Approval in Principle (AiP) from DNV for membrane tanks with a design vapour pressure of up to 2 barg
The maritime sector faces new challenges with the evolution of environmental regulations towards carbon neutrality and the corresponding nascent supply chains. LNG has proved to be the only immediate solution available for a greener shipping. Though a major opportunity for shipping, LNG comes with its challenges of usage of space and boil-off management.
GTT has developed LNG fuel solutions with the Mark III containment system, in particular for large applications like 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. GTT has worked on an improved design which increases the flexibility of membrane tanks for specific applications
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.
For any future newbuilding project, the alternative design process requires the involvement of the relevant Flag Administration at early stage and their approval at Design stage.
This innovation recently passed another major milestone by receiving an Approval in Principle (AiP) from DNV.
The AiP with DNV is based on assessment of safety equivalence with tanks/vessels built within the limitations given by the applicable regulations.
The safety equivalence was evaluated by DNV in a systematic way considering the implications of the increased vapour pressure on the main components constituting the LNG tank and associated systems. The system breakdown was as follows:
- The containment system including safety systems
- Fuel tank domes (area where all pipes penetrate in the tank)
- The fuel gas handling system
- The load carrying hull structure
DNV reviewed not only the technical documentation justifying the capacity of the Containment System to sustain a vapour pressure of up to 2 barg but also the implications in terms of operation as well as in case of potential downgraded situations such as system leaks.
Before construction and installation onboard any particular ship or object subject to classification, a complete set of documentation relevant for the particular ship is to be approved by the Society according to normal classification procedures and the alternative design process.
Prior to being granted an AiP from DNV, the same concept was already Approved in Principle by Bureau Veritas. As well as a general approval when GTT designed the containment system with a design pressure of 2 barg for the exploration cruise vessel of Ponant with two LNG tanks for a total 4,500 m3 capacity.
The alternative design process was indeed 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 Bureau Veritas, and the design was approved by the Flag.
Today, GTT is very happy to propose this solution with enhanced flexibility for many types of LNG fuelled vessel, from Cruise vessels to Car Carriers (PCTC) or Container Feeders, already evaluated by two major Classes.
A small step in design, a major gain in operation!
