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SPECIAL | OFFSHORE TECHNOLOGY The Schottel Combi Drive: suited to offshore application PROPULSION Schottel which develops, designs, manufactures and sells propulsion and manoeuvring systems with power ratings of up to 30 MW for vessels of all types and sizes set new standards for the manoeuvrability in the early 1950s with their Schottel Rudderpropeller (SRP). The SCHOTTEL Combi Drive (SCD) combines the main technical and economic criteria of both mechanical Rudderpropellers and pod drives. Schottel builds combi drives both in a twin-propeller version and as a ducted single-propeller system, at present covering a power range from 2,100 to 3,300 kW. In contrast to pod drives with an electric motor inside the underwater pod, the motor in the Combi Drive is integrated vertically into the support tube of the Rudderpropeller. Neither an above-water gearbox nor a cardan shaft is required, making the system extremely compact and easy for the shipyard to install in the vessel with very low space requirements. Since the market launch, 16 of these innovative propulsion systems have already successfully entered everyday service. Three double-ended ferries built for Fjord1 Fylkesbaatane in Norway by the shipyard Aker Brattvaag AS are each equipped with four gas-electric powered Schottel Combi Drives of type SCD 2020 in twin-propeller confi guration (4 x 2,750 kW). The ferries operate between Bergen and Stavanger. An ecological and safety-oriented concept The SCD has a promising future in the offshore sector. The Norwegian Ulstein Verft, for example, has chosen it as the propulsion system for two platform supply vessels of type PX105. These 4700 dwt ships with the distinctive Ulstein X-Bow were built for the shipping company Bourbon Offshore Norway IS KS, a subsidiary of the French Group Bourbon, based in Marseille. With their new bow, they are “highly-developed, large and reliable multifunctional platform supply vessels, which distinguish themselves particularly in terms of fuel consumption, good sea-going characteristics, positioning, speed, stability and loading capacity”, as the shipyard emphasizes. Both vessels are equipped with two SCD 2020 twin-propeller systems (max. 2,700 kW). The decision to combine a diesel-electric propulsion concept with an azimuth drive system means a signifi - cant improvement to the performance of Special 10 Schiff & Hafen | June 2008 | No. 6 the vessels. The concept allows fl exible power distribution and offers a higher degree of redundant safety. The lower fuel consumption also makes the PSVs more environmentally friendly. The shipping company had stipulated that the vessels were to be equipped in accordance with the requirements of the DNV Clean Design Class. This means that the equipment had to meet strict criteria with regard to its environmental impact. BP Norway, the operator, also stressed that the ecological and safety-related concept of the new vessels had played a decisive role in the awarding of the contract. The economic expectations of Bourbon Offshore Norway with regard to these two innovative vessels were quickly confi rmed with the result that the company has mean while placed a further order with Ulstein Design for the design and technical equipment of four more vessels of the same type, also with Schottel SCD 2020 Combi Drives. The construction contract was awarded to Zhejiang Shipbuilding Co. Ltd. in Zhejiang, China. The shipyard will be delivering the vessels in late 2009 or early 2010. Both Neptune Offshore in Norway and EDT on Cyprus have ordered two further vessels of the same design in the mean time. US owners such as Otto Candies LLC in New Orleans, a major offshore shipping company in the Gulf of Mexico, are also focussing on this innovative propulsion concept. For two of its new offshore supply vessels, built by Dakota Creek Industries (DCI), the company has chosen the Combi Drive. The ships are each to be equipped with two SCD 2020 systems (2 x 2,250 kW and 2 x 2,500 kW). Following the successful market launch of the twin-propeller version, intensive market research culminated in the decision to introduce a ducted variant of the SCD. Ducted variant of the SCD While the twin-propeller SCD is mostly used in vessels with an operating profi le in the transit range at medium to high speeds, the ducted propeller operates at its best in the lower speed range and at static thrust. Especially for vessels with an operating profi le characterized by dynamic positioning (DP) and partial-load operation, the ducted variant represents a particularly effi cient solution. Anchor Handling Tug Supply vessels (AHTS), seismic research vessels, cable ships and other work vessels are ideally suited to this propulsion solution. This presupposes, of course, that the vessels are equipped with a diesel-electric propulsion system. This is usually the case with cable ships or seismic research vessels; AHTS vessels, however, are still an exception – and not always with good reason, as investigations have shown.
Side view of a new Offshore Supply Vessel for Otto Candies LLC in New Orleans Design: MMC Ship Design & Marine Consulting, Ltd, Poland A diesel-electric propulsion system is endowed with a power management system that ensures that only the currently required power is generated and distributed to the various units in the vessel. The connected generators always run at the optimum operating point. In combination with a ducted fi xed-pitch propeller, as in the Schottel Combi Drive, such a system offers very high effi ciency, especially in the low-load range. In a diesel-electric fi xed-pitch system, the required thrust is regulated via the electric motor speed (frequency control). The connected generators run at the optimized operating point – as does the fi xed-pitch propeller if designed accordingly. Furthermore, for low to medium vessel speeds, a well-designed fi xed-pitch propeller with frequency control is more effi cient at the rated speed than a controllable-pitch propeller. Increased economic efficiency The space saved by the diesel-electric concept, together with the fl exible design of the vessel’s interior (the SCD requires no space-intensive shaft lines), result in a substantial increase in the usable volume. This is of particular importance with such complex vessels as AHTS, and increases their economic effi ciency. These arguments convinced the shipowner Great Offshore Ltd., based in Mumbai, India, so that it ordered for the fi rst time a 150 t anchor handling tug with diesel-electric propulsion and two ducted SCD 3030s (2 x 3,300 kW) from Bharati Shipyard Ltd. in Mumbai. The newbuilding is scheduled to go into service at Great Offshore at the end of 2008. The trend towards diesel-electric powered vessels has sharply increased over the last few years. This is due to the fact that particularly work vessels in offshore operations are becoming ever more complex. Moreover, the leading seafaring nations in this segment, such as Norway or the USA, are enforcing more restrictive environmental and climate protection measures. Above all in harbours, the amount of pollutants emitted by conventional diesel engines is considerable, due to operation in the unfavourable partial-load range. Under the working title of “Green Tug”, dieselelectric concepts for harbour tugs have for the fi rst time now been developed in the USA for the harbours in Los Angeles and Houston. For example, in Los Angeles it is planned to allow tugs to operate only under battery power in the city area of the harbour. In the outer area of the harbour, the batteries can then be recharged via the generators. If such plans are implemented – and there are many arguments in their favour – the Schottel Combi Drive undoubtedly represents the optimal propulsion solution for harbour tugs meeting these requirements. Schottel GmbH Spay/Rhein www.schottel.de The platform supply vessel “Bourbon Mistral“ has already fulfi lled the shipping company´s economic expectation in just a short space of time. Photo: Tony Hall Schiff & Hafen | June 2008 | No. 6 11 Special
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