Sébastien Fouques

Statistical properties of mechanically generated surface gravity waves: a laboratory experiment in a three-dimensional wave basin

A wave basin experiment has been performed in the MARINTEK laboratories, in one of the largest existing three-dimensional wave tanks in the world. The aim of the experiment is to investigate the effects of directional energy distribution on the statistical properties of surface gravity waves. Different degrees of directionality have been considered, starting from long-crested waves up to directional distributions with a spread of ±30° at the spectral peak. Particular attention is given to the tails of the distribution function of the surface elevation, wave heights and wave crests. Comparison with a simplified model based on second-order theory is reported. The results show that for long-crested, steep and narrow-banded waves, the second-order theory underestimates the probability of occurrence of large waves. As directional effects are included, the departure from second-order theory becomes less accentuated and the surface elevation is characterized by weak deviations from Gaussian statistics.

Second-Order Random Wave Kinematics and Resulting Loads on a Bottom-Fixed Slender Monopile

The importance of including second-order nonlinear random wave kinematics in the numerical prediction of drag-induced shear forces and moments, at various levels on a bottom-fixed slender monopile in 40m water depth, is investigated. A vertical circular cylinder of diameter 0.5m is considered, representing typical dimensions of members in jacket type foundations of offshore wind turbines. The focus is here on the wave loads only, and wind and a propeller are therefore not included in this study. In particular, the main focus is on the effects from second-order random wave kinematics on the structural quasi-static time-varying loads due to drag forces in heavy storm wave conditions. Comparisons are made to the traditional use of Airy waves with various ways of stretching.An in-house numerical FEM code developed for structural analysis, NIRWANA, is used for this study. Thus one purpose of the present work is also to verify the implementation of the second-order random waves in the code.The results show significant effects, especially in the wave zone. Extreme crests are around 15%–20% increased, free-surface extreme particle velocities increase by around 30%–40%, while the velocities at levels below MWL are, on the other hand, somewhat reduced. The resulting peak shear forces, and in particular the moments, are thereby increased by typically 50%–100% in the upper parts of the column. At the base the peak shear forces are comparable to the traditional methods, while moments are still somewhat higher. Another effect is the generation of more high-frequency load contributions, which may be important to address further with respect to natural frequencies of such towers.Copyright

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

Bivariate Distributions of Significant Wave Height With Characteristic Wave Steepness and Characteristic Surf Parameter

The paper provides a bivariate distribution of significant wave height and characteristic wave steepness, as well as a bivariate distribution of significant wave height and characteristic surf parameter. The characteristic wave steepness in deep water is defined in terms of the significant wave height and the spectral peak period, and is relevant for e.g. the design of ships and marine structures. The characteristic surf parameter is given by the ratio between the slope of a beach or a structure and the square root of the characteristic wave steepness in deep water. The characteristic surf parameter is used to characterize surf zone processes and is relevant for e.g. wave run-up on beaches and coastal structures. The paper presents statistical properties of the wave parameters as well as examples of results typical for field conditions.Copyright

Influence of a nonlinear RAR modulation on the SAR imaging of ocean waves

The present paper investigates the influence of the real aperture radar (RAR) modulation on the imaging of range travelling waves by synthetic aperture radar (SAR). The study is based on a data set of range travelling wave events that consists of Envisat-ASAR single-look complex imagettes collocated with directional wave spectra from the WAM model. SAR image cross-spectra are simulated from the WAM data using various SAR imaging models and then compared to the crossspectra computed from the actual SAR data. We consider RAR modulation transfer functions (MTF) derived from both Bragg theory and the CMOD5 empirical radar cross-section model. In addition, both linear and nonlinear RAR modulations are applied.

Static Stability of Floating Units in Operational Conditions: A Physics-Driven Approach

When designing a new floating unit concept, static stability computations are performed in order to check stability criteria defined in regulations. Calculations for design conditions generally include the estimation of buoyancy force, gravity force and wind force acting on the floater for a given condition and a desired axis of rotation. However, when studying the stability of a floating platform in operational conditions, all external forces acting on the unit should be comprised in the assessment in order to get a more realistic and even physically admissible picture of the platform stability. Those forces include among others wind, current and anchor line system forces. In addition, limiting the study to one axis of rotation may not provide a complete picture of the floater stability, especially when the hull is of a semi-submersible type. Following this physical approach, a numerical tool has been developed based on the SINTEF Ocean’s SIMA software package. The latter package initially includes a time domain simulator of complex multibody systems for marine operations. The developed tool provides accurate physical models for each force component that may have effects on the stability. It opens the possibility to study the operational stability of a floater without restraining the study to one axis of rotation. It also allows the analysis of damaged conditions with large inclination angles. ∗Address all correspondence to this author. †Earlier MARINTEK, SINTEF Ocean from 1st January 2017 through a merger internally in the SINTEF Group This paper describes the model implemented in this numerical tool. Validation work is presented for simple geometries. Results from an operational stability study of a semi-submersible are discussed. Finally, possible further work is discussed. INTRODUCTION Static stability computation is a central part of the design procedure of all floating units. It includes dimensioning the geometry and the mass distribution of the structure so that its inclination angle remains within an acceptable range in various environmental conditions. Acceptance criteria can be found in different standards and regulations, e.g. DNV-GL [1] and IMO [2] respectively. The main loads to be included in the calculation are the forces and moments due to gravity, buoyancy and wind, as well as other physical effects that may be of importance: current, thrusters and ice loads. In the ship industry, dedicated software are used for that purpose. The static stability is often analyzed by considering a one-dimensional problem, where the rotation of the floating unit about a given axis and the corresponding restoring moment about this axis are studied. This is a practical approach for floating units for which it is easy to identify one axis with low stiffness. This is typically the case for ship-shaped structures; the roll axis is much weaker than the pitch axis in the sense that the latter generally benefits from a larger hydrostatic stiffness. 1 Copyright c