Calculation of Structural Damping of the Global Hull Structure from In-Service Measurements

Ships operating in waves are continuously subjected to varying loads from waves. As a result of these loads the ship undergoes rigid body motions and deformations. The latter can be subdivided into two types, a quasi-static and a dynamic response. Two dynamic responses that are of interest for ships are springing and whipping. Springing is the steady-state resonant vibration of a flexural mode due to continuous wave loading. Whipping is the transient elastic vibration of the ship hull girder caused for example by slamming. Springing has an important contribution to increased lifetime consumption, whereas for assessment of ultimate stresses whipping is more important. One of the major uncertainties when examining global flexural response is the amount of damping to be incorporated. Because springing is a resonance phenomenon, damping is especially important. In this paper Operational Modal Analysis is used to retrieve damping from in-service measurements and is tested for a heavily instrumented frigate type vessel. The aim of this paper is both to explain and discuss the methodology and to provide typical damping parameters which are needed in the assessment of whipping and springing. The paper also shows sensitivity of frequency and damping of the flexural modes to selected operational parameters. The research in this paper is restricted to the two node vertical bending mode. Acceleration data is most commonly used to derive mode shapes. For the present application, however, accelerations and strains were combined. In order to obtain useful results stringent filtering on mode shape, frequency, damping and stationarity of operating conditions should be applied. Operating in confined waters also has a noticeable effect. Variations in the observed natural frequency can be as large as 10%. These variations must be related to mass variations. At average operating speeds in moderate environmental conditions, the damping of the two node bending mode is around 0.6 to 1%. However, analysis presented in this paper have indicated that damping is related to speed and wave height. Specifically, the damping in 26 knots increases to 2.5% from the 1% at 15 knots. In beam sea conditions, an increase in damping from 0.7% to 1.2% was found in wave heights ranging from 1 to 2 m. This indicates that when studying extreme conditions different damping ratios are applicable compared to intermediate conditions.