The classification societies offer various notations covering noise and vibrations onboard, but Det Norske Veritas seems to make something of a speciality in this area with 450 DNV-classed vessels having the additional comfort class notation, plus an extensive noise and vibration consultancy service on offer.
Its comfort class, originally introduced in 1995, includes particularly low levels of noise and vibration alongside general standards of comfort and indoor climate in accommodation and working areas. The notation has, naturally enough, particular relevance to cruise vessels and other passenger ships, but has also been widely applied to offshore vessels and even ocean-going cargo ships. In fact offshore vessels form by far the largest number of comfort class ships, with 304 vessels, 181 of these under construction. 16 are passenger ships; the remainder mostly tankers and bulk carriers.
DNV says that comfort onboard is an essential part of safety, health and environment for the crew. In order to work safely, and operate the ship in a safe and efficient manner, the crew needs proper rest.
There is, according to DNV, a further benefit in that ships with comfort class are more attractive among potential crew and can ease recruiting, an important consideration when there is a shortage of competent and wellqualified seafarers at all levels. The comfort class description outlines the benefits of a top rating for vibration: "...comfort on board is an important quality of the vessel. A high level of comfort increases the performance and vigilance of the crew and, consequently, the safety of the ship and cargo. For a passenger ship, the comfort will be significant for the ship's rating and reputation in the market.
Noise and vibration comfort is a significant parameter in this respect." An important element of DNV's comfort class is the work undertaken at the design stage, which begins with a review of the basics, early in the design, to pinpoint what might be possible problem areas. At the same time, any basic problems are investigated further and methods suggested for further investigations.
This stage is followed by an analysis of the known excitation sources, namely the main and auxiliary engines and the propellers. In order to investigate the likely magnitude of the excitation forces, the propeller excitation is fully analysed, as are the forces from the engines. The shaft forces are also examined for their likely effect on the overall noise levels, and the influence of other systems, such as ventilation and hydraulics, is considered. This stage provides data for input into the next part of the procedure.
The third step is a comprehensive analysis of the responses in the structure to excitation; this includes a global vibration analysis, a prediction of noise levels, and local vibration analyses for critical areas. For this, DNV applies advanced mathematical techniques. A 3D global finite element model (FEM) of the vessel is used to analyse vibration modes and vibration levels. The results are documented in a report covering:
Global natural frequencies and mode shapes
The natural frequencies and mode shapes of large substructures
The forced global vibration level
The forced vibration level of large substructures.
The results are presented as mode shapes and coloured plots showing the distribution of the vibration levels on each deck. If any of the specified vibration levels are likely to be exceeded, the problems and consequences will be documented and DNV says that it will propose and discuss suitable structural modifications. Locally, an analytical approach is applied to calculate the natural frequencies of girders, stiffeners and plates. Where required, a FEM of the local structure is prepared. The analyses are based on lists of all relevant excitation sources, and steel drawings of the structure. The results are documented in a report which includes a review of all relevant excitation sources, and calculated natural frequencies of the selected structure. The report also proposes possible modifications in order to avoid the risk of excessive vibration.
The quiet ship As far as predicted noise levels are concerned, DNV applies its semi-empirical noise analysis programme (NV5850) to calculate the contribution to air- and structure-borne sound induced by the main noise sources, in the normal service transit condition of the vessel. This optionally includes (normally includes for certain offshore vessels) operating with the bow thrusters. The results are presented as dB(A)- levels for the various accommodation spaces, open deck areas and engine room/working spaces. The predicted levels are compared to the specified noise limits. Where the required noise levels are not likely to be met, noise reducing measures are proposed.
This, however, is only the beginning of the task of creating a true quiet and comfortable ship. Once a low background noise level from the machinery has been achieved, other aspects of the ship design and building become more apparent than they would be in a vessel with normally-achieved noise levels. Privacy between cabins, and insulation between cabins and public spaces become an issue. Even such a basic aspect as impact noise, resulting from walking on hard floors, can become intrusive. As a result, it becomes necessary for designers to address all of these points when laying-out the accommodation and working spaces. DNV recommends the use of sound-deadening partition walls, which, with a thickness of 50mm offer a noise attenuation of 53dB as well as good thermal insulation and fire protection qualities.
As well as the effects of a noisy ship on those working or travelling onboard, DNV believes that ship noise will become a wider environmental issue, with the need to limit airborne noise in ports and harbours plus underwater noise pollution. As whales and other marine species communicate and navigate using lowfrequency sounds, there is a strong possibility that these signals could be drowned out, not only by ships but by underwater exploration activities. The classification society thinks it possible that future environmental regulations could be introduced to control marine noise pollution, particularly underwater, undertaking considerable research in anticipation of new, rigidly enforced, noise limits.
Vessels which fulfil DNV's noise and vibration requirements are given the additional class notation COMF-V(crn). The requirements for onboard noise and vibration limits are divided into three groups depending on the level of comfort achieved, i.e. comfort rating number (crn) 1, 2 and 3, with 1 representing the highest level of comfort and 3 representing an 'acceptable' level, i.e. equal to the noise levels in IMO's code on noise and the ISO 6954 vibration levels. Compliance with the rules is verified through measurements of defined ship-type specific noise and vibration parameters and criteria. The requirements for the notation are specific for the various ship types, and are divided into groups for specified locations. For offshore service vessels to be operated in DP mode, noise and vibration measurements are carried out with the side thrusters in operation. For passenger ships, the given comfort rating number normally applies to the passenger accommodation only, though crew accommodation has to comply with at least the minimum rating (crn) 3 for cargo ships. The main parameters used to describe the comfort level are: noise, sound insulation, impact sound insulation and vibrations. Before undertaking the verification measurements, DNV applies a test programme which includes at least the specified measurement locations, the required loading conditions, the required operating conditions for the ship's machinery, and the instrumentation to be used.
DNV says that offshore ships offer a particular challenge to designers and buildings hoping to achieve levels 1 and 2, with large machinery installations relative to the size of ship, large tunnel thruster installations, and long periods of operating in manoeuvring and DP condition. However, applying comfort class has clear benefits, with a Statoil project indicating that accidents were significantly reduced after application of even the minimum COMF-V(3) notation.
Incorporating such improvements as resiliently-mounted bow thrusters, 'floating' accommodation and visco-elastic damping can help reduce crew fatigue and sleeplessness, resulting in fewer accidents such as collisions between vessels and rigs.
Among the successful applications of DNV's comfort class in the offshore industry are the Ulstein AX104 supply ship, which is designed to meet COMF-V(3), the latest Vik Sandvik (Wärtsilä) supply vessel designs, and the Rolls-Royce designed well intervention vessel Island Wellserver, which meets the highest COMF-V(1) level.
What was, at the time of delivery, claimed to be the world's quietest ship, the research vessel James Cook, employed DNV as noise and vibration consultant during design and build.
This artickle is published with the permission of The Motorship
Date: 01 July 2009