Answer
Underwater Radiated Noise (URN) refers to the acoustic sound energy generated by commercial ships that transmits into the marine environment, disrupting marine life communication, echolocation, and foraging.
Regulatory Framework & IMO Status
- IMO Revised Guidelines (MEPC.1/Circ.906): Adopted at MEPC 80 (revoking MEPC.1/Circ.833) to provide updated design, technical, and operational strategies to mitigate shipboard noise. Currently voluntary. IMO's MEPC 80 in July 2023 approved revised Guidelines for reducing underwater radiated noise from commercial shipping. They became effective on 1 October 2023.
- Experience Building Phase (EBP): A global monitoring initiative running through 2028 under the IMO SDC Sub-Committee to gather operational trial data and assess potential future mandatory standards.
- URN Management Plan (UMP): The guideline framework recommends carrying a vessel-specific UMP to baseline noise levels, set voluntary reduction targets, and document control measures.
Primary Shipboard Sources of URN
Noise Source | Contribution | Generation Mechanism & Characteristics |
|---|---|---|
Propeller Cavitation | ~80% to 85% | Dominant Source. Sheet, tip vortex, and transient bubble collapse generate high-amplitude broadband noise and low-frequency pressure pulses. |
Machinery Vibration | ~10% to 15% | Structure-borne vibration from main engines, diesel generators, reduction gears, and heavy pumps transmitted through hull plating into sea water. |
Hydrodynamic Flow | ~5% | Turbulent boundary layer flow, bow waves, and vortex shedding around hull appendages (struts, bilge keels, rudder). |
Mitigation Measures (Design & Operational)
- Design & Technical Solutions:
- Propeller Design: High blade skew, optimized pitch distribution, and tip geometry to increase the Cavitation Inception Speed (CIS).
- Energy Saving Devices (ESDs): Pre-swirl stators, ducts, and hub fins (PBCF) equalize non-uniform wake fields, reducing cavitation volume and pulse strength.
- Vibration Isolation: Resilient double-elastic mounts for diesel engines, flexible pipe couplings, acoustic machinery enclosures, and sound-dampening hull coatings.
- Operational Solutions:
- Speed Reduction (Slow Steaming): Operating below CIS significantly drops acoustic power output (often a 3 dB to 6 dB reduction per 2-knot speed drop).
- Hull & Propeller Maintenance: Frequent cleaning and propeller polishing eliminate biofouling, preventing early boundary-layer turbulence and sheet cavitation inception.
- Trim & Ballast Optimization: Maintains favorable inflow angles to the propeller disk.
Class Notations & Commercial Incentives
- Class Rules: Voluntary notations certified via hydrophone sea-trial measurements (e.g., DNV SILENT, LR URN, ABS UW-NOISE, BV URN, IRS URN).
- Port Incentives: Major ports (e.g., Vancouver EcoAction, Rotterdam, Hamburg) offer port dues discounts (up to 20%) to vessels maintaining certified low-URN notations or complying with seasonal slowdown zones.