LNGC concept maintains propeller immersion stability without needing to ballast when unladen
GTT, Lloyd’s Register (LR) and Dalian Shipbuilding Industry Corp (DSIC) embarked on the second phase of their ballast-free LNG carrier joint development project in March by welcoming Belgian shipping company Exmar as a project partner in tandem with the launch of the latest stage in the 30,000 m³ B-Free LNG carrier design initiative.
The project participants have developed a ballast-free, medium-sized membrane tank LNG carrier design that received approval in principle from LR in December 2017. Phase 2 of the project aims to develop the design further and validate results from the first phase by applying more detailed analysis and verification, including model testing.
For mid-size LNGCs, ballast water is more critical to achieving sufficient draught for propeller immersion when the vessel is unladen, rather than for stability. The B-Free design meets this challenge through a triangular shaped lower part of the hull, LR explained. This allows the vessel to sit deeper in the water when unladen, achieving the necessary propeller immersion for its 5.6 m diameter propeller while maintaining stability.
The design also potentially offers lower construction and operational costs because there is no need for a ballast water management system.
The project partners point out that with their B-Free design they have been able to provide the same cargo-carrying capacity as a conventional 30,000 m3 LNGC, but in a much smaller hull. The B-Free LNGC has a lightship weight of about 1,000 tonnes less than that of the conventional ship, resulting in improved harbour access and lower harbour fees, they say.
During Phase 2 of the B-Free LNG carrier design project, DSIC will concentrate on ship layout and performance, including hull line optimisation, utilising computational fluid dynamics; ship model test verification; propulsion selection; and hull structure basic design.
GTT states that its Mark III membrane is the containment system chosen for the B-Free design. A two-cargo-tank configuration is used rather than the three- or four-tank arrangement usually considered for such ships.
Take care over BWMS power supplies, professor warns
Retrofits involving high-power electronic systems, such as ballast water management systems (BWMSs), should take account of potential electromagnetic compatibility (EMC) problems, warned Frank Leferink, professor of EMC at University of Twente in the Netherlands and technical authority on EMC for Thales Nederland.
In an exclusive assessment of these concerns prepared for BWTT and summarised here, Prof Leferink said that electronic equipment such as power electronics for energy-efficient lighting, electric propulsion and BWMSs consume power in a pulsed manner, which is completely different from conventional power supply systems, which were designed for mostly linear loads with a power factor (PF) different from unity.
“Modern electronic loads consume pulsed currents, with a high peak current with respect to effective current and, if continuous, a high total harmonic distortion of the current (THDi).” Because the power supply system on a ship has a high internal impedance, the THDi results in a high total harmonic distortion of the voltages (THDv), which can cause problems with sensitive electronic equipment such as navigation, communication and automation equipment and reduce accuracy in measuring equipment.
“All classification societies are extremely concerned about harmonic voltage distortion and the possible consequences should some critical item of equipment malfunction or fail,” Prof Leferink said, and they impose strict limitations on the magnitude of harmonic voltage distortion.
Due to space restrictions with BWMS and other retrofits, cheaper and smaller systems are often selected without active PF circuits and sufficient filtering, causing many severe problems later on, he said. In BWMSs that use UV lamps, “the applied electronic lamp drivers with active PF circuits, low mains distortion, filtering for low EMC levels and improved immunity to mains voltage surges will lead to superior performance,” he said. Overvoltage peaks can deteriorate or destroy the lamps in the ballast water system, he added.
Using proper power systems can also achieve significant cost savings through reduced energy demand, lower operating temperatures and extended electrical equipment life.