Thomas Dalton, Abe Boughner, C. David Mako and …

1 Thomas dalton , abe boughner , C. David Mako and Cdr. Norbert Doerry, USNLHD 8: A Step Toward the All Electric WarshipABSTRACTThe recently commissioned Iwo Jima(LHD 7) is the last ship withconventional steam propulsion thatthe Navy plans to build. TheLHD 8 is the next ship of the classand will be built as a modified repeatdesign of the LHD 7. The keymodifications are steam propulsionbeing replaced with a hybridpropulsion system of main gas turbineengines augmented with auxiliarypropulsion motors and electricpowered auxiliaries replacing thosepowered by steam. The LHD 8 willalso be the first USN surface ship toimplement a 4160 VAC ZonalElectrical Distribution System (ACZEDS) as well as the integratedpower system concept for electricalpower generation, distribution andpropulsion. These modificationsembody the intent of the all electricwarship concept for the future This paper presents theconstraints and issues involved in thedesign process by addressing majordesign impacts and significant designconcerns.

2 This text explores thedesign options within the availabletrade space and illustrates specificallyhow the fleet / mission requirements,LHD 7 hull design and propulsionshafting constraints, schedule andfunding drove the propulsion The 40,500 ton 844 ft ships of the Wasp(LHD 1) class of amphibious assault ships(Figure 1) are designed to support MarineCorps air and amphibious assaults againstdefended positions ashore. The propulsionplant for the first seven ships of the Waspclass consists of two independent steamboilers and two 35,000 hp steam turbineengines capable of driving the ship at over20 knots. This basic steam propulsionapproach was adopted from the earlier, circa1960 s, steam propulsion plant of theTarawa (LHA 1) class. In the early 1990sthe Navy made a general decision tophase out conventionally powered steamships due to the high cost of maintenanceand manning.

3 During construction of theLHD 5, 6 & 7, the Navy conducted a globalsearch to replace the steam plant withalternative power systems. At that time aGeneral Electric LM2500 gas turbine engine(25,000 hp) was the only gas turbine enginequalified for propulsion of Navy shipsand inadequate by itself to replace a 35,000hp steam turbine 1 - Amphibious Assault Ship (LHD2 Essex)Because gas turbine engine ducting must berouted through the island structure, gasturbine propulsion for an LHD requires atremendous amount of internal volume thatmay displace equipment in many existingspaces. Studies have shown that althoughtwo gas turbine engines per shaft wouldprovide ample power, they would not fit inthe existing machinery spaces without majorimpacts to the surrounding spaces and theengine ducting would severely impact theship arrangements.

4 With commercialdevelopment of the General ElectricLM2500+ gas turbine engine (35,000 hp), itbecame conceivable to fit a single gasturbine engine into a LHD at power levelscomparable to the steam turbine plant itwould replace. Although conceptual studieswere conducted to fit LHD 7 with oneLM2500+ gas turbine engine per shaft, theship was too far into construction to makesuch a major change. Accordingly, IwoJima (LHD 7) is the last ship in the to be built with a conventional steamplant and LHD 8 will be the first ship in Navy to use an LM2500+ gas PLANTDESIGN HISTORYIn preparation for the design andconstruction of LHD 8, the Navyinitiated a series of feasibility studies(References 1 and 2) aimed at developing agas turbine propulsion concept and reducingTotal Ownership Costs (TOC) over theexpected 40 year service life of the results of this study showed that TOCcould be drastically reduced simply throughthe predicted reduction in crew size of atleast 80 personnel and decreasedmaintenance requirements associated withthe removal of steam turbine engines andboilers.

5 To minimize design andconstruction costs, a number of constraintswere placed on the design: - Maintain the existing shaft line rake andskew of the steam propulsion plant to retainthe same Wasp (LHD 1) class hullhydrodynamic characteristics - Limit design changes to the second stageof the reduction gear to maintain themanufacturing lead time needed to supportthe ship construction schedule - No Marine Corps missions could bedegraded - Minimize the impact to adjacent non-machinery spaces - Allow only reasonable machineryarrangement changesThe amphibious assault ship operatingprofile is such that over 75 % of theunderway time the ship requires less than10,000 hp for propulsion to travel at speedsup to twelve knots. Because a single 35,000hp gas turbine engine is lightly loaded at thispower level, the specific fuel consumptionof a gas turbine plant is very unattractiveand well off its most fuel-efficient designpoint over much of the ship's operatingprofile.

6 Although these feasibility studieswere not predicated on supplementing thegas turbine engines with an electricpropulsion system, their analysis didindicate that significant fuel savings couldbe realized by augmenting the gas turbinepropulsion with electric propulsion. As aresult, propulsion motors were integratedwith the gas turbine engines at a relativelymodest 1600 hp per shaft, which is capableof driving the ship to roughly six knotsunder ideal wind and sea recommendation from these studieswas to retain a small auxiliary boiler forheating and hotel services. This designfeature ensured that the 450 VAC electricplant would essentially be maintained atroughly the same size as previous ships inthe class by simply replacing the samenumber of steam turbine generators withdiesel generators of comparable power leveland density.

7 Although the TOC savingswere sufficient to justify the conversion togas turbine propulsion in manpower andmaintenance reductions alone withoutelectric propulsion, the addition of anelectric propulsion system enhanced design concept for the LHD 8 wasfurther refined to totally eliminate allshipboard steam heating for spaces, laundry,cooking and other hotel services, resulting ina greatly increased electric plant , the generation anddistribution system voltage was increased to4160 VAC to meet the additional loaddemand for electric heating on a cold the removal of the steam auxiliaries,the worst case load demand changed from amission scenario of debarking on a 900 Fday to a cruise condition on a 100 F day. Itwas then realized that the ship had as muchas 8 MW of excess generating capacityunder typical climatic conditions of a 700 Fday (Figure 2).

8 All Conditions Total Load Degree 78 Degree65 Degree40 Degree10 DegreeTemperaturekWAnchorCruiseDebarkSho reFIGURE 2 - Predicted LHD 8 ship servicepower demand loading curveswithout anticipated service life load growthmarginThis excess capacity could support electricpropulsion considerably larger than the 1600hp propulsion motors originally plannedanytime it was warmer than the extremedesign condition of a 100 F day. Since thepropulsion motors are not mission essential(the ship can fulfill all mission requirementsusing the gas turbine engines alone), largerpropulsion motors could be incorporatedwithout requiring an increase in the totalelectric plant generating , follow-on study determinedthe optimal propulsion motor rating to be5000 hp per shaft, which was subsequentlyincorporated into the final PLANTDESIGN The propulsion system for each of the twoshafts of the LHD 8 consists of a singlemain gas turbine engine (LM2500+ at35,000 hp) and an auxiliary propulsionmotor (5000 hp) driving a two-stagereduction gear into a controllable pitchpropeller (CPP) (Figures 3 and 4).

9 The gasturbine engines drive the reduction gearthrough an overrunning self-synchronizingclutch. The first stage reduction gear casingis modified from the LHD 7 design to acceptthe single gas turbine engine and propulsionmotor input, in lieu of the high and lowpressure steam turbine engines. The singlegas turbine engine input pinion splits intotwo locked power train drives. Thepropulsion motor drives through a self-synchronizing clutch into one of the twolocked train first stage reduction gears. Theexisting shaft rake and skew were retainedand modifications to the second stagereduction gear have been minimized to onlythat necessary to fit the CPP hydraulic oildistribution box. The length of the gasturbine engine module in the machineryrooms required moving the reduction gearseveral feet aft from the prior ship designlocation.

10 This was beneficial since iteliminated one section of line the reduction gear ratios for the gasturbine engines and propulsion motors areidentical, the maximum shaft speed of 180rpm from the gas turbine engine is twice thatfrom the propulsion motor at 90 is provided from either the gasturbine engine or the propulsion motor, butnot both simultaneously. Transitioning frompropulsion motor to gas turbine engine driveand back is allowed over the entire speed /power range of the propulsion 3 - Conceptual diagram of shaft propulsion power train arrangement for LHD 8 FIGURE 4 - Sketch of shaft propulsionpower train design concept for LHD 8 ELECTRIC PLANT DESIGNHISTORYD uring the LHD 8 feasibility studies severaloptions for the electric plant were exploredthat were based on ship construction cost,life cycle cost, survivability, maintainabilityand feasibility of the design to support theload increase from the removal of steam andincorporation of electric heaters andauxiliaries.