Using HydroComp NavCad

Posted by Irina Tabakina

To resolve static thrust problems on wind farm service vessels

HydroComp NavCad is software for resistance and propulsion that can be found in the toolbox of hundreds of naval architects and marine professionals from around the world. The latest challenge for NavCad is to help resolve a unique problem for propeller-driven wind farm service vessels (WFSVs). Much like the problems of dynamic positioning in offshore and platform support vessels, WFSVs are finding issues of insufficient thrust during static (i.e., bollard) operation. This is a solvable problem, but one which must be evaluated on a case-by-case basis.
The problem exhibited in WFSVs is one of mutually exclusive operational objectives for the propeller – efficient high speed operation versus maximum static thrust for the crew transfer maneuver. Of course, a propeller optimized for one case (high speed) will be less effective at the other (bollard).

Potential Solutions

A controllable-pitch propeller (CPP) would be the ideal solution, but since this is not generally a feasible option, we will set it aside to focus on fixed-pitch propeller (FPP) systems. For FPP propellers, maximum static thrust is determined by an equilibrium relationship between the engine’s ability to generate power and a propeller’s power requirement at a given RPM. There is one point on the engine’s power curve where the propeller cannot be spun any faster as it would require more power than the engine is capable of producing. The thrust is generated by the propeller at this RPM, and in general, more thrust means more RPM.
The solution, therefore, is to:
Increase low end power for the engine so that it can spin the propeller faster,
reduce the propeller’s power requirement (in a way that does not affect high speed operation), or
some combination of both.
So, a solution could come from different engine models with different power curves, or from different propeller characteristics that change the shape of its torque curve.

Propulsion Analysis

Both of these options can be evaluated by NavCad, which includes a “Towing” propulsion analysis that is built upon this equilibrium-power relationship described above. In this analysis, NavCad can find the maximum equilibrium thrust given the engine, transmission and propeller characteristics. It allows the entry (and archiving) of specific engine model power curves, so the effect of different power curves can be evaluated. It also allows for consideration of different propeller types and parameters, including their effect on cavitation breakdown. This offers the ability to look for “WFSV-friendly” propeller geometries. As mentioned above, such an analysis needs to be run on a case-by-case basis, since geometric properties like shaft angle and stern run angle needs to be considered in the analysis.

The following plots illustrate how NavCad can evaluate the effect on static thrust due to differences in engine power curves and propeller characteristics. The static “bollard” condition is represented by a 0.01 knot speed, and the propeller design operating speed is 20 knots.
The plots to the left show engine loading for the given power curves of engines rated at 3,000 kW at 1,000 RPM. The plots to the right are delivered thrust overlaid onto the resistance curve. The top speed is where the two lines intersect, and the static delivered thrust is found at the nominal zero speed position.

Basis – 300 kN thrust

This is the original engine and propeller. Bollard equilibrium occurs at 520 RPM.

New Engine Model – 460 kN Thrust

This is for a new engine model of the same rating but with modestly different engine power curve, using the original propeller. While the engine is not substantially different from the original, the modest increase in low RPM power results in approximately 50% increase in static thrust with increase in RPM to 660. The top speed is shown to be maintained (intersection at 20 knots).

New Propeller – 410 kN Thrust
This is for the original engine with a new propeller that is designed with characteristics to reduce torque at bollard conditions. The engine is not changed, but the new propeller delivers some 35% more static thrust at 720 engine RPM. Top speed is maintained (intersection at 20 knots).
These plots illustrate how NavCad can be used to investigate the differences in engine power curves on static thrust – where an increase in the power delivery curve at low RPM will allow the propeller to spin up to a higher RPM and generate more thrust. It also shows how NavCad can assess the effect of propeller characteristics on static thrust, where for the same power requirement a different propeller might deliver more thrust. Since maintaining top speed is an important mission constraint, NavCad’s robust resistance prediction capabilities are also critical to success.
As has been described herein, finding an acceptable overall solution to the problem of insufficient static thrust requires a tool that has the ability to
• correctly model the influence of engine curve shape and propeller performance on the equilibrium power “towing” condition at bollard, and
• properly predict vessel resistance to insure that top speed is not compromised by any changes that might be proposed.

Other stories from August 2014 issue


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