The rising fuel prices and ongoing efforts to minimize the environmental impact makes reducing fuel consumption one of the main challenges in maritime sector. Simcenter Amesim models can be used within the design process to evaluate drivetrain alternatives and during operations to compare sea routes based on expected weather conditions.

Figure 1: Ship trajectory form port of Hamburg to New Jersey

Amesim in marine

This article gives an example on how Simcenter Amesim could assist during operations. A cargo ship is about to leave the port of Hamburg with and has New Jersey as destination. The following questions need to be answered when planning the trip:

• What would be the Estimated Time of Arrival (ETA)?
• What is the expected fuel consumption?
• How are these influenced by the weather conditions?

Figure 2: Simcenter Amesim simulation model. The engine model and control on the left, the propeller, ship, and sea conditions on the right.

The Amesim model

Within Simcenter Amesim a model of the ship is constructed (Figure 2). The main components are:

• Two stroke diesel engine
  Engine performance are based on user-defined mappings.
• Propeller designed for marine application
  In this model, the thrust and torque are computed using 2D tables of the Ct (thrust coefficient) and Cq (torque coefficient) as function of the pitch diameter ratio and advance angle defined on 360° (all four quadrants). Other options are 2D tables for the first and fourth quadrants or first quadrants only and the theoretical Wageningen B-Series propeller model.
  The influence of the hull on the wake is omitted in this analysis, but can be included and computed with Taylor, Holtrop, Harvald, user defined.
• Ship model considering mass and navigation resistance
  The model determines the longitudinal translational motion of a ship and takes into account its mass and the navigation resistance due to the friction with water. The navigation resistance is computed based on experimental test data of a ship model. The empirical formulas of ITTC 78 methodology are used to scale the result to the real ship dimensions. Other ways to determine the navigation resistance are Statistical Barrass, Holtrop and Mennen method, Savitsky Method, user defined as function of ship velocity.

Varying sea conditions along the sea route are accounted for by apply two levels of representative weather conditions (good and bad). These levels have been established taking the average weather you may encounter on the travel and are split into two extreme levels. Figure 3 and 4 show the varying conditions.

Figure 3: Basic seawater properties for good and bad weather conditions

Figure 4: Wand and wave properties for good and bad weather conditions

Conditions

Three different command speed profiles are imposed. Based on the two levels of expected conditions and three different cruising speeds a total of six simulations are performed to determine the travel times, fuel consumptions and CO2 emissions. Figure 5 shows the real ship speed compared to the command speed. For good weather conditions the real speed follows the 8 and 11knot command speed. However, the real speed does not follow the max speed command due to the maximum load reached by ship engine. For bad weather conditions, the maximum real speed even drops below 11knot and 8knot along the route.

Figure 5: Commanded speed profiles and real ship speeds

Figure 6: Sea resistances of different sources for good (top row) and bad weather conditions

The ship accelerations are computed by the resultant force on the ship divided by the mass of ship. The result force is the difference between the total resistance and propeller thrust. The forces acting on the ship are shown in Figure 6.

The travel time, total fuel consumption and CO2 emission are determined for the six simulations. Figure 7 shows how these evolve along the ship advance position. The varying speeds in the bad weather conditions are clearly visible in the travel time graph, as the slopes are not constant.

Results

The increase in travel time and average fuel consumptions are shown in the table.