Section 1 Introduction

1.1.1 This document is intended to encapsulate the experience and best advice of Lloyd’s Register (LR) that has been assembled since circa 1980 regarding prediction and avoidance of potential problems in relation to structural vibration in ships.

1.1.2 This subject of structural dynamics can appear to be complex in nature and terminology. However, in this document, the intention is to make the subject's presentation as simple, concise and as useful as possible in both the analytical and practical sense.

1.1.3 Vibration is generally not subject to mandatory classification rules, except for the propeller shaft where excessive vibration can cause failure that would compromise the safety of a ship by loss of propulsion. However, classification societies do have optional class notations in relation to vibration and noise standards for habitability. The International Standards Organisation (ISO) also publishes standards. An owner usually specifies such criteria in the build contract, which are normally derived from ISO or a class notation that are with regard to measured values.

1.1.4 The primary objectives in order to meet vibration standards are avoidance of significant resonances (coincidence of structural natural frequencies and excitation frequencies in way of the operating range) and minimisation of excitation forces. Frequencies up to around 20 Hz are usually relevant in relation to global ship structure vibration, most notably 5 – 20 Hz. Somewhat higher frequencies can be relevant for local panels that are adjacent to propeller(s) and main engine(s).

1.1.5 This document does not cover prediction of noise levels. Although noise can be regarded as high frequency vibration, it is in the audible range above 30 Hz evaluated in decibels; and noise prediction methodology for large-scale structures is necessarily completely different to that for vibration.

1.1.6 This document covers structural vibration prediction and therefore does not include predictive methods for propulsion system vibration.

1.2.1 Clients recognise the importance of performing cost-effective and meaningful structural vibration prediction analyses at the ship design stage, with the aim of avoiding potentially expensive and problematical situations occurring on ship trials and/or in service (see Ch 1, 8 Vibration problems and solutions).

1.2.2 Contractual habitability standards are becoming ever more demanding, particularly for passenger ships. This is due to advancing expectations of crew and passengers in relation to comfort.

1.2.3 Invariably due to complicated structures, uncertainty of damping, fluid/structure interaction and excitation forces, it is not possible to guarantee a ship that is completely free of vibration problems by conducting dynamic analysis. The risk of unacceptable vibration occurring in practice can be greatly reduced, and any issues that may arise during ship trials or in service would normally be solvable, often by using an existing computer model.

1.3.1 In the past, non-uniform beam methods were often used to represent global ship structure (such as LRHULVIB). This approach is essentially limited to fundamental vibration modes because three dimensional effects become significant for higher modes.

1.3.2 A development from beam element models was hybrid models such as a coarse three-dimensional model of an aft hull including a deckhouse and beam elements for the remainder of the hull.

1.3.3 However the ship is modelled, for dynamic response analysis it is necessary to represent the whole mass/stiffness system. Partial models can be used to evaluate natural frequencies of local structure, subject to suitable boundary conditions being arranged.

1.3.4 With increasing computer power and use of large scale finite element analysis (FEA), together with global stress analysis finite element models often being available, FEA is now the main vibration analysis method. LR mostly uses the NASTRAN finite element program. Therefore, FEA guidance and definitions in this document are in relation to that program, though comments for other FEA programs would be similar.

1.3.5 A typical ship finite element model created for global vibration analysis is shown in Figure 1.1.1 A typical finite element model, suitable for global vibration analysis:

Figure 1.1.1 A typical finite element model, suitable for global vibration analysis

1.4.1 LR also has a document entitled Ship Vibration and Noise Guidance Notes that primarily covers routine measurement surveys and investigation of problems occurring on ship trials or in service. This is a companion to this vibration prediction document. Content in the Ship Vibration and Noise Guidance Notes relating to vibration acceptance criteria is particularly relevant for this document.