Green Sea and Water Impact on FPSO: Numerical Predictions Validated Against Model Tests

2002 ◽  
Author(s):  
C. T. Stansberg ◽  
O̸. Hellan ◽  
J. R. Hoff ◽  
V. Moe
Keyword(s):  
Structural Integrity ◽  
Design Method ◽  
Model Tests ◽  
Random Wave ◽  
Simplified Approach ◽  
Irregular Wave ◽  
Wave Kinematics ◽  
Numerical Predictions ◽  
Statistical Sense ◽  
Local Pressures

A recently developed numerical design method for analysis of green sea events and resulting impact loads on deck structures of FPSO’s, is validated against model test data. Steep irregular wave conditions are considered, and numerical time series reconstructions are made using the measured wave as input. A second-order numerical random wave description is combined with standard 3D wave diffraction and related vessel motions to predict the relative wave kinematics. A modified shallow water formulation is applied for the prediction of the propagation on deck, and resulting local pressures on the deck-house are estimated by a similarity solution. From this an analysis of the structural integrity can be made. Comparisons to the experiments are made for the relative wave amplitudes, water propagation on deck, and the resulting deck-house loads. A reasonably good agreement is observed for the reconstructions, in a statistical sense, but also for individual events. Thus selected green sea events are investigated in detail, and characteristics identified. The agreement with the model tests is promising especially on the background of the simplified approach used, as well as the expected statistical scatter.

Experimental Assessment of the Performance of CECO Wave Energy Converter in Irregular Waves

2018 ◽  
Author(s):  
Claudio A. Rodríguez ◽  
F. Taveira-Pinto ◽  
P. Rosa-Santos
Keyword(s):  
Wave Energy ◽  
Transfer Functions ◽  
Nonlinear Effects ◽  
Model Tests ◽  
Experimental Results ◽  
Irregular Waves ◽  
Scale Model ◽  
Experimental Assessment ◽  
Simplified Approach ◽  
Wave Basin

A new concept of wave energy device (CECO) has been proposed and developed at the Hydraulics, Water Resources and Environment Division of the Faculty of Engineering of the University of Porto (FEUP). In a first stage, the proof of concept was performed through physical model tests at the wave basin (Rosa-Santos et al., 2015). These experimental results demonstrated the feasibility of the concept to harness wave energy and provided a preliminary assessment of its performance. Later, an extensive experimental campaign was conducted with an enhanced 1:20 scale model of CECO under regular and irregular long and short-crested waves (Marinheiro et al., 2015). An electric PTO system with adjustable damping levels was also installed on CECO as a mechanism of quantification of the WEC power. The results of regular waves tests have been used to validate a numerical model to gain insight into different potential configurations of CECO and its performance (López et al., 2017a,b). This paper presents the results and analyses of the model tests in irregular waves. A simplified approach based on spectral analyses of the WEC motions is presented as a means of experimental assessment of the damping level of the PTO mechanism and its effect on the WEC power absorption. Transfer functions are also computed to identify nonlinear effects associated to higher waves and to characterize the range of periods where wave absorption is maximized. Furthermore, based on the comparison of the present experimental results with those corresponding to a linear numerical potential model, some discussions are addressed regarding viscous and other nonlinear effects on CECO performance.

Evaluation of a Small and Fast Planing Boat’s Seakeeping Performance at the Design Stage

2015 ◽  
Author(s):  
Dong Jin Kim ◽  
Sun Young Kim
Keyword(s):  
Model Tests ◽  
Full Scale ◽  
Scale Model ◽  
Design Stage ◽  
Small Scale ◽  
Irregular Wave ◽  
Seakeeping Performance ◽  
Head Waves ◽  
Scale Ratio ◽  
Free Running

Seakeeping performance of a planing boat should be sufficiently considered and evaluated at the design stage for its safe running in rough seas. Model tests in seakeeping model basins are often performed to predict the performance of full-scale planing boats. But, there are many limitations of tank size and wave maker capacity, in particular, for fast small planing boats due to small scale ratio and high Froude numbers of their scale models. In this research, scale model tests are tried in various test conditions, and results are summarized and analyzed to predict a 3 ton-class fast small planing boats designed. In a long and narrow tank, towing tests for a bare hull model are performed with regular head waves and long crested irregular head waves. Motion RAOs are derived from irregular wave tests, and they are in good agreements with RAOs in regular waves. Next, model ships with one water-jet propulsion system are built, and free running model tests are performed in ocean basins. Wave conditions such as significant heights, modal periods, and directions are varied for the free running tests. Motion RMS values, and RAOs are obtained through statistical approaches. They are compared with the results in captive tests for the bare hull model, and are used to predict the full-scale boat performances.

Development of a Multifactor Regression Model of Ship Maneuvering Forces Based on Optimized Captive-Model Tests

2006 ◽  
Vol 50 (04) ◽  
pp. 311-333 ◽  
Author(s):  
S. Sutulo ◽  
C. Guedes Soares
Keyword(s):  
Experimental Design ◽  
Polynomial Regression ◽  
Design Method ◽  
Model Tests ◽  
Ship Maneuvering ◽  
Captive Model Tests ◽  
Experimental Design Method ◽  
Force Moment ◽  
Force Components ◽  
Multifactor Regression

The paper provides the results of model tests planned with an optimized experimental design method. Captive-model tests have been carried out according to such a design on a computerized planar-motion carriage with a model of a fast catamaran with five varying factors (drift angle, rate-of-yaw amplitude, sinkage, trim and heel angles) and with all six force/moment components measured at each run. The measured values were used after preprocessing for construction of polynomial regression models for all force components acting upon the catamaran's hulls. It is demonstrated that the optimized experimental design method allows rather complicated mathematical models for maneuvering hydrodynamics forces to be obtained from captive model tests at a reasonable level of effort.

Structural Analysis of a Subsurface Cage for Sea Cucumber, Stichopus japonicus, Grow-out Using Numerical Modeling Techniques

2012 ◽  
Vol 46 (5) ◽  
pp. 55-66 ◽  
Author(s):  
Moo-Hwan Oh ◽  
Taeho Kim ◽  
David W. Fredriksson ◽  
Judson DeCew
Keyword(s):  
Numerical Modeling ◽  
Sea Cucumber ◽  
Structural Integrity ◽  
Animal Movement ◽  
Structural Characteristics ◽  
Cage Structure ◽  
Stichopus Japonicus ◽  
Irregular Wave ◽  
Cage System ◽  
Modeling Techniques

AbstractThe structural characteristics of a subsurface cage system for sea cucumber, Stichopus japonicus, grow-out were analyzed by using numerical modeling techniques. The cage system was constructed of polypropylene pipe and netting and weighted to sit on the seafloor bottom. Inside the cage, concrete blocks were used for animal aestivation and weight and a thin-plated device was mounted for animal movement. Environmental loads on the structure, resulting from a prescribed irregular wave field with and without currents, were first determined with a Morison equation-type finite element model. The structural response of beam and truss cage components was then calculated with the software MSC.MARC/Mentat. In addition to the irregular wave and current input forcing parameters of the structure, response was also calculated for possible forces incurred during lifting operations. Reaction loads, bending moments, axial tensions, and von Mises stresses of the sea cucumber cage structure were calculated for evaluation. The results of the study indicate that the combination of numerical model analyses presented can be used to assess structural integrity of these subsurface cage systems. These techniques will become more important as the industry expands and economics of scale promotes the construction of larger sea cucumber containment structures.

Analysis of Semi-Submersible Under Combined High Waves and Current Conditions Compared With Model Tests

2018 ◽  
Author(s):  
Limin Yang ◽  
Arne Nestegård ◽  
Erik Falkenberg
Keyword(s):  
Simulation Model ◽  
Low Frequency ◽  
Model Tests ◽  
Wave Forces ◽  
Viscous Effects ◽  
Numerical Predictions ◽  
Wave Drift ◽  
Zero Current ◽  
Frequency Excitation ◽  
Wave Drift Forces

Viscous effects on the low-frequency excitation force on column based platforms are significant in extreme waves. The wave drift force as calculated by a zero-current potential flow radiation/diffraction code becomes negligible for such waves. In the present study, the effect of current and viscous contributions on the slowly varying wave forces are adjusted by a formula developed in the Exwave JIP, see e.g. [1], which is validated against model test results. This paper presents numerical predictions of low frequency horizontal motions of a semi-submersible in combined high waves and current condition. In the simulation model, frequency dependent wave drift forces from radiation/diffraction code are modified by the formula. Static current forces and viscous damping are modelled by the drag term in Morison load formula using relative velocity between current and floater and with force coefficients as recommended by DNVGL-RP-C205 [2]. Low frequency surge responses calculated by the simulation model are compared with model tests for waves only and for combined collinear and noncollinear wave and current conditions.

“Bigfoot” Direct Vertical Access Semisubmersible Model Tests and Comparison With Numerical Predictions

2016 ◽  
Vol 138 (5) ◽  
Author(s):  
Stergios Liapis ◽  
Yile Li ◽  
Haining Lu ◽  
Tao Peng
Keyword(s):  
Model Tests ◽  
Attractive Alternative ◽  
Force Measurements ◽  
Numerical Predictions ◽  
Order Of Magnitude ◽  
Drilling Operations ◽  
Computational Fluid Dynamics Cfd ◽  
Project Model ◽  
Production Host ◽  
Motion Characteristics

The Bigfoot direct vertical access (DVA) semisubmersible is a novel floating drilling and production host that provides an attractive alternative to the spar. This concept utilizes heave plates (big feet) that improve the motion characteristics of a semisubmersible in all mild environments (Southeast Asia, West Africa, and Brazil). Bigfoot offers riser-friendly motions that enable top-tensioned risers, which is often a project requirement. This floater works in all water depths, in particular ultra-deepwater (5000 + ft) where a tension leg platform (TLP) is not an option, supports top-tensioned risers, and enables drilling and workover operations. The Bigfoot has several advantages over a spar. These include: (1) quayside topsides integration. This eliminates offshore topsides integration, a significant issue for all spar projects in terms of cost, safety, and schedule, (2) a more open deck layout compared to a spar, and (3) no fabrication location restrictions as it can be built by many yards worldwide potentially offering local content to a project. Model tests were undertaken at the Shanghai Jiao Tong University (SJTU) Offshore Basin to assess the dynamic response of the Bigfoot in waves, swell, wind, and current. Five mild non-Gulf of Mexico (GOM) environments were considered. In all the cases, the floater motions are an order of magnitude smaller than those of a conventional semisubmersible for similar deck payload, thus enabling drilling operations and top-tensioned production risers. In a parallel effort, a cosmos numerical model of the Bigfoot was developed for coupled motion analysis. The experimental results and the cosmos numerical predictions are in close agreement. In addition to measuring global motions, two heave plates were instrumented with load cells to measure forces and moments. The force measurements from the model tests are in good agreement with numerical predictions using computational fluid dynamics (CFD).

Performance of a Longitudinal Circular-to-Slot Gas Turbine Exhaust Duct With a 90 Degree Bend

2004 ◽  
Author(s):  
S. Bottenheim ◽  
A. M. Birk ◽  
D. J. Poirier
Keyword(s):  
Gas Turbine ◽  
Structural Integrity ◽  
Ambient Air ◽  
Effective Area ◽  
Loss Coefficient ◽  
Flow Distortion ◽  
Rectangular Slot ◽  
High Loss ◽  
Numerical Predictions ◽  
Gas Turbine Exhaust

In some gas turbine applications, it is desirable to redirect the exhaust flow through 90 degrees and mix this flow with the ambient air for the purposes of structural integrity and heat signature suppression. A method to achieve this is to transform the flow from a circular profile to a rectangular slot of high aspect ratio. The increase in wetted perimeter allows for greater mixing with the ambient air; however the shape of such a duct causes significant amounts of flow distortion and poor pressure recovery. This paper presents preliminary experimental results of the performance of such a duct and discusses the ability of a commercial CFD software package to numerically predict this performance. Significant crossflows and reversed flows were observed at the duct outlet leading to inefficient use of the outlet area, high back pressure and consequently a high loss coefficient. These trends are exacerbated with an increasing inlet swirl angle. The preliminary numerical predictions captured the general trends of the flow but could not capture the extent of the reversed flow, leading to over-prediction of the effective area ratio, E, and under-prediction of the loss coefficient, k.

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