VIBRATION

Excessive vibration of ship structures and equipment can be a serious problem and lead to failure of the propulsion system, structural failures of the primary structure and damage to shipboard equipment. Even if the vibration level is not high enough to result in major damages, it can lead to crew discomfort and fatigue and increase the frequency of maintenance works for ship systems.

Due to the potential serious nature of the vibration problem, limit values that must not be exceeded during operation of the ship are defined in Specifications. The shipyard thus bears the responsibility for ensuring that these limits are not exceeded or – if they are – for taking action with the aim of reducing the vibration level to acceptable value. However, solving vibration problem on a completed ship can be very costly and time-consuming.

Therefore, it is recommended to take all measures in order to avoid such situation. Especially, the Specifications must contain wordings about propeller pressure pulses, tip clearances, main engine elastic mounting, continuity of main structure and proper support of the accommodation deckhouse.

The low-speed diesel main engine and the propeller are the principal vibration exciting sources. The hull structure responds as a both ends free beam when subjected to dynamic loads.

Excitation forces from the propeller are transmitted into the ship via the shaft line and also in the form of pressure pulses acting on the ship hull surface above the propeller. Whereas propeller shaft forces (bearing forces) are the most significant factor for vibrations of shaft lines, the pressure fluctuations on the hull surface (hull surface forces) are the predominant factor for vibrations of ship structures.

Dynamic forces from the shafting system are transmitted to the hull through shaft bearings. The propeller induces fluctuating pressures on the surface of the hull, which induces vibration in the hull structure. The main and auxiliary engines can directly cause vibrations through dynamic forces transmitted through their supports and foundations. The response to this forcing can cause the vibration of the hull girder, deckhouse, deck and other structures, local structures and equipment.

The following measures should be taken in order to avoid excessive vibration of ship structure and equipment:

1. During the initial stern lines fairing and propulsion system arrangement studies,
check the maximum angles and minimum propeller clearances.

2. During the initial engine selection, check the 2nd order vertical moment M2v from
potential vendor and calculate the Power Related Unbalance.

PRU = M2V (Nm)/Engine Power (kW)

If the PRU exceeds 220 Nm/kW, consider either change of engine selection or installation of moment compensators. Also, the installation of engine lateral stays on the engine room structure is to be addressed at the early design stage.

3. The ship is to be designed with particular care to continuity and alignment of bulkheads and walls to create stiff foundations for the accommodation deckhouse.

4. Longitudinal deckhouse walls shall be supported on longitudinal bulkheads running from the fore bulkhead of the Engine Room to the transom.

5. Longitudinal shear stiffness of the deckhouse should be maximised by means of continuous longitudinal walls having as few and small cut-outs as possible.

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