Recalibration of partial load factors in the Canadian offshore structures standard CAN/CSA-S471

2004 ◽  
Vol 31 (4) ◽  
pp. 684-694
Author(s):  
M A Maes ◽  
S Abdelatif ◽  
R Frederking
Keyword(s):  
Offshore Structures ◽  
Process Models ◽  
Pulse Load ◽  
Load Effect ◽  
Load Models ◽  
Code Calibration ◽  
Load Factors ◽  
Partial Load ◽  
Environmental Events ◽  
Load Types

The present paper describes a recalibration of the loading side of all the design check equations in the Canadian offshore structures standard CAN/CSA-S471, General requirements, design criteria, the environment, and loads (offshore structures). The recalibration was prompted by concerns about changing or improved load–load effect models and new load types and by Canada's intention to harmonize its offshore standards with the new International Organization for Standardization (ISO) offshore codes in the near future. Calibration is performed over wide ranges of combinations consistent with the normal application scope of CAN/CSA-S471. Updated load models are based on a more refined zonation of operational loads into loads of short duration and slowly varying live loads. Frequent environmental load processes and operational loads are modeled using Ferry-Borges–Castanheta pulse load models and infrequent environmental events and are based on point process models. The calibration is performed using a nonlinear optimization of an upwardly restrained safety objective function to result in optimal load factors, companion and combination factors, and optimal specified exceedance probabilities for infrequent load processes.Key words: load combinations, code calibration, pulse load models, safety factors, reliability levels.

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.

Reliability-Based Criteria for Fixed Steel Offshore Platforms

1997 ◽  
Vol 119 (2) ◽  
pp. 120-124 ◽  
Author(s):  
M. Efthymiou ◽  
J. W. van de Graaf ◽  
P. S. Tromans ◽  
I. M. Hines
Keyword(s):  
Decision Making ◽  
Structural Reliability ◽  
Offshore Structures ◽  
Sufficient Accuracy ◽  
Design Criteria ◽  
Offshore Platforms ◽  
Reliability Models ◽  
Reliability Level ◽  
Load Factors ◽  
Partial Load

The key issue addressed in this paper is the accuracy of structural reliability models for the case of fixed steel offshore structures under extreme storm loading. The emphasis is on engineering accuracy for the purpose of use in decision-making, and more specifically to achieve sufficient accuracy to enable the use of reliability models in deriving design criteria for fixed offshore platforms. These reliability models are used to derive partial load factors for use in conjunction with API LRFD to achieve a target reliability level appropriate for permanently manned installations. These load factors are location-dependent. Further load factors are proposed for the design of new, not normally manned installations.

scholarly journals Code Calibration of the Eurocodes

2021 ◽  
Vol 11 (12) ◽  
pp. 5474
Author(s):  
Tuomo Poutanen
Keyword(s):  
High Reliability ◽  
High Accuracy ◽  
Variable Load ◽  
Reliability Model ◽  
Safety Factors ◽  
Rounding Errors ◽  
Code Calibration ◽  
Load Factors ◽  
Characteristic Values

This article addresses the process to optimally select safety factors and characteristic values for the Eurocodes. Five amendments to the present codes are proposed: (1) The load factors are fixed, γG = γQ, by making the characteristic load of the variable load changeable, it simplifies the codes and lessens the calculation work. (2) Currently, the characteristic load of the variable load is the same for all variable loads. It creates excess safety and material waste for the variable loads with low variation. This deficiency can be avoided by applying the same amendment as above. (3) Various materials fit with different accuracy in the reliability model. This article explains two options to reduce this difficulty. (4) A method to avoid rounding errors in the safety factors is explained. (5) The current safety factors are usually set by minimizing the reliability indexes regarding the target when the obtained codes include considerable safe and unsafe design cases with the variability ratio (high reliability/low) of about 1.4. The proposed three code models match the target β50 = 3.2 with high accuracy, no unsafe design cases and insignificant safe design cases with the variability ratio 1.07, 1.03 and 1.04.

Reliability Calibration of a Waves–Icebergs Interaction Load Combination Factor

2004 ◽  
Author(s):  
Ricardo O. Foschi ◽  
Michael Isaacson ◽  
Norman Allyn
Keyword(s):  
Numerical Analysis ◽  
Offshore Structures ◽  
Combined Effects ◽  
Limit States ◽  
Sea State ◽  
Load Effect ◽  
Design Load ◽  
Load Combination ◽  
Wave Parameters ◽  
Design And Construction

The Canadian Standards Association [1] has developed and published a code for the design and construction of fixed offshore structures. One of the limit states relates to the combined effects of waves and iceberg collision loading. The Code uses a load combination factor to determine the design load effect. The present paper describes a recent study on the appropriateness of the recommended value of the combination factor. The study involves a numerical analysis in which loads have been calculated, at different probability levels, for a range of iceberg and wave parameters, considering waves alone, an iceberg alone, and an iceberg and waves in combination. The paper thereby makes recommendations for the load combination factor as a function of iceberg and sea state parameters.

EXTREME ENVIRONMENTAL LOAD MODELS FOR WAVE AND CURRENT RESPONSE SURFACE METHOD

2016 ◽  
Vol 3 (2) ◽  
Author(s):  
Zafarullah Nizamani ◽  
Yap Eng Ching ◽  
Mohamed Mubarak Bin Abdul Wahab ◽  
Abdullateef Olanrewaju
Keyword(s):  
Response Surface ◽  
Wave Height ◽  
Limit State ◽  
Offshore Structures ◽  
State Function ◽  
Current Response ◽  
Environmental Load ◽  
Local Conditions ◽  
Load Model ◽  
Load Models

The three most uncertain environmental design loads acting on offshore structures are wave height, current and wind velocity. If the reliability of structure is to be determined, then we need to have limit state function for load which requires that we should transform the environmental loads into load model. Load model, which predicts the load, will ultimately affect the design resistance and thus its final impact on cost could be large. Since, there are many offshore structures located in different offshore regions in Malaysia, the calibration of the environmental load model, should be evaluated to determine which model fits best. To obtain the environmental load model, response surface technique is generally applied. Load models suggested by DNV and ISO code are analysed to determine the best model fit for local conditions of Malaysia. Base shear, wave height and current velocity are used in a linear fit to determine polynomial response surface. The results showed that due to the geography of Malaysia different regions might have to use specific load models instead of a general load model for all regions.

Environmental Load Effect Analysis of Guyed Towers

1985 ◽  
Vol 107 (1) ◽  
pp. 24-33 ◽  
Author(s):  
O. Mo ◽  
T. Moan
Keyword(s):  
Time Domain ◽  
Structural Model ◽  
Dimensional Space ◽  
Time Integration ◽  
Offshore Structures ◽  
Random Waves ◽  
Significant Discrepancy ◽  
Effect Analysis ◽  
Load Effect ◽  
Fluid Field

A general method for dynamic load effect analysis of slender offshore structures subjected to short crested random waves, current and wind, is given. The structure is represented by a three-dimensional space frame model utilizing dash-pots and linear or nonlinear spring elements to represent guy lines and coupling between structure and foundation. The component mode synthesis formulation is adopted for reduction of the number of degrees of freedom. The hydrodynamic forces are computed by Morison’s equation, accounting for finite wave elevation, directionality, and relative fluid-structure motion. Various kinematic models for the fluid field in the splashing zone are compared. To get a reasonable representation of nonlinearities in the loading and the structural model, a Monte Carlo approach is adopted. Starting with simulated samples of the random fluid field and wind forces, time series of structural responses are found by numerical time integration utilizing the Newmark β-family of time integration operators. Numerical results for a guyed tower at 450-m water depth are presented. The statistical uncertainties associated with the stochastic time domain simulations are discussed. A significant discrepancy is found between linearized frequency domain solutions and the present nonlinear time domain formulation. The importance of an adequate representation of superharmonic responses is particularly discussed. The differences in results due to various solution methods are found to vary significantly with sea-state conditions.

Characteristic Levels of Strongly Nonlinear Extreme Wave Load Effects

2016 ◽  
Author(s):  
Thomas B. Johannessen ◽  
Øistein Hagen
Keyword(s):  
Wave Breaking ◽  
Nonlinear Problems ◽  
Offshore Structures ◽  
Short Term ◽  
Nonlinear Load ◽  
Load Effect ◽  
Strongly Nonlinear ◽  
Load Effects ◽  
Term Analysis

Offshore structures are typically required to withstand extreme and abnormal load effects with annual probabilities of occurrence of 10−2 and 10−4 respectively. For linear or weakly nonlinear problems, the load effects with the prescribed annual probabilities of occurrence are typically estimated as a relatively rare occurrence in the short term distribution of 100 year and 10 000 year seastates. For strongly nonlinear load effects, it is not given that an extreme seastate can be used reliably to estimate the characteristic load effect. The governing load may occur as an extremely rare event in a much lower seastate. In attempting to model the load effect in an extreme seastate, the short term probability level is not known nor is it known whether the physics of the wave loading is captured correctly in an extreme seastate. Examples of such strongly nonlinear load effects are slamming loads on large volume offshore structures or wave in deck loads on jacket structures subject to seabed subsidence. Similarly, for structures which are unmanned in extreme weather, the governing load effects for the manned structure will occur as extremely rare events in a relatively frequent seastate. The present paper is concerned with the long term distribution of strongly nonlinear load effects. Using a simple point estimate of the wave elevation correct to second order and a crest kinematics model which takes into account the possibility of wave breaking, the long term distribution of drag load on a column above the still water level is studied and compared with a similar loading model based on second order kinematics which does not include the effect of wave breaking. The findings illustrate the challenges listed above. Model tests are useful in quantifying strongly nonlinear load effects which cannot be calculated accurately. But only a relatively small number of seastates can be run in a model test campaign and it is not feasible to estimate short term responses far beyond the three hour 90% fractile level. Similarly, Computational Fluid Dynamics (CFD) is increasingly useful in investigating complex wave induced load effects. But only a relatively small number of wave events can be run using CFD, a long term analysis of load effects cannot in general be carried out. It appears that there is a class of nonlinear problems which require a long term analysis of the load effect in order for the annual probability of occurrence to be estimated accurately. For problems which cannot be estimated by simple analytical means, the governing wave events can be identified by long term analysis of a simple model which capture the essential physics of the problem and then analysed in detail by use of CFD or model tests.

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