Johannes Palm

• PhD
• Engineer at Sigma Energy and Marine

This paper describes a new high-order composite numerical model for simulating moored floating offshore bodies. We focus on a floating offshore wind turbine and its static equilibrium and free decay. The composite scheme models linear to weakly nonlinear motions in the time domain by solving the Cummins equations. Mooring forces are acquired from a...

This work presents the verification and validation of the freely available simulation tool MoodyMarine, developed to help meet some of the demands for early stage development of MRE devices. MoodyMarine extends the previously released mooring module MoodyCore (Discontinuous Galerkin Finite Elements) with linear radiation-diffraction bodies, integra...

Numerical models based on the linear potential flow equations are of paramount importance in the design of wave energy converters (WECs). Over the years methods such as wave stretching, nonlinear Froude-Krylov and Morrison drag have been developed to overcome the short-comings of the underlying assumptions of small amplitude wave, small motion and...

Accepted conference paper for ISOPE-2023: This paper describes a new high-order composite numerical model designed for the efficient arbitrary-scale simulation of moored floating offshore bodies. The study focuses on static equilibrium and free decay of such structures, particularly a floating offshore wind turbine. The composite scheme models the...

We numerically simulate the hydrodynamic response of a floating offshore wind turbine (FOWT) using CFD. The FOWT under consideration is a slack-moored 1:70 scale model of the UMaine VolturnUS-S semi-submersible platform. This set-up has been experimentally tested in the COAST Laboratory Ocean Basin at the University of Plymouth, UK. The test cases...

We present a hybrid linear potential flow - machine learning (LPF-ML) model for simulating weakly nonlinear wave-body interaction problems. In this paper we focus on using hierarchical modelling for generating training data to be used with recurrent neural networks (RNNs) in order to derive nonlinear correction forces. Three different approaches ar...

High-fidelity simulations using computational fluid dynamics (CFD) for wave-body interaction are becoming increasingly common and important for wave energy converter (WEC) design. The open source finite volume toolbox OpenFOAM is one of the most frequently used platforms for wave energy. There are currently two ways to account for moving bodies in...

High-fidelity viscous computational fluid dynamics (CFD) models coupled to dynamic mooring models is becoming an established tool for marine wave-body-mooring (WBM) interaction problems. The CFD and the mooring solvers most often communicate by exchanging positions and mooring forces at the mooring fairleads. Mooring components such as submerged bu...

There are many uncertainties associated with the estimation of extreme loads acting on a wave energy converter (WEC). In this study we perform a sensitivity analysis of extreme loads acting on the Uppsala University (UU) WEC concept. The UU WEC consists of a bottom-mounted linear generator that is connected to a surface buoy with a taut mooring lin...

The power output from many wave energy converters (WECs) is limited by a finite stroke length in the power takeoff (PTO) mechanism. As the PTO approaches its maximum stroke length, an end-stop system needs to be engaged to avoid damage to the machinery. Still the onset of the end-stop is a nonlinear trigger force, a stiff point in the system. In th...

High-fidelity simulations using computational fluid dynamics (CFD) for wave-body interaction are becoming increasingly common and important for wave energy converter (WEC) design. The open source finite volume toolbox OpenFOAM ® is one of the most frequently used platforms for wave energy. There are currently two ways to account for moving bodies i...

There are many uncertainties associated with the estimation of extreme loads acting on a wave energy converter (WEC). In this study we perform a sensitivity analysis of extreme loads acting on the Uppsala University (UU) WEC concept. The UU WEC consists of a bottom-mounted linear generator which is connected to a surface buoy with a taut mooring li...

Marine cables are primarily designed to support axial loads. The effect of bending stiffness on the cable response is therefore often neglected in numerical analysis. However, in low-tension applications such as umbilical modelling of ROVs or during slack events, the bending forces may affect the slack regime dynamics of the cable. In this paper, w...

Mooring systems often make use of submerged buoys (SBs) in order to make the moorings compliant. In this paper we present the dynamic effects of changing the buoy geometry or the buoy model fidelity on the mooring system response. Three cylindrical SBs with increasing slenderness (height/diameter) are studied for a mooring leg with two polyester ro...

Accurate modeling and prediction of extreme loads for survivability is of crucial importance if wave energy is to become commercially viable. The fundamental differences in scale and dynamics from traditional offshore structures, as well as the fact that wave energy has not converged around one or a few technologies, implies that it is still an ope...

The nonlinear interaction between waves and floating wave energy converters (WECs) has become in the recent years one of the biggest challenges for wave power industry. Large displacements of real structures demand accurate and robust numerical tools to solve the problems more efficiently. However, all this nonlinear interactions between the floati...

Mooring systems for floating wave energy converters often rely on floaters to allow for minimum restraints of the body motion in heavy. However, the inclusion of floaters also introduce possible slack-taut scenarios induced by the dynamic response of the floater in relation to the fair-lead point of the mooring. This can increase the occurrence of...

A coupling between a dynamic mooring solver based on high-order finite element techniques (MooDy) and a radiation-diffraction based hydrodynamic solver (WEC-Sim) is presented. The high-order scheme gives fast convergence resulting in high-resolution simulations at a lower computational cost. The model is compared against a lumped mass mooring code...

In this paper we present and discuss the use of CFD for coupled mooring analysis of floating wave energy converters. We use the two-phase Navier-Stokes finite volume solver in OpenFOAM and a high-order finite element model for the cable dynamics. The implementation of the coupling is described in some detail and we show validation of the scheme aga...

This paper analyses the nonlinear forces on a moored point-absorbing wave energy converter (WEC) in resonance at prototype scale (1:1) and at model scale (1:16). Three simulation types were used: Reynolds Averaged Navier–Stokes (RANS), Euler and the linear radiation-diffraction method (linear). Results show that when the wave steepness is doubled,...

The paper discusses the use of CFD simulations to analyse the parametric excitation of moored, full scale wave energy converters in six degrees of freedom. We present results of VOF-RANS and VOF-Euler simulations in Open FOAM ® for two body shapes: (i) a truncated cylinder; and (ii) a cylinder with a smooth hemispherical bottom. Flow characteristic...

CFD simulations of floating wave energy converters are computationally very heavy. This paper deals with a straightforward attempt to cut down on the computational effort by using adaptive mesh refinement (AMR). We investigate the use of AMR for simulations involving floating bodies inside the open-source finite volume framework OpenFOAM. A simple...

The paper discusses the use of CFD simulations to analyse the parametric excitation of moored, full scale wave energy converters in six degrees of freedom. We present results of VOF-RANS and VOF-Euler simulations in OpenFOAM R for two body shapes: (i) a truncated cylinder; and (ii) a cylinder with a smooth hemispherical bottom. Flow characteristics...

A coupling between a dynamic mooring solver based on high-order finite element techniques (MooDy) and a radiation-diffraction based hy-drodynamic solver (WEC-Sim) is presented. The high-order scheme gives fast convergence resulting in high-resolution simulations at a lower computational cost. The model is compared against a lumped mass mooring code...

Moody is developed as a module for computing cable dynamics. It is primarily aimed at solving problems in marine applications. All examples in this manual are therefore different types of mooring configurations for an offshore structure. This manual aims to explain the usage of Moody as a stand-alone solver for mooring dynamics, and as a plug-in mo...

Recently CorPower Ocean AB presented laboratory tests of a point absorber wave energy converter equipped with a novel technique for passive phase control. The technique, known as WaveSpring, widens the response bandwidth by a negative spring arrangement, and in the tank experiment an up to threefold increase in delivered power as compared to pure l...

The wave loads and the resulting motions of floating wave energy converters are traditionally computed using linear radiation–diffraction methods. Yet for certain cases such as survival conditions, phase control and wave energy converters operating in the resonance region, more complete mathematical models such as computational fluid dynamics are p...

Computational fluid dynamics (CFD) is becoming an increasingly popular tool in the wave energy sector, and over the last five years we have seen many studies using CFD. While the focus of the CFD studies have been on the validation phase, comparing numerically obtained results against experimental tests, the uncertainties associated with the numeri...

This paper focuses on modelling snap loads in mooring cables. Snap loads are a known problem for the established oil and gas industry, and they pose a major challenge to robust mooring design for the growing industry of wave energy conversion. We present a discontinuous Galerkin formulation using a local Lax-Friedrich Riemann solver to capture snap...

This work aims to increase the modelling accuracy of two important problems for the wave energy industry. One concerns the mooring dynamics in the presence of snap loads (shock waves in cable tension). The other is related to nonlinear effects in the resonance region of moored wave energy converters (WECs). The thesis describes the development of M...

In a number of experiments and field tests of point absorbers, snap loads have been identified to cause damage on the mooring cables. Snap loads are basically propagating shock waves, which require special care in the numerical modeling of the mooring cable dynamics. In this paper we present a mooring cable model based on a conservative formulation...

Mooring systems are required to keep floating wave energy converters (WECs) on station. The mooring concept might impact the performance of the WEC, its cost and its integrity. With the aim of clarifying the pros and cons of different mooring designs, we present the results from physical model experiments of three different mooring concepts in regu...

Floating wave energy converters (WECs) operating in the resonance region are strongly affected by non-linearities arising from the interaction between the waves, the WEC motion and the mooring restraints. To compute the restrained WEC motion thus requires a method which readily accounts for these effects. This paper presents a method for coupled mo...

Mooring systems for floating wave energy converters (WECs) must be designed to survive the cyclical loads and motions to which they are subjected as a result of the wave load-WEC interaction and the motions of the WEC in the random elevation of the sea surface. The current study compares simulation procedures for the fatigue analysis of WEC mooring...

The dynamic response of mooring cables for marine structures is scale-dependent, and perfect dynamic similitude between full-scale prototypes and small-scale physical model tests is difficult to achieve. The best possible scaling is here sought by means of a specific set of dimensionless parameters, and the model accuracy is also evaluated by two a...

We present an initial study of using nonlinear and dispersive Boussinesq-type equations for simulating the response of a point absorber in waves. Boussinesq-type models are expressed in horizontal dimensions only, resulting in computationally efficient models with a real possibility to simulate entire sea states in a fully nonlinear sense. The wave...

Mooring systems are required to keep floating wave energy converters (WECs) on station. The mooring concept chosen for this has an impact on the performance of the WEC, on its cost and on its integrity. To clarify the pros and cons of different mooring designs, we present the results of experiments on physical models of three different mooring conc...

The Wave Dragon is a floating wave energy converter (WEC) working by the overtopping principle. The overtopping discharge has been determined by model scale experiments in wave basins. In the present study we numerically simulate the overtopping behavior of the Wave Dragon device using a VOF based incompressible Euler/Navier-Stokes solver in the Op...

A generic point-absorbing wave energy converter is modeled in CFD as a vertical cylinder, moored with a single catenary chain that is fully coupled through a dynamic mooring code. The method of choice is very complete and takes much of the non-linearities in the highly coupled system of the moored body into account. The paper presents numerical res...

Floating point absorbers are a common class of wave-power devices typically designed to move with large amplitude in energetic waves. This stands in sharp contrast to traditional marine design, where the wave-excited motions of the struc- ture are preferably kept to a minimum. Large motions in relatively shallow water increase the risk of slack in...

A numerical model (MOODY) for the study of the dynamics of cables is presented in Palm et al. (2013), which was developed for the design of mooring systems for floating wave energy converters. But how does it behave when it is employed together with the tools used to model floating bodies? To answer this question, MOODY was coupled to a linear pote...

The differences between the task of designing safe mooring systems for large floating platforms of the oil and gas offshore industry, that have severe accident consequences, and the task of designing moorings for ocean energy devices, with reduced accident consequences, are discussed. None of the existing guidelines satisfy the needs of wave energy...

Experiments of a moored floating cylinder in regular waves have been conducted. The buoy had a mass of 35.3 kg and was moored by a 5.9 m long catenary chain. The motions of the buoy were recorded with an infra-red video system and the tension at the fairlead was measured by a load cell. The experiments are used to validate a fast and simple numeric...

bstract—The paper presents incompressible Navier-Stokes simulations of the dynamics of a floating wave energy converter (WEC) coupled to a high-order finite element solver for cable dynamics. The coupled model has very few limiting assumptions and is capable of capturing the effects of breaking waves, green water loads on the WEC as well as non-lin...

A new numerical model for simulating the dynamics of mooring cables is presented. The model uses the hp formulation of the discontinuous Galerkin method. Verification against analytical solutions for a static and a dynamic case is carried out and the model is shown to exhibit exponential convergence with increasing polynomial order of the expansion...

To harness wave energy in a reliable, profitable manner, offshore, high energy areas will have to be explored. In these locations, due to the large water depths, the devices will have to be floating units and secured by mooring systems. In this paper we present the requirements of such mooring systems as well a numerical model developed to study th...