Consistent Design Codes for Anchors and Mooring Lines

2002 ◽  
Author(s):  
Rune Dahlberg ◽  
Jan Mathisen
Keyword(s):  
Structural Reliability ◽  
Mooring System ◽  
Design Codes ◽  
Safety Factors ◽  
Design Code ◽  
Mooring Lines ◽  
Safety Level ◽  
Industry Standard ◽  
Partial Safety Factors ◽  
Anchor Design

As the water depth of hydrocarbon discoveries becomes deeper, the technological challenges related to the design of mooring systems increases. Changing from steel catenary mooring systems (CMS) to fibre rope taut mooring systems (TMS) has been accompanied by an immense focus on how to qualify and approve fibre rope material for use in a TMS. This involves items related to specifications for manufacturing, handling and testing fibre ropes, as well as calibration of safety factors to use in the design of TMSs. One consequence of moving to a TMS is that the anchors will have to take more uplift load than in a conventional CMS, which makes the anchors a more critical component of the mooring system than before. The types of anchor normally available to the designer of a TMS are pile anchors, suction anchors and various types of plate anchors. Anchors of all types are designed and installed in ever-deeper water, but the safety of the designed mooring systems varies with the design code adopted. There is thus an obvious need for an industry standard, a design code for each anchor type that is calibrated based on structural reliability analysis using the current experience and knowledge in the industry. This paper compares anchor design codes that use total safety factors (TSF) with the DNV design code that uses partial safety factors and failure consequence classes. Examples of design codes for station-keeping systems that adopt the TSF format are API RP2SK and (assumed herein) the ISO code, which is under development. The comparison demonstrates that use of the safety format adopted in the DNV code provides more flexibility and ensures a uniform safety level of all components in a mooring system than the TSF format. If all types of anchor were designed to the same safety level it would be possible to compare anchors without worrying about differences in safety. A typical approach for calibration of a design code is described.

Value of information of in situ inspections of mooring lines

2021 ◽  
pp. 1748006X2098740
Author(s):  
Jorge Mendoza ◽  
Jacopo Paglia ◽  
Jo Eidsvik ◽  
Jochen Köhler
Keyword(s):  
Pitting Corrosion ◽  
Value Of Information ◽  
Structural Reliability ◽  
Structural Safety ◽  
Statistical Dependence ◽  
Mooring System ◽  
Mooring Lines ◽  
Corrosion Condition ◽  
Chain Links

Mooring systems that are used to secure position keeping of floating offshore oil and gas facilities are subject to deterioration processes, such as pitting corrosion and fatigue crack growth. Past investigations show that pitting corrosion has a significant effect on reducing the fatigue resistance of mooring chain links. In situ inspections are essential to monitor the development of the corrosion condition of the components of mooring systems and ensure sufficient structural safety. Unfortunately, offshore inspection campaigns require large financial commitments. As a consequence, inspecting all structural components is unfeasible. This article proposes to use value of information analysis to rank identified inspection alternatives. A Bayesian Network is proposed to model the statistical dependence of the corrosion deterioration among chain links at different locations of the mooring system. This is used to efficiently update the estimation of the corrosion condition of the complete mooring system given evidence from local observations and to reassess the structural reliability of the system. A case study is presented to illustrate the application of the framework.

scholarly journals Reliability-based calibration of Brazilian structural design codes used in the design of concrete structures

2019 ◽  
Vol 12 (6) ◽  
pp. 1288-1304
Author(s):  
W. C. SANTIAGO ◽  
H. M. KROETZ ◽  
A. T. BECK
Keyword(s):  
Structural Design ◽  
Concrete Structures ◽  
Transverse Reinforcement ◽  
Longitudinal Reinforcement ◽  
Design Codes ◽  
Safety Factors ◽  
Target Reliability Index ◽  
Live Loads ◽  
Effective Depth ◽  
Partial Safety Factors

Abstract This paper presents a reliability-based calibration of partial safety factors for Brazilian codes used in the design of concrete structures. The work is based on reliability theory, which allows an explicit representation of the uncertainties involved in terms of resistances and loads. Regarding the resistances, this study considers beams with concrete of five classes (C20, C30, C40, C50 and C60), three ratios between base and effective depth (0.25, 0.50 and 0.75), three longitudinal reinforcement ratios (ρmin, 0.5% and ρmax) and three transverse reinforcement ratios ( A s s m i n, 5 . A s s m i n and A s s m a x). In terms of loads, this work considers seven ratios between live loads and permanent loads (qn/gn), and seven ratios between wind loads and permanent loads (wn/gn). The study also adopts a single value for the target reliability index (βtarget = 3.0). Results show that the optimized set of partial safety factors leads to more uniform reliability for different design situations and load combinations.

The application of Structural Reliability Analysis on hull girder ultimate strength of bulk carriers-a trial on Safety Level Approach

2015 ◽  
Author(s):  
Daokun Zhang ◽  
Wenyong Tang
Keyword(s):  
Reliability Analysis ◽  
Ultimate Strength ◽  
Structural Reliability ◽  
Safety Level ◽  
Hull Girder ◽  
Structural Rules ◽  
Structural Reliability Analysis ◽  
Trial Analysis ◽  
Bulk Carriers ◽  
Partial Safety Factors

The International Maritime Organization is developing the Goal Based Standard, in which the Safety Level Approach(SLA) is one of the two parallel ways forward focusing on deriving explicit and reasonable safety level. During the development of Safety Level Approach, the Structural Reliability Analysis(SRA) is recognized as one of the useful tools. The application of SRA on the calibration of partial safety factors for hull girder ultimate strength is so far a typical illustration, which could be very helpful for the application of Safety Level Approach on the structural Rules in the future. China Classification Society (CCS) carries out a trial analysis with co-operation of Shanghai Jiao Tong University.

Reliability-Based Design Criterion for TLP Tendons

2006 ◽  
Author(s):  
Federico Barranco Cicilia ◽  
Edison Castro Prates de Lima ◽  
Lui´s Volnei Sudati Sagrilo
Keyword(s):  
Reliability Analysis ◽  
Structural Reliability ◽  
Limit State ◽  
Design Criterion ◽  
Ultimate Limit State ◽  
Safety Factors ◽  
Extreme Response ◽  
Load Effects ◽  
Partial Safety Factors ◽  
State Models

This paper presents a Load and Resistance Factor Design (LRFD) criterion applied to the design of Tension Leg Platform (TLP) tendons in their intact condition. The design criterion considers the Ultimate Limit State (ULS) of any tendon section along its whole length taking into account both dynamic interactions of load effects and the statistics of its associated extreme response. The partial safety factors are calibrated through a long-term reliability-based methodology for the storm environmental conditions, like hurricanes and winter storms, in deep waters of the Campeche Bay, Mexico. In the reliability analysis, the uncertainties in the definition of load effects and analytic limit state models for calculation of tendon strength and randomness of material properties are included. The results show that the partial safety factors reflect both uncertainty content and the importance of the random variables in structural reliability analysis. When tendons are designed according to the developed LRFD criterion, a less scattered variation of reliability indexes is obtained for different tendon sections across a single or various TLP designs.

Structural Reliability-Based Control for Moored Vessels: Experimental Results

2007 ◽  
Author(s):  
Per I. B. Berntsen ◽  
Ole M. Aamo ◽  
Bernt J. Leira
Keyword(s):  
Structural Properties ◽  
Reliability Index ◽  
Laboratory Experiments ◽  
Structural Reliability ◽  
Weather Conditions ◽  
Dynamic Positioning ◽  
Mooring System ◽  
Reliability Measures ◽  
Mooring Lines ◽  
Surface Vessels

This paper addresses dynamic positioning of surface vessels moored to the seabed via a turret based spread mooring system, referred to as position mooring. The controller utilize a reliability index to determine the actuator force needed to operate safely. The structural reliability measures become an intrinsic part of the controller, automatically adjusting the allowed geographical region based on current weather conditions and structural properties of the mooring lines. The performance of the controller is demonstrated through laboratory experiments on a model vessel named CyberShip III.

ECAs, Probabilities and Axial Misalignment

2018 ◽  
Author(s):  
Andrew Cosham ◽  
Kenneth A. Macdonald
Keyword(s):  
Sensitivity Analysis ◽  
Wall Thickness ◽  
Input Data ◽  
Structural Reliability ◽  
Limit State ◽  
Probabilistic Assessment ◽  
Safety Factors ◽  
Partial Safety Factors ◽  
The Difference ◽  
Girth Welds

An engineering critical assessment (ECA) is commonly conducted during the design of an offshore pipeline in order to determine the tolerable size of flaws in the girth welds. API 579-1/ASME FFS-1 2016 and BS 7910:2013+A1:2015 Incorporating Corrigenda Nos. 1 and 2 give guidance on conducting fitness-for-service assessments of cracks and crack-like flaws. The essential data required for an assessment (nature, position and orientation of flaw; structural and weld geometry; stresses; yield and tensile strength; fracture toughness; etc.) is subject to uncertainty. That uncertainty is addressed through the use of bounding values. The use of extreme bounding values might be overly-conservative. A sensitivity analysis is one way of investigating the sensitivity of the results of an assessment to the input data. A structural reliability-based assessment (a probabilistic assessment) is an alternative. A probabilistic assessment is significantly more complicated than a deterministic assessment. API 579-1/ASME FFS-1 and BS 7910:2013 note that a sensitivity analysis, partial safety factors or a probabilistic analysis can be used to evaluate uncertainties in the input parameters. Annex K of BS 7910:2013 gives partial safety factors for different combinations of target reliability and variability of input data. ISO 16708:2006 gives guidance on the use of structural reliability-based limit-state methods in the design and operation of pipelines. The structural reliability-based assessment of circumferentially-orientated, surface crack-like flaw in a girth weld in a pipeline is used to illustrate the significance of the distributions of the difference between the wall thickness and the ovality (out-of-roundness) of two pipes when calculating the bounding value of the stress concentration factor due to axial misalignment. The (assumed) distributions of diameter, wall thickness, out-of-roundness, yield strength, etc. are based on Annex B of ISO 16708:2006. The (nominal) probability of failure is calculated. It is then used to inform the choice of an appropriate bounding value (i.e. a characteristic value) for axial misalignment.

Reliability-Based Structural Optimization for Positioning of Marine Vessels

2006 ◽  
Author(s):  
Bernt J. Leira ◽  
Per I. B. Berntsen ◽  
Ole Morten Aamo
Keyword(s):  
Control System ◽  
Control Algorithm ◽  
Structural Reliability ◽  
Mooring System ◽  
Response Model ◽  
Marine Structures ◽  
Mooring Lines ◽  
Control Schemes ◽  
Marine Vessels

The present paper is concerned with floating marine structures that are kept in position by means of mooring lines in addition to a thruster system. Different options relevant for construction of automatic control schemes for the thruster system are first investigated based on a simplified response model. In particular, the role of structural reliability criteria applied to the mooring system is investigated. Subsequently, a more refined control algorithm based on such reliability criteria is introduced. The performance of this control system is demonstrated by numerical simulations.

Mooring System Calibration of the Intact Condition, Ultimate Limit State (ULS)

2017 ◽  
Author(s):  
Torfinn Hørte ◽  
Siril Okkenhaug ◽  
Øivind Paulshus
Keyword(s):  
Gulf Of Mexico ◽  
Continental Shelf ◽  
Reliability Analysis ◽  
Time Domain ◽  
Structural Reliability ◽  
Mooring Line ◽  
Mooring System ◽  
Safety Factors ◽  
Reliability Level ◽  
Norwegian Continental Shelf

Structural reliability analysis (SRA) has been used to calculate the probability of mooring line failure in an intact mooring system as a function of the magnitude of the safety factor applied in design. A range of different units have been considered, comprising ship shaped units and semisubmersibles at different water depths from 100 m to 2200 m. Environmental conditions representative for the Norwegian continental shelf and the Gulf of Mexico are used in the analyses, and the characteristics of the results in the different environments are compared and discussed. Analyses for Brazilian environment are currently ongoing, but not included here. Time domain analysis is applied to obtain the short-term, extreme value distribution of line tension, conditional on stationary metocean conditions. A large number of different conditions are considered. A response surface is used to interpolate on the distribution parameters in order to describe the tension response in varying conditions. Joint probabilistic models of the metocean environment corresponding to the different geographical locations have been applied, taking account of wind, wave and current and their respective heading angles. A continuous model is used for the metocean conditions at the Norwegian continental shelf, whereas a hurricane model is used in the Gulf of Mexico. The effects of uncertainties in the response calculation are included. The mooring line component strength is based on strength data from break load tests. Conventional catenary chain-wire chain systems as well as polyester moorings are considered. With the probability of failure as a function of the safety factor, it is shown that present regulations result in a significant scatter in reliability level between the cases. Safety factors have been calibrated considering all cases. Alternative design formats are proposed and tested including a format with 2 safety factors. Calibration results are provided as a function of the target reliability level. The final recommendation on target reliability level is given in an accompanying paper at OMAE 2017, comprising both the ULS and the ALS. It is demonstrated that alternative design formats can provide a more consistent safety level across the cases. A different design philosophy is needed for the Gulf of Mexico in order to achieve acceptable risk. Options for design are discussed. The present work provides a unique and comprehensive set of results, where advanced reliability methods are used in combination with detailed response calculations in the time domain. The results provide a basis for calibration of mooring design for ULS and subsequently for regulators to update their rules. The work has been carried out as part of the NorMoor Joint Industry Project, with participants from oil companies, engineering companies, rig-owners, manufacturers and marine authorities. This paper is the first one in a series of three at OMAE 2017, where the second deals with structural reliability analysis of the ALS and the third one provides summary and recommendations for safe mooring design in ULS and ALS.

Limit states and reliability-based design for a non-codified problem of aqueduct buoyancy

2006 ◽  
Vol 43 (8) ◽  
pp. 869-883
Author(s):  
Gil Robinson ◽  
James Graham ◽  
Ken Skaftfeld ◽  
Ron Sorokowski
Keyword(s):  
Design Methods ◽  
Model Parameters ◽  
Safety Factors ◽  
Design Code ◽  
Limit States ◽  
Reliability Based Design ◽  
Simulation Techniques ◽  
Working Stress ◽  
Engineering Problems ◽  
Partial Safety Factors

Limit states design methods and engineering judgement have been used to assess buoyancy issues for remediation of the 85 year old Shoal Lake Aqueduct in Manitoba. The study demonstrates how these methods can be applied to non-codified engineering problems. Four separate buoyancy analyses were completed using (i) partial safety factors from the Ontario Highway Bridge Design Code, (ii) project-specific partial safety factors, (iii) Monte Carlo simulation techniques, and (iv) working stress design (WSD) methods. Engineering judgement was required to develop a buoyancy model, interpret data for modeling parameters, and provide meaningful values for parameters that could not be measured. Results from the analyses show that more uniform reliability is provided when measured variability of the model parameters is accounted for. The reliability is not quantifiable when working stress design methods are used. Key words: limit states, probability, non-codified problem, aqueduct, buoyancy.

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