Global Structural Loads Induced by Abnormal Waves and Design Storms on a FPSO

2008 ◽  
Vol 130 (2) ◽  
Author(s):  
Nuno Fonseca ◽  
Ricardo Pascoal ◽  
C. Guedes Soares
Keyword(s):  
North Sea ◽  
Time Domain ◽  
Probability Distributions ◽  
Bending Moment ◽  
Nonlinear Effects ◽  
Large Set ◽  
Scatter Diagram ◽  
Northern North Sea ◽  
Vertical Bending Moment ◽  
Classification Society

The structural loads on a floating production, storage, and offloading platform induced by a large set of realistic (measured) abnormal waves are compared with the loads induced by “design storms” and also with the minimum values required by Classification Society rules. The design storms have a duration of 3h and correspond to the 100yr contour of the scatter diagram of the Northern North Sea. Time domain simulations are performed with a time domain seakeeping program that accounts for the most important nonlinear effects. The results are analyzed to obtain probability distributions of the sagging and hogging peaks of the vertical bending moment. Several theoretical distributions are fitted to the simulated realizations.

Global Structural Loads Induced by Abnormal Waves and Design Storms on a FPSO

2006 ◽  
Author(s):  
Nuno Fonseca ◽  
Ricardo Pascoal ◽  
Carlos Guedes Soares
Keyword(s):  
North Sea ◽  
Time Domain ◽  
Probability Distributions ◽  
Bending Moment ◽  
Nonlinear Effects ◽  
Large Set ◽  
Scatter Diagram ◽  
Northern North Sea ◽  
Vertical Bending Moment ◽  
Classification Society

The structural loads on a FPSO induced by a large set of realistic (measured) abnormal waves are compared with the loads induced by “design storms” and also with the minimum values required by Classification Society rules. The design storms have a duration of 3 hours and correspond to the 100 years contour of the scatter diagram of the Northern North Sea. Time domain simulations are performed with a time domain seakeeping program that accounts for the most important nonlinear effects. The results are analyzed to obtain probability distributions of the sagging and hogging peaks of the vertical bending moment. Several theoretical distributions are fitted to the simulated realizations.

scholarly journals Study on Longitudinal Ship Strength Caused by the Placement of Beams and Girders on Upper Deck Side

2018 ◽  
Vol 1 (2) ◽  
pp. 74-80
Author(s):  
Andi Ardianti ◽  
Andi Mursid Nugraha ◽  
Ganding Sitepu ◽  
Hamzah Hamzah ◽  
Ade Khantari ◽  
...  
Keyword(s):  
Structural Response ◽  
Bending Moment ◽  
Ship Model ◽  
Upper Deck ◽  
Deck Plate ◽  
Longitudinal Strength ◽  
Moment Load ◽  
Vertical Bending Moment ◽  
Beams And Girders ◽  
Classification Society

The location of the beam and the deck girder of the ship can be effect on it is strength especially for the longitudinal strength due to the vertical wave bending moment. The objective of this study is to know the structural response of the ship due to vertical bending moment load on hogging and sagging conditions. The analysis is carried out by using Finite Element Method so-called ANSYSTM. The results shows that the stress occurring on the ship model with deck beam above the deck plate is larger than the ship model with deck beam under the deck plate. When the load with the variated of 0.2 x moment of vertical moment load, there is an increase of stress that occurs both on the deck area about 12% while on the bottom area about 0.98%. This study also conducted a stress comparison by using analysis methods with analytical methods. The results show that by the Stress differences that occur in the structure with the longitudinal deck beam and deck girder above are 14.1% on the deck and 7.1 on the bottom. Whereas in the structure with deck longitudinal deck eam and deck girder under there is a difference of 5.7% on the deck area and 3.5% in the bottom area of the ship. The stress that occur in both models have a difference that is not too far away and still under the permisible stress by the classification society so that both can be applied to the construction of a tanker.

Longitudinal Forces and Bending Moments of a FPSO

2007 ◽  
Author(s):  
Gu¨nther F. Clauss ◽  
Andre´ Kauffeldt ◽  
Katja Jacobsen
Keyword(s):  
Time Domain ◽  
Bending Moment ◽  
Time Domain Analysis ◽  
Rogue Waves ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Neutral Axis ◽  
Water Line ◽  
Line Level ◽  
Vertical Bending Moment

For the design of FPSOs the vertical bending moment is a key parameter to ensure safe operation. If analyzed at water line level, however, the unknown influence of longitudinal forces may distort the results. Hence, a segmented FPSO model with midship force transducers at two levels is investigated in various deterministic wave sequences to identify the vertical bending moment and its associated neutral axis as well as the superimposing longitudinal forces. It is shown that the neutral axis is far below the water line level, with the consequence, that extreme cyclic loads at deck level would be expected. However, as the associated longitudinal forces — even if significant — generate a counteracting moment, this effect is largely compensated. Both, frequency- and time-domain results are presented. With frequency-domain analysis the profound data for the standard assessment of structures, concerning seakeeping behaviour, operational limitations and fatigue are obtained. In addition, time-domain analysis in real rogue waves gives indispensable data on extremes, i.e. motions and structural forces.

‘Wet Handshake’: Workability Study of an Offshore Thruster Exchange Operation

2011 ◽  
Author(s):  
Henk Feikens ◽  
Roel Verwey ◽  
Jorrit-Jan Serraris ◽  
Rene´ Huijsmans
Keyword(s):  
Frequency Domain ◽  
Significant Influence ◽  
Time Domain ◽  
Nonlinear Effects ◽  
Domain Analysis ◽  
Scatter Diagram ◽  
Heavy Lift ◽  
Exchange Operation ◽  
Frequency Domain Approach ◽  
Workability Diagram

Thruster exchange operations are performed when an azimuth thruster on a DP operated vessel needs to be replaced for repair or maintenance purposes. At present these operations are performed either during a dry-dock call or in sheltered waters with the assistance of a shearleg or heavy lift vessel moored alongside the DP operated vessel. In order to reduce downtime of the DP operated vessel a clear trend is observed of operations being carried out more offshore and in deeper waters by means of a wet handshake between a heavy lift vessel (HLV) and a DP operated vessel. To get insight into the workability of such offshore thruster exchange operations it is important to study the dynamic interactions between the bodies involved. This paper describes the development of a methodology which accurately determines the workability of an offshore thruster exchange operation. The methodology is developed by BigLift Shipping, which has performed several thruster exchange operations in sheltered waters in the past few years. Furthermore the results of a workability analysis of a thruster exchange operation offshore Ghana are presented. In order to determine workability, first the hydrodynamic behavior including interaction effects of the coupled configuration of the DP operated vessel, the heavy lift vessel and the azimuthing thruster is calculated for a range of seastates, which results in the response spectra of the vessels. Then for each seastate the response is compared with the defined operational criteria to determine whether this seastate is workable, critical or non-workable, which results in a workability diagram. The workability can be quantified by combining this workability diagram with a wave-scatter diagram of the location of interest. In general this is a thoroughly studied topic. In order to achieve a higher level of accuracy of the workability prediction the focus of the methodology described in this paper has been pointed on two aspects that can be of significant influence. • First the persistency of a certain seastate is investigated. In the approach described above any variation of the environmental conditions over the duration of the operation is not considered. The effect of changing weather in time can be of significant influence on the overall workability. In order to take into account persistency information a numerical dataset of 3 hours statistics over a period of 10 years is used. Combining this information with the duration of all consecutive steps of the operation, results in a more realistic workability prediction. Similar approaches in seakeeping of ships are shown by Dallinga et al. (2004) [1], Naito et al. (2006) [2]. • Secondly the influence of a frequency domain approach compared to a time domain approach is analyzed. The nonlinear effects that occur in the coupled configuration of multiple bodies can not be taken into account in the frequency domain approach. Therefore the motion analysis is performed in the time domain. As an example case a thruster exchange operation offshore Ghana has been studied. The results of the study show that the workability prediction based on scatter diagram metocean data is influenced substantially when persistency information of metocean data is taken into account. The effect of the duration and criteria of independent operational steps on the workability are clearly visible in the persistency approach. This enables a to-the-point approach in improving the workability. Although time domain analysis is taking into account nonlinear effects, the difference between time and frequency domain analysis can be neglected for the presently studied configuration.

Time Domain Comparison With Experiments for Ship Motions and Structural Loads on a Container Ship in Abnormal Waves

2011 ◽  
Author(s):  
Suresh Rajendran ◽  
Nuno Fonseca ◽  
C. Guedes Soares ◽  
Gu¨nther F. Clauss ◽  
Marco Klein
Keyword(s):  
Time Domain ◽  
Bending Moment ◽  
Green Water ◽  
Ship Motions ◽  
Container Ship ◽  
Wave Elevation ◽  
Strip Theory ◽  
The Time Domain ◽  
Vertical Bending Moment ◽  
Comparison With Experiments

The paper presents experimental results from model tests with a containership advancing in abnormal wave conditions and comparisons with numerical simulations. A nonlinear time domain method based on strip theory is used for the calculation of vertical ship responses induced by abnormal waves. This code combines the linear diffraction and radiation forces with dominant nonlinear forces associated with vertical response arising from Froude-Krylov forces, hydrostatic forces and shipping of green water. The time domain simulations are compared directly with experimental records from tests with a model of a container ship in deterministic waves for a range of Froude numbers. Extreme sea conditions were replicated by the reproduction of realistic abnormal waves like the New Year Wave and abnormal wave from North Alwyn. Head sea condition is considered and the comparisons include the wave elevation, the vertical motions of the ship and the vertical bending moment at midship.

Short Term Distribution of Loads Acting on a Cruise Vessel in Extreme Seas Using a Body Nonlinear Time Domain With Second Order Froude-Krylov Pressure

2015 ◽  
Author(s):  
Suresh Rajendran ◽  
Nuno Fonseca ◽  
C. Guedes Soares
Keyword(s):  
Free Surface ◽  
Time Domain ◽  
Surface Condition ◽  
Bending Moment ◽  
The Body ◽  
Short Term ◽  
Weakly Nonlinear ◽  
Strip Theory ◽  
Vertical Bending Moment ◽  
Nonlinear Time Domain

Short term probability distribution of the vertical bending moment acting on a cruise vessel in extreme seas is calculated using a body nonlinear time domain method based on strip theory. The hydrodynamic forces are calculated for the exact wetted surface area under the incident wave profile. The incident potential satisfies the weakly nonlinear free surface condition based on the Stokes expansion. The disturbance potential satisfies the linear free surface and body boundary conditions. Certain practical engineering techniques are employed for the calculation of the body nonlinear forces. The statistics and the probability of distribution of the numerical vertical bending moment are compared with the experimental results measured in the wave tank.

Systematic Investigation of Loads and Motions of a Bulk Carrier in Extreme Seas

2009 ◽  
Author(s):  
Gu¨nther F. Clauss ◽  
Andre´ Kauffeldt ◽  
Marco Klein
Keyword(s):  
Time Domain ◽  
Bending Moment ◽  
Rogue Waves ◽  
Frequency Domain Analysis ◽  
Systematic Investigation ◽  
The European Union ◽  
Bulk Carrier ◽  
Precise Knowledge ◽  
Response Amplitude Operators ◽  
Vertical Bending Moment

During their lifetime ships often operate in severe weather and rough sea states. To ensure survival, a precise knowledge of global and local loads is an inevitable integral part for a safe design. One of the key parameters for ship design is the vertical bending moment. Not only vertical forces but also longitudinal forces can contribute to this bending moment. As the overall effect of longitudinal forces is still not fully understood, all structural loads are investigated in detail, especially in extreme seas. Within the project “Handling Waves”, funded by the European Union, three segmented ships, equipped with force transducers, are investigated systematically at several cruising speeds and in various deterministic wave sequences to identify structural loads, i.e. the vertical bending moment as well as the superimposing longitudinal forces. Within this paper a detailed overview of the results for the bulk carrier is given and both, frequency- and time-domain results are presented. With Response Amplitude Operators (RAOs), delivered by frequency-domain analysis, the profound data for the standard assessment of structures, concerning seakeeping behaviour, operational limitations and fatigue are obtained. In addition, time domain analyses in rogue waves such as the so called “New Year Wave” provide essential data for extreme motions and structural loads. Former investigations on a FPSO revealed that, due to the location of the neutral axis, the longitudinal forces are significant and generate a counteracting moment compared to the vertical bending moment [1]. The new results show to what extent the above mentioned conclusions are applicable for various hull designs.

Analysis of Riser Measurements in the North Sea

1984 ◽  
Vol 28 (04) ◽  
pp. 272-281
Author(s):  
P. H. J. Verbeek
Keyword(s):  
North Sea ◽  
Active Zone ◽  
Probability Distributions ◽  
Bending Moment ◽  
The North ◽  
Simulation Techniques ◽  
Theoretical Predictions ◽  
Solution Methods ◽  
The North Sea ◽  
Non Gaussian

Riser measurements in the field have provided a variety of data collected from several locations in the North Sea. Data were obtained by means of instrumented riser joints installed in both a 24-in. and a 16-in. drilling riser. Bending moments in the riser, riser tensions, motions of the semisubmersible, and wave heights were recorded simultaneously. The measurement program was aimed at verifying predictions from theoretical riser models in the typical regions of riser response; namely, the wave-active zone at the top, the "boundary layer" at the bottom, and the riser main section in between. In this paper attention is focused on the data obtained for riser response in the wave-active zone. Measured results are compared with theoretical predictions based on both analytical solution methods and numerical time-domain simulation techniques. In particular, the relationship between the standard deviation of riser bending response and significant wave height is discussed, as is the probabilistic nature of riser response. It is concluded that standard deviations of measured bending moment response agree well with theoretical results. The probability distributions of measured response were found to be non-Gaussian and consistent with theoretical predictions based on Morison-type wave loading.

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