Measure and quantify the URN contribution of a ship’s propulsion unit

A propulsion system manufacturer aimed to secure a contract with a shipyard for a specialized vessel project. The vessel needed to meet strict Underwater Radiated Noise (URN) requirements, comparable to DNV SILENT R.

To mitigate risks, the shipyard specified in its request for proposals (RFP) that the propulsion unit must not exceed a defined noise limit curve. However, the manufacturer lacked a track record of similar projects with proven acoustic compliance. To participate in the tender, they needed to provide a URN study along with measurement results for their propulsion unit.

To verify the propulsion unit’s acoustic performance and ensure compliance with the required noise limits, JASCO ShipConsult developed a systematic measurement and analysis plan.

Identifying a comparable test vessel

Since no prior URN measurements were available, the team identified an operational vessel equipped with a comparable propulsion system from the same manufacturer. By conducting a dedicated URN measurement on this vessel, ShipConsult could isolate and analyze the propulsion unit’s specific noise contribution.

Overcoming measurement challenges

Securing the necessary measurements posed a challenge, as the vessel operator had limited flexibility to alter its route or schedule. To address this, the ShipConsult team devised a creative URN measurement procedure that minimized disruption while ensuring high-quality data collection.

Selecting the optimal measurement location and equipment

The first step was identifying a suitable measurement location near a port—one with adequate water depth and minimal background noise. Next, the team selected the best measurement devices for the project, combining stationary subsea autonomous noise recorders with a floating sound measurement buoy.

Conducting the URN measurement and data analysis

With these devices in place, and under the supervision of JASCO’s acoustic engineers, the ship successfully passed through the pragmatically erected sound range at the required operating conditions. The subsequent data analysis separated the propulsion unit’s noise contribution from other ship-generated noises, such as diesel engine sounds and ambient noise from distant vessels.

The analysis provided a detailed breakdown of which physical effects contributed to specific noise levels at different frequencies. Some of these effects caused the propulsion unit to exceed the specified noise limit curve. By identifying these critical issues, the manufacturer was able to implement targeted engineering solutions to mitigate them.

Now, more than five years later, the propulsion unit has been successfully integrated into the specialized vessel. Upon URN testing, both the vessel and its propulsion system met the required noise limits, complying with the DNV SILENT R-equivalent standard.