Propulsion Plant
Introduction
Hi,Initially I had planned to address ship propulsion plant weight at a single digit weight level, as most of the data I have for real world ships, and other designs, only have the propulsion plant weight reported at this level. However, the further I went into developing this site, the more I began to think that such an approach might be too simplistic, potentially hiding some significant trade-offs that arise in trying to design a ship and/or limiting the options that the user can explore.
Specifically, since many main propulsion units for modern ships, such gas turbines, come in limited set sizes, or in the case of high and medium speed diesels their maximum size may may be limited, a ship designer may find that he/she has limits on total plant power output and configuration that he/she must take into account. Additionally, when calculating ship fuel consumption and endurance at cruise speed, the total amount of power required in comparison to the total amount of power available can play a significant role. Unfortunately until recently I only had limited data available for estimating power plant weights.
Not long ago however I came across several Undergraduate and Graduate Level Theses on the internet at sites such as the MIT D-Space website and the Public Section of Stinet that deal specifically with estimating ship propulsion plant weights at a more detialed level, suitable for addressing some of the issues I noted above. I hope to combine some of the information provided in these Theses with the limited detail propulsion plant weight data that I have on existing ships and design studies, and develop a method suitable for use with the rest of the formulas, equations, and data presented on this site. I then hope to try and validate this method by estimating the weights for some of the vessels, for which I currently only have a single digit weight estimate for and then compare my total propulsion plant weight estimate from this method to the w200 weght estimate for those ships.
Unfortunately, because these methods are more detailed they require more input from the user, some of which he/she will have to make an educated guess at, and I hope to use the validation runs I put together to investigate suitable default values and approximations for some of those required inputs.
In addition to this I also have a paper that I found on the web relating ship propulion plant costs to specific propulsion plant components, and by using the data in this paper and an estimation method for sizing a ship's propulsion plant based on the Theses and other data that I have, I believe that we can also come up with better estimate of propulsion plant costs, than if we were to try and base the costs solely off a single digit propulsion plant weight.
An added benefit of this approach will also be that it will allow the user to investigate a wider range of potential power plant options, whereas if I stayed with a method based on the single digit weight estimates that I have, the user would be somewhat limited to only considering plants that fall within the boundaries of the previous designs.
Overview of Plant Layout
For our purposes we are going to assume that a ship's propulsion plant consists of four main parts, as follows;
- The Power Generation Section
- The Transmission Equipment
- The Propulsor
- The Controls, Auxiliaries, and Support Systems
Group 1 primarily consists of things like the diesels, gas turbines, steam plants, and/or other items like fuel cells etc. For conventional steam plants this includes not just the boilers, but also the steam turbines, and main condsors, etc. For a nuclear powered steam plant this will include the nuclear reactor, the primary and secondary sheilding, the steam turbines, and main condsors, etc.
On some vessels, the same power generation components are used to generate the required power at both cruise speed and at full-speed. Examples of ship's like this would be many of the conventional and nuclear powered surface combatants built by the world's navies, including the USN's CGN 9, CG 16-24, CGN 25, CG 26-34, CGN 35, CGN 36-37, CGN 38-41, DDG 2-24, DDG 37-46, FF 1037-1038, FF 1052-1097, or the RN's Rothesay, Whitby, and Leander class frigates.
On other vessels, different power generation components are used to augment the power generated by the "cruise" plant to provide enough power for the ship to reach full speed. Examples of ship's like this are the Royal Navy's Steam and Gas Turbine powered "County" Class Destroyers, Type 81 "Tribal" Class Sloops, and Type 82 "Bristol" Class Destroyer, or the Soviet Navy's "Kirov" Class Large Strike Cruiser, which is believed to have been configured with a Nuclear Powered Steam Plant for cruising, augmented by conventional oil fired components at high-speed operations. The propulsion plant on the RN vessels mentioned above are described as Combined Steam and Gas Turbine (CoSAG) plant, where;
- the first power generation component term ("Steam") signifies what the vessel uses for cruise speeds
- the second power generation component term ("Gas Turbine") signifies what other power generation components are included on the ship to reach high-speeds, and
- the term and signifies the Gas Turbines and Steam plant operate together to attain these high-speed operations
The propulsion plant on the Soviet vessel is described as a Combined Nuclear and Steam (CoNAS) to signify that the ship is believed to have;
- a nuclear plant for normal cruising,
- additional conventional steam plant components for high-speed operations,
- and these components are believed to all operate together for high-speed operations,
However, there are some complexities that have to be dealt with when trying to combine different plants together on a single shaft, and as such it may be simpler to instead configure a ship's propulsion plant with one set of power generation components for use when "cruising" and a second set of power generation components for use at high-speed, and the only time both sets of components are on line together is when the ship is accelerating from cruise speed to higher speed where the high-speed engines are brought up to speed and clutched into the shaft after which the cruise engines are taken off line, which allows for simpler gearbox etc. Examples of ship's like this are the US Coast Guard's Diesel and Gas Turbine powered 378ft WHEC High-Endurance Cutters and the RN's Gas Turbine powered Type 21, early Type 22, and Type 42 vessels.
On the USCG's 378ft WHEC the ship is configured with one diesel engine/shaft to provide cruise power and one gas turbine/shaft to provide high-speed power for a Combined Diesel or Gas Turbine (CoDOG) plant, where;
- the first power generation component term ("Diesel") signifies what the vessel uses for cruise speeds
- the second power generation component term ("Gas Turbine") signifies what other power generation components are included on the ship to reach high-speeds, and
- the term or signifies the Diesels and Gas Turbines do not operate together to attain these high-speed operations, but that rather the ship is either propelled by its Diesels or Gas Turbines but not both at the same time.
On the RN vessels mentioned above they are configured with a small cruise gas turbine/shaft and a larger High-Speed Gas Turbine/Shaft for a Combined Gas Turbine or Gas Turbine (CoGOG) plant, where;
- the first power generation component term ("Gas Turbine") signifies what the vessel uses for cruise speeds
- the second power generation component term ("Gas Turbine") signifies what other power generation components are included on the ship to reach high-speeds, and
- the term or signifies the cruise Gas Turbines and High-Speed Gas Turbines do not operate together to attain these high-speed operations, but that rather the ship is either propelled by its cruise Gas Turbines at low-speeds or high-speed Gas Turbines for high-speeds but not both at the same time.
Because these or type plants basically have two separate sets of power generation components the total amount of power installed on the ship is more than what is required to meet its high-speed requirements, but the overall plant configuration is relatively simple.
Group 2 consists of the components that connect the Power Generation Section (Group 1) to the Propulsors (Group 3). In general the transmission systems on a modern naval surface combatant will either be a mechanical system (consisting of gearing and shafting) or an electrical system (consisting of generators, power conditioning and converting equipment, and motors, etc).
Group 3 consists of the actual components of the system that convert the energy output from the Power Generation units into thrust. As such, this group consists mainly of items like fixed-pitch or controllable-pitch propellers, waterjets, axipods, voith-schnieder type propulsors, or Z-Drives, etc.
Group 4 then consists of all the remaining Controls, Auxiliaries, and Support Systems associated with the propulsion plant.