Thomas Sauder

Real-time hybrid model testing of a braceless semi-submersible wind turbine. Part I: The hybrid approach

This article presents a method for performing Real-Time Hybrid Model testing (ReaTHM testing) of a floating wind turbine (FWT). The advantage of this method compared to the physical modelling of the wind in an ocean basin, is that it solves the Froude-Reynolds scaling conflict, which is a key issue in FWT testing. ReaTHM testing allows for more accurate testing also in transient conditions, or degraded conditions, which are not feasible yet with physical wind. The originality of the presented method lies in the fact that all aerodynamic load components of importance for the structure were identified and applied on the physical model, while in previous similar projects, only the aerodynamic thrust force was applied on the physical model. The way of applying the loads is also new. The article starts with a short review (mostly references) of ReaTHM testing when applied to other fields than marine technology. It then describes the design of the hybrid setup, its qualification, and discusses possible error sources and their quantification. The second part of the article [1] focuses on the performance of a braceless semisubmersible FWT, tested with the developed method. The third part [2] describes how the experimental data was used to calibrate a numerical model of the FWT. ∗Corresponding author (thomas.sauder@marintek.sintef.no)

Real-time hybrid model testing of a braceless semi-submersible wind turbine. Part II: Experimental results

Real-Time Hybrid Model (ReaTHM) tests of a braceless semi-submersible wind turbine were carried out at MARINTEK’s Ocean Basin in 2015. The tests sought to evaluate the performance of the floating wind turbine (FWT) structure in environmental conditions representative of the Northern North Sea. In order to do so, the tests employed a new hybrid testing method, wherein simulated aerodynamic loads were applied to the physical structure in the laboratory. The test method was found to work well, and is documented in [1]. The present work describes some of the experimental results. The test results showed a high level of repeatability, and permitted accurate investigation of the coupled responses of a FWT, including unique conditions such as blade pitch faults. For example, the influence of the wind turbine controller can be seen in decay tests in pitch and surge. In regular waves, aerodynamic loads due to constant wind had little influence on the structure motions (except for the mean offsets). Tests in irregular waves with and without turbulent wind are compared directly, and the influence of the wave-frequency motions on the aerodynamic damping of wind-induced low-frequency motions can be observed. ∗Corresponding author: maxime.thys@marintek.sintef.no INTRODUCTION Floating wind turbines (FWTs) are an emerging technology which can be used to generate electricity from the significant wind resource in relatively deep water (>50 m). Scaled model tests are an important part of the qualification process for such novel concepts, and such tests may have many different objectives. Model tests can, for example, be used to confirm system behavior, evaluate nonlinear phenomena, assess extreme and detailed loads, validate computer codes, or convince decision makers of the feasibility of a concept [2]. There are, however, significant challenges related to carrying out scaled model tests of FWTs in an ocean basin. Hydrodynamic tests generally follow Froude scaling, but a consistent scaling of the wind turbine will then result in a reduced Reynolds number compared to the prototype, which leads to generally poor aerodynamic performance [3]. Furthermore, there are practical challenges related to generating (and measuring) constant and turbulent wind fields in a wave basin [4, 5]. In order to improve the aerodynamic load modeling in wave basin experiments, several researchers have attempted various forms of “non-geometrical” scaling of the wind turbine rotor. One form of non-geometrical scaling is to replace the wind turbine rotor with a drag disk (e.g. [6, 7]), which gives the correct mean thrust and provides some aerodynamic damping, and 1 Copyright c

Validation of a Hybrid Code Combining Potential and Viscous Flow With Application to 3D Moonpool

When operating with a moonpool, a main concern is the large-amplitude piston-mode motion at resonance. This limits the time-window for operations inside the moonpool. Longer time-windows are desired. Further, the moonpool size is expected to increase for dedicated vessels. There has therefore recently been an increased attention to moonpool design. Potential theory highly over-predicts the water motion at moonpool resonance, and may not be used for analyzing moonpool. Viscous damping has been shown to be important, and hence vital for the moonpool functionality. We present new numerical results with a hybrid method that combines potential and viscous flow. The simulations are done with a newly implemented code called PVC3D (Potential Viscous Code). The free-surface motion is governed by potential theory, while a Navier-Stokes solver provides the solution in the main bulk of the water. With the presently considered set-up with simple geometries, the computational time remains similar to that of pure potential flow time-domain solvers, while the important flow separation that provides viscous damping is captured. The application is to a 3D moonpool set-up. The inlet of the moonpool has sharp corners, and viscous damping is significant. Good agreement with experiments is demonstrated.Copyright

Real-Time Hybrid Model Testing of Moored Floating Structures Using Nonlinear Finite Element Simulations

The paper proposes an application of real-time hybrid model testing (abbreviated ReaTHM testing) for the study of moored offshore structures. The structure under study is a moored axisymmetric floater with various bilge configurations, whose hydrodynamic properties are of interest. The system is partitioned into a physical substructure, consisting of a scaled model of the floater, and a numerical substructure, consisting of 12 mooring lines. All mooring lines are described by a nonlinear finite element model, to capture important phenomena such as geometric stiffness and drag-induced damping. The paper describes the substructuring strategy, the architecture of the test setup, and provides details regarding its components, namely the sensors, kinematic observer, predictor, numerical model, control/allocation system, and actuators. Results from qualification tests in calm water are presented, the main sources of time delays (which are compensated for) are identified, and the presence of jitter induced by Newton-Raphson iterations is discussed.

Integrity Assessment of a Free-Fall Lifeboat Launched From a FPSO

The paper addresses the structural integrity assessment of lifeboat launched from floating production, storage and offloading (FPSO) vessels. The study is based on long-term drop lifeboat simulations accounting for more than 50 years of hindcast data of metocean conditions and corresponding FPSO motions. Selection of the load cases and strength analyses with high computational time is a challenge. The load cases analyzed are those corresponding to the 99th percentile of long term distribution of indicators for large slamming loads (CARXZ) or large submergence (Imaxsub). For six selected cases, the time-varying pressure distribution on the lifeboat hull during and after water impact is calculated by CFD simulations using StarCCM+. The finite element model (FEM) of the composite structure of the lifeboat is modelled by ABAQUS. Quasi-static finite element (FE) analyses are performed for the selected load cases. The structural integrity is assessed by the maximum stress and Tsai-Wu failure measure.In the present study, the load and resistance factors are combined and applied to the response. A sensitivity study is performed to investigate the non-linear load/response effects when the load factor is applied to the load. In addition, dynamic analysis is performed with the time-varying pressure distribution for selected case and the dynamic effect is investigated.Copyright

Human injury probability during water entry of free-fall lifeboats: Operational criteria based on long-term simulations using hindcast data

The paper addresses the safety of occupants in free-fall lifeboats launched from turret-moored floating production, storage and offloading (FPSO) vessels. It presents a methodology for assessing operational limits with respect to acceleration-induced loads experienced by the passengers during water entry. The probability of being injured is estimated by means of numerical simulations for several seat rows and in various sea states described in terms of significant wave height and mean wind velocity. Those results are therefore practical for on-site decisions regarding the use of the free-fall lifeboats. The numerical simulations performed to estimate the 6-degrees of freedom (6-DOF) water entry accelerations in the lifeboats are based on more than 50 years of hindcast metocean data. These consist of sea state parameters provided every third hour and including the significant wave height, the peak period and the direction of both wind-sea and swell as well as the direction and mean velocity of the wind. In a first step, the motion of the FPSO is computed for the whole time period covered by hindcast metocean data, using a state-of-the art numerical model validated against experimental data. The model includes nonlinear excitation forces, a dynamic positioning system with a realistic heading control strategy, mooring line forces as well as turret-hull coupling. The obtained FPSO motion is then used in Monte Carlo simulations of lifeboat launches performed for selected time windows in the original metocean hindcast database corresponding to selected intervals of the significant wave height and mean wind velocity. In addition to the 6-DOF skid motion, the lifeboat launch simulations account for the effects of wind and waves diffracted by the FPSO hull. Finally, a probabilistic model describing the joint-distribution of several injury types and water entry acceleration parameters computed through the launch simulations is used to evaluate the injury probability. The results are presented in term of seating matrices showing critical seat rows, in which the probability of being injured exceeds a pre-defined threshold.

Effect of Wind Loads on the Performance of Free-Fall Lifeboats

The paper presents a model describing the launch of free-fall lifeboats from offshore structures in strong environmental wind.Six-degrees-of-freedom numerical simulations of the lifeboat launch are performed using the free-fall lifeboat simulator VARUNA with a complete set of wind coefficients for the lifeboat. Those wind coefficients are obtained by CFD simulations validated against wind tunnel tests. The lifeboat launch simulations are then verified against time-domain CFD simulations of the whole launch in air until water entry.It is shown by means of numerical simulations that wind-induced loads on the lifeboat have a strong influence on its kinematics until water entry, and subsequently on the acceleration loads experienced by the occupants, on the structural loads on the lifeboat, and on its forward speed after water exit.It is concluded that the effect of wind-induced loads on the lifeboat performances should in general be investigated when establishing the operational limits for a given offshore installation.Copyright

Computing Acceleration Loads on Free-Fall Lifeboat Occupants: Consequences of Including Nonlinearities in Water Waves and Mother Vessel Motions

The safety of occupants in free-fall lifeboats (FFL) launched from a skid is addressed, and the focus is on numerical evaluation of acceleration loads during water impact. This paper investigates the required level of detail when modeling the physics of a lifeboat launch in waves. The first part emphasizes the importance of the non-linearity of the wave surface. Severity of impacts in linear (Airy) waves is compared to impacts in regular Stokes waves of the 5th order. Correspondingly, severity of impacts in irregular waves of the 2nd order is statistically compared to impacts in linear irregular waves. Theory of the two wave models are also briefly presented. The second part discusses the importance of a more detailed modeling of the launching system. This concerns especially cases for which damage to the mother vessel induces major lifeboat heel angles. A three-dimensional skid model is presented, along with validation against experimental measurements. In addition, the wave induced motion of the mother vessel is included. Consequences on the severity of the impact of the lifeboat in regular waves are discussed. This study is based on MARINTEK’s impact simulator for free-fall lifeboats, in which slamming loads are evaluated based on momentum conservation, a long wave approximation, and a von Karman type of approach. It is coupled here to the SIMO software, also developed at MARINTEK. Performance of this coupling is discussed.Copyright

Theoretical Study of the Water Entry of a Body in Waves: Application to Safety of Occupants in Free-Fall Lifeboats

The safety of occupants in free-fall lifeboats (FFL) during water impact is addressed. The first part of the paper describes a theoretical method developed to predict the trajectory in six degrees of freedom of a body entering water waves. Slamming forces and moments are computed, based on momentum conservation, long wave approximation and a von Karman type of approach. The added mass matrix of the body is evaluated for impact conditions by a boundary element method. The second part of the paper focuses on the application of the method to free-fall lifeboats, which are used for emergency evacuation of oil platforms or ships. Acceleration loads on FFL occupants during water impact are dependent on numerous parameters, especially the hull shape, the mass distribution, the wave heading relative to the lifeboat, and the impact point on the wave surface. Assessing operational limits of FFL by means of model tests only has therefore been costly and time consuming. This issue is addressed here by applying the theoretical method described in the first part. The model has been validated for FFL through extensive model testing in calm water and regular waves, and statistical estimates of acceleration levels for lifeboat occupants, as well as acceleration time series were obtained that can be used as inputs to numerical human response models.© 2009 ASME

Reliable and Efficient Injury Assessment for Free-Fall Lifeboat Occupants During Water Entry: Correlation Study Between Lifeboat Acceleration Indicators and Simulated Human Injury Responses

The evacuation of personnel from an offshore installation in severe weather conditions is generally ensured by free-fall lifeboats. During the water entry phase of the launch, the lifeboat may be subject to large acceleration loads that may cause harmful acceleration-induced loads on the occupants. The present/common methodology for assessing the occupant safety of free-fall lifeboats uses one single characteristic launch to perform injury risk analysis for a given free-fall lifeboat launch condition that includes e.g. weather conditions, lifeboat and host installation loading conditions. This paper describes an alternative methodology to fully assess the risk of injury for lifeboat occupants during water entry by introducing a correlation model between acceleration load indicators and injury responses. The results are presented in terms of seating matrices showing critical seat rows, in which the probability of being injured exceeds a pre-defined threshold.