Irregular Analysis Time series of waves, wind, and/or first or second order wave loads (or linear support vessel motions) are required during the dynamic simulation. These time series should be generated such that they are at least as long as the full dynamic simulation and correctly represent the desired frequencies. In other words: The time series length should be greater than or equal to the simulation length (to avoid repetition) The time increment for time series generation should be appropriate for the given process. The time increment for time series generation should also be an integer multiple of the simulation time step. The wave seed is used to generate random phase angles for each wave frequency component. This will give different realizations of the selected wave spectrum for different seeds. If stochastic amplitudes are not selected, deterministic amplitudes are used (=default option). All realizations will have the same spectrum with sampled or interpolated values from the specified wave spectrum. The time series (wave/wind/load/motion) are generated by discretizing the variance spectrum into a finite number of harmonic components with uniformly distributed phases using FFT, see Discretized wave spectrum The phase angles are generated using a pseudo-random number generator with the seed given as user input. Choosing a different seed will give a different time series realization which in turn will yield different simulation results when wave loads or support vessel motions are included in the simulation. Figure 1. Discretized wave spectrum The time increment, frequency increment and maximum duration (without repetition) are interrelated when the addition of the harmonic components is performed by FFT: \[\Delta \omega = \frac{2\pi}{N_t \Delta t} = \frac{2\pi}{T_{rep}}\] where \(\Delta \omega\) is the frequency increment \(N_t\) is the number of time steps (in practice, this will be increased such that \(N_t = 2^N\)) \(\Delta t\) is the time increment \(T_{rep}\) is the length of the generated time series. 1. Simulation parameters Input for a dynamic simulation with irregular waves Simulation length: The length of the simulation. Simulation time step: The simulation time step should be chosen so that the important dynamic effects are captured in the simulation. This time step should be chosen based on the load frequencies, finite element lengths, dynamic response, and iteration procedure. Simulations with contact problems will generally require a short time step. 2. Time series generation parameters The parameters used to generate the irregular wave time series used in the simulation. These are a realization of the wave conditions, which are normally defined by a wave spectrum. Different seeds will give different sets of phase angeles realizations of the wave process. Wave seed: Seed for the random generator used to generate the realization of the waves. Use Stochastic Amplitude: Not selected: At each frequency, the wave component will have an amplitude given by the wave spectrum and a random phase angle. The generated wave time series will have the same spectrum as the specified wave spectrum at the discrete frequencies. Selected: At each frequency, the wave component will have both a random amplitude and a random phase angle. The generated wave time series will not have the same spectrum as the specified wave spectrum. Requested time: Length of the generated wave time series. Should normally be equal or longer than the length of the simulation. Time increment: The time increment for the generated wave time series. It should be short enough to capture the shortest period in the waves. This will normally be significantly longer than the the simulation time step. It should be a multiple of the simulation time step. The values will be adjusted so that the number of steps in the wave time series is a power of 2. FFT can then be used to generate wave time series. The adjusted values and the resulting frequency resolution are shown. 3. Support vessel motions The parameters needed to generate the motions of a support vessel. Irregular motions: Forced irregular motions calculated from waves and vessel motion transfer functions No irregular motions applied for support vessels Forced irregular motions from file: Support vessel motions may be read from a file in either ASCII or STARTIMES format. The motions may be given as displacements from the final static position of the vessel (global dynamic displacement) or as the position/coordinates (global positions ) of the vessel. Additional options for prescribed additional motions or support vessels Low-frequency motions: No low frequency motions Low frequency motion from file Motion Scaling: Enable scaling of motions for support vessels 4. Irregular wave Additional options for generating and using the waves. Simulation Start Time: By default, the simulation starts at the beginning of the wave time times. The simulation may be started at a later point in time; e.g. to simulate an interval wit particularly interesting conditions or response. Irregular wave forces present or not? If not present, no wave kinematics will be used in the simulation. Add wave time series from file: Option for reading a wave time series from file. If this is selected, the wave spectrum given in the environment input will be ignored. The wave time series files format options are ASCII: time in one column, wave elevation in another. STARTIMES: wave elevation time series in a specified channel in a binary STARTIMES files. An arbitrary time step may be used on an ASCII file, while the STARTIMES file has a fixed time step. Linear interpolation is used to get the wave elevation at the kinematics time step given in Time series generation parameters (Time Increment). The wave spectrum will then be found using FFT. The waves elevation time series will be used at the specified location; Global X coordinate, Global Y coordinates. Wave elevations at other locations will be found using the specified Wave direction. The waves will be long-crested; i.e. no wave spreading. 5. Wave kinematics 5.1. Wave kinematics specification The key input is described below. For details, see Wave kinematics specification. Kinematics position: Choice of position and method for calculation of wave kinematics. Options are Kinematics at static position: Kinematics are pre-generated at the static position before the start of the simulation. Kinematics at dynamic position calculated during simulation: Kinematics are calculated at the dynamic position during the simulation. This will normally take significantly longer time. Kinematics at static position calculated during simulation: Kinematics are calculated during the simulation. Normally not recommended as it should give the same results as pre-generated kinematics while taking significantly longer. Kinematics in wave zone: Mean water level: Wave kinematics and wave loads calculated to the mean water level Stretching and compression of wave potential Moving the potential to actual surface Keeping the potential constant from mean water level to wave surface when the surface is above the mean water level Second order wave with integration of wave forces to wave surface Figure 2. Wave kinematics and the wave surface- 1: to mean water level, 2: profile stretched / compressed, 3: profile moved Selection of points for wave kinematics. For the default procedure the input is Node step: Wave kinematics are calculated for every Node step between Z Lower and Z Upper. A negative value will use the absolute value of the step in the upper part and a larger step in the lower part of the Z Lower to Z Upper range. Z Lower and Z Upper: lower and upper vertical coordinate for which wave kinematics will be generated. The simulation will be more efficient if wave kinematics are not generated at depths that the waves do not reach. See Default procedure for more information and examples. See Wave kinematics options for information about the advanced options at the bottom of this tab. 5.2. Specification of wave kinematics points This is detailed specification of the wave kinematics points Diffracted waves : Wether diffracted wave points are to be specified or not Use Wave kinematics time series from a specified file Dynamic Calculation Parameters Regular Analysis