Moss Type B Tank
LNG Fundamental Knowledge and Understanding
General overview
Type ‘B’ tanks are generally spheres.
Construction
The containment system is subjected to a thorough stress analysis and must include fatigue life and crack
propagation analysis. Because of these design factors, the Type B tank requires only a partial secondary
barrier, comprising a drip tray and a splash barrier. When carrying flammable cargoes, the hold space in
this design is filled with either IG or dry air. When dry air is used, the hold space must be capable of being
inerted.
A protective steel dome (weather cover) covers the primary barrier above deck level and insulation is
applied to the outside of the primary barrier surface. Although the spherical Type B tank was originally
designed for LNG carriage, it can also be used for LPG carriers, primarily those carrying ethylene.
Insulation
The cargo tank is a self-supporting, predominantly aluminium sphere that is carried by a cylindrical skirt at
the equator. The containment system is based on the principle of ‘leak before fail’.
The insulation system consists of two layers:
Layer 1 = phenolic resin foam
Layer 2 = polyurethane foam.
The insulating structure is reinforced by wire nets and covered with an AL-PET aluminium foil sheet. The
insulation also doubles as a spray shield.
Any leaked cargo drains by gravity through the annular space, between the cargo tank and the insulation,
to the bottom of the sphere. An opening in the insulation leads to a rupture disc that opens under
cryogenic temperatures, allowing the liquid to drain into the drip-pan in the cargo hold space beneath.
Figure 1.13 Moss Type B tank
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Cargo tank vent systems
LNG Shipping Knowledge 3rd Edition
The IGC Code for Gas Carriers requires at least two pressure relief valves of equal capacity to be fitted to any
cargo tank with a volume greater than 20 m3. On tanks with a volume less than 20 m3, one pressure valve is
considered to be sufficient.
The types of valves normally fitted are either spring loaded or pilot operated relief valves (PORV). PORVs
are commonly used on cargo tanks. Spring-loaded relief valves are generally used on cargo pipework and
equipment.
Although the Moss spherical Type B tank can operate at the slightly higher pressure of 1.9 barG (emergency
discharge), the normal relief valve set pressure for both Type A and Type B tanks is 0.25 barG.
Pressure relief valve design must take account of at least two basic requirements:
• Over-pressurisation
• overheating of the tank contents.
The Classification Societies have defined rules that determine the amount of gas to be discharged as a
function of the liquid volume in the tank, the dimensions of the tank and the amount/effectiveness of the
thermal insulation of the tank.
The rules also create a requirement for high gas flow rates.
Pressure relief valves are, therefore, principally designed to satisfy the following requirements:
• To provide an effective seal until the pre-set opening pressure (set pressure) is reached
• achievement of a precise and clean release of gas, regardless of cargo temperature
• complete opening of the valve to give full flow
• complete closing of the valves at a pressure slightly below the opening pressure
• the operation has to be free from the effects of frosting that may occur within the valve
• the operation has to be unaffected by acceleration due to movement in a seaway, by list or by trim
• the valve must consistently open at the prescribed set pressure
• pressure relief valves should be type tested to ensure that the valves have the capacity required
• pressure relief valves should be set and sealed by a competent authority acceptable to the flag
State and a record of this action, including the values of set pressure, should be retained aboard the
ship
• back pressure in the vent pipe system should not impede full flow of the valve.
What happens if a leak occurs in the Moss containment system?
Such a leak might be caused by a crack in a weld. The containment system is fitted with a gas and leak detection
system.
Any cargo flow from a leak would flow between the tank and the insulation and be collected in the drip pan in the
hold space, which is insulated with polystyrene and a stainless steel layer. Gas and liquid monitoring alarms would
quickly and accurately detect the presence of cargo.
Any liquid leakage from the northern hemisphere of the tank will be collected at the upper ring of the stiffening skirt
and led to one of 4 drain pipes, fore, aft, port or starboard, and then led to the drip pan. In the southern hemisphere
of the tank it will be led to a drain pipe at the south pole and then routed to the drip pan.
Prior to liquid entering the drip pan, it will pass through a rupture disc (bursting disc), designed to fail at cryogenic
temperatures.
The temperature sensor in the drip pan will indicate whether it is a cryogenic temperature, a liquid LNG leakage, or
above 0°C, indicating that it is water.
An eductor system is fitted to the drip pan.
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Membrane containment system
LNG Fundamental Knowledge and Understanding
The membrane cargo containment system is based on a very thin primary barrier, or membrane, that is
supported by a layer of insulation within the confines of the hull of the ship.
The membrane system differs from the self-supporting tanks in that the membrane containment system
must be provided with a complete secondary barrier to ensure cargo integrity in the event of leakage of the
primary barrier. There are two principal types of membrane system, designed by GTT, that are in common
use (Mark III and NO96).
A secondary barrier is a backup protection, outside of the primary tank, that can contain the cargo for a
period of 15 days in the event of an LNG leakage.
Before 2005, the maximum size of LNGC was about 135,000 m3, and the shipowner would choose either
a membrane or Moss Type B spherical construction. However, the advent of far larger LNGCs has seen the
membrane design dominate as it makes better use of the internal hull space.
GTT NO96 membrane system
In the NO96 system, the primary and secondary membranes are identical, providing 100% redundancy.
The primary and secondary layers are constructed of the same materials ie, they comprise of a 0.7 mm thick
invar skin and a 200 mm layer of perlite filled plywood boxes that act as insulation.
Invar is a stainless steel alloy of about 36% nickel and iron with almost no shrinkage factor.
Insulation Layers
At the design stage, the thickness of the insulation layers is adjusted to achieve the desired cargo
boil-off rates for fuelling the boilers, propulsion systems and auxiliaries.
Figure 1.14 The NO96 membrane
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LNG Shipping Knowledge 3rd Edition
Trunk deck space Passageway
Cargo tank
Ballast tank
Duct keel
Figure 1.15 GTT NO96 cross section showing tank structure
GTT Mark III membrane system
In the GTT Mark III membrane system, the primary barrier is stainless steel constructed of ‘cross
corrugations’, or ‘waffles’, that allow for any expansion or contraction.
Figure 1.16 GTT Mark III membrane system
The primary barrier is supported by an insulation layer constructed of reinforced cellular foam. In between
the insulation layers is a cloth made of laminated fibreglass and aluminium that acts as the secondary
barrier (ie triplex cloth). This secondary barrier is able to contain LNG leakage for 15 days.
Figure 1.17 GTT Mark III cross section showing tank structure
Other types of containment system
There are other types of containment system but, to date, construction is very limited. For example, other
types include the ‘CS1’ system, the ‘LNT A-box’ system and the ‘SPB’ system. For more detailed information
on tank construction, see Section 2.1.
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1.3 Regulations and Guidance
LNG Fundamental Knowledge and Understanding
Reference: SIGTTO ‘LNG Shipping Suggested Competency Standards’, Sections:
1.3.1 Have an awareness of the applicable regulations
• IGC Code (Reference 1)
• Classification Society Regulations
• Flag State Regulations
1.3.2 Know the relevant regulatory requirements for containment system and cargo related operations
• IGC Code (Reference 1)
• Classification Society Regulations
• Flag State Regulations
• Code of Safe Working Practices for Merchant Seafarers (Reference 2)
1.3.3 Have an awareness of industry and organisational guidance publications available
• Liquefied Gas Handling Principles on Ships and in Terminals (Reference 3)
• International Safety Guide for Oil Tankers and Terminals (ISGOTT) (Reference 4)
• Tanker Safety Guide – Liquefied Gas (Reference 5)
• Ship to Ship Transfer Guide for Petroleum, Chemicals and Liquefied Gases (Reference 6)
• Safety Management System
• Guidance for the Prevention of Rollover in LNG Ships (Reference 7)
• Liquefied Gas Carriers: Your Personal Safety Guide (Reference 8)
• Crew Safety Standards and Training for Large LNG Carriers (Reference 9)
1.3.4 Have an awareness of industry sources of guidance and advice
• LNG Operations in Port Areas (Reference 10)
• Ship Vetting and its Application to LNG
• Liquefied Gas Fire Hazard Management (Reference 11)
IGC Code
The International Code of the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk
(IGC Code) has been mandatory under SOLAS Chapter VII since 1st July 1986. It applies to ships carrying
liquefied gases with the characteristics described in the Code (listed in Chapter 19 of the 2016 edition).
The Code includes design and construction standards, and equipment requirements. Gas carriers
constructed prior to 1st July 1986 should comply with the requirements of the older Code for the
Construction Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) and the Code for Existing
Ships Carrying Liquefied Gases in Bulk (EGC Code).
The IGC Code 2016 edition has 183 pages and covers the following areas:
Chapter 1 – General
Chapter 2 – Ship survival capability and location of cargo tanks
Chapter 3 – Ship arrangements
Chapter 4 – Cargo containment
Chapter 5 – Process pressure vessels and liquid, vapour and pressure piping systems
Chapter 6 – Materials of construction and quality control
Chapter 7 – Cargo pressure/temperature control
Chapter 8 – Vent systems for cargo containment
Chapter 9 – Cargo containment system atmosphere control
Chapter 10 – Electrical installations
Chapter 11 – Fire protection and extinction
Chapter 12 – Artificial ventilation in the cargo area
Chapter 13 – Instrumentation and automation systems
Chapter 14 – Personal protection
Chapter 15 – Filling limits for cargo tanks
Chapter 16 – Use of cargo as fuel
Chapter 17 – Special requirements
Chapter 18 – Operating requirements
Chapter 19 – Summary of minimum requirements.
Recent amendments
Two amendments to the IGC Code entered into force on 1st January 2020. The first of these was approved
in November 2016 (Resolution MSC.411(97)) to include new ship and fire integrity arrangements. The other
was approved in May 2018 (Resolution MSC.441(99)) to include a revised model form of the Certificate of
Fitness.
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Classification Societies
LNG Shipping Knowledge 3rd Edition
Classification Societies are recognised organisations that establish and apply rules and technical standards
in relation to the design, construction and survey of marine related facilities, including ships and offshore
structures. These are issued by the Classification Society as published rules. A ship that has been designed
and built to the appropriate rules of a society may apply for a Certificate of Classification from that society
and the society will issue a certificate on completion of the relevant surveys and formalities.
Such a certificate does not imply, and should not be construed as, an express warranty of safety, fitness for
purpose or seaworthiness of the ship. The issuance of a certificate by a Classification Society only implies
that the ship was in compliance with their standards at the time it was issued. Certificates are subject to
periodic renewal following appropriate surveys.
As an independent and self-regulating body, a Classification Society has no commercial interests in the
operation of the ship. In establishing its rules, each Classification Society may draw on the advice of
members of the industry considered expert in their field. The principal Classification Societies are members
of the International Association of Classification Societies (IACS). IACS establishes a set of common rules.
Classification rules are developed to ensure the structural strength and integrity of essential parts of the
ship’s hull and its appendages, the reliability and function of the propulsion and steering systems, power
generation and other features and auxiliary systems that have been built into the ship to maintain essential
services on board.
A ship is maintained in ‘Class’ provided that the relevant rules have, in the opinion of the society concerned,
been complied with and that the appropriate surveys have been completed.
Flag State regulations
The role of the flag State is to ensure that its ships comply with the applicable international rules and
regulations. Compliance is then enforced when the ship visits other States under what is known as Port
State Control (PSC) inspections.
Most flag States ensure that ships comply with the applicable international rules and regulations, largely
indirectly through recognised organisations, normally Classification Societies.
The requirements of the flag State can be considered to complement the requirements in the IGC Code and
Class Societies Rules for the Classification of Ships.
For LNGCs, the assigning Class Society will conduct independent calculations for the cargo containment
system to verify its safety, by assessing the following:
• Thermal stress analysis for each cargo containment system/cargo tank and the cargo piping system,
taking into account expansion, contraction, ship movement and flexing
• dynamic cargo pressure calculations based on hull accelerations at sea, indicating local maximum
forces
• fatigue stress analysis caused by thermal stress and forces created by dynamic cargo pressures and
induced forces from the hull
• sloshing calculations based on the impact forces borne by the cargo containment system due to
the cargo free surface movement at sea
• transverse, longitudinal torsional and hull deflection analysis showing stress levels
• local stress analysis for tank domes and their pipe penetrations in addition to pipe supports within
the tank and on the cargo deck
• any filling restrictions, in any or all of the cargo tanks, must be clearly stated in the ship’s
documentation.
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