Title: Landing craft power & propulsion: future evolutions John Buckingham CEng FIMechE 1 Simon Jones CEng MRINA 1 Abstract In an uncertain world, with challenging times ahead, the flexibility and potency of an amphibious capability offers a nation a great many options in the defence of its own interests. A key corner-stone of the logistical support and power chain from the base port to the landing beaches is the modest landing craft. A feature of amphibious landings for over seventy years but one which has evolved slowly and which has arguably not fully accommodated all the possibilities of the 21 st century. One area which is ripe for further development is the power and propulsion system. In many designs this comprises two separate systems where there are possibilities for a more flexible use of the installed prime movers. Using an indicative landing craft design and using analytical methods, the benefits and issues with the adoption of twin-wound motors and novel propulsors are explored. Introduction The mechanised landing craft is a product of the 21 st century. Having witnessed the problems encountered by the British when trying to land their expeditionary force at Gallipoli, the German Army developed solutions which proved more effective in Operation Albion. The US Marine Corps (USMC) took on such lessons themselves especially from the Japanese and then proceeded to work with the Higgins Boat Company to develop a suitable vessel for embarking troops on hostile beaches (Ref 1). The Landing Platform Dock vessel had yet to be created and so such Landing Craft (LC) were designed for longer sea voyages and were also capable of being deployed from support vessels. In USMC landings in WWII, 8 to 18% were killed in action and thus the set of roles and features of the LC emerged from this: On one hand they had to be quickly and easily manufactured in boat yards in emergency and were to be of low complexity and cost in view of their potential disposability. On the other they had to transport a large number of highly valuable troops in a combat situation to an undefended beach. The former drives the initial design but upon analysis the latter indicates a need for adequate protection in respect of armour and separated propulsion rooms. However time at risk is also a key factor and whether the LC is arriving from an LPD or from a home-base further away it is important that the factors relating to time and value are assessed through appropriate operational analysis. Landing Craft Utility (LCU) are designed to carry one or more main battle tanks (MBT) and/or a number of 4 tonne trucks, with other options relating to any number of troops. Although the USN air-cushioned landing craft (LCAC) can carry a 75 tonne MBT at 40 knots, the manoeuvrability, the noise and the fuel consumption are all very poor and as they have four GT engines, they are also 1. BMT Defence Services Limited, Bath, UK 1
very costly at over $40m each. A policy of diminished use of LCAC with a greater focus on the use of LCU was made by Schmitz in 1996, (Ref 2). Whilst the future of amphibious landing may include fast landing craft, there is likely to continue to be a need for those which can carry heavy loads in a wide range of sea states at more modest speeds. 2. Study Basis Landing Craft An extended UK LCU Mk 10 was used as the study vessel. The principal particulars of the basis vessel are shown below. Parameter Length, overall Beam Draught (transit) Value 40.13 m 7.0 m 2.0 m Maximum contracted speed 8.4 knots at sea state 1 Nominal range Ship s non-propulsion electrical load 600nm at 6 knots 40kWe cruise 80kWe landing duties Displacement Nominal Payload 292 tonnes 100 tonnes Standard Power & Propulsion Diesel gensets Propulsion 2 x Cat C44 each 99 kwb 2 x Schottel 82 pump-jets Table 1. Main Particulars Each driven by a MAN D2862 engines, each 735kWb. Option 1(122) Diesel Mechanical drive to Schottel Pumpjets The Landing Craft s baseline propulsion system comprises two Schottel pumpjets (SPJ), each driven by a main engine located in separate engine room spaces. A reduction gearbox reduces the engine speed from 2,100rpm to ~1,200rpm drive shaft speed. Two DG sets provide power to the ship. One of these could be air-cooled to allow for operations when beached. Pump-jets allow for a low external propulsor profile. They are location almost completely with the hull and therefore are less prone to damage due to contact with the breach or other items in shallow water. In operation, water is ingested at a central intake and then pumped out at speed to generate the required thrust. The vessel s assumed operating speed profile in sea state 1 is shown in Figure 1. This profile will vary slightly at higher sea states as the increasing resistance leads to speed loss at the top end. The resistance estimate is based on the Holtrop & Mennen method, (Ref 3), which provides a sufficient basis for comparison purposes between different propulsion options. 2
Figure 1. Assumed vessel-speed operating profile Using the BMT proprietary marine Power & Propulsion (P&P) analysis tool, Ptool (Ref 4), a comparison has been made between a series of design options. 3. Propulsion Studies The options for the propulsion of a landing craft have grown with the introduction of new technologies and the proving of those technologies which have been used elsewhere. Cordia (Ref 5) has addressed the use of a mix of the following propulsors: propeller, waterjets and contra-rotating propellers in a different number of shaftlines. Due to the very limited space at the transom this study limits the number of shaftlines to two. Cordia addresses the use of energy storage for P&P purposes. Most Landing Craft have a battery storage capability so that the vessel can operate as a command and control centre when beached. Energy storage is not addressed in this study, for apart from reducing polluting emissions when in a well-dock, the energy capacity and the rated power is usually too low for propulsion duties. Cordia also considers hybrid solutions which have been used to good advantage elsewhere. The complexity of such solutions is merited when it offers significant fuel savings which can be offset against the increased initial costs. In an analysis of the BMT Aegir design as presented in Ref 6 and Ref 7, the economy of hybrid systems is demonstrated, both for fuel economy and reduced engine running hours, together with the ability to have four power generators and four independent means for driving the two propellers. Couch & Fisher, 3
Ref 8, describe the design methodology for achieving the hybrid P&P design and outline its operating envelope and inherent flexibility. Simmonds et al, Refs 9 & 10 explain that to achieve a robust hybrid design where there are many changes from one set-up to another requires a great deal of design analysis and integration which may require considerable insight and effort. For landing craft, it is suggested that a hybrid solution cannot be easily inserted into the limit space due to the space required for the power conversion equipment. The study has considered the following set of configurations for a two propulsor solution. a. Option 1 (122) Diesel Mechanical (DM) drive to SPJ (baseline design described above) b. Option 2. (150) DM drive to propellers c. Option 3. (151) DM drive to Voith Linear Jet (VLJ) d. Option 4. (610) DM drive to waterjets e. Option 5. (253) Diesel Electric (DE) with VLJ f. Option 6. (480) DE: Twin-wound motor (TWM) driving Controllable Pitch Propellers (CPP) through a reversing gearbox g. Option 7. (490) TWM driving CPP through a reversing gearbox (EGB). Option 2.(150) Diesel Mechanical drive to propellers This comprises two open high-skewed Fixed Pitch Propellers (FPP) each of 0.7m diameter, PD ratio 1.1 and blade area ratio of 1.0. When running at full load at top speed they are subject to over-speed due to the high cavitation as is to be expected in such a high-load compact design. As FPP are used, the gearbox is to have a reversing capability and may be an EGB design. Each propeller is driven by a MAN D2868 rated at 597kWb via a reversible gearbox. Figure 2. DM propeller drive: Engine characteristic 4
Figure 2 shows the main diesel engine power-speed characteristic with the individual ship speed point indicated. At speeds below 6 knots, the engine risks being under loaded and the time in such conditions may be limited. This is a potential issue with all diesel mechanical drives but is partly offset by the match with the operating profile. Option 3. (151) Diesel Mechanical drive to VLJ The VLJ has been used on a windfarm support vessel where high static thrust and good efficiency over the operating speed range is required (Ref 11). Figure 3 shows a VLJ installation indicating how the thruster rotor is protected by the nozzles. Figure 3. Voith Linear Jet Installation Design 151 has the same arrangement as 150 but the main engines drive two VLJ each of diameter 0.7m to allow for a valid comparison with 150. The gearbox is a reversing gearbox which may be an epicyclic or simpler design. This is required because the VLJ cannot provide reverse thrust on its own. The aft end is redesigned to accommodate the VLJ but in this study the resistance is assumed to be the same and the thrust deduction factor is 0.01. Figure 4 shows the VLJ and propeller efficiency curves against the advance coefficient. The current design of the VLJ (Ref 12) has a maximum possible propulsive efficiency of ~70% which is comparable to the propeller design for option 150. However in practice, both propulsors will have much lower efficiencies as they will be highly loaded in this application. 5
Figure 4. Propulsor Efficiencies v Advance Coefficient The VLJ has a good efficiency across a broader range of advance coefficient values and therefore loads. This makes it a robust design solution for LC type applications. Option 4. (610) Diesel Mechanical Waterjets Two 78cm waterjets are each driven by an MAN D2840 engine rated at 434kWb through a reduction gearbox. Figure 5 shows the waterjet power-thrust-speed characteristic. Figure 5. DM Waterjets: Power on thrust v ship speed Figure 6 shows how the efficiency varies with ship speed and load. The maximum efficiency is over 40% at or near full speed. This is a much better performance than for most other propulsors (see also Figure 14). 6
Figure 6. DM Waterjets: Power on thrust v speed Option 5. (253) Diesel Electric driving VLJ The two 0.7m diameter VLJ are driven by variable speed electric motors each rated at 440kWb. The electric motors provide direct drive to the two VLJ at 640rpm. Power is from 2 x Cat C18 DG sets with each engine rated at 452kWb. Option 6. (480) Diesel Electric Geared TWM to CPP Maillardet (Ref 13) makes a strong case for the use of standard affordable medium speed motors to drive propellers through reduction gears on the basis of acquisition cost, size and weight. He addresses the reliability issues that are often associated with gearboxes and observes that an electric motor-gearbox arrangement does not require clutches, actuators and interlocks which are often the source of unreliability. Geared electric propulsion is employed in the Dutch Landing Craft (Ref 5). In Landing Craft there are significant space and weight penalties with a directdrive electric propulsion motor. For that reason the main drive motor, M1, proposed is a geared Twin-Wound Motor (TWM) solution with CPP. Such a design offers a smaller weight as well as space and also permits commercial size motors to be employed where shock and other military standards permit. The Norwegian company, Stadt (Ref 14) offers geared electric motors solutions with no converter losses. The TWM has two sets of windings: a supply to one winding with a given number of pole pairs provides one synchronous speed, with the supply of power to the other winding gives a different synchronous speed. Such a design, in essence has a simple make-break switch isolator between the switchboard and the selected motor windings with a sinusoidal waveform power supply with no losses associated with the rectifier and power converter (Ref 15). However such a design would mean that the ship could only operate at one of two speeds in a given speed-resistance situation, where that is defined by 7
displacement, sea state and hullform fouling amongst other possible factors. To achieve variable thrust and thus variable speed, a controllable pitch propeller (CPP) is used. By varying the propeller blade pitch angle for a given shaft speed, a range of thrust and thus ship speeds can be achieved. Thus the losses are transferred from the PWM to the CPP with the additional efficiency loss between CPP and fixed pitch propeller (FPP) which is assumed to be 1% across the whole pitch range. The use of a TWM with a CPP design allows the main motor, M1, to operate at two main speeds at loads above ~14% with the speed being varied at full propeller pitch at lower powers. This design avoids the use of power electronics for the upper power range and the losses and harmonic distortion they can create. A smaller motor, M2, drives up to ~14% power is controlled by standard marine power electronics. Reverse thrust is provided by the CPP blades being reversed. Figure 7 shows the TWM, M1 as part of the overall machinery layout. M2 is a separate PTI variable-speed motor which can supply up to 66kW. This machine can also be used as a PTO and this is the subject on current studies. Motor M1 can supply 470kW at 900rpm or 282kW at 720rpm. The motors can also be combined in different ways as shown in the table in Figure 7. Figure 7. Twin-wound Motor Machinery Layout The TWM and gearbox layout are shown in Figure 7. The speeds of the TWM are dictated by the relative number of pole pairs. This leads to a step-up gearbox for this application. 8
Figure 8. Motor & converter efficiencies Figure 8 shows the small efficiency difference between the standard motor and TWM and the assumed standard converter efficiency versus load. The Stadt converter is 100% efficient as there is no power conversion equipment (Ref 16). The analysis uses a fixed value gearbox efficiency for the assessment of the designs. Figure 9. DE-Geared-TWM: Propeller shaft speed v ship speed Figure 9 shows how the propeller shaft speed increases steadily up to 430rpm at the lower rated power setting before it moves to the constant speed regime of 1,040 rpm at 6 to 7 knots. At 7 knots and above, the propeller shaft speed is at the second steady speed condition of 1300rpm. 9
Figure 10. DE Geared-TWM: Motor power v speed with propeller pitch ratios Figure 10 shows operation of the TWM design with the fixed pitch operations at powers up to 66kW before the first set speed operations are used between 66kW and 282kW per shaft. At full speed the second fixed-speed is used for powers up to 470kW per shaft. Work is still going on to investigate and compare performance with the standard variable speed drives. Option 7. (490) Diesel Electric VLJ A twin 70cm VLJ solution with a geared drive TWM employs a reversing gearbox to allow for thrust propulsor reversal for manoeuvring and going astern. The gearbox could be an epicyclic design but these are seldom used at sea due their considerable costs and currently they would be used for specialist applications where space was at a premium. A standard reversing gearbox would have to operate quickly enough to allow the vessel to be handled in a prompt fashion in confined waters. The benefits of this design are the good efficiency of the VLJ and the low electrical losses of the electrical power transmission. 10
4. Results The principal results for all design options are presented together below with a short commentary to each. Figure 11. Fuel consumption in kg/h baselined to SPJ Figure 11 shows how all designs above 6 knots have a lower fuel consumption than the SPJ baseline design. The propeller fuel consumption becomes worse near top speed due to excessive cavitation. For the vessel s principal range of operating speeds, the VLJ-based solutions are the top three designs for fuel consumption with the waterjets in fourth place. Figure 12. Fuel consumption in % baselined to SPJ Figure 12 shows the comparative fuel consumption as a percentage of the SPJ baseline design. The DM-VLJ is better than the baseline at all speeds. 11
Figure 13. Annual fuel consumption baselined to the SPJ Figure 13 shows the total relative annual fuel consumption based on the operating profile. All designs are better than the baseline design with the VLJbased designs all show the better annual fuel consumption with the waterjets in fourth place. Figure 14. Propulsor Efficiencies Figure 14 shows how the VLJ design (DM only shown) offers a higher efficiency across the speed range with the waterjet the next best solution. A key aspect of military performance of any naval vessel is the ability to be as autonomous as possible and less dependent on logistic support such as fuel supplies. The legend box of Figure 15 shows the fuel burn for the given range 12
and speed and the figure shows how that fuel permits different ranges at different speeds. Figure 15. Range v Speed Figure 15 shows that the DM-VLJ design has the lowest fuel burn of 5.2 tonnes at 6 knots but at higher speeds this fuel burn leads to a lower range. Clearly it is important to choose a speed and range which is a good representation of the intended use of the vessel. Weights The sum of the principal P&P machinery weights for each option are shown in Figure 16. Engine data is taken from suppliers catalogues and the rest has been estimated parametrically from similar size equipment. The figure shows that the ship-sets of the DE designs are up to ten tonnes heavier than the direct mechanical designs. The propeller and VLJ mechanical drives are the lightest. 13
Figure 16. Weight Summary of Principal Machinery Figure 16 therefore shows that the use of electric propulsion leads to a weight penalty which could only be balanced out by a significant fuel economy. 5. Other Considerations The study and results presented above has focussed on fuel efficiency, this is an important consideration for a Landing Craft design because it results in the following benefits: reduced fuel load for a given range resulting in reduced fuel costs or increased payload; the improved propulsive efficiency may also allow smaller power generation plant or increased speed. Although these are all important benefits, the following factors should also be considered: 1. Acquisition cost; 2. Speed operating profile; 3. Space and weight (including payload and range requirements); 4. Availability (reliability) and maintenance requirements; 5. Robustness of the propulsor, especially considering use in Humanitarian Aid and Disaster Relief (HADR) scenarios where the water is likely to contain debris; 6. Manoeuvrability and rudder: the SPJ and the waterjets do not require a rudder. This is a consideration when the risk of Foreign Object Damage (FOD) is to be considered as above; 7. Autonomy and endurance requirements. Such issues have been qualitatively assessed to create Table 2. 14
Table 2. Summary of Comparisons Table 2 shows the best designs in green and the worst in red with those which are median in orange. The only design which does not have a red box is the waterjet but it does lead to an aft extension of the vessel which may cause issues if the landing craft is used in a well-dock. The mechanical drive VLJ relies on rudders which limits its manoeuvrability. If the landing craft were to be designed for longer transits with a less demanding manoeuvrability requirement then this may be acceptable. 6. CONCLUSIONS This study has explored the use of a range of propulsors and drives for the propulsion of a typical landing craft design. The use of geared electric propulsion with twin-wound motors and CPPs has been considered. The design reduces the need for sizeable electrical power conversion equipment but the powers in Landing Craft are low enough that this may not be an issue. It is likely that such a design is too complex for a Landing Craft even though the use of CPP allows for rapid changes in thrust. BMT continues to assess this design. The standard motor-converter solution with VLJ is an attractive solution as it allows good system response with an efficient propulsor. Although the VLJ is a new product and has not been used in a littoral naval vessel to-date, it clearly has an attractive efficiency in such applications. Waterjet and propeller based mechanical drives are both better than the pumpjet baseline design but the propellers are vulnerable to damage on inshore obstacles and the beach. This study has explored the fuel consumption benefits of a range of propulsion solutions. Work continues in ship integration so that a holistic assessment of their match to a Landing Craft can be better understood. 7. ACKNOWLEDGEMENTS It is BMT s intention to claim copyright for this work. The kind permission and resources granted to the author by BMT are acknowledged with thanks. All findings, ideas, opinions and errors herein are those of the author and are not necessarily those of BMT Defence Services Limited. 15
8. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Piscitelli. The United States Marine Corps way of war. 2014 University of Glasgow. Schmitz K L. LCAC v LCU: Are the LCAC worth the expenditure? CSC 1996. Holtrop, J & Mennen, GGJ. An Approximate power prediction method. Netherlands Ship Model Basin (MARIN). Wageningen, The Netherlands Ptool: Fast performance and cost modelling of propulsion powering systems, J. E. Buckingham. AES 2000, Paris. October 2000. Cordia, M J & CJCM Posthumus Faster RNLN s landing craft utilities in a new concept of operations INEC 2016. April 2016. Bristol, UK Kimber, A. Future Naval Tankers - Bridging The Environmental Gap - The Cost Effective Solution, Pacific International Maritime Conference.. Sydney, Australia. 2006 Buckingham, J E Hybrid Drives For Naval Auxiliary Vessels Pacific International Maritime Conference. Sydney, Australia. 2013 Couch T, Fisher, J Power & Propulsion Meeting the concept. INEC 2016 Simmonds, OJ. Advanced hybrid systems and new integration challenges, INEC Bristol, 2016. Simmonds, OJ. Couch, Fisher Meeting the design concept: The integration challenges of advanced hybrid systems. IMDEX-INEC-Asia. Singapore. May- 2017. Voith Linear Jet: The Efficient, Reliable and Low-Noise Propulsion Solution for Fast Windfarm Support Vessels 30Aug2016 Steden, M. "Optimisation of a Linearjet" First International Symposium on Marine Propulsors. smp 09, Trondheim, Norway."June-09 Maillardet, P & Hoffman, D. The geared medium speed induction motor. JNE 39(2) 2000. Stadt AS. Norwegian Electrical engineering company Motor drives from Stadt. Marine Engineers Review (MER). November 2009., pg 16. Stadt AS document The guideline to electric propulsion. 3 rd April 2017 16