*Electric Load Balance*

The term propulsion system (as we considered it in this work) is not only referred to the power required to move the ship, produced by two main diesel engines and two small electric motors, but it takes into account the electric production as well. This electrical power is needed not only for the electric propulsion motors, but also for the bow or stern thrusters, auxiliary propulsion systems, hotel loads, and the scientific equipment. In the original vessel configuration, this power demand is ensured by three diesel gen-sets (please refer to Figure 4 and Table 1) that satisfy the total electrical power demand in all ship operative conditions. The electric load balance of the reference vessel is available in Table 4. Utilization factors in Table 4 appear to be far from ideal figures, and, in particular, 95% and 88% relevant to summer harbor and winter harbor conditions are not in principle practicable, even though such values refer to a ship currently in operation. This configuration is, therefore, suitable for further improvement.


**Table 4.** Electric Balance of a research vessel.

Harbor, Maneuver, and Navigation are the three phases considered in the electric balance. Each operational phase is split up in summer and winter condition to have an overview of power demand during different seasons.

For each column in Table 5, an indication of several gen-sets, in use, and their utilization factor has been provided. All this information gives an overview of how the propulsion configuration works in different scenarios during its working life. Besides, Table 5 shows for each scenario the number of working hours in a year of each WBS considered and described in paragraph 5.


**Table 5.** Working hours in a year for each WBS analyzed (see Abbreviation List at the end of the paper). Reference configuration.

Tables 4 and 5 refer to reference propulsion layout, and the information would be used as a solid base for the further considerations discussed in paragraph 6. All this conclude the preliminary phase.

#### **6. Application Case: Design Alternatives**

Starting from information identified in paragraph 5, it is possible to evaluate few alternative propulsion layouts to achieve a more efficient system, with possible advantages on building costs and operative costs, including all maintenance actions. In the following lines, three alternatives configurations are proposed: they have been designed focusing on a reduction in WBS working hours and/or maintenance costs.

## *6.1. First Design Alternative—Power Take-O*ff

The first alternative layout (identified as S1) proposes the introduction of a Power Take-Off (PTO) in the original propulsion system. In particular, the PTO is supplied by the main propulsion Diesel with the double aim to reduce the working hours of one or more diesel gen-sets, and to achieve a better working point both for diesel engines and for the gen-sets (compared to the original layout). Engines power size are the same as reference vessel, and the PTO is active only from 9 kn to 17 kn; up to 8 kn, the configuration works exactly as the original one. So, for the harbor, maneuvering, and navigation activities up to 8 kn, the power demand is supplied by two electric motors (for propulsion and maneuvering) and by the diesel gen-sets, as reported in Table 6. After 8 kn, the propulsion starts to be supported by the diesel engines, so, after this point, it is possible to use the electric motors (used as PTO) instead of the diesel gen-sets to supply the power needed for hotel services. Figure 6 clarifies this working operation.


**Table 6.** First alternative configuration power-use analysis results. Focus on changed WBS working hours.

**Figure 6.** Power generation layout in the first alternative configuration proposed. (**a**) How configuration works up to 8 kn: the two electric motors supply the total propulsion power, while the gen-sets supplies the hotel power. The Diesel engines are disconnected. (**b**) How configuration works from 9 kn to 17 kn: the two diesel engines supply both propulsion power and the PTO (Power Take-Off) units. The PTO delivers electrical power to the main switchboard, while the surplus in the power demand is covered by one or more gen-sets.

Table 6 reports the configuration analysis in terms of propulsion power (PB), PTO, number of active gen-sets and their working point (%), and WBS voices that have changed their working hours. The analysis put in evidence the PTO power impact during navigation phase: after 8 kn, the PTO supplies more electrical power in the network, so it is possible to use only one gen-set, compared to two in the original configuration. There in an increase in for WBS 235 working hours, while a decrease is evident for WBS 311 working hours.

It is important to underline that the gen-set in use works at 41.5% of its nominal power, and for diesel gen-sets, it is not a good solution in terms of fuel consumption and maintenance. This consideration took us to consider a second alternative configuration to solve this problem.

#### *6.2. Second Design Alternative—Power Take-O*ff *with Higher Power Size*

The second alternative (S2) layout proposes the introduction of a higher power size PTO and also a higher power size for gen-sets to obtain an efficient working point. This solution could be identified as an evolution of the previous one, with the main aim to reduce to zero the number of gen-sets used during the navigation phase. So, the configuration layout is the same, as defined in 6.1, and differs only for the items size and working point. Figure 6 continues to represent the system layout. The new proposal increases the electric motors size (also used as PTO) from 250 kW to 390 kW and gen-sets size from 650 kW to 850 kW. This option permits to satisfy the total amount of power demand in the navigation phase using the PTO only. The total number of simultaneous active gen-sets, in the worse scenario, decreases from three to two, but from a reliability and redundancy perspective, the third gen-sets are used as a stand-by element. The results achieved in this way have been represented in Table 7. It puts in evidence how the PTO supplies the total electric power demand during the navigation phase over 8 kn. The two diesel engines are the only active power generators during the navigation from 9 to 16 kn, both in summer and winter.


**Table 7.** Second alternative configuration power-use analysis results. Focus on changed WBS working hours.

Comparing Table 7 to Table 5 (S0 configuration), the higher power sizes ensured a better gen-sets working point in all conditions, less working hours, a reduction in fuel consumption, and stress on the mechanical elements. A reduction of working items was ensured: from three to two in maneuver; from two to 0 during navigation. The gen-sets total working hours were also reduced (from 4449 to 2623), but the working hours of electric motors were increased from 1699 to 3763. Main diesel working hours were the same, as described in Tables 5 and 6, but the working point was higher due to the PTO supply during navigation from 8 kn to 17 kn.

The remaining systems have the same working hours as S0, but the active units are lesser with a positive influence on maintenance tasks. All this information are the input data to the maintenance tool described in paragraph 4. The comparison in terms of a maintenance plan, between reference layout S0 and layout S2, is shown in Figure 7. The blue line is referred to reference layout (S0), while the red one to second alternative layout (S2): over 20 years, the S2 maintenance actions are, on average, less expensive than the original layout. The MTBM correction affects not only the distribution of the costs over the years but also the maintenance periodicity, that is out of phase than the original maintenance plan. From the owner point of view, this type of results presentation provides an overview of M&R, permitting to schedule when could be better to sell the ship to prevent excessive M&R costs.

Paragraph 7 provides a costs comparison between all alternative solutions described here.

**Figure 7.** Second layout M&R costs.

## *6.3. Third Design Alternative—Total Electric*

The third alternative layout (S3) proposes a complete change in the propulsion layouts through the introduction of fully electric propulsion. Instead of the two main Diesel engines, two main electric motors have been installed, supplied by four diesel gen-sets. This configuration could be an optimal choice for ships that have a special environment request, as a research vessel, and need a large amount of electric power available during the operational phases. A comparison between hybrid propulsion and fully electric propulsion, on maintenance perspective, could be interesting. As synthesized in Figure 8, usually this configuration is composed of diesel gen-sets; main switchboards; propulsion transformers and frequency converters; electric engines for propulsion; gearboxes (optional) and propellers.

**Figure 8.** Full electric propulsion system layout.

This configuration ensures high power flexibility, and so it is not necessary to divide the speed range as done before. From 1 to 17 kn, the total electrical power demand will be satisfied by a variable number of gen-sets, regulated by onboard automation. The electrical motors are only used as main propulsion engines: a PTO is not considered in this configuration.

The two electric propulsion engines have a power size of 1800 kW each, to ensure a 17 kn maximum speed. The gen-sets have to satisfy the power requested at 17 speed (by propulsion) and the power requested by the hotel loads during navigation, equal to 760 kW in summer navigation (worse condition). So, the gen-sets are four units of 1150 kW each. This layout ensures the redundancy of power generation units and, at the same time, permits a high control over the electrical power generation during the different operational phases. Table 8 proposes an overview of WBS that changed their working hours.


**Table 8.** Third alternative configuration power-use analysis results. Focus on changed WBS working hours.

Figure 9 provides an overview of the comparison of a maintenance plan between S0 and S3. M&R costs are, on average, less expensive than the original ones with a better peaks distribution over the years. Plus, a fully electric solution is eco-friendly, that is not a negligible aspect for a research vessel.

The costs comparison is proposed in paragraph 7.

**Figure 9.** Third layout M&R costs.
