Battery-Electric Ferry: Centralised or Distributed Power Conversion
A 120 m battery-electric ferry will carry 600 passengers and 180 cars across a 4.2 km crossing. A 4.5 MWh battery feeds a 900 V DC bus; two azimuth thrusters draw 2 x 1.8 MW. Hotel load is 450 kW and may grow 20% when shore-charging equipment is added. Two architectures are on the table. Option A: one converter room with two 2.5 MW converters and long DC cable runs to the thrusters. Option B: four 1.25 MW converters distributed fore and aft near the thrusters, with a shared DC bus and more cooling interfaces. Class rules require return to port after any single failure at reduced speed. The yard prefers A because it simplifies commissioning; the operator prefers B because it reduces cable weight by an estimated 6 tonnes. The mass margin is 4%, and the schedule has two weeks of float.
1. Compare the two architectures against the operator's drivers and recommend one with conditions.
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Evaluate against explicit measures: single-failure return-to-port, mass, energy efficiency, maintainability, commissioning risk and growth margin. Option A concentrates conversion, so commissioning is simpler and cooling is in one place, but a converter-room fire or bus fault is a common-mode threat, and long DC runs add cable mass and losses. Option B reduces cable mass by about 6 tonnes against a 4% margin, and distributing converters makes a single converter loss a partial loss. It also creates four cooling interfaces, more electromagnetic-compatibility work, and harder maintenance access.
Given a 4% mass margin and a class requirement for return to port after any single failure, B is the stronger architectural candidate, but only conditionally: the shared DC bus must be protected against a single fault propagating to all converters, cooling must tolerate loss of one branch, and the yard must accept a joint commissioning plan. If those conditions cannot be met within the two-week float, A is defensible with a fire-containment and bus-segregation case. Record the conditions in the decision record rather than choosing on preference.
2. How should the single-failure requirement and the 20% hotel-load growth be treated in the trade study?
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Make single-failure return-to-port an MOE with a defined failure set: converter loss, cooling loss, bus section fault, control loss. For each architecture, analyse whether the remaining capacity supports reduced speed and hotel load, and whether a fault can propagate. A design that meets the requirement only for the failures the team happened to list is not compliant.
Treat the 20% hotel-load growth as a range, not a point. Size the hotel-load converter and cooling for the upper bound, or show how growth is deferred. Run sensitivity on mass, cable length and hotel load; if B's advantage disappears when cable routing is longer than estimated, the recommendation should say so. Include the battery's usable state-of-charge and the crossing schedule in the reduced-speed case. The output should show which assumptions drive the ranking, not only a weighted score.
3. What should the architecture decision record contain, and how would you verify the chosen architecture at integration?
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The decision record should state the problem, the options, the criteria and weights, the evidence, the assumptions (cable routing, cooling capacity, failure behaviour), the residual risks, the conditions of approval and the accountable owner. It should also state what would cause the decision to be reopened, for example a measured cable mass above the estimate or a class objection to the bus topology.
At integration, verify the architecture's claims rather than the equipment alone. Test converter loss at full and reduced load, bus-section fault propagation, cooling-branch loss, and electromagnetic compatibility with thrusters running. Measure actual cable mass and losses against the estimates. Demonstrate reduced-speed return to port with hotel load applied. These tests verify the design; validation with the operator should confirm that the reduced-speed crossing and charging schedule are acceptable in service.