Storm peak validation and analysis of uncertainty in estimates of extreme sea states

Storm peaks are often under-estimated in numerical models, as is widely acknowledged. Yet, for many sites where estimates of extreme storm conditions are needed for engineering design, numerical models are the best – if not only – source of information. We discuss uncertainty in estimates of extreme sea states based on numerical model datasets . A method of validation for extreme conditions is presented based on matched pairs of independent storm peak events between modelled time series and buoy based observations, and found to be preferable to exceedance based validation techniques. Systematic biases in the storm peaks of the CFSR, ERA- Interim and NORA10 datasets, when compared to buoy data, are discussed. Sea states at the peaks of storms are compared and contrasted to the general population of sea states. We demonstrate a calibration scheme designed to remove bias from estimated storm peaks using a minimal set of parameters, in order that parameters may be mapped and estimated at sites where no observations are available. Bias corrected model estimates of storm peaks have a remaining uncertainty associated with model precision. Probability distributions of extreme sea states are estimated using Markov Chain Monte Carlo and Bayesian techniques. Estimates of model precision, based on peak-focussed validation, are used to predict extreme sea states with associated uncertainty representing both model precision and sampling of the long-term distribution. The analysis allows investigation of the relative contribution to estimate uncertainty of model precision and sampling/length of record. Once uncertainty is considered in the estimation of extreme sea states, a contribution to the mean estimate from precision based uncertainty in the source data becomes apparent. This contribution is also relevant to estimates of extreme conditions based on buoy data with measurement and short-term sampling uncertainties, and provides a baseline level of achievable uncertainty in extreme conditions. Validating uncertainty in estimates requires analysis at a large number of sites, and depends to a great extent on the probability associated with the most extreme events in a series, the so-called plotting position. The formulation of the plotting position has been debated for many years. Numerical experiments are presented that suggest that the correct formulation lies within a narrow subset of the debated schemes

Robust control volume finite element methods for numerical wave tanks using extreme adaptive anisotropic meshes

Multiphase inertia‐dominated flow simulations, and free surface flow models in particular, continue to this day to present many challenges in terms of accuracy and computational cost to industry and research communities. Numerical wave tanks and their use for studying wave‐structure interactions are a good example. Finite element method (FEM) with anisotropic meshes combined with dynamic mesh algorithms has already shown the potential to significantly reduce the number of elements and simulation time with no accuracy loss. However, mesh anisotropy can lead to mesh quality‐related instabilities. This article presents a very robust FEM approach based on a control volume discretization of the pressure field for inertia dominated flows, which can overcome the typically encountered mesh quality limitations associated with extremely anisotropic elements. Highly compressive methods for the water‐air interface are used here. The combination of these methods is validated with multiphase free surface flow benchmark cases, showing very good agreement with experiments even for extremely anisotropic meshes, reducing by up to two orders of magnitude the required number of elements to obtain accurate solutions

Damage to rubble mound breakwaters – Extracting design guidance from ‘old’ test data

The aim of this project has been to expand the concept of the CLASH overtopping database to include information on armour damage to rubble mound breakwaters and seawalls using a similar format. Response data have been entered under different armour types, and further distinction is made between different parts of the structure. To date, 2000 tests have been entered and proved the capacity of the database to summarize different rubble mound structures. This database can describe damage progression, thus improving analysis of lifetime risks for armoured rubble mound structures. Initial trials demonstrate the potential utility as a tool for validation and pre-design analysis

Wave Spectra Revisited – New guidelines based on observations

Metocean studies often experience a lack of wave observation data at sites of interest. This leads to assumptions generally accommodated in maritime engineering but not necessarily accurate or correct. This is the case for the wave spectral shape and peakedness, often approximated to a standard JONSWAP spectrum with gamma (peakedness) of 3.3 (Hasselman et al., 1973) when no spectral data are available. This assumption leads to a set of spectral parameter ratios widely used in coastal engineering. Offshore peakedness variation however leads to different Tm02/Tp and Tm-1,0/Tp ratios (Goda, 2010). This paper presents the findings arising from the analysis of observed wave energy spectra at a number of offshore wave buoys, and compares the results against JONSWAP 1D (frequency) spectral profiles. Resulting averaged fitted gamma values at each location remain within a range between 1.4 and 2.5, lower than the standard 3.3. A Deviation Index (D.I.) proposed by Liu (1983) and Pires Silva (1988) to evaluate the goodness of spectral profile fits against observations is also discussed

Robust control volume finite element methods for numerical wave tanks using extreme adaptive anisotropic meshes

Multiphase inertia‐dominated ow simulations, and free surface ow models in particular, continue to this day to present many challenges in terms of accuracy and computational cost to industry and research communities. Numerical Wave Tanks (NWT) and their use for studying wave‐structure interactions are a good example. FEM with anisotropic meshes combined with dynamic mesh algorithms have already shown the potential to significantly reduce the number of elements and simulation time with no accuracy loss. However, mesh anisotropy can lead to mesh quality‐related instabilities. This paper presents a very robust FEM approach based on a CV discretisation of the pressure field for inertia dominated ows, which can overcome the typically encountered mesh quality limitations associated with extremely anisotropic elements. Highly compressive methods for the water‐air interface are used here. The combination of these methods is validated with multiphase free surface ow benchmark cases, showing very good agreement with experiments even for extremely anisotropic meshes, reducing by up to two orders of magnitude the required number of elements to obtain accurate solutions

Physical model tests on spar buoy for offshore floating wind energy conversion

status: accepte

SparBOFWEC Spar Buoy for Offshore Floating Wind Energy Conversion - Data Storage Report

The present work describes the experiences gained from the design methodology and operation of a 3D physical model experiment aimed to investigate the dynamic behaviour of a spar buoy (SB) off-shore floating wind turbine (WT) under different wind and wave conditions. The physical model tests have been performed at Danish Hydraulic Institute (DHI) off-shore wave basin within the European Union-Hydralab+ Initiative, in April 2019. The floating WT model has been subjected to a combination of regular and irregular wave attacks and wind loads.The present work describes the experiences gained from the design methodology and operation of a 3D physical model experiment aimed to investigate the dynamic behaviour of a spar buoy (SB) off-shore floating wind turbine (WT) under different wind and wave conditions. The physical model tests have been performed at Danish Hydraulic Institute (DHI) off-shore wave basin within the European Union-Hydralab+ Initiative, in April 2019. The floating WT model has been subjected to a combination of regular and irregular wave attacks and wind loads.

PHYSICAL MODEL TESTS ON SPAR BUOY FOR OFFSHORE FLOATING WIND ENERGY CONVERSION

La domanda globale di energia eolica sta aumentando rapidamente e sta acquisendo sempre più importanza come risorsa energetica, dato l'interesse crescente per le energie rinnovabili. Le risorse eoliche offshore hanno attirato un'attenzione significativa e, rispetto alle risorse eoliche terrestri, sembrano essere più promettenti per lo sviluppo. I venti marini sono generalmente più forti e più affidabili e grazie agli enormi miglioramenti della tecnologia, il mare è diventato un hot spot per nuovi design e metodi di installazione per le turbine eoliche. C'è molto interesse in questo campo, poiché si ritiene che svolga un ruolo importante nel futuro dell'eolico offshore. Vari carichi dinamici vengono trasmessi dalla torre della turbina eolica alla sua piattaforma: carico del vento, carico delle onde del mare, carico dinamico dovuto al rotore, effetti di schermatura del vento della pala sulla torre che crea un carico ciclico. Per una turbina eolica offshore che opera sulla superficie del mare in continua evoluzione, è quindi fondamentale studiare il comportamento dinamico a cui è soggetta la struttura e in che modo la complessa interazione dei carichi delle onde e del vento influisca sul sistema. Un robusto processo di progettazione deve garantire che la frequenza naturale dell'intero sistema non si avvicini alle frequenze dei carichi ambientali imposti. In caso contrario, si potrebbe amplificare la risposta dinamica della struttura, portando a maggiori deflessioni della torre che possono compromettere le prestazioni della turbina eolica. Le turbine eoliche galleggianti sono supportate da strutture galleggianti e, quindi, hanno 6 gradi di libertà, che possono essere eccitate da carichi di onde, vento e correnti oceaniche. L'intero sistema è ormeggiato e stabilizzato mediante un sistema di molle e contrappesi. Sono strutture relativamente grandi che variano tra 5000 e 10.000 tonnellate per un'unità da 2 a 5 MW. Le turbine eoliche galleggianti sono soggette a carichi di vento e anche la struttura di supporto è soggetta a carichi idrodinamici, innescando un comportamento non lineare complesso dovuto alla combinazione di questi carichi. I carichi del vento che agiscono sulle pale della turbina eolica vengono trasmessi alla piattaforma galleggiante attraverso i componenti rotore- navicella e le torri; allo stesso tempo, anche la tensione di ormeggio ricevuta dal sistema di ormeggio viene trasmessa alla piattaforma galleggiante. Questi influenzano la risposta dinamica della piattaforma mobile. D'altra parte, il movimento della piattaforma galleggiante a sua volta provoca un movimento relativo tra la piattaforma e le pale della turbina eolica, influenzando le forze aerodinamiche sulle pale. Le reazioni tra le varie componenti della struttura sono complesse e accoppiate, rendendo la risposta dinamica del sistema FOWT difficile da prevedere e piena di sfide. Il sistema di accoppiamento pale-torre-piattaforma non è lineare, elemento di novità rispetto alle strutture tradizionali. Sono stati condotti esperimenti di alta qualità per esaminare la risposta dinamica della turbina eolica offshore. Sono stati condotti esperimenti utilizzando le strutture del DHI (Danish Hydraulic Institute) nell'ambito dell'iniziativa EU-Hydralab + Integrated Infrastructure Initiative utilizzando una turbina eolica di riferimento NREL 5MW in scala 1:40 (RWT) posta su piattaforma galleggiante OC3-Hywind, un concetto sviluppato da Statoil della Norvegia costituito da un unico cilindro verticale di grande diametro. Sono stati simulati diversi carichi dinamici che agiscono sulla turbina eolica galleggiante offshore, derivanti da una combinazione di attacchi di onde regolari e irregolari a cresta lunga ortogonali (0°) e obliqui (20°) alla struttura e diversi carichi del vento. Gli effetti delle alte frequenze non sono stati considerati in questo documento e la ricerca considera la torre della turbina eolica come un corpo rigido, quindi solo i sei gradi di libertà della piattaforma sono considerati per calcolare la risposta a bassa frequenza della piattaforma. Sono state implementate tecniche di elaborazione del segnale sui dati acquisiti al fine di valutare le principali proprietà dinamiche della struttura offshore. In primo luogo, è stata studiata la risposta strutturale dopo l'applicazione di un impulso, consentendo di ottenere la frequenza naturale di ciascun grado di libertà a cui è soggetta la struttura offshore e di identificare i rapporti di smorzamento modale. Successivamente, sono stati presentati i risultati relativi alle onde regolari, con incidenza ortogonale alla struttura; i test selezionati hanno considerato un'onda regolare con lo stesso periodo al fine di analizzare l'influenza dell'altezza dell'onda senza carico del vento e con un carico del vento che consente al rotore della turbina eolica di raggiungere la condizione nominale. Sono state effettuate analisi di spostamenti, rotazioni, accelerazioni, risposta delle forze della struttura galleggiante e delle linee di ormeggio. I risultati mostrano che la maggior parte della risposta dinamica longitudinale si verifica alla frequenza dell'onda con un contributo minore, ma non trascurabile, alle frequenze del corpo rigido, in contrasto con la risposta dinamica laterale in cui le frequenze del corpo rigido sono predominanti. The present paper describes the experiences gained from the design methodology and operation of a 3D physical model experiment aimed to investigate the dynamic behaviour of a spar buoy floating offshore wind turbine. The physical model consists in a Froude-scaled NREL 5MW reference wind turbine (RWT) supported on the OC3-Hywind floating platform. Experimental tests have been performed at Danish Hydraulic Institute (DHI) offshore wave basin within the European Union-Hydralab+ Initiative, in April 2019. The floating wind turbine model has been subjected to a combination of regular and irregular wave attacks and different wind loads. Measurements of displacements, rotations, accelerations, forces response of the floating model and at the mooring lines have been carried out. First, free decay tests have been analysed to obtain the natural frequency and the modal damping ratios of each degree of freedom governing the offshore. Then, the results concerning regular waves, with orthogonal incidence to the structure, are presented. The results show that most of longitudinal dynamic response occurs at the wave frequency and most of lateral dynamic response occurs at rigid-body frequencies