FPSO Conference—Estimating Wind-Sea and Swell for FPSO Operability

2005 ◽  
Vol 128 (4) ◽  
pp. 314-321 ◽  
Author(s):  
K. C. Ewans ◽  
L. Vanderschuren ◽  
P. S. Tromans
Keyword(s):  
State Of The Art ◽  
Wave Spectrum ◽  
Sea State ◽  
Art Program ◽  
Motion Responses ◽  
Term Variation ◽  
Directional Wave ◽  
Wind Sea ◽  
One Year

The motion response of an FPSO is sensitive to the relative intensities and directions of the wind-sea and swell components in a sea state, and the operability of the FPSO is a function of the long-term variation in these components. Estimations of the operability are therefore dependent both on how the sea state is described in terms of its constituent wind-sea and swell components, and on how the long-term variability of the sea state is captured. However, there is currently no consensus on how either the sea state or its long-term variability should be described. We investigate these issues by means of a study of the responses of a typical FPSO to the wave fields at a location offshore Namibia and a location off the west coast of New Zealand. We make use of a state-of-the-art program for splitting a directional wave spectrum into wind-sea and swell components, and we examine the effect on the motion responses of allowing the spectra to be split into many swell partitions or constraining the spectral split to a maximum of two partitions, as is often assumed in response calculations. The resulting decompositions are used to examine the effects of swell on hull motions and, hence, to identify methods for generating sea state criteria for operability. In addition, one-year metocean conditions are estimated; these are relevant for analysis of the limits on operations.

Directional Wave Partitioning and Its Applications to the Structural Analysis of an FPSO

2008 ◽  
Author(s):  
Ruth Lawford ◽  
Jill Bradon ◽  
Thomas Barberon ◽  
Claude Camps ◽  
Richard Jameson
Keyword(s):  
Time Series ◽  
Structural Analysis ◽  
Wave Spectrum ◽  
Wave Spectra ◽  
Offshore Structure ◽  
Sea State ◽  
Environmental Force ◽  
Directional Wave ◽  
Wind Sea ◽  
The Individual

A full characterisation of the individual components of a sea-state is key to enabling the response of an offshore structure to be accurately calculated. This paper discusses the partitioning of a time series of directional wave spectra into wind-sea and swell components with distinct frequency and direction characteristics. Once the wave data have been partitioned, JONSWAP or Pierson-Moskowitz parameters can be fitted to each spectrum using ‘best-fit’ techniques. The result of the partitioning and fitting analyses is a time series of wave parameters defining the wave spectrum for each component of the sea state. A 10-year site specific time series of directional wave spectra has been partitioned in this way and used in the analysis of the Triton FPSO, a turret moored FPSO in the central North Sea. The representation of the directionality and magnitude of each environmental force acting simultaneously on the vessel, allows the relative heading of the vessel to be determined and the mooring and hydrodynamic analyses to be performed. These analyses provided input to a structural analysis of the FPSO, which resulted in an inspection plan for monitoring the effects of the metocean conditions on the unit.

Estimation of Wind-Sea and Swell Components in a Bimodal Sea State

2004 ◽  
Vol 128 (4) ◽  
pp. 265-270 ◽  
Author(s):  
K. C. Ewans ◽  
E. M. Bitner-Gregersen ◽  
C. Guedes Soares
Keyword(s):  
Wave Spectrum ◽  
Low Frequency ◽  
Wave Spectra ◽  
Spectral Parameters ◽  
High Frequency Peak ◽  
Wave Measurements ◽  
Frequency Peak ◽  
Directional Wave ◽  
Wind Sea ◽  
Complete Frequency

Methods for separating the spectral components and describing bimodal wave spectra are evaluated with reference to wave spectra from directional wave measurements made at the Maui location off the west coast of New Zealand. Two methods involve partitioning bimodal wave spectra into wind-sea and swell components and then fitting a spectral function to each component, while the third assigns an average spectral shape based on the integrated spectral parameters. The partitioning methods involve separating the wave spectrum into two frequency bands: a low-frequency peak, the swell component, and a high-frequency peak, the wind-sea. One partitioning method uses only the frequency spectrum while the other analyzes the complete frequency-direction spectrum. Comparison of the spectral descriptions and derived parameters against the measured counterparts provides insight into the accuracy of the different approaches to describing actual bimodal sea states.

A Practical Spectral Fatigue Analysis Procedure for Topside of Floating Offshore Structures in West Africa

2015 ◽  
Author(s):  
Sungwook Chung ◽  
Minsung Chun ◽  
Kibok Jang ◽  
Youngsuk Suh
Keyword(s):  
West Africa ◽  
Wave Spectrum ◽  
Normal Wave ◽  
Offshore Structures ◽  
Fatigue Analysis ◽  
Analysis Procedure ◽  
Sea State ◽  
Commercial Program ◽  
Wind Sea ◽  
Wrapped Normal

In most offshore projects recently ordered, spectral fatigue analysis is required for design integrity. However, the spectral fatigue analysis is very complicated to implement since it has many variations for parameters and forms of input data, and the classification and commercial software packages are exposing limit to support all those variations. A topside fatigue analysis for a FPSO design in West Africa is one of such a challenging project due to the fact that the specification of the project requires spectral fatigue analysis considering 3-peak Ochi-Hubble wave spectrum, Wrapped normal wave spreading and sea state data with 3 wave components, main swell, secondary swell and wind sea. In this study, a practical spectral fatigue analysis procedure is introduced in order to implement the fatigue analysis using a commercial program SACS. Applying adaptive cosine spreading wave distribution which can approximate Wrapped normal wave spreading is devised for each sea state and the comparison between two wave spreading is carried out. Finally, the proposed methodology is justified by analyzing the characteristics of the sea state in West Africa.

PARAMETRIC ESTIMATION OF THE DIRECTIONAL WAVE SPECTRUM FROM SHIP MOTIONS

2021 ◽  
Vol 158 (A2) ◽  
Author(s):  
M A Hinostroza ◽  
C Guedes Soares
Keyword(s):  
A Priori ◽  
Wave Spectrum ◽  
Estimation Method ◽  
Parametric Estimation ◽  
Ship Motions ◽  
Offshore Platforms ◽  
Sea State ◽  
Directional Wave Spectrum ◽  
Directional Wave ◽  
Full Scale Tests

A parametric estimation of the directional wave spectrum based on ship motions is presented. The estimation of the sea- state parameters is essential to have an updated data base of the main characteristics of the sea-state, which are useful for several applications on open-sea such as offshore platforms installations and safe ship navigation. The sea-state parameters at a fixed position can be obtained using a traditional waverider buoy. The analogy between the ship and the buoy is clear thus, it is possible to obtain an estimate of the wave spectrum at the location of an advancing ship by processing its wave-induced responses similarly to the traditional waverider buoy. In the parametric procedure the estimated wave spectrum is a-priori assumed to be composed of one parameterized spectrum or by the summation of several parameterized spectra, e.g. the generalized JONSWAP spectrum. Genetic algorithms are applied to found the best estimation of wave parameters. The wave estimation method is validated against numerical simulations and full scale tests in a patrol ship.

Observation of high-resolution fetch-limited wave growth using CFOSAT near-nadir measurements

2020 ◽  
Author(s):  
Alexey Mironov ◽  
Bertrand Chapron
Keyword(s):  
High Resolution ◽  
Wind Waves ◽  
Wave Spectrum ◽  
Surface Wind ◽  
Third Party ◽  
Sea State ◽  
Spectral Parameters ◽  
Wave Growth ◽  
Directional Wave ◽  
Ku Band

<p>CFOSAT is the joint space mission of the French (CNES) and Chinese (CNSA) space agencies dedicated to the observation of ocean surface wind and waves. Two main on-board payloads, the Ku-band near-nadir wave scatterometer SWIM and dual-polarization Ku-band wind scatterometer SCAT for the first time provide regular synchronized surface wind vector and sea state observations on a global scale. <br> <br>After the first year of the mission, SWIM innovative wave scatterometer conception proved to be suitable for space-born directional wave spectra measurements. The overall performance of the instrument and the quality of inverted data are close to the planned specifications. Moreover, in particular cases, precise wave parameters can be estimated even in limited coastal seas with varying wave and wind conditions.<br> <br>In this work, we will show examples of high-resolution directional wave spectrum fetch evolution as observed by CFOSAT mission. The present analysis was performed for areas with fetch-determined conditions, during periods when waves were generated by strong coastal winds. The dataset includes spectra starting from young wind waves (~20 km fetch distance) to mature almost developed sea state (>450 km fetch). The unique multi-beam configuration of SWIM provides multiple estimates of wave spectral parameters all over the sensor footprint along the satellite track. This allows the capturing of the very fine details of spectral variability with distance from the coast. It will be shown, the observed deviation of direction and wavelength of the measured spectral peak from a wave growth fetch law can be attributed to time-varying or topography-induced coastal wind field inhomogeneities. In some cases, this can be explained by local surface current configuration. The obtained results can be directly compared with third party remote sensing observations, numerical model outputs, classical wave fetch-growth laws, and existing empirical parametrizations. </p>

scholarly journals A Practical Method of Extracting Wind Sea and Swell from Directional Wave Spectrum

2015 ◽  
Vol 32 (11) ◽  
pp. 2147-2159 ◽  
Author(s):  
Zezong Chen ◽  
Longgang Zhang ◽  
Chen Zhao ◽  
Xi Chen ◽  
Jianbo Zhong
Keyword(s):  
Doppler Radar ◽  
Wave Spectrum ◽  
Weather Conditions ◽  
Practical Method ◽  
Wave Age ◽  
Directional Wave Spectrum ◽  
Overshoot Phenomenon ◽  
Mean Wave Period ◽  
Directional Wave ◽  
Wind Sea

AbstractWind sea and swell representing different weather conditions generally coexist in both open waters and coastal areas, which results in bimodal or multipeaked features in directional wave spectrum. Because they make wave parameters such as significant wave height and mean wave period of the mixed sea state less meaningful, the processes of separation and identification of wind sea and swell are crucial. Consistent wind sea and swell results can be obtained by a commonly used method based on wave age (WA) with the directional wave spectrum and wind velocity. However, the subjective dependence of wave age threshold selection and the required wind information restrict the application of this method. In this study, a practical method based on the overshoot phenomenon (OP) in wind-generated waves is proposed to extract wind sea and swell from the directional wave spectrum without any other meteorology information. Directional wave spectra derived from an S-band Doppler radar deployed on the coast of the South China Sea have been utilized as the datasets to investigate the performance of both methods. The proposed OP method is then validated by comparing it with the WA method and the verifying results are presented.

scholarly journals Investigation on Spectrum Estimation Methods for Bimodal Sea State Conditions

Sensors ◽  
2021 ◽  
Vol 21 (9) ◽  
pp. 2995
Author(s):  
Giovanni Battista Rossi ◽  
Francesco Crenna ◽  
Marta Berardengo ◽  
Vincenzo Piscopo ◽  
Antonio Scamardella
Keyword(s):  
Weather Forecasting ◽  
Wave Spectrum ◽  
Least Square ◽  
Estimation Methods ◽  
Random Wave ◽  
Reconstruction Method ◽  
Spectrum Estimation ◽  
Sea State ◽  
Benchmark Study ◽  
Wind Sea

The reliable monitoring of sea state parameters is a key factor for weather forecasting, as well as for ensuring the safety and navigation of ships. In the current analysis, two spectrum estimation techniques, based on the Welch and Thomson methods, were applied to a set of random wave signals generated from a theoretical wave spectrum obtained by combining wind sea and swell components with the same prevailing direction but different combinations of significant wave heights, peak periods, and peak enhancement factors. A wide benchmark study was performed to systematically apply and compare the two spectrum estimation methods. In this respect, different combinations of wind sea spectra, corresponding to four grades of the Douglas Scale, were combined with three swell spectra corresponding to different swell categories. The main aim of the benchmark study was to systematically investigate the effectiveness of the Welch and Thomson methods in terms of spectrum restitution and the assessment of sea state parameters. The spectrum estimation methods were applied to random wave signals with different durations, namely 600 s (short) and 3600 s (long), to investigate how the record length affected the assembled sea state parameters, which, in turn, were assessed by the nonlinear least square method. Finally, based on the main outcomes of the benchmark study, some suggestions are provided to select the most suitable spectrum reconstruction method and increase the effectiveness of the assembled sea state parameters.

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