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to offer optimised payload capacity and greater operating efficiency compared to similar-sized vessels, while facilitating simplified construction methods to reduce unit production costs. Diesel-electric is favoured by Rigdon


on account of the benefits it offers PSVs in terms of overall fuel efficiency, redundancy and flexibility of installation. The advantages are especially apposite to the typical operating profile of a PSV, which spends much of its time at reduced power, as when holding station off a platform. Each of the 654s is powered by two 1235kW generators, driven by Cummins KTA50 diesels, and by one 410kW generator, incorporating a Cummins KTA19 engine. Larger Cummins prime movers are employed for the more potent, three generators in the 640-series PSVs. The three gensets in Rigdon’s new


class of offshore support ships supply electrical energy to two Steerprop azimuthing drives, one Steerprop fixed drive and two bow thrusters. The level of redundancy meets and exceeds DP2 dynamic positioning standards. Even if two of the three generators were to fail, the third would still be able to provide power for propulsion. The design advantages of diesel-electric


are demonstrated by the fact that the main engines for the powering and propulsion system can be fitted at main deck level. This has allowed the underdeck volume to be maximised for revenue-earning, while also reducing the amount of ancillary installation work, such as the length of runs of exhaust piping, relative to more conventional arrangements. Commissioned in 2007, Østensjø


Rederi’s diesel-electric Edda Fram marked a milestone in the evolution of offshore support vessels, both as the first platform supply ship to employ the Voith Schneider Propeller (VSP) system and as the first commercial recipient of Brunvoll Rim Driven Thruster (RDT) technology. The decision to employ Voith Schneider cycloidal propellers, in conjunction with a diesel-electric power and propulsion arrangement, was founded on considerations of overall efficiency, manoeuvrability, redundancy and safety. VSP affords stepless control of thrust


in both magnitude and direction. Voith claims that a platform support vessel such as Edda Fram using two 2500kW cycloidal propellers requires some 10-20% less power, depending on load conditions and speed, compared to alternatives such as contra-rotating azimuth thrusters.


Hybrid solutions Hybrid propulsion systems are finding increased favour in the offshore support sector, conferring flexibility and efficiency across the full mission profile of vessels


such as anchor-handlers, which are subject to extensive periods of high- and low-load operation. Hybrid powering combines both diesel-mechanical and diesel-electric propulsion, facilitating three main operating modes, namely drive to the propellers by the diesel engines only, by the electric motors only, or by the diesel engines and electric motors together.


AHTS Wärtsilä has developed a new, hybrid propulsion system for different designs


SCD SCHOTTEL Combi Drive


A highly efficient azimuth drive system


• Combination of electric drive system with proven mechanical components


• Utmost manoeuvrability due to 360° steering


• No shaftline required • Extremely compact design • Easy to install by the shipyard


2 x SCD 2020


2 x SCD 2020


Our product range com- prises azimuth propulsion systems, manoeuvring and take-home devices, and also complete con- ventional propulsion packages rated at up to 30 MW. Through our worldwide sales and service network we offer economical and reliable solutions for vessels of all kinds and sizes. So we can provide the right thrust for your vessel.


Innovators in propulsion technology


SCHOTTEL GmbH Mainzer Str. 99 · D-56322 Spay/Germany Tel.: +49 (0) 26 28 / 6 10 · Fax: +49 (0) 26 28 / 6 13 00 eMail: info@schottel.de · www.schottel.com


Ship & Boat International November/December 2008 39


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