Effects of mooring compliancy on the mooring forces, power production, and dynamics of a floating wave activated body energy converter

The paper aims at investigating the interactions between a floating wave energy device (WEC) and its mooring system under a variety of wave conditions (regular and irregular, perpendicular and oblique, ordinary and extreme). The analyzed WEC is the DEXA, a wave activated body point absorber, of the type that performs better when aligned to the incident wave direction. Two typologies of mooring systems were studied: for limited depths, the spread system, with a disposition of the lines that do not constrain the yaw movements; for large depths, the catenary anchor leg mooring (CALM) system. The spread system was experimentally investigated, including a realistic power take-off system, to capture non-linear behaviors and assess device motions, power production, and forces on mooring lines. The CALM system was numerically simulated, as mooring modelling is more reliable in deep waters and allows testing of a number of different configurations, by changing the number of the mooring lines and the mooring layout. The experiments showed that a reduction of the mooring compliancy increases the power production. The numerical simulations showed that a redundancy on the number of chains allows a better distribution of the loads, with advantages on reliability and costs

Le reti neurali artificiali in idraulica marittima

Le reti neurali artificiali (Artificial Neural Networks, ANN) sono algoritmi di analisi dati e di modellazione numerica. Vengono dette “data driven models”, ossia modelli di calcolo basati sul processamento di dati – ottenuti sperimentalmente o prodotti artificialmente – al fine di testare la sensibilità di un certo fenomeno ad alcuni parametri. Le ANN sono particolarmente utili nelle analisi di fenomeni in cui il numero di parametri in gioco è molto elevato, e le relazioni fisiche tra essi sono fortemente non lineari e molto complesse. Dal punto di vista della tipologia di modellazione, le ANN costituiscono una “black-box”, dunque l’algoritmo di calcolo prescinde completamente dallo studio del fenomeno fisico che sta alla base della relazione di causa-effetto tra dati di input e di output; pertanto, le ANN sono in grado di elaborare i dati sperimentali a disposizione (input) e produrre delle predizioni dei valori desiderati (output), ricostruendo artificialmente gli effetti della complessa relazione di causa-effetto incognita. Obbiettivo di tale lavoro è esaminare la struttura caratteristica di una rete neurale e proporre alcuni esempi di ANN impiegati per la modellazione della portata di tracimazione e del coefficiente di trasmissione di strutture costiere, per sottolineare l’adeguatezza di tali strumenti all’impiego in idraulica marittima e presentarli per eventuali successivi utilizzi, quali, ad esempio, la stima del coefficiente di riflessione ondosa, ad oggi non ancora modellato accuratamente

Le interazioni onda – struttura: la riflessione ondosa

L’inserimento di una qualsiasi struttura preposta alla protezione costiera (o portuale) incide notevolmente sull’idrodinamica e la morfologia della zona circostante. Conoscere gli effetti che un’opera determinerà sul territorio è il primo elemento che consente di definire il tipo stesso di opera da progettare e governa alle fasi di progettazione stessa. Da ciò nasce l’esigenza di studiare l’interazione delle strutture artificiali con le onde marittime, le correnti e il trasporto solido. Tuttavia, l’analisi dell’interazione tra le onde e le strutture costiere non è uno stru-mento fondamentale soltanto per la progettazione delle opere, ma anche per la messa in sicurezza delle zone di battigia o portuali protette dalle opere stesse. Valutare l’affidabilità di tali opere di difesa, ai fini della rappresentazione dei meccanismi del processo di inondazione delle zone costiere in occasione di mareggiate, necessita della conoscenza e della stima accurata dei fenomeni principali che nascono dall’interazione delle opere con il moto ondoso: la risalita delle onde sul paramento lato mare, la traci-mazione e trasmissione di acqua a tergo delle strutture, la riflessione delle onde che, dopo essersi frante sull’opera, ritornano verso il largo

Semi-automatic detection of the overtopping waves and reconstruction of the overtopping flow characteristics at coastal structures

This paper proposes a semi-automatic and customizable procedure for the identification of the overtopping waves based on a threshold-down-crossing analysis of the sea surface elevation signals. The procedure can be applied to 2D experimental and numerical signals, to emerged and submerged structures, with the same accuracy of a human-supervised analysis. The procedure includes an original and innovative algorithm to compare the water level signals at consecutive gauges and couple the waves propagating in between. The coupling algorithm implies a series of original applications of practical relevance, such as: i) the computation of the wave celerity, which is a crucial parameter for the assessment of the structural stability and the hydraulic vulnerability of the landward area; ii) the estimation of the wave overtopping discharge, which can be obtained by integrating the wave celerities with the surface elevations; iii) the description of the wave overtopping characteristics and their evolution over the structure crest; iv) the evaluation of the volumes lost for percolation in permeable structures. The application to new and literature data and the comparison with well-established formulae prove that the results obtained from the identification and coupling procedures are accurate and reliable

Structural Stability Of Detached Low Crested Breakwaters

The aim of the paper is to describe hydraulic stability of rock-armoured low-crested structures on the basis of new experimental tests and prototype observations. Rock armour stability results from earlier model tests under non-depth-limited long-crested head-on waves are reviewed. Results from new 2-D and 3-D model tests, carried out at Aalborg University, are presented. The tests were performed on detached low-crested breakwaters exposed to short-crested head-on and oblique waves, including depth-limited conditions. A formula that corresponds to initiation of hydraulic damage and allows determining armour stone size in shallow water conditions is given together with a rule of thumb for the required stone size in depth-limited design waves. Rock toe stability is discussed on the basis of prototype experience, hard bottom 2-D tests in depth-limited waves and an existing hydraulic stability formula. Toe damage predicted by the formula is in agreement with experimental results. In field sites, damage at the toe induced by scour or by sinking is observed and the volume of the berm is often insufficient to avoid regressive erosion of the armour layer. Stone sinking and settlement in selected sites, for which detailed information is available, are presented and discussed

Benchmarking of Numerical Models for Wave Overtopping at Dikes with Shallow Mildly Sloping Foreshores: Accuracy versus Speed

To accurately predict the consequences of nearshore waves, coastal engineers often employ numerical models. A variety of these models, broadly classified as either phase-resolving or phase-averaged, exist; each with strengths and limitations owing to the physical schematization of processes within them. Models which resolve the vertical flow structure or the full wave spectrum (i.e. sea-swell (SS) and infragravity (IG) waves) are considered more accurate, but also more computationally demanding than those with approximations. Here, we assess the speed-accuracy trade-off of six well-known wave models for overtopping (q), under shallow foreshore conditions. The results demonstrate that: i) q is underestimated by an order of magnitude when IG waves are neglected; ii) using more computationally-demanding models does not guarantee more accurate results; and iii) with empirical corrections to account for IG waves, phase-averaged models like SWAN can perform on par, if not better than, phase-resolving models but with far less computational effort

Participatory Design of Multi-Use Platforms at Sea

European oceans are subject to rapid development. New activities such as aquaculture and ocean energy have gained importance. This triggers interest in “multi-use platforms at sea” (MUPS), i.e., areas at sea in which different activities are combined. MUPS are complex features with regards to technology, governance, and financial, socioeconomic, and environmental aspects. To identify realistic and sustainable solutions and designs for MUPS, the MERMAID project applied a participatory design process (PDP) involving a range of stakeholders representing companies, authorities, researchers, and NGOs. This paper evaluates if and how the participatory design process contributed to the design of multi-use platforms. It is based on interviews with the managers of the case study sites and a questionnaire administered to all stakeholders participating in the PDP workshops. Analyzing the four case studies, we conclude that the participatory design process has had a valuable contribution to the development of the four different designs of MUPS, even though the preconditions for carrying out a participatory design process differed between sites. In all four cases, the process has been beneficial in generating new and shared knowledge. It brought new design issues to the table and increased knowledge and understanding among the different stakeholders

Overtopping Wave Energy Converters: general aspects and stage of development

Wave power is the transport of energy by ocean surface waves, and the capture of that energy to do useful work, for example for electricity generation, water desalination, or the pumping of water (into reservoirs). For these and many other reasons, technologies to exploit wave resource are increasingly developing, especially in the recent years, and the wave power could potentially represent a very practical solution. Devices able to generate electricity exploiting the energy of the waves are commonly called Wave Energy Converter (WEC). At the moment exist a variety of technologies to capture the energy from waves; one of these is the one of the Overtopping Converters. An Overtopping device captures sea water of incident waves in a reservoir above the sea level, then releases the water back to sea through turbines connected to a generato