Airgap Statistics for a Tension Leg Platform

2012 ◽  
Author(s):  
Oleg Gaidai ◽  
Arvid Naess ◽  
Carl Trygve Stansberg
Keyword(s):  
Safety Issue ◽  
Extreme Value Statistics ◽  
Critical Parameters ◽  
Offshore Platforms ◽  
Random Waves ◽  
Tension Leg Platform ◽  
Wave Impact ◽  
Large Wave ◽  
Norwegian Sea ◽  
Wave Basin

The paper discusses a method for estimating extreme value statistics of the airgap for floating offshore platforms subjected to random events. Extensive model tests were performed in a large wave basin for a TLP (Tension Leg Platform) operating in the Norwegian Sea. Among several critical parameters, the airgap was measured at a number of locations under the platform deck. The wave in deck impact is a critical safety issue with respect to the deck damage and occurrence of extreme tether tensions. The authors have utilized experimental data to look at critical airgaps under the deck in random waves. Conclusions are drawn about extreme airgap statistics, and consequently about the wave impact probability in severe seas. This paper can be seen as continuing the efforts in the challenging study of the airgap issue, [2, 6–10].

Airgap Statistics for a Tension Leg Platform

2014 ◽  
Vol 137 (1) ◽  
Author(s):  
Oleg Gaidai ◽  
Arvid Naess ◽  
Carl Trygve Stansberg
Keyword(s):  
Safety Issue ◽  
Extreme Value Statistics ◽  
Critical Parameters ◽  
Offshore Platforms ◽  
Random Waves ◽  
Tension Leg Platform ◽  
Wave Impact ◽  
Large Wave ◽  
Wave Basin ◽  
Extreme Response

The paper discusses a method for estimating extreme value statistics of the airgap for floating offshore platforms subjected to random seas. It is an adaptation of a recently developed method, which is based on the mean upcrossing rate (MUR) function for univariate time series combined with an optimization procedure that allows prediction at extreme response levels by extrapolation. Extensive model tests were performed in a large wave basin for a tension leg platform (TLP) operating in the Norwegian Sea. Among several critical parameters, the airgap was measured at a number of locations under the platform deck. The wave in deck impact is a critical safety issue with respect to the deck damage and occurrence of extreme tether tensions. The authors have utilized experimental data to look at critical airgaps under the deck in random waves. Conclusions are drawn about extreme airgap statistics, and consequently about the wave impact probability in severe seas.

Statistics of Extreme Events in Airgap Measurements

2009 ◽  
Vol 131 (4) ◽  
Author(s):  
A. Naess ◽  
C. T. Stansberg ◽  
O. Gaidai ◽  
R. J. Baarholm
Keyword(s):  
Extreme Values ◽  
Optimization Procedure ◽  
Extreme Value Statistics ◽  
Alternative Methods ◽  
Scale Model ◽  
Random Waves ◽  
Mean Values ◽  
Fitting Procedure ◽  
Wave Basin ◽  
Extreme Response

The paper presents a study of the extreme value statistics related to airgap measurements on a scale model of a semisubmersible platform subjected to random waves in a wave basin. Relative wave elevation records corresponding to totally 24 h storm duration are considered, made up by 8×3 h realizations. The focus is on a comparison of two alternative methods for the prediction of extreme values from finite recordings at two different locations at the platform. One is a standard method used in the wave basin, making use of a Weibull-tail fitting procedure. The other is a novel method based on the level upcrossing function combined with an optimization procedure that allows prediction at extreme response levels. Similar results are obtained in the mean values by the two methods, while the latter shows less variability in the predictions from single 3 h records.

Statistics of Extreme Events in Airgap Measurements

2008 ◽  
Author(s):  
A. Naess ◽  
C. T. Stansberg ◽  
O. Gaidai ◽  
R. J. Baarholm
Keyword(s):  
Extreme Values ◽  
Optimization Procedure ◽  
Extreme Value Statistics ◽  
Alternative Methods ◽  
Scale Model ◽  
Random Waves ◽  
Mean Values ◽  
Fitting Procedure ◽  
Wave Basin ◽  
Extreme Response

The paper presents a study of the extreme value statistics related to airgap measurements on a scale model of a semisubmersible platform subjected to random waves in a wave basin. Relative wave elevation records corresponding to totally 24 hours storm duration are considered, made up by 8 × 3-hours realizations. The focus is on a comparison of two alternative methods for the prediction of extreme values from finite recordings at two different locations at the platform. One is a standard method used in the wave basin, making use of a Weibull-tail fitting procedure. The other is a novel method based upon the level up-crossing function combined with an optimization procedure that allows prediction at extreme response levels. Similar results are obtained in the mean values by the two methods, while the latter shows less variability in the predictions from single 3-hours records.

Green Water Flow on a Fixed Model Structure in a Large Wave Basin Under Random Waves

2018 ◽  
Author(s):  
Wei-Liang Chuang ◽  
Kuang-An Chang ◽  
Richard Mercier
Keyword(s):  
Velocity Distribution ◽  
Flow Behavior ◽  
Green Water ◽  
Wave Crest ◽  
Breaking Wave ◽  
Model Structure ◽  
Random Waves ◽  
Large Wave ◽  
Geometry Model ◽  
Wave Basin

Green water generated by random waves on a fixed, simplified geometry model structure was measured in a large wave basin. The velocity field of the flow that is aerated and highly turbulent was quantified using the bubble image velocimetry (BIV) technique. BIV utilizes shadow textures created by air-water interfaces as tracers in backlit images recorded by a high speed camera. The tracers in consecutive images are then cross-correlated to obtain the corresponding two-dimensional velocities. Random waves were generated by the JONSWAP spectrum with a significant wave height close to the freeboard. An image-based triggering method was employed to detect the green water events and trigger image acquisition. A total of 179 green water events were collected and categorized into three different types, based on the flow behavior. That includes the collapse of overtopping wave, fall of bulk water, and breaking wave crest. Statistical distributions of maximum green water velocities under random waves were developed, while the lognormal distribution was found as the best fit. By modeling the green water as a dam break flow, the Ritter solution was found to be able to capture the horizontal velocity distribution for the random green water events. A prediction equation for the green water velocity distribution under random waves was also obtained.

Spectral Analysis of Irregular Wave Impact on the Structure in Splash Zone

2002 ◽  
Author(s):  
Bing Ren ◽  
Yongxue Wang
Keyword(s):  
Spectral Analysis ◽  
Wave Height ◽  
Incident Wave ◽  
Impact Pressure ◽  
Splash Zone ◽  
Spectral Moment ◽  
Wave Impact ◽  
Large Wave ◽  
Irregular Wave ◽  
The Impact

The spectral analysis from experimental data of irregular wave impact on the structures with large dimension in the splash zone is presented. The experiments were conducted in the large wave-current tank in the State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology. In the experiment, the target spectrum is JONSWAP spectrum, the significant wave height H1/3 is in the range from 0.1m to 0.3m, and the peak period of spectrum Tp in the range from 1.0s to 2.0s. The ratio of s/H1/3, which refers to the clearance of the subface of the structure above still water level (s) to the incident wave height, is between −0.1 and 0.4. The spectral analysis results of the irregular wave impact pressure on the subface of the structure under various case studies are presented. The distribution of spectral moment of the impact pressure on the structure along the subface is given. And the influence of different incident wave parameters and relative clearance s/H1/3 on the average spectral moment of impact pressure are discussed.

Experimental Study of Slow-Drift Ship Motions in Shallow Water Random Waves

2011 ◽  
Author(s):  
Carl Trygve Stansberg ◽  
Trygve Kristiansen
Keyword(s):  
Spectral Analysis ◽  
Shallow Water ◽  
Transfer Functions ◽  
Second Order ◽  
Ship Motions ◽  
Random Wave ◽  
Random Waves ◽  
Low Frequencies ◽  
Wave Basin ◽  
Drift Forces

Slowly varying motions and drift forces of a large moored ship in random waves at 35m water depth are investigated by an experimental wave basin study in scale 1:50. A simple horizontal mooring set-up is used. A second-order wave correction is applied to minimize “parasitic” long waves. The effect on the ship motion from the correction is clearly seen, although less in random wave spectra than in pure bi-chromatic waves. Empirical quadratic transfer functions (QTFs) of the surge drift force are found by use of cross-bi-spectral analysis, in two different spectra have been obtained. The QTF levels increase significantly with lower wave frequencies (except at the diagonal), which is special for finite and shallow water. Furthermore, the QTF levels frequencies at low frequencies increase significantly out from the QTF diagonal. Thus Newman’s approximation should preferrably not be used in these cases. Using the LF waves as a direct excitation in a “linear” ship force analysis gives random records that compare reasonably well with those from the cross-bi-spectral analysis. This confirms the idea that the drift forces in shallow water are closely correlated to the second-order potential, and thereby by the second-order LF waves.

Wave Impact Load and Corresponding Nonlinear Response of a Semi-Submersible

2019 ◽  
Author(s):  
Yinghao Guo ◽  
Longfei Xiao ◽  
Handi Wei ◽  
Lei Li ◽  
Yanfei Deng
Keyword(s):  
Spatial Distribution ◽  
Structural Integrity ◽  
Impact Load ◽  
Wave Crest ◽  
Impulsive Effect ◽  
Offshore Platforms ◽  
Wave Impact ◽  
Global Response ◽  
Local Wave ◽  
Incident Waves

Abstract Offshore platforms operating in harsh ocean environments often suffer from severe wave impacts which threaten the structural integrity and staffs safety. An experimental study was carried out to investigate the wave impact load and its effect on the global response of a semi-submersible. First, two typical wave impact events occurring successively in the wave test run are analyzed, including the characteristics of incident waves, relative wave elevations and the spatial distribution of the wave impact load. Subsequently, the corresponding global response of the semi-submersible under these two wave impacts are investigated in time domain. It reveals that compared with the incident wave, the relative wave elevation has a more straightforward relationship with the wave impact load. The relative wave crest height is associated with the spatial distribution of the wave impact load, while the local wave steepness matters more in the magnitude of the wave impact load. The impulsive effect of the wave impact load on the motion behaviors is not obvious. But severe wave impacts can introduce excessive horizontal accelerations and nonlinear behaviors like ringing in the acceleration response.

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