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Shuttle Tanker Propulsion

Introduction
Shuttle tankers, which load their cargo from storage facilities at the field or directly from the production platform, are widely used to serve offshore oil fields from which pipeline connections are not feasible. In 1994, for example, just over 50 per cent of the oil brought ashore from Norwegian offshore oil fields was transported by shuttle tankers.MAN B&W Diesel has gained valuable experience in the laying out of propulsion machinery for shuttle tankers by supplying the main engines for all dieselmechanically driven shuttle tankers built with two-stroke main engines during the last decade.

ters to match the above requirements are in operation. The large side thruster power installed on the vessels calls for equipment that can providesufficient electricity production, i.e. large diesel generators or large shaft generators need to be installed on all shuttle tankers intended for dynamic positioning operation. Consequently, it is often decided to install cargo pumps driven by electric motors. The cargo pumps are primarily used for unloading the cargo in port but may also be used in the field to distribute oil among the segregatedcargo tanks. The maximum power consumption of the cargo pumps is typically around 4-5,000 kW. The MCR of the main engine or propulsion motor of a typical 125,000 dwt shuttle tanker is normally between 15,000 and 20,000 kW. The electrical load during steaming is typically around 700-1000 kW and is mainly used to cover the power consumption of the auxiliary equipment related to the ship’s machinery.Estimated average values of the power requirements of a 125,000 dwt shuttle tanker are illustrated in Fig. 1.
Load condition © Propeller load (kW) Electricity production (kW) Time in service (days/year) Sea 15,000 900 140 Port 0 5,500 55 Dynamic positioning 2-4,000 3-6,000 170

Propulsion Machinery
Several different propulsion systems meet the above-mentioned requirements.
• diesel-mechanicalsystem with

low speed main engine without shaft generator
• diesel-mechanical system with

low speed main engine and shaft generator
• diesel-mechanical system with

medium speed main engine without shaft generator
• diesel-mechanical system with

medium speed main engine and shaft generator
• diesel-electric system.

Operating Power Requirements
The operation of a shuttle tankerrequires that the ship has a quite unique manoeuvrability. High performance manoeuvring equipment is made necessary by the operating profile which, during loading of the ship, includes long periods of accurate dynamic positioning at the field, by using bow and stern thrusters as well as the main propeller. The time used for loading the oil depends on the loading facilities and may vary from one toten days in each round trip. A state-of-the-art shuttle tanker can be specified to keep the loads on the anchoring system below the maximum permitted values, even during the 100year storm conditions, when the ship is moored at the field. These are defined as significant wave heights of 15.5 metres, a mean wind speed of 37 m/sec (72 knots) and a wind-generated current of 0.7 m/sec (1.4 knots). Whenconnecting to the loading and anchoring system, the ship’s propeller and thrusters alone may be required to keep the exact ship position in wave heights of up to 6.0 metres. Obviously, such extreme conditions place the highest demands on both the main propulsion system and the thruster equipment. Shuttle tankers with 3 x 1750 kW bow thrusters and 2 x 1750 kW stern thrus-

The ship may beequipped with one or two propellers. The number of engines depends on the propulsion system selected and the number of propellers. The number and the capacity of the auxiliary engines depend on whether shaft generators are applied or not. The most typical configurations are diesel-mechanical systems with low speed main engines, with or without a shaft generator, driving one or two propellers, or...
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