Long-Term Fatigue and Extreme Design of Steel Risers

2009 ◽  
Author(s):  
Michele A. L. Martins ◽  
Andre´ S. Do´ria ◽  
Eduardo S. S. Silveira
Keyword(s):  
Limit State ◽  
Joint Probability ◽  
Offshore Structures ◽  
Joint Probability Distribution ◽  
Full Time ◽  
Extreme Statistics ◽  
Limit States ◽  
Sea State ◽  
Zero Crossing

Environmental loads are the main source of dynamic excitation of offshore structures. Due to the random nature of these loads, one should consider statistical properties when designing such structures. Recent codes for the design of steel risers address the use of environmental contours or full long-term analysis to take into account the statistics of load effects. Moreover, sea states for fatigue design should also be carefully selected so as to include the region of the scatter diagram which most contributes to the total damage. This paper looks into fatigue and ultimate limit states of steel risers. The first part shows a comparative study of three design criteria for extreme statistical analysis. These criteria are based on design storm, environmental contour, and full long-term statistics. A joint probability distribution found in the literature for a Brazilian location was used to describe the correlation between the significant wave height and the zero crossing wave period. The second part deals with the analysis of the coefficient of contribution for fatigue and long-term extreme statistics. Several full time domain analyses were performed and the most important sea state region for each limit state is shown in two examples of steel catenary risers. A discussion about sea state selection is then presented.

scholarly journals Probability Calibration Of Load Duration Modification Factor For Timber Roofs In The Polish Mountain Zones

2014 ◽  
Vol 60 (2) ◽  
pp. 195-208
Author(s):  
T. Domański
Keyword(s):  
Limit State ◽  
High Stress ◽  
Life Time ◽  
Probabilistic Methods ◽  
Limit States ◽  
Term Limit ◽  
Load Duration ◽  
Maximum Likelihood Methods ◽  
Short And Long Term

Abstract The resistance parameters of timber structures decrease with time. It depends on the type of load and timber classes. Strength reduction effects, referred to as creep-rupture effects, due to long term loading at high stress ratio levels are known for many materials. Timber materials are highly affected by this reduction in strength with duration of load. Characteristic values of load duration and load duration factors are calibrated by means of using probabilistic methods. Three damage accumulation models are considered, that is Gerhard [1] model, Barret, Foschi[2] and Foshi Yao [3] models. The reliability is estimated by means of using representative short- and long-term limit states. Time variant reliability aspects are taken into account using a simple representative limit state with time variant strength and simulation of whole life time load processes. The parameters in these models are fitted by the Maximum Likelihood Methods using the data relevant for Polish structural timber. Based on Polish snow data over 45 years from mountain zone in: Zakopane – Tatra, Świeradów – Karkonosze, Lesko – Bieszczady, the snow load process parameters have been estimated. The reliability is evaluated using representative short – and long –term limit states, load duration factor kmod is obtained using the probabilistic model.

Reliability of TLP Tendons Under Storm Sea States

2005 ◽  
Author(s):  
Federico Barranco Cicilia ◽  
Edison Castro Prates de Lima ◽  
Lui´s Volnei Sudati Sagrilo
Keyword(s):  
Reliability Analysis ◽  
Limit State ◽  
Nonlinear Dynamic Analysis ◽  
Analytical Models ◽  
Cumulative Probability ◽  
Wave Forces ◽  
Ultimate Limit State ◽  
Sea State ◽  
Extreme Response

This paper presents a methodology for reliability analysis of Tension Leg Platform (TLP) tendons subjected to extraordinary sea state conditions like hurricanes or winter storms. A coupled approach in time domain is used to carry out TLP random nonlinear dynamic analysis including wind, current and first and second order wave forces. The tendons Ultimate Limit State (ULS) condition is evaluated by an Interaction Ratio (IR) taking into account dynamic combination among tension, bending and hydrostatic pressure. Expected long-term extreme IR is obtained through the integration of cumulative probability functions (CPFs) fitted to response maxima associated to individual short term sea states. The reliability analysis is performed using a time-integrated scheme including uncertainties in loads, tendon strength, and analytical models. Failure probabilities for the most loaded tendon of a TLP in Campeche Bay, Mexico, considering a 100-yr design sea state and the 100-yr extreme response generated by long-term observed storms are compared.

scholarly journals Probability calibration of deformation factor for timber roofs in the Polish mountain zones

2018 ◽  
Vol 163 ◽  
pp. 07003
Author(s):  
Tomasz Domański ◽  
Kamil Kmiecik
Keyword(s):  
Limit State ◽  
High Stress ◽  
Life Time ◽  
Probabilistic Methods ◽  
Limit States ◽  
Term Limit ◽  
Load Duration ◽  
Maximum Likelihood Methods ◽  
Short And Long Term

The resistance parameters of timber material structures decrease with time, depending on the type of load and timber classes. Strength and Modulus of Elasticity reduction effects, referred to as creeprupture effects, due to long term loading at high stress ratio levels are known for many materials. Timber materials are highly affected by this reduction in strength and deflection with duration of load. Characteristic values of load duration and deformation factors are calibrated by means of using probabilistic methods. The reliability is estimated by means of using representative short-and long-term limit states. Time variant reliability aspects are taken into account using a simple representative limit state with time variant strength and simulation of whole life time load processes. The parameters in these models are fitted by the Maximum Likelihood Methods using the data relevant for Polish structural timber . Based on Polish snow data over 45 years from mountain zones in: Zakopane – Tatra, Świeradów – Karkonosze, Lesko – Bieszczady, the snow load process parameters have been estimated. The reliability is evaluated using representative short – and long –term limit states. The deformation factor kdef is obtained using the probabilistic model.

scholarly journals Reliability Updating of Offshore Structures Subjected to Marine Growth

Energies ◽  
2022 ◽  
Vol 15 (2) ◽  
pp. 414
Author(s):  
Franck Schoefs ◽  
Thanh-Binh Tran
Keyword(s):  
Limit State ◽  
Dynamic Bayesian Network ◽  
Offshore Structures ◽  
Drag Forces ◽  
The North ◽  
The North Sea ◽  
Inspection And Maintenance ◽  
Inspection Data ◽  
Atlantic Area

Marine growth is a known problem for oceanic infrastructure and has been shown to negatively impact the reliability of bottom-fixed or floating offshore structures submitted to fatigue or extreme loading. Among other effects, it has been shown to change drag forces by increasing member diameters and modifying the roughness. Bio-colonization being highly random, the objective of this paper is to show how one-site inspection data increases reliability by decreasing uncertainties. This can be introduced in a reliability-based inspection framework for optimizing inspection and maintenance (here, cleaning). The modeling and computation are illustrated through the reliability analysis of a monopile in the European Atlantic area subjected to marine growth and according to the plastic collapse limit state. Based on surveys of structures in the North Sea, long-term stochastic modeling (space and time) of the marine growth thickness is first suggested. A Dynamic Bayesian Network is then developed for reliability updating from the inspection data. Finally, several realistic (10–20 measurements) inspection strategies are compared in terms of reliability improvement and the accuracy of reliability assessment.

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.

Comparative and Uncertainty Assessment of Design Criteria for Stiffened Panels

2004 ◽  
Vol 48 (03) ◽  
pp. 231-247
Author(s):  
Ibrahim A. Assakkaf ◽  
Bilal M. Ayyub
Keyword(s):  
Failure Modes ◽  
Prediction Models ◽  
Limit State ◽  
Uncertainty Assessment ◽  
Offshore Structures ◽  
Paper Strength ◽  
Limit States ◽  
Ship Structures ◽  
Steel Panels ◽  
Strength Models

Stiffened and gross steel panels (plates) are very important components in ship and offshore structures, and therefore they should be designed for a set of failure modes that govern their strength. They form the backbone of most ships' structure, and they are by far the most commonly used element in a ship. They can be found in bottom structures, decks, side shell, and superstructures. To evaluate the strength of a stiffened or gross panel element, it is necessary to review various strength-predicting models and to study their biases, applicability, and limitations for different loading conditions acting on the element. In this paper, strength limit states for various failure modes of ship panels are presented. For each limit state, commonly used strength models were collected from many sources for evaluating their limitations and applicability and to study their biases and uncertainties. Wherever possible, the different types of biases resulting from these models were computed. The bias and uncertainty analyses for these strength models are needed for the development of load and resistance factor design (LRFD) rules for stiffened and gross panels of ship structures. The uncertainty and biases of these models were assessed and evaluated by comparing their predictions with ones that are more accurate or real values. The objective of this paper is to summarize strength prediction models of stiffened and gross panels that are suitable for LRFD development for ship structures. Monte Carlo simulation was used to assess the biases and uncertainties for these models. Recommendations for the use of the models and their biases in LRFD development are provided.

On Efficient Long-Term Extreme Response Estimation for a Moored Floating Structure

2018 ◽  
Author(s):  
HyeongUk Lim ◽  
Lance Manuel ◽  
Ying Min Low
Keyword(s):  
Wave Train ◽  
Offshore Structures ◽  
Offshore Structure ◽  
Floating Structure ◽  
Sea State ◽  
Peak Period ◽  
Frequency Wave ◽  
Extreme Response ◽  
Term Response

This study investigates the use of efficient surrogate model development with the help of polynomial chaos expansion (PCE) for the prediction of the long-term extreme surge motion of a simple moored offshore structure. The structure is subjected to first-order and second-order (difference-frequency) wave loading. Uncertainty in the long-term response results from the contrasting sea state conditions, characterized by significant wave height, Hs, and spectral peak period, Tp, and their relative likelihood of occurrence; these two variables are explicitly included in the PCE-based uncertainty quantification (UQ). In a given sea state, however, response simulations must be run for any sampled Hs and Tp; in such simulations, typically, a set of random phases (and deterministic amplitudes) define a wave train consistent with the defined sea state. These random phases for all the frequency components in the wave train introduce additional uncertainty in the simulated waves and in the response. The UQ framework treats these two sources of uncertainty — from Hs and Tp on the one hand, and the phase vector on the other — in a nested manner that is shown to efficiently yield long-term surge motion extreme predictions consistent with more expensive Monte Carlo simulations, which serve as the truth system. Success with the method suggests that similar inexpensive surrogate models may be developed for assessing the long-term response of various offshore structures.

Estimating Long-Term Extreme Slamming From Breaking Waves

2016 ◽  
Vol 138 (5) ◽  
Author(s):  
Gunnar Lian ◽  
Sverre K. Haver
Keyword(s):  
Limit State ◽  
Gumbel Distribution ◽  
Nonlinear Problems ◽  
Breaking Waves ◽  
Offshore Structures ◽  
Exceedance Probability ◽  
Ultimate Limit State ◽  
Breaking Wave ◽  
Large Variability

Characteristic loads for design of offshore structures are defined in terms of their annual exceedance probability, q. In the Norwegian Petroleum Regulations, q = 10−2 is required for the ultimate limit state (ULS), while q = 10−4 is required for the accidental limit state (ALS). In principle, a full long-term analysis (LTA) is required in order to obtain consistent estimates. This is straightforward for linear response problems, while it is a challenge for nonlinear problems, in particular if they additionally are of an on–off nature. The latter will typically be the case for loads due to breaking wave impacts. In this paper, the challenges related to estimation of characteristic slamming loads are discussed. Measured slamming loads from a model test are presented, and the observed large variability is discussed. The stochastic nature of slamming loads is studied using a simplified linear relation between the sea states and the Gumbel distribution parameter surfaces. The characteristic slamming loads with q-annual probability of exceedance are estimated from an LTA using the short-term distribution of the slamming loads and the long-term distribution of the sea states. The effect of integrating over a smaller area of the scatter diagram of the sea states is studied. The uncertainties in response from slamming loads are compared to a more common response process, and the relation between variability and the number of realizations in each sea state is looked into.

Efficient Evaluation of Long-Term Response for Design of Components of Offshore Structures

2017 ◽  
Author(s):  
Paulo Mauricio Videiro ◽  
Luis Volnei Sudati Sagrilo
Keyword(s):  
Limit State ◽  
Offshore Structures ◽  
Ultimate Limit State ◽  
Wave Load ◽  
Wave Effects ◽  
Load Effects ◽  
Bracing System ◽  
Term Response ◽  
Tubular Component

This paper compares two approaches for the estimation of long-term response of wave load effects on offshore structures. These approaches are applied to estimate the extreme value of the cross section interaction ratio of a tubular component of the bracing system of a semisubmersible platform. The tubular component is subjected to axial loads and bending moments due to static loads and wave effects. The iteration ratio in the ultimate limit state is defined by applying design criteria derived from API RP-2A LRFD [6]. The approaches are also applied to estimate the long-term response of a single degree of freedom system due to wave actions. The first approach is based on the proposals of Videiro and Moan [3]. The results of the first approach are compared with a new model of long-term response estimation, based on the up-crossing rate distribution of the response process.

Estimating Long Term Extreme Slamming From Breaking Waves

2015 ◽  
Author(s):  
Gunnar Lian ◽  
Sverre K. Haver
Keyword(s):  
Coefficient Of Variation ◽  
Limit State ◽  
Breaking Waves ◽  
Contour Method ◽  
Short Term ◽  
Sea State ◽  
Response Process ◽  
Term Response ◽  
Short Term Variability

Characteristic loads for design are defined in terms of their annual exceedance probability, q. For ultimate limit state (ULS) q = 10−2, while q = 10−4 for accidental limit state (ALS). In principle a full long term analysis is required in order to obtain consistent estimates. This is straight forward for linear response problems, while it is a challenge for non-linear problems in particular if they additionally are of an on-off nature. The latter will typically be the case for loads due to breaking wave impacts. The Contour line approach is an alternative convenient method to estimate the long term extreme response, based on short term statistics from an appropriate sea state. The consequence of very large short term variability (large coefficient of variation for 3-hour extreme value) on the application of the contour method will be discussed. The long term integral is carried out over all sea state combinations. The lowest sea states will of course not affect the extremes. However, for the impact problem the short term variability is much larger than for most response cases. The coefficient of variation of the 3-hour maximum impact pressure is often between 0.5 and 1, while for a typical response process it is between 0.1 and 0.2. Due to the large variability, lower sea states than normal will contribute to the long term response. In this paper the irregularity of the response surface, and the uncertainties related to the number of seeds used in each sea state is looked into. The focus is on slamming loads from breaking waves, and some results from a model test are presented. The uncertainties in long term response from slamming loads are compared to a more common response process. The effect on the long term response when integrating over a reduced area of sea states in the scatter diagram is discussed.

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