DECK EQUIPMENT Describing the various options available today
and their advantages and disadvantages, Mr Toven noted that the major advantage of low-pressure hydraulic drives is the dynamic performance and high speeds possible, and the high level of safety that low-pressure hydraulics afforded. Low- pressure winches have also proved to be reliable in service, and have a 30-40 year working life. High-pressure systems are also inexpensive,
but do not offer the high speed and dynamic performance provided by low-pressure equipment. With high-pressure units, operating at or around 350bar all the time, there is also a risk of leakages occurring and of environmental problems; whereas with low-pressure units, which, in the case of Rolls-Royce low-pressure designs, might operate at around 5bar until they actually start working, leakages are less likely. Pole-change technology is also reliable and relatively inexpensive but the disadvantages include the fact that the technology does not offer stepless control, does not offer the same high level of dynamism as low-pressure hydraulic systems, and braking is therefore required.
Frequency-control benefits More recently developed electrically driven options include the frequency-controlled systems mentioned above. These have proved popular in the cruise ship market, notes Mr Toven, particularly on the new generation of very large cruise vessels, primarily because they are extremely quiet compared with conventional hydraulic machinery, and there is no possibility of environmental problems through leakages. The newer technology is, however, more expensive to acquire, and maintenance is less straightforward; whereas a conventional hydraulic system might have an operating life of 30-40 years, a frequency controlled system might only have a lifetime of 10-20 years, and as Mr Toven noted, whereas most owners feel comfortable with the maintenance requirements of hydraulic machinery they are less familiar with electric drives. TTS Kocks’ assessment of the advantages and
disadvantages of the various types of system available broadly concurs with that of Rolls- Royce. However, when comparing low-pressure
General layout of a Rolls-Royce self-contained hydraulic winch/windlass.
one winch does not affect any of the other winches onboard. An anchor-windlass can be operated by a single motor/pump unit, with a second in standby mode, each motor being connected with two independent power-supply lines. Another advantage of this kind of arrangement is optional simultaneous operation for high anchor heaving speed or deep-sea operation. Comparing stepless electric drives and self
contained systems, TTS Kocks has the following observations to make. Firstly, because stepless control is required by terminal-operations, this means that a frequency controlled drive or electric slip-ring type system is required. TTS Kocks claims that these types of devices are susceptible to breakdown due to overheating of different components, particularly boosters, in lowering- mode. The second drive in such systems can be very
TTS Kocks says it believes self-contained winches are more reliable, safer, more environmentally friendly, and easier to install.
complicated, claims TTS Kocks, and an additional interchange control and blocking device are required, which may cause confusion in operation because combined mechanical/electrical controls are necessary. In some applications, such as onboard gas and
hydraulic and self-contained systems, TTS Kocks notes that with a low-pressure system the oil supply is ‘in line’ for several winches and that, this being the case, one failure may affect all of the winches in line. Typically, in a low-pressure system, one anchor-windlass requires two pumps (and/or a boost device) if exceeding a certain anchor-length, and it is not possible to run a mooring winch with more than one pump. Switch-over valves – if not adequately maintained – can also be problematic, claims the company. In a TTS Kocks self-contained system, each winch works completely independently, and failure on
oil tankers, electrical components are, of course, safety critical, because of the risk of explosion, and electric remote controls are not generally available or are extremely expensive if explosion-proof equipment is required. Compared with stepless electric, the self-contained systems have easy and efficient heat control by air cooler for the hydraulic oil; integration of a second drive is straightforward by means of a hydraulic connection; and offering an explosion-proof system is simple. The CEH type also has several important
environmental advantages, notes Mr Meldal, including: a significant reduction in the total amount of oil required, by around 80%-90%, whereas, claims TTS Kocks, any low-pressure equipment sooner or later shows leakages, particularly at flange-joints (there are around 150 flange-points
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THE NAVAL ARCHITECT FEBRUARY 2007 47
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