Data-Driven Condition Monitoring for Mooring Systems of a Multi-Float Wave Energy Converter with Two Configurations

Monitoring the condition of mooring systems is essential for the safe operation and timelymaintenance of the wave energy converter. However, mooring dynamics are nonlinearity related to the wave forcing, and are complicated by its coupling with the wave energy converter. These nonlinear and coupling effects pose challenges in accurately identifying the condition of the mooring. In this study, we introduce a data-driven approach for condition monitoring that uses wavelet filtering and dynamic modeling to address these challenges. Initially, a wavelet filter is applied to separate the low-frequency surge motion, attributed to the nonlinear and coupling effects, from the surge at the wave frequencies. Then a linear ARX model is used to build the dynamic relationship between the filtered surge motion and wave surface elevation for monitoring purposes. The effectiveness of this method is demonstrated through its application to two different mooring system configurations within a multi-float wave energy converter. Comprehensive testing across eleven wave conditions confirms the effectiveness of the proposed method

On the re-creation of site-specific directional wave conditions

Wave tank tests facilitate the understanding of how complex sea conditions influence the dynamics of man-made structures. If a potential deployment location is known, site data can be used to improve the relevance and realism of the test conditions, thus helping de-risk device development. Generally this data is difficult to obtain and even if available is used simplistically due to established practices and limitations of test facilities. In this work four years of buoy data from the European Marine Energy Centre is characterised and simulated at the FloWave Ocean Energy Research Facility; a circular combined wave-current test tank. Particular emphasis is placed on the characterisation and validation processes, aiming to preserve spectral and directional complexity of the site, whilst proving that the defined representative conditions can be effectively created. When creating representative site-specific sea states, particular focus is given to the application of clustering algorithms, which enable the entire spectral (frequency or directional) form to be considered in the characterisation process. This enables the true complex nature of the site to be considered in the data reduction process. Prior to generating and measuring the resulting sea states, issues with scaling are explored, the facility itself is characterised, and emphasis is placed on developing measurement strategies for the validation of directional spectra. Wave gauge arrays are designed and used to characterise various elements of the FloWave tank, including reflections, spatio-temporal variability and wave shape. A new method for directional spectrum reconstruction (SPAIR) is also developed, enabling more effective measurement and validation of the resulting directional sea states. Through comparison with other characterisation methods, inherent method-induced trade-offs are understood, and it is found that there is no absolute favourable approach, necessitating an application specific procedure. Despite this, a useful set of ‘generic’ sea states are created for the simulation of both production and extreme conditions. For sea state measurement, the SPAIR method is proven to be significantly more effective than current approaches, reducing errors and introducing additional capability. This method is used in combination with a directional wave gauge array to effectively measure, correct, and validate the resulting directional wave conditions. It is also demonstrated that site-specific wave-current scenarios can be effectively re-created, thus demonstrating that truly complex ocean conditions can be simulated at FloWave. This ability, along with the considered characterisation approach used, means that representative site-specific sea states can be simulated with confidence, increasing the realism of the test environment and helping de-risk device development

The loading on a vertical cylinder in steep and breaking waves on sheared currents using smoothed particle hydrodynamics

Waves and currents coexist in a wide range of natural locations for the deployment of offshore structures and devices. This combined wave–current environment largely determines the loading of vertical surface piercing cylinders, which are the foundations typically used for offshore wind turbines along with many other offshore structures. The smoothed particle hydrodynamics (SPH) code DualSPHysics is used to simulate focused waves on sheared currents and assess subsequent loading on a vertical cylinder. Outputs from another numerical model are used to define the SPH inlet–outlet boundary conditions to generate the wave–current combinations. A modified damping zone is used to damp the waves, but allow the currents to exit the domain. Numerical results are validated against experimental measurements for surface elevation and associated loading on the cylinder. Four phase repeats are used in the SPH model to understand the harmonic structure of the surface elevation at the front face of the cylinder and associated loading. It is shown that the SPH model provides agreement with experimental measurements of harmonic components for both force and elevations. Taking advantage of the SPH method, wave amplitudes were increased up to, and beyond, the breaking threshold highlighting a complex relationship between peak force and wave phase, requiring detailed investigation. The numerical modeling of interactions of steep and breaking waves on sheared currents with the cylinder demonstrates the SPH model's capability for modeling highly nonlinear fluid–structure interaction problems

Resolving combined wave-current fields from measurements using interior point optimization

This is the author accepted manuscript. the final version is available on open access from Elsevier via the DOI in this recordComplex wave and wave-current conditions exist in the natural world, and are increasingly emulated in advanced experimental facilities to de-risk the deployment, operation and maintenance of onshore structures and renewable energy devices. This can include combinations of ocean swell, multi-directional wind-driven seas, and reflected wave conditions interacting with a current field. It is vital to understand the full nature of these potentially hazardous conditions so they can be properly simulated in numerical models, to contextualize measurements made in field, and experimental programmes. Here, a numerical framework is presented for isolating both the wave systems and the mean current velocities from measured data using an interior point optimizer. A developed frequency domain solver is used to resolve, from experimentally obtained wave gauge measurements, two opposing wave systems on a collinear current, and used to electively isolate the wave systems and predict the current velocity using only wave gauge measurements. Thirty five test cases are considered; consisting of five wave spectra interacting with seven different current velocities ranging from 0:3ms1 to 0:3ms1. Comparisons between the theoretical and derived wave numbers and current velocities show good agreement and the performance of the method is similar to that of existing methodologies while requiring no a priori knowledge of the current velocity impacting the wave field required. Although results are presented for the collinear problem, the presented method can be applied to a wide range of wave and current combinations, and provides a useful tool for increasing understanding of both ocean and experimental conditions.Engineering and Physical Sciences Research Council (EPSRC)Energy Technologies Institut

Why rogue waves occur atop abrupt depth transitions

Abrupt depth transitions (ADTs) have recently been identified as potential causes of ‘rogue’ ocean waves. When stationary and (close-to) normally distributed waves travel into shallower water over an ADT, distinct spatially localized peaks in the probability of extreme waves occur. These peaks have been predicted numerically, observed experimentally, but not explained theoretically. Providing this theoretical explanation using a leading-order-physics-based statistical model, we show, by comparing to new experiments and numerical simulations, the peaks arise from the interaction between linear free and second-order bound waves, also present in the absence of the ADT, and new second-order free waves generated due to the ADT

Experimental Data of Bottom Pressure and Free Surface Elevation including Wave and Current Interactions

From MDPI via Jisc Publications RouterHistory: accepted 2021-09-23, pub-electronic 2021-09-30Publication status: PublishedFunder: Engineering and Physical Sciences Research Council; Grant(s): EP/S000747/1Force plates are commonly used in tank testing to measure loads acting on the foundation of a structure. These targeted measurements are overlaid by the hydrostatic and dynamic pressure acting on the force plate induced by the waves and currents. This paper presents a dataset of bottom force measurement with a six degree-of-freedom force plate (AMTI OR6-7 1000, surface area 0.464 m × 0.508 m) combined with synchronised measurements of surface elevation and current velocity. The data cover wave frequencies between 0.2 to 0.7 Hz and wave directions between 0∘ and 180∘. These variations are provided for current speeds of 0 and 0.2 m/s and a variation of the current in the absence of waves covering 0 to 0.45 m/s. The dataset can be utilised as a validation dataset for models predicting bottom pressure based on free surface elevation. Additionally, the dataset provides the wave- and current-induced load acting on the specific load cell at a fixed water depth of 2 m, which can subsequently be removed to obtain the often-desired measurement of structural loads