Propeller Shaft Resonance Mitigation on Offshore Support Vessel

Case Study 1: Propeller Shaft Resonance Mitigation on Offshore Support Vessel

Scenario: An offshore support vessel (OSV) operating in the North Sea experienced excessive torsional vibration at 120 rpm during transit, causing premature coupling wear and alarm-triggering vibration spikes. The project involved retrofitting the main propulsion shafting to avoid resonance with the 3rd engine firing order (180 rpm × 3 = 540 rpm → 9 Hz excitation). Location context: Harsh marine environment with limited dry-dock window (<72 hrs); constraints included no change to engine speed range (0–180 rpm), strict weight limits (+50 kg max), and mandatory compliance with DNV-OS-E401 and ISO 14566-1.

Given data:

• Torsional stiffness (k): 62,500 Nm/rad (measured via static twist test on 8.2 m stainless steel shaft, Ø320 mm, G = 79 GPa)
• Polar moment of inertia (J): 12.4 kg·m² (calculated from flywheel + propeller hub mass distribution; validated via pendulum test)

Calculation: Natural frequency is computed as:

$$ f_n = \frac{1}{2\pi} \sqrt{\frac{k}{J}} $$

Substituting values:

• $ k = 62{,}500 , \text{Nm/rad} $
• $ J = 12.4 , \text{kg·m}^2 $
• $ \frac{k}{J} = \frac{62{,}500}{12.4} \approx 5040.32 $
• $ \sqrt{5040.32} \approx 70.99 $
• $ f_n = \frac{70.99}{2\pi} \approx \frac{70.99}{6.2832} \approx 11.30 , \text{Hz} $

Rounded to two decimal places per tool specification: 11.30 Hz.

Result and decision: The calculated natural frequency (11.30 Hz ≈ 678 rpm equivalent) lies within 3% of the 3rd firing order excitation at 120 rpm (9.0 Hz) — but critically, it avoids overlap with dominant excitations at 6 Hz (2nd order @ 120 rpm) and 12 Hz (4th order @ 180 rpm). More importantly, modal analysis confirmed no amplification near 11.3 Hz under load. Therefore, the existing shaft configuration was retained with enhanced damping: a tuned viscous damper (TVD) tuned to 11.3 Hz was installed at the forward end of the shaft. No geometry or material changes were needed.

Lesson: Measured torsional stiffness and polar inertia—not design nominal values—are decisive for resonance avoidance; field validation prevents over-engineering and unnecessary dry-dock time.