Measurements of Wave Induced Hull Girder Vibrations of an Ore Carrier in Different Trades

[+] Author and Article Information
Gaute Storhaug

Hydrodynamics, Structures and Stability, DNV, Veritasveien 1, 1322 Hoevik, Norway; CeSOS, NTNU, Marine Technology Centre, 7491 Trondheim, NorwayGaute.Storhaug@dnv.com

Erlend Moe

Bulk Carriers and Container Ships, DNV, Veritasveien 1, 1322 Hoevik, NorwayErlend.Moe@dnv.com

Gabriel Holtsmark

Class Services and Newbuilding, DNV, Veritasveien 1, 1322 Hoevik, NorwayGabriel.Holtsmark@dnv.com

Note that 107 is forgotten in the half-hour damage in (1).

Note that the response matrix for cargo condition is now disregarded.

J. Offshore Mech. Arct. Eng 129(4), 279-289 (Mar 18, 2007) (11 pages) doi:10.1115/1.2746398 History: Received May 16, 2006; Revised March 18, 2007

Currently, the conventional wave loading is the only effect considered in fatigue assessment of ships. Det Norske Veritas (DNV) has recently confirmed that fatigue damage from wave induced vibrations may be of similar magnitude as from the conventional wave loading (Moe, 2005, RINA, International Conference, Design and Operation of Bulk Carriers, London, Oct. 18–19, pp. 57–85). A 40% contribution to the total fatigue damage in deck amidships is documented through extensive measurements onboard an ore carrier (the reference ship) trading in the North Atlantic. The effect of strengthening the vessel, i.e., increasing the natural frequency by 10%, is ineffective in reducing the relative magnitude of the vibration damage. The wave induced vibration, often referred to as whipping and/or springing, also contributes to fatigue damage for other ship types and trades (Moe ). This paper considers the effect of trade. It indicates when the wave induced vibrations should be accounted for in the design phase with respect to fatigue damage. A second ore carrier (the target ship) is monitored with respect to the wave induced hull vibrations and their fatigue effect. Stress records from strain sensors located in the midship deck region are supplemented by wave radar and wind records. Based on the measurements, the vibration stress response and associated vibration induced fatigue damage are determined for varying wind and wave forces and relative headings. While the reference ship operates in the Canada to Europe ore trade, the target ship trades between Canada and Europe, Brazil and Europe, and South Africa and Europe. A procedure is suggested by Moe to estimate the long term fatigue damage for different trades by utilizing the measured data from the reference ship. The vibration and wave damage are considered separately. By comparing the measured wave environment and the DNV North Atlantic scatter diagram, the effect of routing indicated a reduction of the fatigue damage by one-third. A slightly revised procedure is applied to estimate the effect of trade for the second ore carrier, comparing the long term predicted fatigue damage with the measured fatigue damage. The importance of trade is confirmed. However, the relative contribution of the vibration damage is shown to increase in less harsh environments. The target ship vibrates more than the reference ship for the same trade and Beaufort strength. The vibration damage of the target ship constitutes 56% of the total measured damage, and the high natural frequency is observed to have no significant effect.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Illustration of the different trades of the target ship

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Figure 2

Damping ratio estimates from target ship in ballast condition. The pair of dots represents port and starboard sensors.

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Figure 3

Upper: High pass and wave pass filtered stress in a sea state with 30deg head sea: HS=6m, TP=11.5s, U=5.4kn. Lower: Fit of decay curve to high pass filtered response for a single whipping event in ballast condition of the reference ship.

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Figure 4

Speed reduction for reference (R) and target (T) ship in ballast condition in head wind. For the reference ship, data from the whole year and the winter months are compared.

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Figure 5

Probability distribution of measured significant wave height of the target ship versus the reference ship, UK Met Office (10), and Argoss (11) data. Both ships were equipped with wave radars.

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Figure 6

Probability distribution of measured wind of the target ship versus the reference ship, UK Met Office (10), and Argoss (11) data

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Figure 7

Measured wind distribution for target and reference ship in NA during the period January to April

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Figure 8

Probability distribution of measured Beaufort strength in different trades for the target ship

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Figure 9

Average wave height as a function of wind in NA and summer/tropical based on Argoss (11) data

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Figure 10

Average zero up-crossing periods as a function of the significant wave height for two trades based on Argoss (11) data

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Figure 11

Measured wave height as a function of measured wind speed during all year and winter months for the reference ship in NA. The standard deviation refers to the thin lines.

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Figure 12

Normalized fatigue damage for target ship based on data from all trades

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Figure 13

Extrapolated fatigue damage (20 years) based on all measurements of the target ship

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Figure 14

Extrapolated fatigue damage (20 years) in NA and summer/tropical trade of the target ship. SCF=2.04.

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Figure 15

Extrapolated fatigue damage of the reference ship in NA trade for whole year and winter season. SCF=2.0.

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Figure 16

Vibration damage compared to fatigue damage from high-pass filtered signal

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Figure 17

Standard deviation wave and vibration stress for target and reference ship as a function of Beaufort strength in ballast condition at head seas in NA between January and April (no SCF is included)

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Figure 18

Average half-hour wave and vibration damage for target and reference ship as a function of Beaufort strength in ballast condition at head seas (includes SCF of 2.0)

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Figure 19

Standard deviation wave and vibration stress for target and reference ship as a function of Beaufort strength in cargo condition in following seas (no SCF is included)

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Figure 20

Spreading of wave direction from measurements and Argoss (11) for 5m significant wave height in NA trade




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