Numerical simulations of a novel offshore floating solar system

Abstract

The development of renewable energy applications has become increasingly important in the past couple of years due to a growing global energy demand and increasing consciousness of global warming. A recent development in renewable energy is the application of offshore floating solar systems. A prototype design for this application, as developed by the company ’Oceans of Energy’, consists of multiple floating platforms (floaters), moored to the sea bed. One of the challenges for offshore floating solar systems is the continuous wave induced force acting on the floater that can become significant in severe weather conditions. Numerical simulations are often used to analyse and predict these forces. Combined frequency-time domain solvers are the most promising tool for this design to obtain reliable results within reasonable computational time. However, it is unknown what the capabilities of this solver is for the novel offshore floating solar system. The goal of this study is therefore to determine to what extend the frequency-time domain solver is able to accurately simulate the behavior of the multi-body, offshore floating solar system. A numerical model was developed in the simulation tool ’aNySIM’. Experiments conducted in the offshore basin at MARIN were used for the model structure development and validation. Additionally, viscous damping effects were implemented from empirical data based on the experiments. Decay tests were used for damping determination. This resulted in inaccurately determined forces and motion amplitudes for regular and irregular wave tests. A new damping determination method was introduced, using regular wave results as input. The accuracy of the model improved with 4% using this method. Nonetheless, inaccuracies in mooring line forces, oscillating behaviour and motion amplitudes are present. These inaccuracies have been addressed with an uncertainty analysis and sensitivity studies. Trends within the accuracy of the simulation model were linked to physical phenomena that occurred during the experiments. Analysis showed that several effects, related to the novel design characteristics, influence the model accuracy. Multi-body (e.g. shielding of floaters) and mooring system aspects (e.g. hydrodynamic loads on mooring lines) have been identified, influencing the viscous damping of the system. Additionally, nonlinear effects due to, amongst others, breaking waves have been identified. For regular wave tests and lower irregular wave tests, the simulation model gives reliable results with errors in the mooring line forces below 10%. These errors are mainly ascribed to viscous damping effects. For higher sea states the simulation behaviour shows discrepancies and errors in the mooring line forces increase. In these tests, nonlinear effects due to breaking waves become more relevant. For future work, different uncertainty causes must be further isolated in the simulation model. This can be done by obtaining more experimental data. To quantify the effects, studies in other numerical tools can be performed (e.g. CFD calculations) and more extended sensitivity studies must be executed in aNySIM. We present a structured