Characteristics of abnormal waves in North Sea storm sea states

Dramas of the sea: episodic waves and their impact on offshore structures

The paper presents an approach to determine the global load effects induced on ship structures by abnormal, freak, or episodic waves. It refers to the present procedure of determining extreme values of wave-induced responses, including the recent advances of adopting time series of wave elevation as reference design conditions to calculate the wave-induced structural loads on ships in heavy weather. It is shown how this procedure can be extended to account for abnormal or episodic waves. Reference is made to what is presently known about abnormal or freak waves, showing that although it is possible to determine the loads induced by these waves in floating and fixed structures, the present knowledge about the probability of occurrence of these waves is not enough to allow a wave design criterion to be defined in a way consistent with the present probabilistic approaches. However, it is suggested that at the present stage of knowledge it is possible to determine the loads induced by abnormal waves similar to ones that have been measured at various ocean locations and that are thus realistic; a method is described to perform such calculations. Although this information cannot replace the wave-induced loads calculated with the presently established procedures, it can serve as guidance for the design. An application example is presented of a containership subjected to a wave trace that includes an episodic wave that was measured during a severe storm in Central North Sea. The measured wave time history is modified in order to investigate the influence of the wave steepness on the induced vertical motions and loads. The loads induced by the abnormal wave are compared for the first time with extreme values from long-term distributions.

Abnormal Wave-Induced Load Effects in Ship Structures

In developing a wave energy converter (WEC), assessing and rating the device is a difficult, but important issue. Conventionally, a large scaled device (maybe large enough for accommodating a power takeoff (PTO) system) or prototype device is needed to be tested in wave tanks or in seas in different wave conditions so that a power matrix for the device can be defined using scaling or interpolation/extrapolation methods. Alternatively, a pure numerical simulation in time-domain may be used for assessing the power capture capacities of wave energy devices. For the former, it is convincing, but can be especially difficult in the early stages of development, when small scaled models are normally used; and for the latter, the pure numerical simulation may not be very reliable and convincing, especially when the dynamic problem is very complicated. In this paper, a method for assessing the captured wave power for a device from its power capture response is presented. In the proposed method, a measured or calculated linear power capture response of the device is combined with wave spectrum to compute the average captured power function. Once the average captured power function is obtained, the overall average captured power corresponding to the wave state can be easily calculated. If a linear power capture response is obtained from a model test, the power assessment based on this proposed method can be very convincing and reliable. To illustrate the application of the proposed method, an example of a fully linear dynamic system, including the linear hydrodynamics of the floating structure and a linear power takeoff, is considered. For such a system, the frequency-domain analysis can be employed to obtain the performance of the floating device under waves and the power takeoff system. The hydrodynamic performance of the wave energy converter is then used to define the power capture response and to calculate the average captured power functions in different sea states. Then, the captured power of the device in different sea states, i.e, the power matrix, can be calculated, and accordingly, the device can be assessed and rated. To validate the proposed method, a time-domain analysis is also performed for a cross-check. In the time-domain analysis, the hydrodynamic coefficients and responses are first assessed in frequency-domain, and then transformed into the relevant terms by means of impulse response functions for establishing the time-domain (TD) equation. By comparing the results from frequency-domain and time-domain analyses of irregular waves, it can be concluded that the proposed wave energy capture assessment method can be used in assessing or rating the device.

Assessment of Wave Energy Extraction From Seas: Numerical Validation

The increasing numbers of reported rogue waves with extreme crest and wave heights and unusual group pattern with the consequence of severe damages raise the question if such exceptional events have to be considered routinely for the design of ships and offshore structures. For the investigation of the effects of rogue wave impacts time domain simulation methods are required in addition to traditional frequency domain methods which may not be sufficient to consider these extreme events. In this paper the vertical bending moments at the midship section of an FPSO are investigated using state of the art numerical simulation tools in combination with experiments. For the seakeeping tests the extremely high New Year Wave (registered in the North Sea) is generated in the wave tank, and motions and structural forces are analyzed at model scale. For validation the results are evaluated deterministically and compared to numerical simulations. The time domain calculation allows to artificially change local wave characteristics. The steepness of the selected rogue wave is varied and the influence on wave induced loads is studied. A comparison with standard procedures of seakeeping analysis and classification rules closes the paper.Copyright © 2004 by ASME

Bending Moments of an FPSO in Rogue Waves

For the design of safe and economic offshore structures and ships the knowledge of the extreme wave environment and the related wave/structure interactions is required. In general frequencydomain analysis has been regarded as an adequate tool for the assessment of motions and loads which are needed to derive stresses, fatigue and wear as well as operational limitations. However, the increasing number of reported rogue waves with unexpected large wave heights (Hmax/Hs > 2), crest heights (ζmax/Hmax > 0.6), wave steepness and group pattern (e.g. Three Sisters) may suggest a reconsideration of design codes. For investigating the response to specific extreme sea conditions this paper analyzes the motion behavior and resulting splitting forces of a semisubmersible of type GVA 4000 in a reported rogue wave, the Draupner New Year Wave embedded in irregular seas. Response evaluations in time-domain provide information of the significance of rogue wave impact. The numerical time-domain investigation using a panel-method and potential theory is compared to frequency-domain results. For time-domain analysis the commercial code TiMIT is used to provide the motions and forces on the wetted body of the semisubmersible in rogue waves as time series. Corresponding response amplitude operators are also calculated with WAMIT, a program system for zero-speed problems. The resulting response spectra, e.g. the splitting forces, are then transformed into time-domain, using Fourier transformation. For validation, the selected sea condition is generated in a physical wave tank, and the seakeeping behavior as well as the (splitting) forces on the semisubmersible are evaluated at model scale. In conclusion, WAMIT and TiMIT results compare quite well as far as significant and maximum motions and forces are evaluated by a frequency-domain analysis. Also the time-domain analysis of the dynamics of the selected semisubmersible agrees satisfactorily with experimental results. Thus, the effect of rogue wave sequences is adequately predicted by TiMIT evaluations. The specific freak wave itself excites motions and forces, i.e. which are directly correlated to the maximum wave heights, resonance phenomena are not observed in the investigated time-domain case. As higher responses are observed as compared to standard frequencydomain methods, design procedures and codes should be reconsidered if more comprehensive data on freak waves and their impact has been analyzed.

Freak Wave Impact On Semisubmersibles - Time-domain Analysis of Motions And Forces

Heave, pitch and roll motions as well as airgap are key characteristics of semisubmersibles in extreme seas which are defined by Ultimate Limit State design conditions (ULS) with a specified 100-year design wave height Hs and peak period Tp . The increasing number of reported rogue waves with unexpected large wave heights (Hmax /Hs > 2), crest heights (ζmax/ /Hmax > 0.6), wave steepness and group patterns (e.g. Three Sisters) may suggest a reconsideration of design codes by implementing an Accidental Limit State (ALS) with a return period of 104 years. For investigating the consequences of specific extreme sea conditions this paper analyses the seakeeping behaviour of a semisubmersible in a reported rogue wave, the Draupner New Year Wave embedded in irregular sea states. The numerical time-domain invegstigation using a panel method and potential theory is compared to frequency-domain results. In particular, the characteristics of the embedded rogue wave is varied to analyse the dynamic response of the semisubmersible in extreme wave sequences For validation, the selected sea condition is generated in a physical wave tank, and the sea-keeping behaviour of the semisubmersible is evaluated at model scale. In conclusion, the results deomstrate the consequences of rogue wave impacts, with respect to the relevance of present design methods and safety standards.Copyright © 2002 by ASME

Time-Domain Investigation of a Semisubmersible in Rogue Waves

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Rogue wave impact on offshore structures

Broaching, surf-riding and capsizing of ships and offshore structures are transient wave-structure interactions which imply high risks for crew, vessel and cargo. As nonlinear effects are of great importance time-domain investigations are indispensable. For unveiling the associated driving mechanism of these critical motions it is desirable to analyse the cause-reaction chains in detail: Depending on the transient wave elevation we obtain an instationary pressure distribution on the wetted surface of the cruising vessel. Resulting forces and moments excite vessel motions in six degrees of freedom. Based on the linear panel-method program for transient wave-body interactions, TiMIT [1], this paper investigates seakeeping characteristics of offshore structures with forward speed. Results are presented in frequency and time domain. The procedure allows to identify critical seaways, and to analyse cause-reaction chains in deterministic wave sequences where critical and steep wave packets are embedded in random seas. The detailed evaluation reveals that large roll and pitch motions are easily reduced by variation of course and speed. For investigating the mechanism of wave/structure interactions, this paper introduces the relevant time-domain methodology, and indicates how non-linear wave characteristics can be introduced in the time-stepping analysis. In subsequent steps non-linear wave/structure interactions will also be considered.

https://mt.dms.tu-berlin.de/publikationen/2001/OMAE2001_OFT-1072_Time-Domain_Analysis.pdf

Katja Stutz

Papers

Freak Wave Impact On Semisubmersibles - Time-domain Analysis of Motions And Forces

Time-Domain Investigation of a Semisubmersible in Rogue Waves

Rogue wave impact on offshore structures

Time-Domain Analysis of Floating Bodies with Forward Speed