Physical Modeling of Wind Load on a Floating Offshore Structure

2001 ◽  
Vol 123 (4) ◽  
pp. 170-176 ◽  
Author(s):  
Bertrand Bobillier ◽  
Subrata Chakrabarti ◽  
Poul Christiansen
Keyword(s):  
Dynamic Effect ◽  
Feedback System ◽  
Offshore Structures ◽  
Wind Load ◽  
Design Criterion ◽  
Wind Effect ◽  
Offshore Structure ◽  
Floating Structure ◽  
Floating Structures ◽  
Wave Basin

Wind is an important environmental parameter that influences the design of floating offshore structures, particularly in harsh environment. Because wind spectrum is broad-banded, computation of wind load on the floating structure is complicated. Moreover, the wind-induced slow-drift oscillation is an important design criterion. Simulated environment in a model test often includes wind effect. Accurate modeling of wind in a laboratory environment is, however, a difficult task. The wind tunnel provides a steady load on the superstructure quite accurately, but fails to show the effect of the changing free surface as well as dynamic effect. Therefore, simultaneous simulation of wind in the wave basin is desirable. A weight representing the steady wind load with a string and pulley arrangement at the center of the application of the superstructure is inadequate since it fails to simulate the variation of the wind spectrum. The generation and control of the design wind spectrum by an overhead bank of fans facing the model superstructure is an extremely difficult task due to large windage area. This paper presents an accurate and highly controllable method of the generation of variable wind simultaneously with waves and current in the wave basin that can be used with a variety of floating structure model. The concept was originally proposed by Kvaerner Oil & Gas International and implemented by the offshore model basin (OMB). In this method, a fan equipped with a constant-speed motor and blades with an adjustable pitch angle is directly mounted on the model deck above water. A digital signal generated from the specified wind spectrum is used to run the fan much like the wavemaker. A feedback system ensures the proper generation of the wind with the model motion. The method was successfully applied in several model tests of deepwater floating structures in which broad-banded wind spectra were generated. An example from an earlier such test is given here. The importance of the effect of the simulated wind spectrum on floating structures should be clear to a design engineer from this example.

Very Large Floating Structures

2007 ◽  
Author(s):  
H. Suzuki ◽  
H. R. Riggs ◽  
M. Fujikubo ◽  
T. A. Shugar ◽  
H. Seto ◽  
...  
Keyword(s):  
Design Procedure ◽  
Offshore Structures ◽  
Floating Structure ◽  
Floating Structures ◽  
Design Procedures ◽  
Novel Technology ◽  
Hydroelastic Analysis ◽  
Behavior Design ◽  
Hydroelastic Response ◽  
Near Future

Very Large Floating Structure (VLFS) is a unique concept of ocean structures primary because of their unprecedented length, displacement cost and associated hydroelastic response. International Ship and Offshore Structures Congress (ISSC) had paid attention to the emerging novel technology and launched Special Task Committee to investigate the state of the art in the technology. This paper summarizes the activities of the committee. A brief overview of VLFS is given first for readers new to the subject. History, application and uniqueness with regard to engineering implication are presented. The Mobile Offshore Base (MOB) and Mega-Float, which are typical VLFS projects that have been investigated in detail and are aimed to be realized in the near future, are introduced. Uniqueness of VLFS, such as differences in behavior of VLFS from conventional ships and offshore structures, are described. The engineering challenges associated with behavior, design procedure, environment, and the structural analysis of VLFS are introduced. A comparative study of hydroelastic analysis tools that were independently developed for MOB and Mega-Float is made in terms of accuracy of global behavior. The effect of structural modeling on the accuracy of stress analysis is also discussed. VLFS entails innovative design methods and procedure. Development of design criteria and design procedures are described and application of reliability-based approaches are documented and discussed.

Planning and Design of Floating Offshore Architecture

2020 ◽  
Author(s):  
Akane Takahashi ◽  
Ikuo Yoshida
Keyword(s):  
Offshore Structures ◽  
Tokyo Bay ◽  
Offshore Structure ◽  
Floating Structure ◽  
Sea Levels ◽  
Motion Response ◽  
Response Characteristics ◽  
Planning And Design ◽  
Movement Characteristics ◽  
Rising Sea Levels

Abstract Floating offshore structures are attracting increasing attention as a method for addressing problems such as rising sea levels due to global warming and the increasing global populations. However, unlike ground structures, floating offshore structures must consider the effects of waves. The movement characteristics of the floating offshore structure have been reported. However, no studies have compared variations in motion response characteristics according to the scale of floating structures or buildings atop them, so it is currently difficult during the initial planning and design stages to estimate the size of superstructures that can be designed for a given marine area. Therefore, with the aim of obtaining basic data for planning floating offshore structures, in this study we developed floating structure modules (a square 36m on a side) according to their superstructure and investigated the basic motion response characteristics for each. We furthermore derived tendencies for horizontal acceleration and inclination occurring in individual modules according to design waves for Tokyo Bay.

Computing Large Amplitude Motions of a Floating Offshore Structure in the Time Domain

2008 ◽  
Author(s):  
Wei Qiu ◽  
Hongxuan Peng
Keyword(s):  
Time Domain ◽  
Large Amplitude ◽  
Offshore Structures ◽  
The Body ◽  
Surface Boundary ◽  
Offshore Structure ◽  
Floating Structure ◽  
Time Step ◽  
Large Amplitude Motions ◽  
The Time Domain

Based on the panel-free method, large-amplitude motions of floating offshore structures have been computed by solving the body-exact problem in the time domain using the exact geometry. The body boundary condition is imposed on the instantaneous wetted surface exactly at each time step. The free surface boundary is assumed linear so that the time-domain Green function can be applied. The instantaneous wetted surface is obtained by trimming the entire NURBS surfaces of a floating structure. At each time step, Gaussian points are automatically distributed on the instantaneous wetted surface. The velocity potentials and velocities are computed accurately on the body surface by solving the desingularized integral equations. Nonlinear Froude-Krylov forces are computed on the instantaneous wetted surface under the incident wave profile. Validation studies have been carried out for a Floating Production Storage and Offloading (FPSO) vessel. Computed results were compared with experimental results and solutions by the panel method.

On Efficient Long-Term Extreme Response Estimation for a Moored Floating Structure

2018 ◽  
Author(s):  
HyeongUk Lim ◽  
Lance Manuel ◽  
Ying Min Low
Keyword(s):  
Wave Train ◽  
Offshore Structures ◽  
Offshore Structure ◽  
Floating Structure ◽  
Sea State ◽  
Peak Period ◽  
Frequency Wave ◽  
Extreme Response ◽  
Term Response

This study investigates the use of efficient surrogate model development with the help of polynomial chaos expansion (PCE) for the prediction of the long-term extreme surge motion of a simple moored offshore structure. The structure is subjected to first-order and second-order (difference-frequency) wave loading. Uncertainty in the long-term response results from the contrasting sea state conditions, characterized by significant wave height, Hs, and spectral peak period, Tp, and their relative likelihood of occurrence; these two variables are explicitly included in the PCE-based uncertainty quantification (UQ). In a given sea state, however, response simulations must be run for any sampled Hs and Tp; in such simulations, typically, a set of random phases (and deterministic amplitudes) define a wave train consistent with the defined sea state. These random phases for all the frequency components in the wave train introduce additional uncertainty in the simulated waves and in the response. The UQ framework treats these two sources of uncertainty — from Hs and Tp on the one hand, and the phase vector on the other — in a nested manner that is shown to efficiently yield long-term surge motion extreme predictions consistent with more expensive Monte Carlo simulations, which serve as the truth system. Success with the method suggests that similar inexpensive surrogate models may be developed for assessing the long-term response of various offshore structures.

Comparison of Response Amplitude Operator Curve Generation Methods for Scaled Floating Renewable Energy Platforms in Ocean Wave Basin

2020 ◽  
Vol 1 (2) ◽  
Author(s):  
Deirdre O’Donnell ◽  
Jimmy Murphy ◽  
Vikram Pakrashi
Keyword(s):  
Numerical Models ◽  
Offshore Structures ◽  
Full Scale ◽  
Response Amplitude ◽  
Physical Models ◽  
Ocean Wave ◽  
Design Development ◽  
Floating Structures ◽  
Wave Basin ◽  
Physical Testing

Abstract Response amplitude operator (RAO) curves are commonly employed to assess the dynamic behavior of floating offshore structures in the frequency domain. There are multiple methods used to obtain RAOs for numerical models, scaled physical models, and full-scale tests. While for numerical modeling many studies detail the precise methods used, the literature around experimental RAO curves often do not detail them or leave methodological information incomplete. There exists inadequate experimental evidence in assessing the differences in results obtained by following different RAO generation methods from scaled physical testing. This paper addresses this gap by comparing two most popular RAO generation methods: the energy spectra (ES) and the cross spectral auto spectra (CSAS) method. These are experimentally compared on scaled semisubmersible and spar-buoy platforms in an ocean wave basin. Differences of heave and pitch RAOs generated by different methods are investigated. A method for reasonably collating multiple tests to create a representative RAO is also presented. RAO amplitudes vary significantly and how they decay off beyond certain frequencies is dependent on the method adopted to create them. This variation can be a source of significant uncertainty for floating structures for further analysis, design, control, or repair. Some RAOs (e.g., pitch) are sensitive to scaling and should be considered when converting scaled tests to full-scale equivalent. Detailing methods of RAO generation and comparing approaches of developing them can be important for crucial decisions from scaled physical testing of floating structures at design/development stages.

Application of Wave Devouring Propulsion Technology to Support Positioning of Floating Structure

2018 ◽  
Author(s):  
Hisafumi Yoshida ◽  
Keiichi Yamasaki ◽  
Shunji Sunahara
Keyword(s):  
Incident Angle ◽  
Offshore Structures ◽  
Vertical Position ◽  
Wave Forces ◽  
Offshore Structure ◽  
Gas Emission ◽  
Floating Structure ◽  
Column Type ◽  
Key Issues ◽  
Plate Type

It becomes one of the key issues to reduce capacity of generators for the dynamic positioning (DP) system on offshore structures for the reduction of Green House Gas emission. For the purpose, to decrease environmental forces, wave, wind and current forces, are very important for the DP system design. One of the concepts to reduce wave forces is the “wave devouring propulsion technology” (WDPT) installed vessels. WDPT employs wings attached to the vessel and they produce thrust in waves as a propulsor of vessel. WDPT concept seems to be applicable for offshore structures. In this paper, application of WDPT for three-column type offshore structure is experimentally investigated. Flat plate type wings are installed between columns, and vertical position of wings, frequency response of thrusts in waves and directional particulars of wave incident angle are studied.

Two-Dimensional Hydroelastic Analysis of Very Large Floating Structures

1992 ◽  
Vol 29 (01) ◽  
pp. 13-24
Author(s):  
Xiling Che ◽  
Dayun Wang ◽  
Minglun Wang ◽  
Yingfan Xu
Keyword(s):  
Deformable Body ◽  
Vertical Direction ◽  
Offshore Structures ◽  
Integrated System ◽  
Flexible Beam ◽  
Two Dimensional ◽  
Floating Structure ◽  
Floating Structures ◽  
Strip Theory ◽  
Very Large Floating Structures

We have reached a stage at which we are capable of building very large floating structures to meet the steadily increasing needs of ocean resource utilization or to fulfill some special industrial or civil purpose. When such a structure is large enough, its behavior in waves may be substantially different from that of ordinary offshore structures due to low resonant frequencies of the deformable body, and its analysis may require different techniques. In this paper, a two-dimensional hydroelastic theory is applied to a very large floating structure that may be multimodule and extend in the longitudinal direction. A revised strip theory is employed to analyze the hydrodynamic coefficients, but some modifications are introduced to allow for multibody cross sections. The structure is considered to be a flexible beam responding to waves in the vertical direction. Numerical examples are presented with reference to an integrated system of semisubmersibles. A simple model for engineering estimation is also presented.

Downtime Analysis Techniques for Complex Offshore and Dredging Operations

2004 ◽  
Author(s):  
Remmelt J. van der Wal ◽  
Gerrit de Boer
Keyword(s):  
Wind Waves ◽  
Weather Forecast ◽  
Offshore Structures ◽  
Operational Mode ◽  
Offshore Structure ◽  
Hydrodynamic Models ◽  
Time Step ◽  
Made In

Offshore operations in open seas may be seriously affected by the weather. This can lead to a downtime during these operations. The question whether an offshore structure or dredger is able to operate in wind, waves and current is defined as “workability”. In recent decades improvements have been made in the hydrodynamic modelling of offshore structures and dredgers. However, the coupling of these hydrodynamic models with methods to analyse the actual workability for a given offshore operation is less developed. The present paper focuses on techniques to determine the workability (or downtime) in an accurate manner. Two different methods of determining the downtime are described in the paper. The first method is widely used in the industry: prediction of downtime on basis of wave scatter diagrams. The second method is less common but results in a much more reliable downtime estimate: determination of the ‘job duration’ on basis of scenario simulations. The analysis using wave scatter diagrams is simple: the downtime is expressed as a percentage of the time (occurrences) that a certain operation can not be carried out. This method can also be used for a combination of operations however using this approach does not take into account critical events. This can lead to a significant underprediction of the downtime. For the determination of the downtime on basis of scenario simulations long term seastate time records are used. By checking for each subsequent time step which operational mode is applicable and if this mode can be carried out the workability is determined. Past events and weather forecast are taken into account. The two different methods are compared and discussed for a simplified offloading operation from a Catenary Anchor Leg Mooring (CALM) buoy. The differences between the methods will be presented and recommendations for further applications are given.

A Numerically Efficient Method for Analysis of Very Large Articulated Floating Structures

1998 ◽  
Vol 42 (03) ◽  
pp. 174-186
Author(s):  
C. J. Garrison
Keyword(s):  
Hydrodynamic Interaction ◽  
Near Field ◽  
Three Dimensional ◽  
Computational Effort ◽  
Floating Structure ◽  
Complete Structure ◽  
Field Interaction ◽  
Floating Structures ◽  
Computing Effort ◽  
The Individual

A method is presented for evaluation of the motion of long structures composed of interconnected barges, or modules, of arbitrary shape. Such structures are being proposed in the construction of offshore airports or other large offshore floating structures. It is known that the evaluation of the motion of jointed or otherwise interconnected modules which make up a long floating structure may be evaluated by three dimensional radiation/diffraction analysis. However, the computing effort increases rapidly as the complexity of the geometric shape of the individual modules and the total number of modules increases. This paper describes an approximate method which drastically reduces the computational effort without major effects on accuracy. The method relies on accounting for hydrodynamic interaction effects between only adjacent modules within the structure rather than between all of the modules since the near-field interaction is by far the more important. This approximation reduces the computational effort to that of solving the two-module problem regardless of the total number of modules in the complete structure.

Launching of Ships and Floating Structures from Horizontal Berth by Tipping Table

1998 ◽  
Vol 14 (04) ◽  
pp. 265-276
Author(s):  
Ivo Senjanovic
Keyword(s):  
Structural Dynamics ◽  
Review Paper ◽  
Offshore Structures ◽  
Model Tests ◽  
Full Scale ◽  
Strength Analysis ◽  
Test Results ◽  
Floating Structures ◽  
Measurement Results

This review paper covers extensive investigations which were undertaken in order to verify the idea of launching of ships and other floating structures from a horizontal berth by a set of turning pads. This includes structural dynamics during launching, model tests and strength analysis of the structure and the launching system. The most important results, which were used for the design of the launching system, are presented. The preparation of a barge for side launching is described, and the full-scale measurement results are compared with the test results. The advantages of building ships and offshore structures on a horizontal berth are pointed out in the conclusion.

Sign in / Sign up

Export Citation Format

Share Document