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Innovation fills the Doughnut Tanker

The Naval Architect: March 2016

The Cubic Doughnut Tank System (CDTS) was originally developed in 1973 and a patent was awarded in 1976. The main design principle behind the initial design concept was the construction of a near prismatic shape using 12 substantially identical cylinders that were interconnected to each other so that it would have the structural efficiency of a spherical tank and the volumetric efficiency of a membrane tank. 

Unfortunately, this was the time when the US broke off diplomatic relations with Algeria cancelling all contracts to import LNG. Six new LNG Carriers were immediately laid up and interest in LNG containment technology in the USA waned until recently.

In 2005, Altair Engineering, Inc. was providing advanced structural engineering analysis tools to the University of Michigan and applied it to the CDTS, as a case study.  This in turn developed into an interest to develop this Type ‘B’ tank concept further resulting in the awarding of multiple improvement patents to Altair’s Product Design team in 2013, 2014 & 2015, and Approval-in-Principle (AIP) by ABS in March 2015. Since 2005, the development has been continuous, as shown in Figure 1, with the design efforts focused on the use of CDTS for both the marine transport (LNG Carriers) and Floating LNG Processing and Storage Platforms (FLNG).

Alcoa joined this effort in 2012 providing material recommendations as well as manufacturability assessments, and Dongsung Finetec in 2014, providing insulation solutions. The details of the CDTS have been presented at the Offshore Technology Conference in 2009 and 2010 as well as LNG Conferences in London in 2009 and ICCAS 2009. Since the original concept was patented:

  • The average LNG ship size has increased, increasing the need for sloshing mitigation
  • Altair has identified and solved functional and structural problems with the original concept and patented the improved design
  • The economics of transport costs have made efficiency an even more significant economic opportunity

The marine transport of LNG is a mature technology that is almost 50 years old. In the early years of LNG Carrier designs, a number of containment systems were developed and the most successful were the Moss Rosenberg Spherical Tank and the Gas Transport Membrane Lining System.

The CDTS has much better volumetric efficiency (0.85) than the Spherical Tank (0.52) and is close to the membrane system (0.88). This high hold space volumetric efficiency coupled with the CDTS being of an independent tank construction and self-standing allows for higher utilisation of available space above and below the main deck resulting in a significantly smaller ship length for the same cargo capacity. Recent development of the extended Spherical Tanks has improved the Volumetric Efficiency to 0.56.

The major benefit of the CDTS is that for the same LBP it offers 35% more capacity than Spherical and 24% more than a Membrane and IHI SPB ship, or a reduction in ship or platform length for equivalent Cargo Capacity; 12% compared to Spherical and 6% compared to Membrane ships.

The use of the CDTS results in the following additional advantages:

  • Significant reduction in Gross Tonnage
  • Significant reduction in the overall construction schedule compared to the membrane tank system
  • Eliminates the restriction on partial filling of tanks for sloshing, allowing multiple discharge locations
  • Reduced estimated cost of LNG carrier by 10% compared to Spherical and 5% Membrane
  • Provides ease of construction and ease of installation in the ship
  • Offers superior structural efficiency
  • Less installed power and thus fuel savings in service 
  • Utilises a simple support system
  • Better protection from side collision damage ­ 4m versus 2.5m for Membrane Ships and 3.25m for Spherical Tank Ship
  • Better protection from bottom damage ­ 4m versus 2.75m for Membrane Ships
  • Excellent Boil off Gas performance
  • Internal Connectivity provides Natural Conductive Cool-Down Pathways ­ low pre-loading spray down time with low spray-in mass flow-rate

Furthermore, the CDTS can be constructed using typical shipyard rolling and forming equipment. While the CDTS offers benefits just from the tank design, construction and installation in the ship, it also offers unique benefits in the design of the ship including a significant reduction in length, providing a significant reduction in the longitudinal bending moment, which results in construction benefits in reduced steel weight and less work content for the same capacity ship compared with any other system.

A comparison of tank outlines are shown in Figure 3 for the spherical, spherical extended, membrane, SPB and CDTS tanks of equal volume. Table 1 shows the capacity benefit of the CDTS over the other containment systems for the same Length Overall (LOA) of 300m and within the new Panama Canal beam restriction of 49m. The spherical extended tank would enable the LNG Capacity to increase to 180,000m3 within the 300m LOA and 49m beam restrictions. Note that the pure spherical tank LNG Carrier would have a beam of 52m and that the largest total spherical LNG Carrier capacity for the new Panama Canal would be 155,000m3 compared with the CDTS LNG Capacity of 210,000m3.

Altair ProductDesign provided the structural design expertise and technology that was used to complete the development of the CDTS design, and enabled the Approval-In-Principle (AIP) from ABS.

Altair’s HyperWorks Suite of software was used to ideate design solutions, and subsequently to analyse and optimise the CDTS tank. Altair’s HyperWorks suite is a computer-aided engineering (CAE) simulation software platform that allows businesses to create superior, market-leading products efficiently and cost effectively. The HyperWorks platform offers modelling, visualisation as well as ideation, analysis and optimisation solutions.

Although the constituent parts of the CDTS comprise of simple shapes, the overall geometry is structurally complex with significant design improvement opportunities. Starting from 2005, the HyperWorks suite of advanced structural design, analysis and optimisation tools were used to improve the design to meet the structural objectives which could not otherwise be attained by the proposed original design. An earlier paper RAMOO, 2009 describes the finite element analysis and optimisation of the CDTS as applied for LNG applications. The design tools (software) used included:

  • Optimisation Techniques: Topology Optimisation, Free-Size Optimisation, Size/Gage Optimisation
  • RADIOSS Non Linear Transient Dynamic Analyses for ALE & SPH analyses to predict sloshing loads
  • AcuSolve CFD Analyses for  Fluid & Heat Transfer analyses
  • HyperStudy (MDO) was used to develop the thermal management strategy such as position, locations, and volume of LNG

Altair and its partners are currently actively marketing the CDTS to shipowners and working with shipyards and tank fabricators to investigate fabrication and integration solutions.

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