Converteam UK has released initial details of a revolutionary new concept in the field of electrical ship propulsion which looks set to push the power envelope of what is achievable in low cost, low voltage (690V) systems. The new configuration, known as Dual Active Front End (D-AFE), will feature in a Gusto MSC designed NG-9000C-HPE wind turbine installation jack up vessel which is under construction at Drydocks World SE Asia's Graha yard. Converteam's delivery responsibility for this vessel includes for the design supply and commissioning of an integrated electrical package which comprises major elements of:
3 x azimuth thruster drive trains
3 x tunnel thruster drive trains
Dual redundant C-Series Dynamic Position (DP) control system
Vessel Control System (VCS)
The D-AFE concept relates to the configuration of the propulsion drive trains and the subsequent steps taken to optimise all related elements of the power system. For many years, manufacturers of variable frequency drives have offered transformerless active front end solutions. Converteam's MV3000 is one such drive and it features the characteristic IGBT power semiconductors which are used in a fully controlled rectifier instead of traditional uncontrolled diode bridges. AFE solutions are noted for their intelligent control properties which result in extremely low harmonic distortion and in the provision of a sinusoidal current being drawn by the converter. As a result of this the requirement for additional transformers is eliminated and the size and bulk of switchboards is minimised.
D-AFE builds on the advantages of AFE and is particularly suited to offshore vessels in which reliability and resistance to single point failures are critical considerations. Dynamically positioned vessels in the DP2 or DP3 category are vessels which are likely to derive the most immediate benefits from D-AFE. Each inverter being fed by both switchboards is capable of operating based on a supply from either one of them or from both together. In consequence thrusters, generators and switchboards can be rated such that a worst case failure of, for example, an entire switchboard results in the loss of no thrusters but does mean that 50% of total available power is lost. By wholly separating the two switchboards, and eliminating all physical links (including expensive exploding fuses, between them, each thruster drive is configured such that it can operate continuously even in the event of the loss of an entire switchboard.
"The benefits of this approach are potentially massive," says Neil Barford, offshore and merchant business manager at Converteam UK. He continued, "D-AFE allows us to greatly extend the power range that's available in a low voltage configuration that not only cuts the initial costs of system purchase but it also means that through life costs are lower too since on board personnel costs are lower for LV trained crew."
Interestingly the benefits don't end there, when thinking about advanced DP vessels there are some really tantalising possibilities in terms of installed thrust and installed power generation capacities. Barford observes, "Class 2 and Class 3 DP vessels always focus on what happens to DP station keeping in the event of a worst single case equipment failure. In the case of a Converteam D-AFE system a worst case failure of a whole switchboard would no longer result in the loss of 50% of thrusters. Now the worst case single point failure typically reduces to being the loss of the single most significant thruster."
DP vessels that adopt this concept could benefit from significantly improved station keeping capability than would be possible for with an equivalent conventional arrangement. Alternatively these benefits may be taken in terms of reduced equipment ratings. For example, since 50% thrust can be obtained from 35% power, the same station keeping capability as the conventionally arranged ship may be obtained with a much reduced power consumption. "The potential for savings in either fuel consumption, thruster installation, generator installation or some combination of all of them are becoming apparent" says Barford.