2 Module propulsion systems Propulsion Systems
Summary of Regulations
Main and auxiliary machinery essential for the propulsion, control and safety of the ship shall be provided with effective means for its operation and control. All control systems essential for the propulsion, control and safety of the ship shall be independent or designed such that the failure of one system does not degrade the performance of another system.
Where the main propulsion and associated machinery, including sources of main electrical supply, are provided with various degrees of automatic or remote control and are under continuous manual supervision from a control room the arrangements and controls shall be so designed, equipped and installed that the machinery operation will be as safe and effective as if it were under direct supervision for this purpose. Particular consideration shall be given to protect such spaces against fire and flooding.
In general, automatic starting, operational and control systems shall include provisions for manually overriding the automatic controls. Failure of any part of such systems shall not prevent the use of the manual override.
Ships Equipped with an Auxiliary Propulsion System
New Rules to Assign the Special Notations THS or APS
In order to increase the level of safety on board a ship, new rules have come into force. They provide technical requirements for the design and construction of an auxiliary propulsion system able to guarantee adequate sailing conditions should the main propulsion engine fail.
One of the following special notations is assigned to ships, which have an auxiliary propulsion system:
THS (Take Home System)
If the auxiliary propulsion system allows the ship to reach the first suitable landing, simultaneously ensuring the availability of essential services for navigation, safety and the minimum conditions of habitability;
APS (Alternative Propulsion System)
If the auxiliary propulsion system allows the ship to maintain its normal operating conditions unchanged (safety, habitability and preservation of cargo) except speed which, in any case, is not to be less than 7 knots (a knot is a colloquial term for a nautical mile = 1,852 metres or 2025 yards or 6080 feet).
Optimising Storage Space
When considering production costs for a vessel, the most cost influencing parameter is the length of the ship. Assuming certain practical limitations on breadth and draft, it is important to achieve the accommodation of the longest possible cargo space within a given ship length. When considering a vessel of a bulk carrier / container type, the goal is to create a prismatic cargo space. With regard to the bow, the lines define the loss in "pay-length" at this end. These can be optimized in various ways including the introduction of bulbs or 'bulbous bows', which is a common feature today.
The stern end including the machinery space, steering gear platform, aft peak, rudder and propeller shaft boss is an area where one can see a number of solutions to minimize the distance from the transom to the forward machinery bulkhead.
When considering conventional propulsion machinery, the length of the engine plus reduction gear (where applicable) is the most dominant factor for determining the above-mentioned distance. The selection of a suitable engine type and gearbox configuration is therefore important. The length occupied by the tail-shaft, propeller and rudder can be marginally influenced. Necessary auxiliary machinery is positioned on platforms, above and on the sides of main machinery. To achieve full breadth in the aft end, a barge-type stem should be considered.
The design of the stern end around conventional optimized main machinery will finally have a limitation when it comes to the possibility of minimizing the length of the machinery space, i.e. the station where the forward machinery bulkhead can be located and thus how long the prismatic load box will be. In the modern bulk / container vessel design of today, it may be assumed that one has considered most of the possibilities with regard to optimizing the stern when utilizing a conventional machinery concept.
Layout and General Features
For any normal self-propelled ship the following functions are essential:
A Propulsion System
This provides the necessary thrust to give the vessel in question a specified speed (in some cases the static thrust or bollard pull is also of importance). The normal answer to this function is a conventional marine propeller with a shaft line, stern tube with seals, and line shaft bearing and reduction gearbox (in the case of medium or high-speed diesels). Remote control of the machinery from the bridge is normally incorporated as well (Refer Figure 17.1).
A Steering System
This provides for control of the ship's steering and course keeping at ship speeds, at which the rudder is efficient. The standard solution is of course a conventional rudder with a rudderpost, bearings and steering gear with auxiliaries. In the wheelhouse, a means for rudder control is available (a wheel, lever or a similar device) with a control link to the steering gear system. An interface for an autopilot system is normally arranged.
Figure 17.1 - Machinery Arrangement - Conventional Propeller
A Manoeuvring System
This is for safe and rapid control of the ship in confined waters (harbours, canal, etc. where the combination of the propulsion and steering systems is inadequate. To achieve better control in this respect, a transverse (tunnel) thruster system can be installed in the bow and in some cases in the stern. Such a thruster system includes a tunnel thruster with a drive system (normally an electric motor), auxiliaries and remote controls.
In comparison with the conventional systems described here, the Rotatable Thruster concept offers a unique package (Refer Figures 17.2 and 17.3). The three systems and functions described are all (with the exception of the bow tunnel thruster) generally available as one supply from one manufacturer.
Electrical Propulsion
Electrical propulsion systems offer numerous advantages for ships that are subject to specific requirements. They are rated as particularly economical, environmentally friendly and reliable, offer considerable comfort in terms of operation and control, have optimal manoeuvring and positioning properties, low vibration and noise levels and additionally suitable the best possible utilisation of space owing to their reduced volume.
The electrical side of the system will be based on a direct current or an alternating current motor, coupled to the ship's propeller shaft, with speed and direction of propeller rotation being governed by electric control of the motor itself or by the alternation of the power apply.
An electrical propulsion arrangement for a ship is often described as a diesel-electric or turbo-electric system. It is characterized only by the type of prime mover, with no reference to lie type of electrical propulsion motor, the generator or the electrical power system.
The most commonly used diesel electric propulsion systems are not a new concept. In the past these systems were usually diesel engine driven d.c. generators that supplied d.c. motors. Their applications were generally limited to vessels that required a degree of low-speed manoeuvring.
Vessels such as ferries, harbour tugs and various other applications used diesel electric systems for features that were not available in mechanical systems at that time. Till date, electrical propulsion systems have been used mainly for specialized vessels rather than for cargo ships in general. These include dredgers, tugs, trawlers, lighthouse tenders, cable ships, icebreakers, research ships, floating cranes and vessels for the offshore industry.
Electric-drive systems have made substantial progress in recent years. The two systems dominating the market today are frequency controlled a.c. motors and SCR controlled d.c. motors. Frequency controlled a.c. motor drive systems were generally more cost effective below 500 H.P. and SCR controlled d.c. motor systems more cost effective at the higher end. The offshore drilling industry favours SCR controlled DC drives. Modern SCR and frequency controlled systems have efficiencies approaching 97% in power conversion. The selection of one over the other is an application issue.
The deep draft cruise ship industry, due to the very high hotel power requirements, is adopting high-power diesel-electric (a.c.) propulsion systems in most of its new builds. Both technologies have a proven record of efficiency and reliability. For a direct current propulsion motor, the electrical power may be from one or more d.c. generators or may be from an alternator and then delivered through a rectifier as a d.c. supply. The power for direct current motors is limited to about 8 MW and so a.c. machines are used for higher outputs unless an effort is made to install d.c. motors in tandem. The rectification scheme can incorporate speed control and the means of reversing. Power for an a.c. propulsion motor is supplied obviously by an alternator. The prime mover may be a diesel engine, a gas turbine or a boiler and steam turbine installation.
The choice of a diesel electric system as the power source for a propulsion system of a vessel has nothing to do with hydro-dynamic efficiency. A propulsion system of a vessel provides thrust to move the vessel and is still chosen by the designer based on its merits for the vessel's application. Conventional propellers, controllable pitch propellers, azimuthing Z drives, transverse tunnel thrusters and low speed water jet systems can all be driven with equal effectiveness by a diesel-electric system.
Diesel-electric systems become viable when the installed KW for propulsion approaches or is exceeded by the installed KW for other purposes. The convenience of electric power distribution makes it possible to locate the primary power source i.e. diesel generators exclusive of consideration as to where the power must be applied, whether it be propulsion, thrusters or cargo handling purposes. A large variation in propulsion power requirements i.e. long periods of low speed or a necessity to shift power from main propulsion to thrusters for dynamic positioning purposes can also justify diesel electric systems.
Modern turbo-charged diesel engines are efficient over a relatively narrow operating load and RPM range. They are not suitable for long periods of low speed, low load, or low RPM, high torque requirements for reversing large propellers. Modern generator systems with load sharing, auto-start, and load shedding features make it possible to efficiently utilize the installed horsepower of a diesel electric system.
Diesel electric propulsion can overcome the following design problems:
When propulsive or station-keeping power requirements are a small or relatively small percentage of the total power requirements. Drill rigs, offshore vessels with special positioning requirements, research vessels with special manoeuvring requirements, and gaming vessels where speed is inconsequential such as gaming vessels operating on a river.
When space and/or propulsion machinery limitations either exclude the use of direct diesels or adversely affect the construction cost resulting from using direct diesels.
a. Vessels with hulls and/or struts too small to accommodate diesel engines, access, ventilation, etc.
b. Vessels with potential trim problems, such as stern wheelers, where machinery needs to be located forward to avoid trim problems.
c. Vessels that require, due to space limitations, more than one machinery space are subject to increased construction cost due to duplication of or increased in systems such as:
i. Engine cooling
ii. Space ventilation
iii. Control facilities
iv. Exhausts, etc.
Vessels that require maximum torque at minimum propeller speeds.
Vessels that have a large variation in power consumption.
The fact that the propulsion power may be supplied by an electric motor instead of a direct driven diesel engine does not make equipment aboard the vessel any less familiar to the operator. The utilization of the diesel engines is transferred from direct propulsion power to generator power. This provides greater flexibility of the use of the installed KW and in some instances reduces the number of diesel engines installed. The ability to generate only the power required to meet the needs of the duty cycle of the vessel utilizing multiple generator sets reduces fuel consumption and maintenance costs. It also provides redundancy in power capacity.
Advantages of Electrical Propulsion
As seen above, the main advantage lies with the flexibility and absence of physical constraints on machinery layout. Support ships for the offshore industry, particularly those with two submerged hulls, can use electrical propulsion motors to give high propulsive power in the restricted pontoon space, while generators and their prime movers are housed in the large platform machinery space. Electric power can be used for self-positioning thrusters and other equipment as well as for main propulsion. Passenger ships with electrical propulsion benefit because the number of generators in operation can be matched to the speed and power required.
The large amount of electric power available for main propulsion can be diverted for cargo or dredge pump operation as well as for bow or stern thrusters and fire pumps of emergency and support vessels (ESVs) described.
There is potential for reduction in the size of propulsion machinery spaces, because machinery is smaller and the generators, whether diesel, steam or gas-turbine-driven, can be located anywhere.
One proposal for a liquefied natural gas carrier was a pair of gas-turbine-driven generators located at the deck level, with electrical propulsion motors of 36000-horsepower (26845 kW) situated in a very small machinery space. Electrical propulsion separates the shaft and propeller system from the direct effect of a diesel prime mover and from transmitted torsional vibrations.
Disadvantages of Electrical Propulsion
No system is without its drawbacks. Higher installation cost also limits its usage. Lower Efficiency in comparison with a conventional diesel propulsion system is also likely. Frequency control and SCR systems produce harmonic distortion that can affect a.c. circuits. This can cause problems with equipment that is sensitive to harmonic distortion such as computers, navigation equipment and other electronic devices. This is not a problem if dealt with in the early design stages of the system. Systems aboard vessels that are sensitive to Harmonic distortion generally operate at low power requirements. The simplest and most economical method is to apply harmonic filters to the sensitive equipment rather than the power source.
There is also the fear on the part of some operators of additional complexity. The fact remains that almost all vessels operating today that meet the criteria for a diesel electric system, already have this equipment aboard even if on a smaller scale i.e. generator sets, switchboards, and electric motors.
Thus the desirable features of diesel electric propulsion do not suit all applications. Diesel electric propulsion is only suitable for displacement applications. High speed vessels that are weight sensitive, and displacement vessels in which the installed horsepower is primarily for propulsion with a constant duty cycle, have no need for a diesel electric system. However, diesel-electric propulsion systems will play an increasing role in the propulsion of vessels in the future.