Marine Auxiliary Machinery 7E Episode 9 pdf

310 Steering gears Figure 9.21 Hydraulic circuit diagram for hand and power steering. For key to letters, see text Steering gear with constant output pump A steering gear as with any hydraulic system, can be operated by a constant delivery pump as an alternative to the conventional variable delivery type. Output from the pump, which runs continuously, is circulated through a bypass until required for steering gear movement. Small hand and power gears A simpler variant of the electro-hydraulic gear, for small ships requiring rudder torques below say, 150 kNm is shown in Figure 9.20. The hydraulic circuit is shown diagrammatically in Figure 9.21. The rams (U) in the double-acting steering cylinders G, which are free to oscillate on chocked trunnions, are linked directly to the tiller. F is a double-acting control cylinder, linked to the floating lever V by rod O. J is a directional control valve linked to the mid point of the floating lever by a spring link Z. W is the cut-off link from tiller to floating lever. H is a Socking valve and I a bypass valve. Valves C, D and S are solenoid controlled. When not energized, C is open and D is closed to through flow but acts as a bypass between the ends of control cylinder F when solenoid S is closed. When the solenoids are energized, C is closed, D and S are open. J is only operative when steering by power. When steering by hand, C, D and S are not energized. A pump B in the steering pedestal, coupled to the wheel, deliver fluid under pressure direct to Steering gears 311 the steering cylinders G, so moving the tiller in the sense and to the extent appropriate to the movement of the wheel. There is no hunting action. To change to power steering, the power pump is started and C, D and S are energized, i.e. C is closed, D ceases to be a bypass and it connects the steering pedestal pump to the control cylinder F. S, now open, allows fluid to pass from the power pump to J, which now comes under the influence of the floating lever. Steering-wheel movement now moves the piston in F, the floating lever pivots on its attachment to W, and J opens to allow the power pump to discharge to the appropriate ends of the steering cylinders G. As the tiller moves, a hunting movement occurs, the cut-off link W acting on the floating lever which, pivoting on its attachment to O, closes I and brings the gear to rest with the rudder at the angle required. Rudder angle indicators, either mechanically linked or electrically powered, are fitted as required. For local control, the ends of the control cylinder are made common by opening a bypass valve and the control piston is moved by a hand lever (shown dotted) or, for example, by wheel, rack and pinion. It will be seen that in the off-loaded condition, the pump discharge circulates through J. Emergency steering is by relieving tackles, fitted when the rudder is locked by closure of the valves H. If hand steering only is required, the gear is reduced to tiller, cylinders and • rams, locking and bypass valves, rudder indicator and steering pedestal with pump. A simple form of this gear for torques below 11 kNm is shown in Figure 9,22. Figure 9.22 Hydraulic steering gear 312 Steering gears Steering gear failures and safeguards The vital importance of the steering gear is reflected in the regulations of the government department with responsibility for shipping (usually Department of Transport or Coastguard) the requirements of the classification societies (Lloyd's Register, American Bureau of Shipping, Bureau Veritas and others) and the recommendations of the International Maritime Organization (IMO). Some general requirements for steering gears, based on the various regulations and SOLAS 1974, are given below: (1) Ships must have a main and an auxiliary steering gear, arranged so that the failure of one does not render the other inoperative. An auxiliary steering gear need not be fitted, however, when the main steering gear has two or more identical power units and is arranged such that after a single failure in its piping system or one of its power units, steering capability can be maintained. To meet this latter alternative the steering gear has to comply with the operating conditions of paragraph 2 — in the case of passenger ships while any one of the power units is out of operation. In the case of large tankers, chemical tankers and gas carriers the provision of two or more identical power units for the main steering gear is mandatory. (2) The main steering gear must be able to steer the ship at maximum ahead service speed and be capable at this speed, and at the ship's deepest service draught, of putting the rudder from 35° on one side to 30° on the other side in not more than 28 sees. (The apparent anomaly in the degree of movement is to allow for difficulty in judging when the final position is reached due to feedback from the hunting gear which shortens the variable delivery pump stroke.) Where the rudder stock, excluding ice strengthening allowance, is required to be 120 mm diameter at the tiller, the steering gear has to be power operated. (3) The auxiliary steering gear must be capable of being brought speedily into operation and be able to put the rudder over from 15° on one side to 15° on the other side in not more than 60 sees with the ship at its deepest service draught and running ahead at the greater of one half of the maximum service speed or 7 knots. Where the rudder stock (excluding ice strengthening allowance) is over 230 mm diameter at the tiller, then the gear has to be power operated. (4) It must be possible to bring into operation main and auxiliary steering gear power units from the navigating bridge. A power failure to any one of the steering gear power units or to its control system must result in an audible and visual alarm on the navigating bridge and the power units must be arranged to restart automatically when power is restored. (5) Steering gear control must be provided both on the bridge and in the steering gear room for the main steering gear and, where the main steering gear comprises two or more identical power units there must be two independent control systems both operable from the bridge (this does not mean that two steering-wheels are required). When a hydraulic telemotor is used for the control system, a second independent system need not be fitted except in the case of a tanker, chemical carrier or gas carrier of 10 000 gt and over. Auxiliary steering gear control must be arranged in the steering gear Steering gears 313 room and where the auxiliary gear is power operated, control must also be arranged from the bridge and be independent of the main steering gear control system. It must be possible, from within the steering gear room, to disconnect any control system operable from the bridge from the steering gear it serves. It must be possible to bring the system into operation from the bridge. (6) Hydraulic power systems must be provided with arrangements to maintain the cleanliness of the hydraulic fluid. A low level alarm must be fitted on each hydraulic fluid reservoir to give an early audible and visual indication on the bridge and in the engine room of any hydraulic fluid leakage. Power operated steering gears require a storage tank arranged so that the hydraulic systems can be readily re-charged from a position within the steering gear compartment. The tank must be of sufficient capacity to recharge at least one power actuating system. (7) Where the rudder stock is required to be over 230 rnm diameter at the tiller (excluding ice strengthening) an alternative power supply capable of providing power to operate the rudder, as described in paragraph 3 above, is to be provided automatically within 45 seconds. This must supply the power unit, its control system and the rudder angle indicator and can be provided from the ships emergency power supply or from an independent source of power located within the steering compartment and dedicated for this purpose. Its capacity shall be at least 30 minutes for ships of JOOOOgt and over and 10 minutes for other ships. Steering gear testing Except in the case of ships regularly engaged on short voyages, the steering gear should be thoroughly checked and tested within 12 hours before departure. These tests should include testing of power unit and control system failure alarms, the emergency power supply (when relevant) and automatic isolating arrangements. Every three months an emergency steering drill should be held and should include direct control from within the steering compartment at which time the use of the communications procedure with the navigating bridge should be practised. Further reading Cowley, J. (1982) 'Steering Gear: New Concepts and Requirements', Trans I Mar E, 94, paper 23. The International Convention for the Safety of Life at Sea (1974) as amended in November 1961, Chapter 11-1: Regulation 29. The Merchant Shipping (Passenger Ship Construction and Survey) Regulations 1984, HMSO. The Merchant Shipping (Cargo Ships Construction and Survey) Regulations 1984, HMSO, 'Steering a Safer Course', MER, October 1988 p. 31. 10 Bow thrusters, stabilizers and stabilizing systems Bow thrusters The transverse thruster, installed in the bow and/or the stern, has become an essential item of equipment on many vessels. It enables the normal process of docking to be managed without tug assistance because the vessel is made more manoeuvrable at low speeds. Safety is increased when berthing in adverse weather conditions provided that the required thruster capacity has been correctly estimated. Transverse thrusters are installed to facilitate the positioning of some types of workboats. Some craft have thrust units for main propulsion and azimuth thrusters with computer control for position holding. Thrust calculations must be based on the above water profile of a ship as well as the under water area. For passenger ships and ferries, the above water area may be three times that of the under water lateral area. For loaded tankers and bulk carriers, the situation is reversed but the unloaded profile must also be considered. The regular and frequent use of electrically driven bow thrust units on ferries and other vessels operating on short sea routes means that motor windings are kept dry by the heating effect of the current. This helps to maintain insulation resistance. There are potential problems with the electric motors and starters of infrequently used units, particularly where installed in cold, forward bow thrust compartments. They are subject to dampness through low temperature and condensation. Insulation resistance is likely to suffer unless heaters are fitted in the motor and starter casings. Space heaters may be fitted also. A fan is beneficial for ventilation before entry by personnel, but continuous delivery of salt laden air could aggravate the difficulties with insulation resistance. Bow thrust compartments below the waterline should be checked frequently for water accumulation and pumped out as necessary to keep them dry. Vertical ducts for drive shafts should also be examined for water and/or oil accumulation. Flexible couplings with rubber elements quickly deteriorate if operating in oily water. Thruster shaft seals must be inspected carefully during preliminary filling of a drydock. Failure to detect and rectify leakage at this stage can be expensive later. Bow thrusters, stabilizers and stabilizing systems 315 Bow thrusters with diesel drive By installing diese! drives various problems are avoided, for example the very large power demand of electrically driven bow thrusters, the insulation problems associated with the windings and the complications involved with starting, speed control and reversing. For a conventional thruster in an athwartship tunnel, the diesel engine may be mounted at the same level as the propeller to provide a direct drive through a reverse/reduction gear. An alternative diesel arrangement (Figure 10.1) where space is limited, has the diesel mounted above the thruster. Both of the units shown, have horizontally mounted diesel engines with simple speed control through the fuel rack, and a reverse/reduction gearbox. The second arrangement requires an extra gearbox with bevel gears to accommodate change of shaft line. Flexible couplings are also fitted. The reversing gearbox has ahead and astern clutches, with one casing coupled to the diesel engine shaft and a drive to the other clutch casing, through external gear teeth. The clutch casings rotate in opposite directions and whichever is selected, will apply drive, ahead or astern, to the output shaft. The engine idles when both clutches are disengaged. Alternating current electric motor drives with pitch control An alternating current (a.c.) induction motor of the (squirrel) cage type is used for many bow thrust units, with the motor being mounted above the Figure 10.1 Diesel bow thruster drive 316 Bow thrusters, stabilizers and stabilizing systems athwartships tunnel (Figure 10.2). Thrust is varied in direction and strength through a controllable pitch propeller. This arrangement permits the use of a simple and robust induction motor, which operates at one speed. Starting current for a large induction motor tends to rise to about eight times the normal full load figure and to reduce this a star-delta or other low current starter is used. Low current starting implies low starting torque as well. It is important that the hydraulic system is operative and holding the propeller blades at neutral pitch when starting. Pitch control for a thruster, is very similar to that for a controllable pitch propeller. The shaft of the lips arrangement shown, is hollow and has a flange to which the one-piece hub casting is held by bolts. The hub is filled with lubricating oil and there is free flow from the hub to the pod through the hollow shaft. The four blades are bolted to the blade carrier and have seals to prevent oil leakage. The pitch of the blades is altered by means of a sliding block, fitted between a slot in the blade carrier and a pin on the moving cylinder yoke. A piping insert in the hollow shaft connects the cylinder yoke to the oil Figure 10.2 Electric bow thruster drive Bow thrusters, stabilizers and stabilizing systems 317 transfer unit which contains a servo valve for follow-up pitch control. A mechanical connection between the oil transfer unit and the inboard servo cylinder facilitates accurate pitch settings and provides feedback for remote control The hydraulic power unit is supplied with two safety valves, suction and pressure filters, a pressure gauge and pressure switch, as well as an electrically driven pump with a starter. To complete the equipment an electric switch is supplied which, in combination with the pressure switch, prevents the prime mover from starting when the pitch is in an off-zero position and/or no hydraulic pressure is available. Hydraulic thruster An external hydraulic drive motor can be used as the alterative to an electric motor but a design with the hydraulic unit within the bow thruster pod, was produced by Stone Manganese Marine. The variable displacement hydraulic pump (Figure 10.3) is powered by a constant speed, uni-directional electric motor or diesel prime mover connected through a flexible coupling. Pump output is controlled by means of a servo-control operated direct from the bridge (or locally) to give the required speed and direction to the hydraulic motor inside the thruster. The pod and propeller are suspended in a conventional athwartship tunnel below the waterline. Other considerations and designs The customary transverse thruster has a limited application because it is based in an athwartships tunnel. It cannot contribute to forward or reverse motion of the ship and ship speed must be less than four knots for it to be effective. Some schemes to improve performance have variously used double entry tunnels, shallow vee or curved tunnels and different flap arrangements. The White Gill type thruster which is fitted on a number of existing ships, can provide thrust in any direction and is also used as the propulsion unit for some small craft. The White Gill type thruster This type of thruster (Figure 10.4) is positioned at the bottom of the hull so that the suction and discharge are at bottom shell plate level. Water is drawn in and discharged by a propeller through static guide vanes, much as with an axial pump. The guide vanes remove swirl and the water passes out as a jet through a rotatable deflector. The latter can be turned through 360°. The deflector has curved vanes, resembling in section a turbine nozzle, which produces a near horizontal jet of water. The deflector is rotated by a steering shaft which passes through a gland in the casing. This in turn is controlled from the bridge. No 318 Bow thrasters, stabilizers and stabilizing systems Figure 10.3 Hydraulic drive bow thruster 1. Prime mover output shaft 6. Running lights 10. Hydraulic motor 2. Variable delivery pump 7. Propeller 11. End cover 3. Servo valve assembly 8. Fairing cover 12. Mounting plate 4. Bridge control unit 9. Main casing 13. Tacho generator 5. Thrust indicator reverse arrangements are needed because thrust is available in any horizontal direction. The drive for the propeller may be applied vertically (Figure 10.4a) or horizontally (Figure 10.4b) depending on the design of unit installed. Stabilizers and stabilizing systems A ship at sea has six degrees of freedom, i.e. roll, heave, pitch, yaw, sway and surge (Figure 10.5). Of these, only roll can effectively be reduced in practice by fitting bilge keels, anti-rolling tanks or fin stabilizers. A combination of fins and Bow thrasters, stabilizers and stabilizing systems 319 Figure 10.4 (a) Vertically driven White Gill bow thruster; (b) Horizontally driven White Gill bow thruster tanks has potential advantages in prime cost and effective stabilization at both high and low speeds. Since a ship is a damped mass elastic system, it has a natural rolling period and large rolling motions may be induced by resonance with relatively small wave forces. Large resonant rolls can be avoided by generating forces equal and opposite to the impressed sea force. Figure 10.6 shows that the roll amplitude at resonance is much greater than that at long wave periods. The ratio of these amplitudes is the dynamic amplification factor which is limited by [...]... stabilizer 6 7 8 9 PERIOD OF SHIP ROLL (SECS) APPLIED BY PENDULUM Typical performance curves for Muirhead-Brown tank Further reading Bell, ], ( 195 7) Ship stabilization controls and computation, Trans RINA, July Conolly J E ( 196 9) Rolling and its stabilisation by active fins, Trans RINA Mitchell, C C and Stewart, D ( 196 6) Hydraulic power applied to ship stabilizers, Proc Inst Mech Engrs, 180, pt 3L 196 5-66 Rorke,... Refrigeration Table 11.1 Apples Bacon Cured un cured Bananas ButterCheese Eggs frozen chilled Fish Grapes Figure 11.1 Carrying temperatures, °C 1-2 1-5 -9 11-14 -9 1-10 -9 0-1 -10 0-1 Beef frozen chilled Lamb Pork Lemons Oranges Pears Plums 4 days at thereafter _o -2 _9 -9 12-13 2 J 7 / . SOLAS 197 4, are given below: (1) Ships must have a main and an auxiliary steering gear, arranged so that the failure of one does not render the other inoperative. An auxiliary. reading Cowley, J. ( 198 2) 'Steering Gear: New Concepts and Requirements', Trans I Mar E, 94 , paper 23. The International Convention for the Safety of Life at Sea ( 197 4) as . Sea ( 197 4) as amended in November 196 1, Chapter 11-1: Regulation 29. The Merchant Shipping (Passenger Ship Construction and Survey) Regulations 198 4, HMSO. The Merchant Shipping (Cargo