BİLGE AKUSTİK
Noise and Vibration Control Engineering
“I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.”
Examples of Ship Vibration
1. Introduction
The vast majority of all shipboard vibration problems the reader may
encounter have occurred at other times on other ships. The objective of this section of
the site is to give examples of these commonly occurring ship vibration problems.
Through these examples it is hoped that the reader may gain some practical
background and insight into the subject of ship vibration in general, and also the
methods and techniques used to analyze and solve ship vibration problems.
The examples contained in this section are arranged by ship element
as hull girder, major substructure, local structure, main propulsion machinery, and
equipment. Each example will include a brief description of the problem, commonly
observed symptoms, a discussion of the cause(s), and potential solutions. Many of the
examples may be applied directly to solve identical or similar problems encountered by
the reader. Others will provide the reader with a feeling for the nature of ship vibration,
Interpretation of observed symptoms, and the various methods used to find solutions.
With the Information contained elsewhere in this examples the reader, confronted with a
vibration problem, should be able to determine the cause and potential solution for
relatively simple or routine cases and may be able to implement a solution with
equipment and material onboard the ship. In the case of more complex or nontypical
vibration problems, these examples provides sufficient information for the reader to
approach the problem intelligently through the discovery of the symptoms and
possible causes, and to plan a suitable course of action to be taken to solve the
problem. Such a plan of action may include the application of a simple solution,
determining where, how, and when vibration measurements should be taken or, at least,
acknowledgement that an outside consultant is required, whether he be an equipment
manufacturer’s representative or a vibration consultant.
Before getting into the examples it is useful to look at some statistics
on ship vibration problems in order to determine what the most common problems are
and into what ship element types they fall. Table 1 shows a breakdown of where
vibration problems occurred on 70 ships reporting problems. As can be seen from the
table, the most frequently reported problems are associated with local and major
structures.
Of these cases, the greatest number are in the superstructure (or
accommodation spaces). This might be expected as it is where the crew spends most of
their time and, hence, they are more sensitive to and critical of vibration in this area.
Superstructure vibration problems can be caused by excessive global hull girder
vibration, superstructure resonance or near resonance due to insufficient stiffness, or
local resonances. These local resonances may be in the structure such as decks or
bulkheads, but they may also be in equipment installed in the superstructure, such as
Joiner panels and doors, ventilation ducting, pipes, lighting fixtures, furniture, navigation
equipment, etc. In light of the latter case, the number of ships with structural problems
in the superstructure Is probably overstated In the table, while the number with
equipment problems is understated. In either case, the table does point out the very
high susceptibility of the living accommodations to vibration problems, whether they be
due to structure or equipment.
The second most frequently reported problems are those associated with the
main propulsion system. While excessive vibration in the superstructure Is very
annoying, excessive vibration In the propulsion system can prove very damaging and
may. In fact, limit operating speeds or render the ship inoperable.
The third most frequently reported problems, according to Table 1, are with
equipment. If the vibration problems associated with equipment in the superstructure
were included in this category then it would rival structural problems for the most
frequent. Excessive vibration of equipment when it forms a part of an essential system
can prevent proper operation of the ship just as with the main propulsion system.
The least frequent problems are associated with hull girder vibration. This is
probably due to the ship designer's ability to accurately calculate the lower hull girder
natural frequencies and to select propulsion systems which will not excite these lower
hull girder modes in the operating range.
The two most frequent forms of hull girder problems are resonances of the aft
end of the hull girder with blade frequency and springing. The former often manifests
itself in excessive superstructure vibration at or near blade frequency. The latter is a
two-noded vertical (first mode) vibration excited by wave encounter forces.
The above serves as a useful guide in determining the number and type of
examples to be included for each ship element type. Major emphasis will therefore be
placed on local and major substructure, main propulsion machinery, and active and
passive equipment. Hull girder problems will receive the least amount of attention in the
examples.