Estimating Iceberg-Wave Companion Loads Using Probabilistic Methods

2015 ◽  
Author(s):  
Mark Fuglem ◽  
Paul Stuckey ◽  
Somchat Suwan
Keyword(s):  
Hydrodynamic Pressure ◽  
Offshore Structures ◽  
Probabilistic Methods ◽  
Wave Loads ◽  
Wave Loading ◽  
Wave Load ◽  
Sea State ◽  
Load Factors ◽  
Induced Velocity ◽  
The Impact

Many challenges arise when designing offshore structures for iceberg loads in arctic and subarctic regions. To help the designer, the ISO 19906:2010 standard provides guidance for the calculation of design ice loads using both deterministic and probabilistic approaches. In determining design loads for different environmental factors, both principal and companion actions must be taken into account; an example is iceberg actions and companion wave actions. ISO 19906 allows the designer to calculate the companion wave action as a specified fraction (combination factor) of the extreme level (EL) design wave load. Alternatively, the designer can calculate appropriate companion wave loads explicitly. A methodology has been developed at C-CORE in which representative iceberg actions are determined using a software package, the Iceberg Load Software (ILS). This is a probabilistic tool which uses Monte Carlo simulation to obtain a distribution of global impact forces based on the expected range of iceberg and environmental conditions that a structure would likely encounter. The software provides a reasonably accurate representation of the iceberg loading situation, following the provisions of ISO 19906:2010, without introducing unnecessary conservatism in the design load. In the software, the influence of waves on the iceberg actions are considered, but companion wave loads must be calculated and added externally to the software, The software accounts for the probability of different sea state conditions and the influence of the sea state on the probability and severity of iceberg impact, given the correlations between the sea state, iceberg management effectiveness and iceberg drift and wave-induced velocity. The additional hydrodynamic pressure due to the wave during the period of the impact; is not considered. This wave loading will be complicated by the influence that the presence the iceberg and structure have on the local sea state. In this paper, brief descriptions are provided of background studies on companion wave loading and the application of companion load factors in ISO 19906. The companion load factors allow the designer to apply the design wave load, which is calculated for situations with no iceberg present, to the case of iceberg impacts. In this study, a methodology is presented for determining companion wave loads based on the distribution of sea states expected during an iceberg impact. These sea states are significantly less severe than that associated with the design wave load as iceberg impacts are rare events. The companion wave loads are determined without accounting for the influence of the iceberg; this is thought to be quite conservative. An example application of the methodology is presented for a hypothetical platform located on the Grand Banks, off the east coast of Newfoundland. Iceberg actions, wave actions and combined iceberg-wave actions are estimated using the described methodology. Comparisons are provided for the resulting companion loads and those based on ISO 19906:2010 companion load factors applied to the extreme level wave load.

System Identification of a Six-Legged Semisubmersible Subjected to Wave Loads through Frequency Domain Analysis

2013 ◽  
Vol 373-375 ◽  
pp. 770-784
Author(s):  
Guo Zheng Yew ◽  
M.S. Liew ◽  
Mohd Shahir Liew ◽  
Cheng Yee Ng
Keyword(s):  
System Identification ◽  
Transfer Function ◽  
Transfer Functions ◽  
Structural Response ◽  
Offshore Structures ◽  
Frequency Domain Analysis ◽  
Random Wave ◽  
Wave Loads ◽  
Sea State ◽  
Wave Heights

Sea state conditions such as wind, wave and current vary in different ocean waters. Two similar offshore structures installed in two different ocean regions will yield different responses. Determining the transfer function of the structure is a system identification exercise that yields the structural response and behaviour given any sea state condition. The transfer function can be determined using available measured sea state data and structural response data. In this paper, a six-legged semisubmersible physical model is developed to a scale of 1:100 and is tested in a wave tank to measure its responses due to simulated random wave loads. The transfer functions of the semisubmersible model are then determined using the measured responses and the measured wave heights.

Environmental Load Factors for Offshore Structures

1999 ◽  
Vol 121 (4) ◽  
pp. 261-267
Author(s):  
H. P. Hong ◽  
M. A. Nessim ◽  
I. J. Jordaan
Keyword(s):  
Model Uncertainty ◽  
Offshore Structures ◽  
Specific Model ◽  
Load Factor ◽  
Environmental Load ◽  
Load Effect ◽  
Environmental Loads ◽  
Load Factors ◽  
Environmental Process ◽  
The Impact

An analysis of the impact of model uncertainties on the design factors for environmental loads on offshore structures was carried out. Considering uncertainties in environmental processes, the load effect models and the member resistance, an approach was developed that gives explicit consideration to model uncertainty in codified design. For frequent environmental load effects, a two-factor approach was developed that defines the overall load factor as the product of two components: a basic factor accounting for uncertainty in the environmental process and a separate factor accounting for model uncertainty. The overall load factor is to be applied to the specified load, which is defined as the load corresponding to the environmental process value associated with a specified return period. This load can be calculated from the environmental process without considering model uncertainty. The model uncertainty factor was defined as a linear function of the mean and the standard deviation of the model uncertainty parameter. It can be estimated based on a specific model and reliability analysis. This two-factor approach has two advantages: (a) it allows for reductions in the load factor if conservative or more accurate models are used; and (b) it eliminates the need for the designer to consider model uncertainty in estimating the specified load. The approach was used to develop a set of load factors for environmental loads on offshore structures. These factors were calibrated to produce reliability levels consistent with those implied by the load factors in CSA-S471.

Efficient Computations of Second-Order Low-Frequency Wave Load

2009 ◽  
Author(s):  
Xiao-Bo Chen ◽  
Fla´via Rezende
Keyword(s):  
Low Frequency ◽  
Second Order ◽  
Wave Loads ◽  
Wave Loading ◽  
Wave Load ◽  
Frequency Wave ◽  
New Developments ◽  
Novel Method ◽  
The Difference ◽  
New Formulation

As the main source of resonant excitations to most offshore moored systems like floating LNG terminals, the low-frequency wave loading is the critical input to motion simulations which are important for the design. Further to the analysis presented by Chen & Duan (2007) and Chen & Rezende (2008) on the quadratic transfer function (QTF) of low-frequency wave loading, the new formulation of QTF is developed by the series expansion of the second-order wave loading with respect to the difference-frequency upto the order-2. It provides a novel method to evaluate the low-frequency second-order wave loads in a more accurate than usual order-0 approximation (often called Newman approximation) and more efficient way comparing to the computation of complete QTF. New developments including numerical results of different components of QTF are presented here. Furthermore, the time-series reconstruction of excitation loads in the motion simulation of mooring systems is analyzed and a new efficient and accurate scheme is demonstrated.

scholarly journals A Comparative Study of Breaking Wave Loads on Cylindrical and Conical Substructures

Water ◽  
2021 ◽  
Vol 13 (7) ◽  
pp. 924
Author(s):  
Eirinaios Chatzimarkou ◽  
Constantine Michailides
Keyword(s):  
Comparative Study ◽  
Pressure Distribution ◽  
Dynamic Pressure ◽  
Load Ratio ◽  
Navier Stokes ◽  
Breaking Wave ◽  
Wave Loads ◽  
Wave Loading ◽  
Wave Load ◽  
Sst Turbulence Model

In the present paper, a comparative study of different cylindrical and conical substructures was performed under breaking wave loading with the open-source Computational Fluid Dynamics (CFD) package OpenFoam capable of the development of a numerical wave tank (NWT) with the use of Reynolds-Averaged Navier–Stokes (RANS) equations, the k-ω Shear Stress Transport (k-ω SST) turbulence model, and the volume of fluid (VOF) method. The validity of the NWT was verified with relevant experimental data. Then, through the application of the present numerical model, the distributions of dynamic pressure and velocity in the x-direction around the circumference of different cylindrical and conical substructures were examined. The results showed that the velocity and dynamic pressure distribution did not change significantly with the increase in the substructure’s diameter near the wave breaking height, although the incident wave conditions were similar. Another important aspect of the study was whether the hydrodynamic loading or the dynamic pressure distribution of a conical substructure would improve or deteriorate under the influence of breaking wave loading compared to a cylindrical one. It was concluded that the primary wave load in a conical substructure increased by 62.57% compared to the numerical results of a cylindrical substructure. In addition, the secondary load’s magnitude in the conical substructure was 3.39 times higher and the primary-to-secondary load ratio was double compared to a cylindrical substructure. These findings demonstrate that the conical substructure’s performance will deteriorate under breaking wave loading compared to a cylindrical one, and it is not recommended to use this type of substructure.

Benchmarking of a Computational Fluid Dynamics-Based Numerical Wave Tank for Studying Wave Load Effects on Fixed and Floating Offshore Structures

2017 ◽  
Vol 139 (3) ◽  
Author(s):  
Ali Nematbakhsh ◽  
Zhen Gao ◽  
Torgeir Moan
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Offshore Structures ◽  
Numerical Results ◽  
Wave Loads ◽  
Numerical Wave Tank ◽  
Wave Load ◽  
Wave Tank ◽  
Load Effects ◽  
Numerical Wave

A computational fluid dynamics (CFD) based numerical wave tank (NWT) is developed and verified to study wave load effects on fixed and free floating offshore structures. The model is based on solving Navier–Stokes equations on a structured grid, level set method for tracking the free surface, and an immersed boundary method for studying wave–structure interaction. This paper deals with establishing and verifying a CFD-based NWT. Various concerns that arise during this establishment are discussed, namely effects of wave reflection which might affect the structure response, damping of waves in downstream, and three-dimensional (3D) effects of the waves. A method is described and verified to predict the time when incoming waves from wave generator are affected by reflecting waves from the structure which can help in better designing the dimensions of NWT. The model is then used to study sway, heave, and roll responses of a floating barge which is nonuniform in density and limited in sway direction by a spring and damper. Also, it is used to study wave loads on a fixed, large diameter, surface piercing circular cylinder. The numerical results are compared with the experimental and other numerical results, and in general very good agreement is observed in all range of studied wave frequencies. It is shown that for the studied fixed cylinder, the Morison equation leads to promising results for wavelength to diameter ratio larger than 2π (kD < 1), while for shorter wavelengths results in considerable over prediction of wave loads, due to simplification of wave diffraction effects.

Extremes of Morison-Type Wave Loading on a Single Pile

1978 ◽  
Vol 100 (1) ◽  
pp. 100-104 ◽  
Author(s):  
G. Moe ◽  
S. H. Crandall
Keyword(s):  
Unit Length ◽  
Wave Theory ◽  
Wave Force ◽  
Offshore Structures ◽  
Random Motion ◽  
Linear Wave ◽  
Wave Loading ◽  
Sea State ◽  
Steady Current ◽  
Wave Heights

A statistical estimate of the extreme wave force per unit length acting on a section of a fixed cylindrical pile in a random sea-state is derived. The random motion of the sea is described by a spectrum of wave heights in conjunction with linear wave theory. The wave force is assumed to depend linearly on the water particle acceleration and non-linearly on the water velocity according to the Morison formula. The interaction of the velocity and acceleration contributions and the contribution of a small steady current are accounted for by an asymptotic approximation valid for large forces. The expected rate of occurrences of extremes based on a simple peak definition agrees satisfactorily with a more elaborate result based on a true maximum definition. The formulas derived here provide a basis for a design-force procedure which could provide an improvement over the design-wave procedure commonly used for the analysis of offshore structures.

Design Criteria for Offshore Structures Under Combined Wind and Wave Loading

1995 ◽  
Vol 117 (1) ◽  
pp. 1-11 ◽  
Author(s):  
M. A. Nessim ◽  
H. P. Hong ◽  
V. R. Swail ◽  
C. A. Henderson
Keyword(s):  
East Coast ◽  
Offshore Structures ◽  
Environmental Data ◽  
Design Criteria ◽  
Separate Analysis ◽  
Return Periods ◽  
Wave Loading ◽  
Wave Load ◽  
Load Combination ◽  
Coast Region

Offshore codes do not give sufficient guidance regarding design criteria for loads resulting from combinations of stochastic environmental processes such as wind and waves. To assist design engineers in defining such criteria, a suite of methods that use environmental data to calculate the probability distributions of load effects resulting from combination of stochastic loads were investigated. An approach has been developed for using the results to calculate structure-specific and generalized load combination criteria. Extensive application of this approach in connection with Environment Canada’s wind and wave data bases for the Canadian East Coast region formed the basis for some interesting conclusions regarding the process of estimating combined extreme loads on offshore structures. It was found that data based on actual measurements of wave height and wind speed are preferable to hindcast data, since the latter have artificially high correlations that lead to overly conservative results. External analyses are most reliable when 20 or more years of data are used with analysis methods based on distribution tails. Reasonably good results can be achieved with 10 yr of data. Methods based on the point-in-time data and using mathematically convenient assumptions regarding distribution types and process characteristics can lead to large errors if the assumptions made are not substantiated by appropriate data. Load combination solutions are highly dependent on the geographic location and data base. Therefore, a separate analysis should be carried out for the structure and location being considered if possible. Wind and wave load combination solutions are sensitive to correlations and assumed distribution types; closed-form solutions for independent and Gaussian correlated processes can lead to significant errors. If site-specific analyses are not practical, companion action factors of 0.65, 0.60, and 0.55 for return periods of 20, 100, and 1000 yr, may be used for wind and wave loading on slender offshore structures in the Canadian East coast region. For wide structures in the same region, the suggested companion factors for the same return periods are 0.75, 0.70, and 0.65.

Conditional Stochastic Processes Applied to Wave Load Predictions

2015 ◽  
Vol 59 (01) ◽  
pp. 1-10
Author(s):  
Jørgen Juncher Jensen
Keyword(s):  
Stochastic Processes ◽  
Nonlinear Problems ◽  
Offshore Structures ◽  
Computational Time ◽  
Wave Loads ◽  
Wave Load ◽  
Parametric Rolling ◽  
Reliability Method ◽  
Wave Induced ◽  
Uniform Accuracy

The concept of conditional stochastic processes provides a powerful tool for evaluation and estimation of wave loads on ships and offshore structures. This article first considers conditional waves with a focus on critical wave episodes. Then the inherent uncertainty in the results is illustrated with an application where measured wave responses are used to predict the future variation in the responses within the next 5–30 seconds. The main part of the article is devoted to the application of the First Order Reliability Method for derivation of critical wave episodes for different nonlinear wave-induced responses. A coupling with Monte Carlo simulations is shown to be able to give uniform accuracy for all exceedance levels with moderate computational time, even for rather complex nonlinear problems. The procedure is illustrated by examples dealing with overturning of jackup rigs, parametric rolling of ships, and slamming and whipping vibrations.

scholarly journals Analysis of changes in the survivability of building structures reinforced after an emergency dynamic impact in the context of assessing the security of buildings in the oil and gas industry

2021 ◽  
Vol 8 (12) ◽  
pp. 25-35
Author(s):  
Sarkisov et al. ◽  
Keyword(s):  
Shock Wave ◽  
Oil And Gas ◽  
Energy Parameter ◽  
Oil And Gas Industry ◽  
Building Structures ◽  
Building Structure ◽  
Wave Loads ◽  
Wave Load ◽  
Short Term ◽  
The Impact

The relevance of the subject matter is conditioned by the technical complexity of the oil and gas facilities due to the increase in the volume and rate of raw materials production, which may be affected by shock-wave loads in emergency situations. The causes of the impact can be explosions, heavy cargo falls, terrorist attacks, natural and anthropogenic disasters, etc. These situations are very likely to cause significant damage to the building structures of industrial facilities, which necessitates their reinforcement. For further safe operation of the facility, reinforced structures must have survivability under repeated impacts no less than before the reinforcement. Given the fact that the survivability of buildings is a complex characteristic influenced by many factors, and it itself is a component of the security of a hazardous production facility, research in this area is topical. The purpose of the study is to test the developed method for assessing the survivability of a building structure under short-term shock-wave load based on the energy parameter and to analyze the results obtained in the context of assessing the security of critical oil and gas facilities. Research methods: Measurement of accelerations, deflections, and loads by strain measurement methods, graphoanalytical method of study using the Microsoft Excel software. A method for assessing the level of survivability of a building structure under shock-wave loading for critical oil and gas facilities using the survivability coefficient is developed. Using specific tests of conventional and cage-reinforced bending concrete elements for short-term dynamic load, the values of the specified coefficient are obtained. The values are compared and conclusions are drawn.

Adaptive Stretching of Dynamic Pressure Distribution in Long- and Short-Crested Sea States

2009 ◽  
Author(s):  
Gu¨nther Clauss ◽  
Sascha Kosleck ◽  
Florian Sprenger ◽  
Florin Boeck
Keyword(s):  
Pressure Distribution ◽  
Dynamic Pressure ◽  
Load Condition ◽  
Wave Structure ◽  
Wave Theory ◽  
Offshore Structures ◽  
Fast Fourier Transformation ◽  
Wave Loads ◽  
Sea State ◽  
Precise Knowledge

During their lifetime, marine structures and ships are frequently exposed to severe weather and rough, sometimes extreme sea states. To ensure survival, the precise knowledge of global and local loads is an inevitable integral prerequisite for the design of safe offshore structures and marine vessels. Wave-structure interaction and the associated pressure induced wave loads are key parameters for the definition of design load cases. Once the complete surrounding sea state for the identified load condition is known, the pressure induced loads can be computed by calculating the pressure distribution along the hull, using an appropriate wave theory. As 1st-order AIRY-Theory as well as 2nd- and 3rd-order STOKES-Theory excessively overestimate the dynamic pressure above still water level, especially in high wave crests, a variety of stretching terms has been applied to common wave theories to correct the pressure distribution. This paper presents a second-order stretching approach to describe the distribution of the dynamic pressure in regular wave crests. In combination with FFT (Fast Fourier Transformation), the applicability of this method can be extended to irregular sea states and even extreme waves. Calculations for several regular and irregular sea states are shown and compared to calculations with existing stretching methods. The results are validated by measurements conducted in a wave tank at the Technical University Berlin. The paper concludes with an example for the calculation of the wave induced pressure field along a ship hull operating in a short-crested, multidirectional sea state.

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