The maritime sector faces new challenges with the evolution of environmental regulations towards carbon neutrality and the corresponding nascent supply chains. Ship-owners require more than ever a maximum flexibility over the lifespan of their next new-buildings.
GTT recently announced two innovations which will help ship-owners to future-proof their assets and investments. These innovations received the first Approvals in Principle (AiP) from Bureau Veritas, a world leader in testing, inspection and certification.
Barg Tank Design Pressure
GTT has developed, together with several yards, LNG fuel solutions with the Mark III containment system, in particular for Very and Ultra Large Container Vessels. The LNG tanks installed in those vessels usually have a design pressure or Maximal Allowable Relief Valve Setting (MARVS) of 0.7 barg in accordance with the IGF Code limitation for atmospheric tanks.
From our decades of experience of in-service vessels, this standard 0.7 barg design is compatible with the LNG supply chain and offers sufficient operational flexibility as long as the Boil-off Gas (BOG) and relative tank pressure can be reasonably managed. The recent ULCVs equipped with GTT membrane tanks are a good example of this.
However, most LNG fuel tanks being significantly smaller than LNG cargo tanks, there is a margin – an opportunity – to offer more flexibility to operators with a few modifications to the system.
Offering an increase pressure rating brings two main advantages:
- Bunker LNG with warmer temperatures (from "lower quality" supply chain)
- Increase pressure holding time (with and without gas consumption)
Setting a membrane tank design pressure above 0.7 barg is beyond the stipulations of the IGF Code. However, the IGF Code allows modifications to the prescriptive requirements as long as the alternative design meets the goals and the functional requirements of the IGF Code and provides a level of safety at least equivalent to that of a prescriptive design.
Before GTT investigated the feasibility of increased design pressure for large LNG tanks on-board large merchant ships, GTT designed a containment system with higher design pressure for the exploration cruise vessel of Ponant with two LNG tanks with a total 4,500 m3 capacity,
The alternative design process was successfully applied for Le Commandant Charcot of Ponant, which is currently close to completion. It mainly consisted in submitting a technical validation to the French Flag, with justification of the equivalent level of Safety between the new design and a conventional design at 0.7 barg. This dossier had been reviewed by the Class Bureau Veritas, and the design was approved by the Flag.
What happens with larger tanks?
The solution must accommodate several parameters/constraints:
- large tank volumes (up to 20 000 m3) with significant height (high hydrostatic pressure),
- vessel accelerations equal or higher than LNGCs (higher dynamic pressure),
- necessity to keep overall maximum pressure equivalent to existing designs of LNGCs and FSRUs (0.7 barg),
- necessity to keep design of pump tower and liquid dome similar to existing designs on LNGCs and FSRUs.
To meet all these parameters, GTT opted for a new design pressure increased to 1 barg.
The first alternative design process for such a configuration will be performed during the next project. This process is expected to be fairly smooth, as most of the analyses carried out in the framework of the previous Alternative Design can be re-used. Due to the tank height difference (approx. 20 m instead of 10m for the Ponant project), the only relevant gap identified for the 1 barg configuration is the use of a large “seat dome” (associated with a classical pump tower). This same type of design is seen on the LNG Carriers and the first ULCVs. The dome is the stainless steel piece located on top of the tank gathering all the pipe penetrations to the Fuel Gas or Cargo Handling System (FGHS / CHS). For the 1 barg concept, the dome stiffeners and welds will be adequately reinforced to sustain the increase of the vapour pressure.
Based on our preliminary design works, GTT presented the same concept to Bureau Veritas, who issued on February 15th 2021 an Approval in Principle related to the design pressure of “1 barg” for Mark III tank for LNG as fuel applications, such as large container vessels.
A small step in design, a major gain in operation!
Although GTT is convinced that the LNG is the solution of choice to reduce ship emissions, widely available now, ammonia (NH3) is considered as a credible alternative fuel in the medium term to enable further decarbonisation.
During 2020, GTT started the compatibility assessment of its technology with ammonia, looking at the potential for membrane tank to store liquid ammonia as a drop-in fuel with little or no cost of conversion.
Bureau Veritas is the first classification society to approve in principle this "NH3 Ready" notation for the Mark III membrane tanks. The AiP recognizes that Mark III technology is suitable for the containment of ammonia as a fuel, without major design changes compared with LNG fuel tanks. For this AiP, GTT has demonstrated the compatibility of the Mark III system and defined the methodology to address the necessary design adaptations. Among the items which have been reviewed:
- The chemical compatibility of Mark III stainless steel primary barrier with ammonia,
- The potential design adaptations of the containment system and the supporting hull structure in order to sustain the higher sloshing loads of liquid ammonia due to higher density (0.682) compared with LNG (0.470),
- The selection of the steel grade of the supporting hull structure to potentially sustain the low temperature (-33°C) of liquid ammonia.
At project stage, the granting of an NH3 Ready notation may require an alternative design process since ammonia is not yet part of the IGF code. Interestingly, all major classification societies are currently working on their guidelines for NH3 as a fuel, in order to ease the process. GTT is thus discussing similar approvals with the other classification societies to provide the best reactivity should a ship owner request the notation.
In its recent past, GTT already validated the use of the same Mark III containment system for multi-gas applications, in particular denser liquid gas such as LPG and LEG. The first Very Large Ethane Carriers (VLEC) are equipped with Mark III and have the flexibility to carry LNG thanks to their LNG Ready notation.
Today, GTT can propose the only “real” NH3 Ready notation for any type of LNG fuelled vessel, from Very Large Container vessels (VLCV), Tankers (VLCC), Bulkers (Newcastlemax) to smaller Car Carriers (PCTC) and Cruise vessels. Keeping the door open as well to carbon neutral LNG with bio-LNG and synthetic LNG later with no cost of conversion.
These recent innovations and first Approvals in Principle demonstrate GTT's ability to offer a flexible and scalable solution that allows ship owners securing their investment with respect to the potential evolution in the supply chain and environmental regulations.