GTT Mark III, Flex and Flex+ Explained

What Mark III is

Mark III combines containment and insulation in a membrane system supported by the inner hull. Its primary barrier is corrugated 304L stainless steel, 1.2 mm thick; a complete composite secondary barrier sits within prefabricated insulation panels.^S1

The thin liner is supported rather than functioning as an independent structural tank. Cargo loads pass through the containment assembly to the ship’s structure.^S2

Inside the tank wall

Conceptual wall section

From cargo toward hull: primary membrane → primary insulation → composite secondary barrier → secondary insulation → support interfaces → inner hull. Heat flows toward the cargo; structural loads pass toward the hull. Simplified educational description, not a construction drawing. Schematic not to scale, not construction detail. Sources: ^S1^S2

The secondary barrier

GTT’s technical paper describes a laminated barrier with aluminium between glass-cloth-and-resin layers. Its role is to keep cryogenic liquid away from the inner hull following a primary-barrier leak.^S2

A complete secondary barrier is a protective layer, not permission to continue normal operation after damage.

The insulation also carries load

Reinforced polyurethane panels provide thermal resistance and load-bearing support. Bonding and support interfaces form part of the load path to the hull.^S2

The engineering question is not only how much heat enters, but how the assembly transmits loads without losing containment integrity.

Mark III, Flex and Flex+

GTT publishes the following family-level reference figures for a 170,000 m³ vessel. These are project-dependent design values, not universal measurements of ships in service.^S1

GTT family-level reference · 170,000 m³ vessel
Variant	Panel thickness primary + secondary	Published BOR % volume/day
Mark III	270 mm 100 + 170 mm	0.125–0.15
Mark III Flex	400 mm 100 + 300 mm	0.085–0.10
Mark III Flex+	480 mm 100 + 380 mm	0.07

GTT’s 2017 release identifies both thicker insulation and an enhanced secondary-barrier arrangement in Flex+. It reports general approval at that time.^S3

Read the conditions before comparing the number.

GTT identifies vessel size, tank arrangement and reinforcements as factors affecting BOR. The actual project specification and guarantee conditions are needed for a like-for-like assessment.^S1

Why containment and BOG belong in the same discussion

Heat entering the cargo contributes to natural evaporation. Insulation limits that heat transfer and protects the hull from cryogenic temperatures.^S2 The resulting vapour has to be managed within the ship’s gas-handling and energy system. DNV highlights the coupling between BOG fuel use and reliquefaction, which itself consumes energy.^S4

Engineering question

When gas demand is high

How does natural vapour generation compare with the intended gas demand? If extra gas is required, what does the approved fuel-gas arrangement provide, and at what energy cost?

Engineering question

When gas demand falls

What happens to surplus vapour during slow steaming, waiting or a change in machinery load? Compare the actual gas-management arrangement, not just the advertised BOR.

Sloshing: an assessment of the complete tank system

Liquid motion creates impact loads that must be considered alongside the strength of the containment assembly. DNV’s historical methodology description includes insulation, inner hull, pump tower and tower supports in that assessment. It also discusses model-test scaling and statistical treatment of impact loads.^S5

GTT’s offshore application guidance explains that FSRU and FSU designs require consideration of intermediate filling levels, site exposure and mooring arrangement; reinforcement is selected for those loads.^S6

No universal filling window on this page.

Do not turn a generic explanation into a permitted loading range. Filling restrictions and operating limits must come from the approved documentation for the particular vessel and service. The cited 2014 DNV announcement is historical methodology background, not the current rulebook.

Mark III and NO96: different construction principles

Construction principles, not a performance ranking
Feature	Mark III family	NO96 family
Primary membrane	Corrugated 304L stainless steel, 1.2 mm	Invar, 36% nickel-steel alloy, 0.7 mm
Secondary barrier	Complete composite barrier	Second Invar membrane
Insulation description	Reinforced foam-and-plywood panels	Construction depends on the NO96 variant; compare the named version
Comparison boundary	Specify the generation, tank arrangement, reference conditions and service profile before comparing thermal performance.

Construction facts: GTT Mark III and NO96 descriptions.^S1^S7

Applications: check the actual project

Delivered vessel

Al Shelila · Mark III Flex

DNV reported delivery of the 175,000 m³ carrier on 29 November 2024 and identified Mark III Flex with a published BOR of 0.085%/day.^S8

A delivered example confirms application. The announcement is not a voyage-by-voyage performance dataset.

Floating infrastructure

FSRU and FSU

GTT identifies both Mark III and NO96 as options for floating storage and regasification projects. The service and exposure determine the assessment required.^S6

A carrier-to-FSRU conversion needs its own engineering review; the original containment name does not establish suitability.

Developments worth following

Dated technical context selected for this article. These cards are researched static background, not a connected live Daily News feed.

20 May 2026 · Announced project

Three tanks, 177,000 m³, Mark III Flex

GTT announced four BW LNG newbuilds with a three-tank arrangement, a project BOR guarantee of 0.08%/day and deliveries scheduled from Q4 2028. It attributes improved thermal performance partly to the cargo-volume-to-insulation-area relationship.^S9

3 June 2025 · Design approval milestone

Pressure-rated LNG fuel tanks are a separate application

GTT reported DNV Approval in Principle for a 1 barg membrane fuel-tank design and referenced earlier 2 barg approval for smaller tanks, notably cruise-ship applications.^S10

19 January 2023 · Monitoring concept

Digital sloshing assessment and survey planning

GTT reported Lloyd’s Register AiP for a solution combining operational data, a tank digital twin and risk analysis to support alternative survey planning.^S11

Questions professionals should ask

Which Mark III variant and which tank arrangement are actually specified?
What are the reference conditions behind the BOR guarantee?
How is containment matched to gas demand and vapour recovery?
What operating limits apply to the intended loading pattern?
Does a reported benefit come from a model, approval, guarantee or measured service result?

Frequently asked questions

Sources and evidence

All sources below were opened during preparation on 12 October 2026. Product-page publication dates are unspecified. Manufacturer claims are identified as such; classification announcements are not independent operating-performance tests.

S1GTT · Mark III technologies
Product description and reference table. Undated.
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S2Gavory and de Sèze / GTT · Sloshing in membrane LNG carriers and its consequences from a designer’s perspective
Historical technical paper (2009; hosted filename includes later dates). Used for construction principles, not current dimensional specifications or operating limits.
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S3GTT Inside #11
25 October 2017. Flex+ development history.
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S4DNV · Overcoming the EEXI challenge for LNG carriers
Used only for machinery/BOG integration context, not a current compliance ruling.
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S5DNV · Revised version of Class Note 30.9
5 September 2014. Historical methodology announcement, not the current class standard.
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S6GTT · FSRU & FSU
Undated application guidance.
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S7GTT · NO96 technologies
Undated product description.
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S8DNV · Delivery of Al Shelila
29 November 2024.
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S9GTT · Three-tank large-scale LNG carriers
20 May 2026. Scheduled newbuild project.
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S10GTT · DNV AiP for 1 barg membrane fuel tanks
3 June 2025.
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S11GTT · Lloyd’s Register AiP for maintenance optimisation
19 January 2023.
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