

temperatures it is important that temperature sensing

equipment is well maintained and accurately

calibrated. Hazards associated with low temperatures

include (Bainbridge, 2003):

Brittle Fracture: Most metals and alloys become

stronger but less ductile at low temperatures (i.e. the

tensile and yield strengths increase but the material

becomes brittle and the impact resistance decreases)

because the reduction in temperature changes the

material’s crystal structure. Normal ship building

steels rapidly lose their ductility and impact-strength

below 0°C. For this reason, care should be taken to

prevent cold cargo from coming into contact with

such steels, as the resultant rapid cooling would

make the metal brittle and would cause stress due to

contraction. In this condition the metal would be

liable to crack. The phenomenon occurs suddenly

and is called ‘brittle fracture’. However, the ductility

and impact resistance of materials such as

aluminum, austenitic and special alloy steels and

nickel improve at low temperatures and these metals

425

are used where direct contact with cargoes at

temperatures below -55° C is involved.

Spillage: Care should be taken to prevent spillage

of low temperature cargo because of the hazard to

personnel and the danger of brittle fracture. If

spillage does occur, the source should first be

isolated and the spilt liquid then dispersed. (The

presence of vapor may necessitate the use of

breathing apparatus). If there is a danger of brittle

fracture, a water hose may be used both to vaporize

the liquid and to keep the steel warm. If the spillage

is contained in a drip tray the contents should be

covered or protected to prevent accidental contact

and allowed to evaporate. Liquefied gases quickly

reach equilibrium and visible boiling ceases, this

quiescent liquid could be mistaken for water and

carelessness could be dangerous. Suitable drip trays

are arranged beneath manifold connections to

control any spillage when transferring cargo or

draining lines and connections. Care should be taken

to ensure that unused manifold connections are

isolated and that if blanks are to be fitted the flange

surface is clean and free from frost. Accidents have

occurred because cargo escaped past incorrectly

fitted blanks. Liquefied gas spilt onto the sea will

generate large quantities of vapor by the heating

effect of the water. This vapor may create a fire or

health hazard, or both. Great care should be taken to

avoid such spillage, especially when disconnecting

cargo hoses (International Tanker Owners Pollution

Federation, 2007).

Cool down: Cargo systems are designed to

withstand a certain service temperature; if this is

below ambient temperature the system has to be

cooled down to the temperature of the cargo before

cargo transfer. For LNG and ethylene the stress and

thermal shock caused by an over-rapid cool down of

the system could cause brittle fracture. If tanks are

fitted with spray equipment it should be used, and

the liquid distributed around the inside of the tank as

evenly as possible to avoid thermal stresses. Pressure

build-up in the tanks will restrict the rate at which

liquid is introduced. The use of a vapor return line is

recommended to avoid cool down and loading rates

being dictated by the capacity of the re-liquefaction

plant. Cargo pipe-work and equipment should be

cooled down by circulating liquid at a controlled

rate. The system should reach liquid temperature

sufficiently slowly to prevent undue thermal stresses

in materials or expansion

contraction fittings. The

temperature sensors will indicate when liquid is

present on the tank bottom, but the liquid should be

introduced slowly until the bottom is completely

covered. The cool down of tanks may cause a

pressure reduction in sealed hold or inter-barrier

spaces, and dry air, inert gas or dry nitrogen should

be introduced in order to maintain a positive

pressure. This is usually done by automatic

equipment. Pressure gauges should be observed

regularly during cool down to ensure that acceptable

pressures are maintained.

Ice Formation: Low cargo temperatures can

freeze water in the system leading to blockage of,

and damage to, pumps, valves, sensor lines, spray

lines etc. Ice can be formed from moisture in the

system, purge vapor with incorrect dew point, or

water in the cargo. The effects of ice formation are

similar to those of hydrates, and anti-freeze can be

used to prevent them.

Rollover: Rollover is a spontaneous rapid mixing

process which occurs in large tanks as a result of a

density inversion. Stratification develops when the

liquid layer adjacent to a liquid surface becomes

denser than the layers beneath, due to boil-off lighter

fractions from the cargo. This obviously unstable

situation relieves itself with a sudden mixing, which

the name ‘rollover’ aptly describes. Liquid

hydrocarbons are most prone to rollover, especially

cryogenic liquids. LNG is the most likely by virtue

of the impurities it contains, and the extreme

conditions of temperature under which it is stored,

close to the saturation temperatures at storage

pressures. If the cargo is stored for any length of

time and the boil-off is removed, evaporation can

cause a slight increase in density and a reduction of

temperature near the surface. The liquid at the top of

the tank is therefore marginally heavier than the

liquid in the lower levels. Once stratification has

developed rollover can occur (Pitblado et al. 2004).

No external intervention such as vibration,

stirring or introducing new liquid is required to

initiate rollover. The response to a small temperature

difference within the liquid (which will inevitably

occur in the shipboard environment) is sufficient to

provide the kinetic energy to start rollover, and

release the gravitational driving forces which will

invert the tank contents. The inversion will be

accompanied by violent evolution of large quantities

of vapor and a very real risk of tank over-pressure.

Rollover has been experienced ashore, and may

happen on a ship that has been anchored for some

time. If such circumstances are foreseen the tank

contents should be circulated daily by the cargo

pumps to prevent rollover occurring. Rollover in a

ship on passage is most unlikely. Essentially,

stratification and the subsequent rollover process are

confined to shore LNG storage. However, if the use

of LNG carriers for floating storage were to be

introduced, personnel manning such vessels would

need to be as aware of the problem and as vigilant to

avoid rollover as their counterparts managing shore

based storage.

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4 CONCLUSION

LNG is an extremely cold, nontoxic, non-corrosive

substance that is stored at atmospheric pressure. It is

refrigerated, rather than pressurized, which enables

LNG to be an effective, economical method of

transporting large volumes of natural gas over long

distances. LNG itself poses little danger as long as it

is contained within storage tanks, piping, and

equipment designed for use at LNG cryogenic

conditions.

This study is mainly concentrated on safety and

environmental awareness issues for LNG carriers

taking into account the expanding tendency of world

LNG shipping capacity. This would enable the not

only the industry itself but the Maritime Training

and Education (MET) institutions that are providing

relevant training on LNG safety management

system. Therefore due to its nature and the core

activity of LNG shipping, the overview of

methodology on LNG hazardous and their mitigation

is discussed firstly to clarify the major concern on

the assurance of safety and the protection of

environment. Then the emergency procedures for

low temperature effects and pressure are discussed in

advanced signifying the brittle fracture, spillage,

cool down, ice formation and roll over which are

only applicable for the transportation of LNG with

their probable consequences. These interpretations

could be utilized during the planning of training

needs of LNG ship operators which could be

designed in terms of basic and specific familiariza-

tion in accordance with STCW convention and for

MET institutions for the enhancement of their

curricula related with LNG operations.

Consequently the LNG industry has an excellent

safety record. This strong safety record is a result of

several factors. First, the industry has technically and

operationally evolved to ensure safe and secure

operations. Technical and operational advances

include everything from the engineering that

underlies LNG facilities to operational procedures to

technical competency of personnel. Second, the

physical and chemical properties of LNG are such

that risks and hazards are incorporated into

technology and operations. Third, the standards,

codes, and regulations that apply to the LNG

industry further ensure safety.

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