Time-domain Numerical Model Research Articles

Numerical study of slamming and whipping loads in moderate and large regular waves for different forward speeds

This paper describes the development of a time-domain hydroelastic numerical model for investigating the slamming and whipping loads imposed on a ship traveling at different forward speeds. The numerical model integrates a beam model, the 3-D Rankine panel model, and the 2-D modified Logvinovich model. The tilt angle of the 2-D profile in the slamming model is determined by the direction of relative velocity, which takes the forward sailing speed into account. The computational results are validated against a segmented model test of a large cruise ship. The convergence of the model with different numbers of elastic modes and slamming profiles is studied to ensure that the numerical results are stable. The main contributor to the whipping response is found to be the vibration of the first-order elastic mode. The slamming force is sensitive to the longitudinal interval of the slamming profiles, especially in the stern. The characteristics of the slamming pressure under bow flare and the whipping response at midship are also investigated. The numerical and experimental whipping responses are found to be in good agreement, and the local slamming pressure based on the slamming profile is more accurate when considering the effect of the ship's forward speed.

Mechanistic research on the complex motion response of a TLP in a tender-assisted drilling state

In this study, nonlinear motion characteristics have been studied in the coupled behavior of a Tension Leg Platform (TLP) combined with a Tender-assisted Drilling Unit (TADU). A time-domain numerical model has been established that accounts for the nonlinear behavior of mooring systems and hydrodynamic interference. Experiments are performed to verify the numerical model. Nonlinear motion characteristics of the steady-state response of the time-domain results are analyzed using Poincaré mapping and fast Fourier transform. The focus of this study is on investigating the motion response of the TLP platform in three degrees of freedom under coupled states. The surge motion is found to mainly consist of the wave frequency component and the half-wave frequency subharmonic component, while the wave-frequency, subharmonic, and super harmonic vibration components are observed in both heave and pitch motions. Three types of nonlinear regimes, namely chaos, transition, and multiple periodic, are identified in the heave and pitch motions under different wave conditions.

INVESTIGATING DREDGE PLACEMENT OPTIMISATION TO BENEFIT SURF AMENITY

Dredge material removed from the Tweed River Entrance is deposited offshore from Gold Coast beaches to assist in mitigating erosion. Research was undertaken by the Queensland Government Hydraulics Laboratory (QGHL), in collaboration with Tweed Sand Bypassing (TSB), to investigate the potential of targeting dredge depositions to also improve surf amenity. A time-domain numerical wave model (SWASH) was developed as a design tool and calibrated using physical model data. Overall, SWASH reproduced wave heights over and in the lee of the mound with reasonable accuracy. However, tuning of the breaking parameters (α and β) is necessary to accurately simulate the onset and persistence of the breaking process. The results of both physical and numerical modelling highlight the importance of sand mound shape in disrupting the wave field, resulting in breakpoint variability in the mound's lee, which could potentially create more favorable surfing conditions.

Numerical simulations of towing a jacket foundation with triple buckets

Offshore wind energy is developing fast as a promising renewable energy resource that can be helpful in achieving the global net-zero emissions goal. In recent years, offshore wind turbine projects have gradually moved towards far-offshore sea areas, where massive jacket foundations with suction buckets are suitable options. To reduce transportation costs, the wet-towing method is usually favourable since the required buoyancy of the foundation can be provided by the air cushions in the suction buckets. This engineering background brings us a research topic that involves towing and air cushions in waves. This study considers the towing problem of a jacket foundation with triple suction buckets. First, numerical analysis in the frequency domain is conducted using WAMIT®. In particular, the NEWMODES subroutine is extended to consider three separate motion modes in the buckets. Numerical predictions of the natural period show satisfactory agreement with the available experimental data in the literature. Moreover, a time-domain numerical model is developed to simulate towing of the jacket foundation in regular waves based on the potential flow theory. The time-domain model numerically solves the towing force in regular waves, the body motions and the air pressures in the buckets. The simulation results generally agree well with the published experimental data. Besides, the present simulation results suggest that the transit towing force and the forward speed of towed triple-bucket jacket may be considerable in the initial towing stage.

Analysis of FPSO Motion Response under Different Wave Spectra

A variety of floating structures at sea play a vital role in the exploitation and utilization of marine resources. The study about interactions between waves and structures is necessary for the impact of the harsh marine environment on the motion and service life of structures. Currently, most studies about the seakeeping of structures are based on simplified regular waves. Because the regular waves do not truly restore the actual wave conditions at sea, the simulation of irregular waves has great practical importance to the study of interactions between waves and structures. Based on the potential flow theory and high-order boundary element method (HOBEM), a Fortran code is developed in this paper and named as SWBI (Solver for Wave–Body Interactions). This program consists of the following two parts: a time–domain numerical model about interactions between waves and 3D structures is based on weakly nonlinear method, and a numerical model about simulation of the nonlinear regular waves, the long-crested irregular waves, and the short-crested irregular waves. This Fortran code is used to simulate the motion of Floating Production Storage and Offloading (FPSO) under three different ocean wave spectra (including ITTC two-parameters spectrum, JONSWAP spectrum and the most likely spectral form of Ochi-Hubble) and found that: To a certain extent, the difference in the motion of FPSO under different wave spectra have a connection with different type of wave, sea conditions and incident angle. The difference in roll of FPSO is quite significant in short-crested irregular waves. The range of FPOS’s roll under the JONSWAP spectrum is the largest when the incident angle is 30°, and range of FPOS’s roll under the most likely spectral form of Ochi-Hubble is the largest when the incident angle is 90°.

How Ultrasonic Pulse-Echo Techniques and Numerical Simulations Can Work Together in the Evaluation of the Elastic Properties of Glasses

This paper presents the numerical simulation of the ultrasonic wave transmittance utilizing the elastodynamic finite integration technique (EFIT). With this methodology, it is possible to simulate the propagation of the ultrasound in a medium with a relatively low computational cost. The capability of this technique for determining the elastic properties of fluorophosphate and the aluminosilicate glasses is described in detail. The elastic constants of the glasses were calculated from the theoretically predicted longitudinal and transversal sound velocities and compared with the corresponding experimental data. Furthermore, the calculated and experimental elastic properties of the fluorophosphate and aluminosilicate glasses were correlated with the structural peculiarities of these glasses. This simulation technique is also suitable for unveiling the existence of possible defects in the glasses by comparing the experimental and simulation data. The EFIT technique is shown to be a very useful tool in order to provide fast and easy-to-acquire data regarding also the structural characteristics of various glassy systems. This can be used in conjunction with other spectroscopic techniques which can prove to be extremely useful for the non-destructive testing of vitreous materials. The latter can prove very important when vitreous materials used in optical or optoelectronic applications need continuous monitoring in order to ensure their optimum operation and functionality with limited intervention. The main contribution of this paper is the treatment of numerical time-domain modeling of 2D acoustic wave propagation in a viscoelastic medium by implementing the elastodynamic finite integration technique (EFIT).

Spectral-domain modelling of a non-linear wave energy converter: Analytical derivation and computational experiments

Harvesting energy from waves is gaining popularity in the last decades, due to growing technological maturity and reduction of manufacturing costs of Wave Energy Converter (WEC) devices. However, the optimal design of WECs represents a challenge to its large-scale commercialisation. Behind this multi-objective optimisation, a significant number of simulations are necessary to identify the optimal configuration of the device that minimise certain techno-economic parameter. Frequency-domain models are suggested in an early stage since they provide a simple and fast simulation tool. On the other hand, non linear time-domain numerical models are more suitable during an advanced design phase where it is essential to assess performance and acting loads with greater reliability. The aim of this work is the development of a spectral-domain model of a WEC device that combines the computational speed of frequency-domain models with the accuracy of non-linear models to improve the design in early stage. Three different models of the ISWEC, a WEC developed by the MOREnergy Lab of Politecnico di Torino (Italy), are derived and compared: a fully non-linear time-domain model, a frequency-domain model, and a spectral-domain model. The spectral-domain modelling of the ISWEC system represents an important novelty for the technological development of the system and an example framework for similar technologies. Moreover, in the energy harvesting field, no previous work addressed the analytical representation of a realistic electro-mechanical converter with both torque and power saturation, and the representation of a gyro-pendulum system in spectral-domain remains untreated. The numerical experiments demonstrate that the spectral-domain model shows an improved accuracy compared with the frequency-domain model, with a reduction of percentage error of the computed gross power in the order of two times. Despite a not perfect accuracy of the spectral-domain model against the time-domain one, the spectral-domain framework is considered as a valid solution due to its computational efficiency, that is demonstrated to be, at least, three order of magnitude higher than the time-domain one.

Efficient Recursive Convolution Schemes for the Accurate FDTD Modeling of Graphene Interband Contribution via Complex Conjugate Pairs

The accurate time-domain numerical modeling of graphene surface conductivity due to the interband contribution is presented in this paper. Initially, the lower frequency limit for the interband conductivity inclusion is studied, highlighting that even in the far-infrared regime it should not be ignored. Then, a precise vector-fitting technique is utilized to decompose the analytical conductivity function into complex conjugate pairs. Every pair is connected to a complex surface current, while the frequency dispersion is evaluated via a proper recursive convolution scheme. Finally, a straightforward algebraic manipulation is conducted to eliminate the complex terms from the update equations of the electromagnetic components, concerning the finite-difference time-domain (FDTD) algorithm. The precision and efficiency of the proposed methodology are thoroughly validated by comparing the numerically extracted surface wave propagation characteristics to those obtained in terms of analytical expressions.

A Coupled Hydrodynamic–Structural Model for Flexible Interconnected Multiple Floating Bodies

Evaluating the structural safety and seakeeping performance of very large floating structures (VLFS) using the rigid module flexible connector (RMFC) method remains challenging due to the complexity of the coupled hydrodynamic–structural responses in this system. In this study, a coupled hydrodynamic–structural frequency–time domain model is developed based on the RMFC method employing the planar Euler–Bernoulli beam elements to investigate the dynamic responses of multi-module floating systems. To reveal the dynamic characteristics of the systems, the coupled hydrodynamic–structural responses are investigated using a frequency–time-domain numerical model with viscous correction, in which the mass and stiffness attributes of connectors are incorporated into the system. Given the effects of hydrodynamic interaction, consideration is given to the case of three modular boxes connected by flexible beams aligned in series in shallow water to validate the present model. Higher efficiency and accuracy can be found in the system using viscous correction in potential flow theory and introducing state–space model to replace the convolution terms in the Cummins equation for the time domain. Moreover, this model can be extended to a considerable number of floating modules, which provides possibilities to analyze N-module floating systems.

Feasibility of integrating a very large floating structure with multiple wave energy converters combining oscillating water columns and oscillating flaps

The installation of wave energy converters (WECs) along the facing-wave side of in-development or pre-existing offshore structures provides an effective cost-sharing solution. In this study, a time-domain numerical model based on the modal expansion theory and the nonlinear potential flow theory, is applied to optimize the size and layout of an in-line array of oscillating water column (OWCs) and oscillating flaps (OFs) deployed along the facing-wave side of a very large floating structure (VLFS). The front/back-wall end of each OWC chamber is hinged by an OF device, and thus the number of OWCs is identical with that of OFs. Comparison with a typical OWC-VLFS integration shows that the addition of OFs can significantly reduce the vertical hydroelastic response of the VLFS. Additionally, wave energy extraction from the design of OFs hinged at the back-wall end of OWCs is enhanced due to more wave energy gathered in the chamber. Under the premise of the same space, the array layout demonstrates both higher conervsion efficiency and hydroelastic reduction. The energy conversion is generally enhanced by increasing the gap, the OWC width, the OF height. The maximum efficiency increases firstly with wave height for weak wave nonlinearity and then decreases for strong wave nonliearity.

Performance characteristics and parameter analysis of a multi-DOF wave energy converter with hybrid power take-off systems

A multi-degree-of-freedom (multi-DOF) wave energy converter (WEC) with hybrid power take-off (PTO) systems may capture more wave energy than a traditional single-DOF WEC, which usually obtains energy from pitch or heave. Compared with the existing WECs with a single PTO, this paper presents the concept design of a multi-DOF WEC with hybrid PTOs named PUWEC, which has prismatic pairs in the hydraulic cylinder and universal pairs in the gear transmission system. Two corresponding PTOs are matched to the motion of the buoy in heave and pitch to enhance wave energy conversion. This work focuses on establishing a time-domain numerical model to simulate the coupled dynamics and power capture performance of the proposed PUWEC. The model tests are carried out in a wave tank to verify the validity of the numerical model. The results show that the numerical model is efficient in predicting the dynamic performance of the PUWEC. Regular and irregular wave conditions are considered in the numerical simulations to explore the performance characteristics of the devices. It is found that the maximum improvement of the captured power of PUWEC is 49.8 % compared with single-DOF PWEC under the regular wave frequency 0.42 Hz, and the capture width ratio of PUWEC is close to 30 % under three wave conditions. In addition, the motion trajectory of the center of gravity of the buoy is analyzed. With the increase in U-PTO damping, the trajectory of the float changes from horizontal bowl-shaped irregular path to inclined regular hollow elliptical orbit. The trajectory coefficient is introduced to provide insights into how the PUWEC improves the energy conversion. The actual working space of the PUWEC is then calculated, which is less than the design working space, indicating that the actual performance of the PUWEC is fully developed. The proposed PUWEC and constructed numerical model which considers the coupled dynamic performance of dual PTOs, can provide valid reference for the design of other multi-DOF WECs with hybrid PTOs.

A Time-Domain Simulation Framework for the Modeling of Jitter in High-Speed Serial Interfaces

We report on the development of a time-domain numerical modeling framework to estimate timing jitter in High-Speed Serial Interfaces (HSSI) including a wide range of effects such as Inter-Symbol Interference (ISI) due to channel dispersion, phase noise of transmitter (TX) and receiver (RX) frequency synthesizers and use of bang-bang phase detector in the Clock and Data Recovery (CDR) loop. Based on the step response of the channel, the numerical model computes the response of the system to a random sequence of bits and provides information in terms of eye diagram, bathtub plot and jitter histogram. Different definitions of jitter are investigated, and for each of them we consider the physical effects that are included, eventually identifying that the most complete definition is based on the time distance between the edges instants defined by the CDR and the crossing points of the sampled signal. We also found that this definition is consistent with the features of the bathtub plot. A channel compliant with the PCIe 4.0 standard is used as test vehicle, including the main equalization strategies. The numerical model is then compared to a simple analytical model for the CDR jitter that can be augmented by including Data-Dependent Jitter (DDJ). An approach to system-level analysis based on proper combination of the above elements is eventually proposed to provide quick although accurate aid to the design of CDRs in HSSIs.

Incorporating the Nearly Constant Q Models Into 3-D Poro-Viscoelastic Anisotropic Wave Modeling

The Earth is often characterized by viscoelastic rocks, porous sediments and anisotropic structures. Poro-elasticity with Biot’s theory is considered fundamental to describe the interaction between the deformation of the elastic porous solid and the flow of fluid in the porous structure. The quality factor ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> ) in the theory of viscoelasticity relates seismic wave attenuation and dispersion to physical properties of the Earth’s interior, e. g. temperature, stress and composition. However, the constant <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> wave equation in its time-domain differential form remains difficult to solve when describing the attenuation in an explicitly specified <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> parameter. Here, we introduce the first-and second-order nearly constant <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> models capable of describing the attenuation of the solid skeleton, thereby extending the Biot and Biot-squirt (BISQ) models to poro-viscoelastic media. The bulk and shear moduli of the solid frame are represented by the modified relaxation function. By presenting examples with finite-difference time-domain (FDTD) numerical modeling for seismic wavefields in anisotropic, viscoelastic porous media including transversely isotropic media with a vertical symmetry axis (VTI) and orthorhombic meida, we demonstrate that the extended Biot and BISQ models provide good descriptions of the wave propagation in poro-viscoelastic anisotropic media and can thus help better understand the Earth’s interior.

Enhancing Thermal Performance, Exergy and Thermodynamics Efficiency of Premixed Methane/Air Micro-Planar Combustor in Micro-Thermophotovoltaic Systems

The present work numerically investigates the effect of a cavity implemented in a premixed methane/air micro-combustor on enhancing its thermal performances and thermodynamic efficiencies for micro-thermophotovoltaic applications. The 3D time-domain numerical model is first validated by comparing its predictions with the experimental data available in the literature. Then it is applied to examine the effects of the cavity dimensionless axial location (xc/L), cavity volume (Vc), the equivalence ratio ϕ and hydrogen blended ratio (α) on the temperature uniformity and enhancement of the combustor outer wall and exergy efficiency. It is found that implementing a cavity in the combustion chamber increases the outer wall mean temperature (OWMT) and the exergy efficiency up to approximately 65 K and 10%, respectively. The optimal cavity dimensionless axial location (xc/L) is set to 1/9, and the height (Hc_dims) is 1/5, respectively. However, the cavity length Lc and angle θc are found to play negligible roles on improving thermal performance. Additionally, increasing the inlet velocity leads to a higher OWMT but a low exergy efficiency, regardless of the equivalence ratio. In general, this work confirms the feasibility of applying a cavity structure to enhance energy efficiency for micro-power generation systems.

Time domain numerical modeling of wave propagation in poroelastic media with Chebyshev rational approximation of the fractional attenuation

In this work, we investigate the poroelastic waves by solving the time‐domain Biot‐JKD equation with an efficient numerical method. The viscous dissipation occurring in the pores depends on the square root of the frequency and is described by the Johnson‐Koplik‐Dashen (JKD) dynamic tortuosity/permeability model. The temporal convolutions of order 1/2 shifted fractional derivatives are involved in the time‐domain Biot‐JKD model, causing the problem to be stiff and challenging to be implemented numerically. Based on the best relative Chebyshev approximation of the square‐root function, we design an efficient algorithm to approximate and localize the convolution kernel by introducing a finite number of auxiliary variables that satisfy a local system of ordinary differential equations. The imperfect hydraulic contact condition is used to describe the interface boundary conditions and the Runge‐Kutta discontinuous Galerkin (RKDG) method together with the splitting method is applied to compute the numerical solutions. Several numerical examples are presented to show the accuracy and efficiency of our approach.

Hydrodynamic Analysis of a Novel Modular Floating Structure System Integrated with Floating Artificial Reefs and Wave Energy Converters

A novel modular floating structure (MFS) system moored by tension legs was proposed, which is composed of hexagonal floating modules, floating artificial reefs and wave energy converters (WECs). The integration of floating artificial reefs and WECs into the MFS can improve the marine environment and produce considerable electricity. The effects of both wave characteristics and the module quantity on the hydrodynamic responses of the MFS system were studied in depth, based on a time-domain numerical model. Both the modules’ hydrodynamic interaction effect and the connectors’ mechanical coupling effect were considered. The results indicate that floating artificial reefs combined with WECs can effectively reduce wave loads and convert wave energy into electricity for the MFS system. More modules involved in the MFS system could significantly reduce motion response and produced more wave energy output, which indicates that the MFS system is suitable for large-scale expansion. The effect of different power take-off (PTO) damping coefficients on the WECs’ performance was further investigated, and the optimal damping coefficient was recommended for the MFS system. Finally, the main extreme responses of the MFS system were further investigated, and its safety was checked thoroughly. One survival strategy was proposed, which could efficiently reduce extreme connector loads by more than 50%.

Assessment of allowable sea states for offshore wind turbine blade installation using time-domain numerical models and considering weather forecast uncertainty

Installation of offshore wind turbines, particularly blades, is challenging and should be executed within the allowable limit of sea states to ensure the safety and the efficiency of installation. During the execution phase where weather forecast will be used in the decision-making process, uncertainty in weather forecast is an important issue required to be dealt with. The purpose of this paper is to assess allowable sea states for offshore blade installation, with emphasis on both considering weather forecast uncertainty and using time-domain numerical models for installation response analysis. The general procedure is presented, which includes generation of the response-based alpha-factor αR (that is a reduction indicator reflecting weather forecast uncertainty) in the planning phase based on time-domain modelling and analysis of blade installation, and assessment of corresponding allowable sea states in the execution phase. Single blade installation by a semi-submersible crane vessel at the North Sea center is studied in this paper, and two typical limiting response parameters, namely the blade root radial motion and velocity, are considered. The αR factors correspond to these two limiting parameters are first generated respectively, based on the forecast uncertainty quantification of sea states, the quantitative assessment of system dynamic responses and the estimation of their characteristic values through probabilistic analysis. The characteristic values of the responses that correspond to a certain exceedance probability (10−2 or 10−4) could be generated with and without the consideration of the weather forecast uncertainty. Finally, the allowable sea states with and without the weather forecast uncertainty are obtained when the characteristic value reaches the allowable response limit. These could be further used to identify overall workable weather windows for the offshore blade installation and support operation decision-making. Results indicate that there is a significant difference between the allowable sea states with and without considering weather forecast uncertainty. As the forecast lead time increases, the allowable sea states gradually decrease. Hence, it is necessary to apply αR to involve and quantify the effect of weather forecast uncertainties on operations. Moreover, since time-domain simulations are used for dynamic response analysis of the installation system, the effect of statistical uncertainty related to the use of a limited number of simulations to derive the characteristic response values is also investigated and is found small.

An accurate and efficient time-domain numerical model for simulating water-axisymmetric structure subjected to horizontal earthquakes

The seismic response of offshore structures is generally influenced by water-structure interaction. This study focuses on the earthquake-induced hydrodynamic forces on axisymmetric offshore structures. Based on the finite element method, an accurate and efficient time-domain numerical model for simulating water-axisymmetric structures subjected to horizontal earthquakes is developed. First, according to interface boundary condition and using the method of separation of variables, the three-dimensional (3D) wave equation governing the compressible water is transformed into a two-dimensional (2D) governing equation in the vertical and radial directions, where the solution in the circumferential direction is analytical. Secondly, an exact artificial boundary condition (ABC) in frequency domain is developed by using the separation of variables method. Third, a high-accuracy ABC that is local in time but global in space is obtained by applying the temporal localization method. Furthermore, the high-accuracy artificial boundary condition is discretized and coupled with the finite element equation of the near field, leading to a symmetric second-order ordinary differential equations. Finally, the coupled finite element equation for the water-axisymmetric cylinder is presented. The proposed method is used to study the effect of water compressibility on the seismic response of and evaluate the seismic responses of a composite bucket foundation.

Development of MOST, a fast simulation model for optimisation of floating offshore wind turbines in Simscape Multibody

Abstract the paper presents the development of an innovative non-linear, time domain numerical model for the simulation of offshore floating wind turbines, named MOST. The model is able to evaluate the movement of the platform in six degrees of freedom, the power production and the load cycles acting on the blades. MOST is implemented in Matlab-Simulink environment using Simscape Multibody. The aerodynamics is modelled with the blade element momentum theory and the hydrodynamics is modelled using WEC-Sim, a Simscape library developed by NREL and SANDIA. The use of Simscape offers great flexibility to quickly introduce complex dynamic systems such as hybrid wave-wind platforms, flexible platforms, or sea water active ballast systems. Additionally, Matlab provides useful toolboxes and extensive libraries for advanced control systems, linearisation analysis, parallelisation and generation of C code. The results of MOST are then compared to FAST, an open-source code widely used in the academic research. The case study is a 15 MW reference wind turbine installed on the Volturn US platform. The comparison shows a good agreement between the two codes with a significant reduction of the simulation time.

Time-domain numerical modelling of the connector forces in a modular pontoon floating breakwater under regular and irregular oblique waves

Floating breakwaters are an interesting alternative to bottom-fixed structures due to their lower cost and environmental impact. Nonetheless, structural failures in the module connections are frequent. Accurate analysis of the hydrodynamic response is therefore crucial to their structural design. This article reports on numerical modelling of the forces on the module connectors in a modular pontoon breakwater. The study case was an array of five pontoons with elastic mooring lines under regular and irregular oblique waves. A 3D Boundary Element Method (BEM) solver was applied to find the velocity potentials and to characterize the frequency-domain response. The time-domain response of the floating breakwater is calculated along with the forces in the elastic mooring lines and module joints. Nonlinear hydrodynamic effects were reproduced by calculating the Froude-Krylov and hydrostatic forces over the instantaneous wetted surface at each time step. The response of the modular array under regular waves was then compared with previous experiments. After calibrating connector stiffness, the numerical and experimental results were in good agreement. Finally, the floating structure's response to regular and irregular waves was compared. The results revealed that simulation of only regular waves underestimates maximum connection forces by one order of magnitude.