Fixed-bottom offshore wind turbines (OWTs) are typically located in shallow to intermediate water depth, where waves are likely to break. Support structure designs for such turbines must account for loads due to breaking waves, but predictions from breaking wave models often disagree with each other and with observed behavior. This variability indicates the need for a better understanding of each model’s strengths and limitations, especially for different ocean conditions. This work evaluates and improves the accuracy of common breaking wave criteria through comparison to Computational Fluid Dynamics (CFD) simulations of breaking waves. The simulated ocean conditions are representative of potential U.S. East Coast offshore wind energy development sites, but the discussion of model accuracy and limitations can be applied to any location with similar ocean conditions. The waves are simulated using CONVERGE, a commercial CFD software that uses a Volume of Fluid (VOF) approach and includes adaptive mesh refinement at the evolving air-water interface. First, the CFD model is validated against experimental data for shoaling and breaking wave surface elevations. Second, 2D simulations of breaking waves are compared to widely-used breaking wave limits (McCowan, Miche, and Goda) for different combinations of wave height, wavelength, water depth, and seafloor slope. Based on these comparisons, the accuracy and limitations of each breaking limit model are discussed. General usage guidelines are then recommended.

The Floating Offshore Wind Turbine (FOWT) is a fairly new concept. There are limited number of full-scale prototypes to provide real data. Therefore, most of the research today relies on numerical models. This is required, so that an adequate amount of confidence can be gained before venturing into large scale production. The major challenge ahead is proving their reliability and robustness. There needs to be supporting studies that consider most factors that can go wrong. The computer program FAST was a groundbreaking contribution from NREL in this regard. FAST is capable of doing combined loading analysis of FOWTs. The numerical model used for the hydrodynamics can, however, be improved further. Non-linear hydrostatic and wave forces on floating structures become very important during large amplitude waves. The computer program SIMDYN is a blended time domain program developed by Marine Dynamics Laboratory at TAMU and is capable of capturing the role of non-linear fluid forces. SIMDYN has previously been used to demonstrate that nonlinear hydrostatics become very important in the problem of parametric excitation. In the current work, SIMDYN is coupled with FAST. The FAST-SIMDYN is now a tool that is capable of studying large amplitude motions of FOWTs in extreme seas. FAST-SIMDYN was then used to study the classic instability of negative damping that occurs in FOWTs that use conventional land based control. The development of platform pitch and platform surge instability are studied in relation to different wave and wind scenarios. The intent was to do an analysis to see if the non-linear forces do play a significant role in large amplitude motions induced by negative damping. This study gives an indication of whether the development of an even more sophisticated hydrodynamic modules is justified.

Making use of theoretical approximations for the computation of the wave-induced slow-drift forces is a common procedure in the early stages of design of a new floating unit. They can help reducing the computational burden in two different fronts: for generating the QTFs in a frequency domain analysis, and during the subsequent execution of time-domain simulations. In a previous paper, we have discussed a simple procedure for making use of the white-noise approximation in FAST, without the need for any modification of the software. The proposal only requires restricting the computation of the QTFs to pairs of frequencies that are indeed essential to the slow-drift dynamics. For this, however, an additional assumption is made, considering that each motion is decoupled from those in the other dofs. In the present paper, a more detailed analysis of the subject is made, in order to clarify the theoretical aspects of the procedure and supplement the previous analysis. Once again, the results are based on the data available for the OC4 FOWT. The accuracy obtained with the procedure is discussed not only in terms of the resulting motions, but also comparing its effects on the second-order force spectra. A more detailed evaluation of the dynamic couplings is presented, and comparisons with the results obtained with Newman’s approximation are made in simulations involving waves only.

Floating offshore wind turbines (FOWT) have many pros and cons. Among the pros, we can mention the availability of more constant winds and a velocity more suitable to the use of turbines in their optimum efficiency. Among the cons, there are the high costs for installation, mooring lines and the large length of cables required for the energy transmission. In this context, saving structural weights of the floater is, certainly, very welcome. This paper describes the results of an experimental campaign of a 1/80th scale model FOWT performed in a wave basin. The model consists of a central column connected by pontoons to three equally spaced columns by an angle of 120 degrees. The offset columns are connected to the central tower by guywires. Being structurally light, both pontoons and guywires are subject to the effects of hydroelasticity. In these preliminary tests of the concept, only waves were considered, hence wind effects are not yet addressed. The analysis is based on the first order motions of the FOWT, the tension at the guywires, and the strain at the pontoons. The results are compared with numerical simulations obtained with the software NK-UTWind, developed at the University of Tokyo, and METiS - USP, currently being developed at the University of São Paulo. Furthermore, the floater motions are also analyzed using the commercial software WAMIT, to provide a better insight on the physics involved by using a different approach to the calculation of hydrodynamic forces.

There are several challenges facing the design of mooring system of floating offshore wind turbines (FOWTs), including installation costs, stability of lightweight minimalistic platforms, and shallow water depths (50–300m). For station keeping of FOWTs, a proper mooring system is required in order to maintain the translational motions in surge and sway and the rotational motions in yaw of the platform within an adequate range. A combination of light pre-tension, shallow water depth and large platform motions in response to a survival storm condition can result in snap-type impact events on mooring lines, thus increasing the line tension dramatically. In this paper, we present a new snap load criterion applicable to a catenary mooring system and compare it with Det Norske Veritas’ criterion for marine operations. As a case study, we examine the extreme tension on a catenary mooring system of a semi-submersible FOWT exposed to a 100-year storm condition. The software OrcaFlex was used for numerical simulations of the mooring system. NREL’s FAST software was coupled to OrcaFlex to obtain aerodynamic loads along with hydrodynamic loads for FOWT analyses. Snap-type impact events were observed in the numerical simulations and were characterized by two criteria. Tension maxima were fitted using composite Weibull distributions (CWDs) and comparisons of probability exceedance were made for the two different snap load criteria.

Floating wind solutions have developed significantly in the recent years, moving from single demonstrators to having several floating wind pilot wind farms currently under development and even in operation. This is an important step for the industry allowing the market to gain confidence in these solutions for offshore wind. Ideol is a leading floating platform designer and they have been working on a demonstration project for their innovative platform in France. The Floatgen demonstration project consists of a 2MW wind turbine mounted on the Damping Pool platform. During the design phase of the project, the coupled analysis of the full system — turbine, tower, floating platform and moorings needs to be carried out to verify the loading on the turbine and platform, adapt the turbine controller for the floating application and re-design the tower and transition piece. For this project, DNV GL performed the aforementioned analysis in Bladed whilst Ideol performed parallel analysis in OrcaFlex, focusing on the platform and mooring design. It is crucial that both numerical models used in the different software tools and parallel analysis workflows are equivalent and lead to the same overall system behavior. This paper describes the numerical model used for coupled analysis in Bladed and its verification against Ideol’s OrcaFlex model, with emphasis on the aspects related to the platform modelling. For the hydrodynamic loading of the platform, boundary element method was considered together with global and local viscous drag terms. To compare and verify the coupled model results in Bladed to Ideol’s own numerical results, a set of static and dynamic tests were run and the resultant kinematics were compared. Ideol’s model was previously validated against tank test experiments giving confidence in its behavior. The viscous drag coefficients in the Bladed model were adjusted to ensure a good agreement between the kinematics of Ideol’s model of the system and the Bladed model. This paper summarizes the results of this verification exercise, along with some recommendations on areas of further research in the floating wind modelling domain.

This paper describes a fully coupled numerical simulation methodology which is tailored towards floating offshore wind turbines. The technique assembles three key components; an aerodynamic model of the applied wind loads based on blade element momentum theory, a structural model of the floating platform and its associated mooring lines based on the nonlinear finite element method, and a hydrodynamic model of the wave-induced forces based on potential flow theory. The simulation methodology has been implemented in a commercial software product called ‘Flexcom Wind’, and the technical validation involves comparisons with experimental data derived from model-scale tank test facilities. The validation process centres on an innovative floating wind turbine concept developed by Eolink. Unlike most wind turbines in industry which are supported by a single mast, this patented design uses four separate pillars to connect the turbine structure to the corners of the floating platform. This unique configuration offers several advantages over conventional designs, including a more even stress distribution in structural members, reduced dynamic vibration, smaller floater size and lower overall capital expenditure. Data obtained from the numerical simulations combined with the empirical tests is helping to optimise the device, with a view to further improving its structural design and performance.

A Tension Leg Platform supporting a wind turbine was tested in combined waves, wind and current in the Offshore Basin of MARIN. The wind turbine loads were simulated in two ways for these tests. Firstly, the wind turbine was modelled by a physical wind turbine following the approach of performance scaling. This approach intends to scale down the wind, the wind turbine inertia, the rotation speed of the rotor according to Froude’s scaling laws while it keeps the thrust that is representative of the considered wind turbine. The Marin Stock Wind Turbine (MSWT) was used in these tests. Secondly, the wind turbine was replaced by a system actuating the wind turbine loads in which the aerodynamic loads are calculated by a numerical code and applied through a system of winches to a frame mounted on the floater. Lift and drag coefficients of the blades of the MSWT were used by the computer program during these tests. The TLP was tested with both techniques for the same environmental conditions. Tests were done in waves, wind and current with the two techniques in the same basin, enabling an objective comparison of these two methods. The experimental results of both methods are analyzed and compared to each other in this study.

The wind engineering community relies on multiphysics engineering software to run nonlinear time-domain simulations (e.g., for design-standards-based loads analysis). Although most physics involved in wind energy are nonlinear, linearization of the underlying nonlinear system equations is often advantageous to understand the system properties and exploit well-established methods and tools for analyzing linear systems. This paper presents the development of the new linearization functionality of the open-source engineering tool OpenFAST for floating offshore wind turbines, as well as the concepts and mathematical background needed to understand and apply it.

This work investigates the implementation of a novel, NASA-developed Fluid Harmonic Absorber (FHA) technology to mitigate platform motions and structural loads that can lead to lighter platforms, increased turbine performance, and ultimately, a lower LCOE. The novel damping strategy takes advantage of existing water ballast in the VolturnUS semi-submersible platform to achieve significant performance gains with minimal additional equipment and complexity. NREL’s FOWT software FAST is modified to include the primary features of the FHA technology. A study of the University of Maine-developed VolturnUS semi-submersible FOWT augmented with FHA technology is undertaken to quantify global performance of the system. When compared to the baseline technology, numerical simulations of a redesigned platform utilizing the FHA dampers indicate a reduction of 15.8% in hull structural material. Finally, the improvements in LCOE resulting from this mass reduction are assessed to demonstrate the advantages of NASA’s FHA technology for FOWT applications.

The use of synthetic fiber ropes for subsea installation is extending, as the offshore industry explores deeper waters, but there are few data available to evaluate the lifetime of these materials. In a previous OMAE presentation the authors described results from the first phase (2010–2013) of a JIP aiming to understand the mechanisms controlling the long term behavior of HMPE fibre ropes [1]. This presentation will describe the results from the second phase of this study (2014–2018) in which predictive models have been developed and applied to a range of improved braided rope materials. Two modeling approaches will be discussed, an empirical method based on residual strength after cycling, and a numerical approach using finite element software specifically adapted to fibre materials [2]. An extensive test program, which has generated a database of CBOS (cyclic bend over sheave) results for various grades of HMPE and different constructions, will be described. Comparisons have been made with steel wire handling lines in order to quantify the benefits of fibre ropes for these deepwater applications.

Vertical pipes are extensively applied on offshore petroleum operations, such as subsea wellbore drilling, wellbore re-entry, installations of equipment on the sea bottom, hydrocarbon production as the conductor for oil and gas from the wellhead at the sea bottom, among other applications. Recent oil and gas field discoveries in ultra deepwater brought an increase of the amount of produced and processed petroleum onboard of floating production system. Consequently, the process plant demands more cooling capacity. One of the alternatives is to elevate cold sea water. The present study aims to improve the understanding of the behavior of a vertical pipe forced to move at its upper end in a water tank. Displacements along the pipe length are measured, and main features of the behavior of the pipe model with its bottom end free are observed. Motions imposed at the vertical pipe intent to represent motions of the surface from the floating platform or ship, due to waves, currents and wind. Pipe model vibrations are analyzed, and results allowed improvements for the input in numerical model simulations to Orcaflex software. Developments of the analysis procedure and evaluations of the experiment allow more reliable design of riser operations with its bottom end free in actual sea operations. In the present work, a physical model with a pipe of small diameter has been developed for the laboratory experiment, and an optical measurement system is used to measure the pipe model displacement in the water tank. Motions are imposed at the top termination by a mechanical device. Experiment is described in details and comparative analyses between experiment and numerical simulations were carried out. Results from numerical simulations as well as from the experiment brought important contributions for description and understanding of the vertical pipe behavior.

Most of the ocean energy technologies are considered to be in a pre-commercial phase and need technical development. This study focuses on design of mooring solutions and compares array systems of a specific floating point-absorbing wave energy converter (WEC) developed by the company Waves4Power. A full-scale prototype of the WEC is installed in Runde (Norway) where it is moored with three polyester mooring lines, each having one floater and one gravity anchor. Based on this reference installation, the method of systems engineering was used to propose twenty-two conceptual mooring solutions for different array systems. They were compared and reduced to four top concepts in a systematic elimination procedure using Pugh and Kesselring matrices. The top concepts were assessed in detail by means of LCOE (levelised cost of energy), LCA (life cycle analysis) and risk analyses. The fatigue life of the mooring lines and the energy capture were calculated using results obtained from coupled hydrodynamic and structure response analyses in the DNV-GL DeepC software. Two final concepts were proposed for the water depths 75 and 200 m.

Mooring systems constitute an important element to secure the stability and survival of floating structures. In the last years, its use has increased potentially linked to the growth of marine renewable energies, about all, offshore wind and waves. Traditionally, fixed foundations such as gravity, monopiles or jackets have been installed. However, new alternatives have been developed based on floating structures looking to take advantage of the potential resource in deep waters. This paper involves an exhaustive mooring system study based on catenary configuration from different points of view. Physical modelling was performed by means of different experimental tests under different loading conditions including prescribed movements and natural forcing (waves and currents) considering two different types of seabed: a rigid seabed using the glass of the flume simulating a rocky bottom and a deformable seabed through sandy seabed. Different types of numerical approaches were implemented and they were validated with laboratory tests. Quasi-static and dynamic models were included. Also, a commercial software called SESAM was used to verify the results. Finally, the effect of wave groupiness on extreme loads was analysed using a Floating Offshore Wind Turbine (FOWT).

A methodology for the initial dimensioning and evaluation of semi-submersible offshore accommodation units by use of a novel optimization procedure is presented. The method developed has two main steps. First, the optimization procedure itself, in which a set of different configurations is considered and one by one optimized following a genetic algorithm in terms of cost estimates, whereas several other characteristics of the units are evaluated according to an embedded synthesis model. Second, the set of optimized geometries are run in commercial software for accurate seakeeping and stability analysis, where requirements’ compliance are checked. The final solution may then be chosen considering the trade-offs between cost and performance. In the last section of this paper, a case study is presented and the results obtained are discussed.

Large underwater equipment used in deep-sea engineering, which needs mooring line in order to hang down while using pendulous installation method in 1500m deep-water installation. During the procession, the mooring line will suffer the vessel’s pull, large equipment’s traction, fluid force leading to elongation, bending and other deformation, so need to analyze its motion. Aiming at the numerical simulation problem of underwater equipment installed by pendulous installation method in deep-water environment, this paper combines lumped mass method, three-dimensional potential theory and Morison equation to establish the analysis model of the whole installation system. Under the different horizontal dropping offsets of the equipment, this paper computes not only tension of mooring line, but motion performance of underwater equipment and working vessel during the pendulous lowering phase. The Pendulous Installation Method (PIM) puts a working vessel, a mooring line and a underwater equipment installed in succession, and there is a coupling interaction among them, so it is essential to do time-domain coupling analysis. So the Orcaflex Software is used to simulate the entire deep-water installation system. Using different horizontal dropping position as a variable, we can get the contrast about the different movement states of the mooring line and underwater equipment, and finally we will get the conclusion.

A code-to-code comparison of the dynamic characteristics of an end-anchored floating pontoon bridge is presented for two commercial software commonly used in the offshore industry, i.e. SIMO-RIFLEX and ORCAFLEX. Furthermore, a second comparison of the hydrodynamic properties of the floating pontoons are compared between two commercial Boundary Element Method software, i.e. AQWA and WADAM. The pontoons are modelled with an extended bottom to change the dynamic properties of the bridge structure. Viscous effects of this so-called heave plate is not considered in the study. The study shows good comparisons between the natural periods of the floating bridge structure and both software yield good convergence in the time domain. However, significant differences in the stochastic response has been found.

The trend in our days seems to drive to hybrid renewable offshore platforms. Several European projects have advanced in this sense. One of these hybrid configurations is obtained by combination of wind and wave energy which allows higher production of electric power. W2Power platform combines offshore wind and wave energy in a semi-submersible floating platform. In this work we present a preliminary study of platform W2Power carried by UCA and collaboration with EnerOcean. A first step of the study, is presented, in this first step the platform and the wave converter are separately studied. A model of these components is created in the software SeaFEM and they are calibrated in the software using previous experimental test tank results. Taking into account this preliminary work, a study of a complete fully coupled model of the W2Power including wind and WECs under different control laws [11] platform will be made in future works.

The influence of the gas-to-liquid density ratio (DR) on the global wave shape before impact is studied through numerical simulations of the propagation of two different waves in a rectangular wave canal. Two different codes are used: the first one, named FSID, is a highly non-linear 2D bi-fluid potential code initially developed in the frame of SLOSHEL JIP (Kaminski et al. (2011)) to simulate incompressible inviscid free-surface flows without surface tension thanks to a desingularized technique and series of conformal mappings; the second one, named CADYF, is a bi-fluid high-fidelity front-tracking software developed by Ecole Polytechnique Montreal to simulate separated two-phase incompressible viscous flows with surface tension. The first studied wave leads to a flip-through impact while the second one leads to a large gas-pocket impact. Each condition is studied with water and three different gases with increasing densities corresponding to DR = 0.001, 0.003 and 0.005. The global wave shapes are compared a few tenths of second before the impact, before free surface instabilities triggered by the shearing gas flow have developed and also before any gas compressibility matters. Both codes give precisely the same global wave shapes. Whatever the condition studied, it is shown that DR has an influence on these global wave shapes. The trends observed from the simulations are the same as those described in Karimi et al. (2016) obtained from sloshing model tests with Single Impact Waves (SIW) in a 2D tank with a low filling level. A small part of the mechanical energy of the liquid is progressively given to the gas. The larger the DR, the larger this transfer of energy from the liquid to the gas. This explains an increasing delay of the wave front for increasing DRs.

The purpose of this work is to examine a three-legged jacket tower support system subjected to wave loading. To this end, linear as well as nonlinear wave scenarios are investigated. The structure was designed for offshore wind turbines installed in intermediate water depths. The phenomenon of the wave-structure interaction is examined experimentally with a 1:18 scaled model as well as numerically with the use of Finite Element Model (FEM). The structural calculations were performed using the structural analysis software SAP2000, which was enhanced by a special programming interface that was developed to calculate the wave loading and to directly apply the wave loads on the structural members. The FEM model in combination with the key parameters that are taken into account, provides a good correlation with the experimental results. The wave theories of Airy and Stokes 5 th are employed for the calculation of the wave particle kinematics. The resulting wave forces are examined both in the frequency and in the time domain.