Extreme Storm Wave Histories for Cyclic Check of Offshore Structures

2010 ◽  
Author(s):  
O̸istein Hagen ◽  
Gunnar Solland ◽  
Jan Mathisen
Keyword(s):  
Structural Reliability ◽  
Failure Modes ◽  
Structural Integrity ◽  
Offshore Structures ◽  
Load History ◽  
Load Effect ◽  
Design Point ◽  
Design Storm ◽  
Existing Structures ◽  
Load Effects

Offshore platform resistance to cyclic storm actions is addressed. In order to achieve the best economy of the structure especially when assessing existing structures, the ultimate capacity of the structure is utilized. This means that parts of the structure may be loaded into the non-linear range and consequently the load-carrying resistance of the structure against future load cycles may be reduced. In such cases it is required to carry out a check of the cyclic capacity of the structure. Such checks are required in the ISO 19902 code for Fixed Steel Offshore Structures. The paper presents a proposal for how a load history for cyclic checks can be established. The method is in line with what is included in the NORSOK N-006 standard on “Assessment of structural integrity for existing load-bearing structures”. The load-history for the waves in the design storm may be expressed as ratio of the dimensioning wave. The ratio will be different for check of failure modes where the entire storm will be relevant such as crack growth, compared to failure modes like buckling where only the remaining waves after the dimensioning wave need to be accounted for. Using simple order statistics and simulation, the statistics for the ith (Hi), i = 1, 2, 3, 4 etc. highest wave in the storm is studied in some detail, assuming that the maximum wave (H1) is equal to an extreme wave obtained by a code requirement. Environmental contours for the pair (H1,H2) are established by Inverse FORM for design conditions. Further, the long term statistics for load effects that are expressed as a function of H1, .., H4, i.e. L = f(H1, .., H4), are determined. The R-year value LR for the load effect L is determined by structural reliability techniques, and the most probable combination (design point) (H1*, .., H4*) for L = LR is determined. The design point values Hi*, as well as the design point value for the significant wave height, are determined for different load effects, and their characteristics for different types of load effects are discussed. The paper gives advice also on how to establish the magnitude for the remaining waves in the storm.

Evaluation of the Effect of Yield to Tensile (Y/T) Ratio on the Structural Integrity of Offshore Pipeline by Limit State Design Approach

2006 ◽  
Author(s):  
Gianluca Mannucci ◽  
Giuliano Malatesta ◽  
Giuseppe Demofonti ◽  
Marco Tivelli ◽  
Hector Quintanilla ◽  
...  
Keyword(s):  
Structural Reliability ◽  
Failure Modes ◽  
Structural Integrity ◽  
Limit State ◽  
Minor Effect ◽  
Offshore Pipelines ◽  
Limit State Design ◽  
A Minor ◽  
Probabilistic Failure Analysis ◽  
Failure Probabilities

Nowadays specifications require strict Yield to Tensile ratio limitation, nevertheless a fully accepted engineering assessment of its influence on pipeline integrity is still lacking. Probabilistic analysis based on structural reliability approach (Limit State Design, LSD) aimed at quantifying the yield to tensile strength ratio (Y/T) influence on failure probabilities of offshore pipelines was made. In particular, Tenaris seamless pipe data were used as input for the probabilistic failure analysis. The LSD approach has been applied to two actual deepwater design cases that have been on purpose selected, and the most relevant failure modes have been considered. Main result of the work is that the quantitative effect of the Y/T ratio on failure probabilities of a deepwater pipeline resulted not so big as expected; it has a minor effect, especially when Y only governs failure modes.

Structural Reliability Analysis Method for Assessing the Fatigue Capacity of Subsea Wellhead Connectors

2020 ◽  
Author(s):  
Torfinn Hørte ◽  
Lorents Reinås ◽  
Anders Wormsen ◽  
Andreas Aardal ◽  
Per Gustafsson
Keyword(s):  
Reliability Analysis ◽  
Structural Reliability ◽  
Failure Modes ◽  
Fatigue Loading ◽  
Load Effect ◽  
Structural Reliability Analysis ◽  
Structural Capacity ◽  
Drilling Operations ◽  
Male Part ◽  
External Loads

Abstract Subsea Wellheads are the male part of an 18 3/4” bore connector used for connecting subsea components such as drilling BOP, XT or Workover systems equipped with a female counterpart — a wellhead connector. Subsea wellheads have an external locking profile for engaging a preloaded wellhead connector with matching internal profile. As such connection is made subsea, a metal-to-metal sealing is obtained, and a structural conduit is formed. The details of the subsea wellhead profile are specified by the wellhead user and the standardized H4 hub has a widespread use. In terms of well integrity, the wellhead connector is a barrier element during both well construction (drilling) activities and life of field (production). Due to the nature of subsea drilling operations, a wellhead connector will be subjected to external loads. Fatigue and plastic collapse due to overload are therefore two potential failure modes. These two failure modes are due to the cyclic nature of the loads and the potential for accidental and extreme single loads respectively. The safe load the wellhead connector can sustain without failure can be established by deterministic structural capacity methods. This paper outlines how a generic and probabilistic engineering method; Structural Reliability Analysis, can be applied to a subsea wellhead connector to estimate the probability of fatigue failure (PoF). As the wellhead connector is a mechanism consisting of a plurality of parts the load effect from cyclic external loads is influenced by uncertainty in friction, geometry and pre-load. Further, there is a inter dependence between these parameters that complicates the problem. In addition to these uncertainties, uncertainties in the fatigue loading itself (from rig and riser) is also accounted for. This paper presents results from applications of Structural Reliability Analysis (SRA) to a wellhead connector and provides experiences and learnings from this case work.

Reliability and (Re)Assessment of Fixed Steel Structures

2011 ◽  
Author(s):  
Mike Efthymiou ◽  
Jan Willem van de Graaf
Keyword(s):  
Structural Reliability ◽  
Structural Integrity ◽  
Steel Structures ◽  
Performance Standards ◽  
Offshore Structures ◽  
Design Standards ◽  
Component Strength ◽  
Post Buckling ◽  
Exposure Levels ◽  
Joint Occurrence

This paper reviews the structural integrity and reliability of fixed steel offshore structures with a focus on improved models and incorporation of these models in design standards. Technical achievements in four key areas are reviewed which, when combined, resulted in a step improvement in the calculation of structural reliability. The first area is the extreme environmental loading on an offshore platform; the second area is the joint occurrence of waves, winds and currents, i.e. accounting for the fact that these do not, in general, peak at the same time and do not act in the same direction. The third area is the estimation of the ultimate strength of a fixed steel platform, accounting for component strength, including the buckling and post-buckling behaviour and the uncertainty in system strength. The fourth and final area is the integration of the above models to estimate the probability of failure. The historical performance of platforms and the improvements in successive editions of API RP 2A are reviewed; reliability targets appropriate for different exposure levels and corresponding performance standards are developed, aimed at harmonizing design practices worldwide. A differentiation is recommended between permanently manned L-1 installations and manned-evacuated L-1 installations in the Gulf of Mexico; this is because the consequences of failure are considerably different.

scholarly journals Development of Risk-Reliability Based Underwater Inspection for Fixed Offshore Platforms in Indonesia

2018 ◽  
Vol 147 ◽  
pp. 05002
Author(s):  
Ricky L. Tawekal ◽  
Faisal D. Purnawarman ◽  
Yati Muliati
Keyword(s):  
Structural Reliability ◽  
Structural Integrity ◽  
Damage Mechanism ◽  
Offshore Structures ◽  
Probability Of Failure ◽  
American Petroleum Institute ◽  
Offshore Platform ◽  
Risk Level ◽  
Offshore Platforms ◽  
Fixed Offshore Platform

In RBUI method, platform with higher risk level will need inspection done more intensively than those with lower risk level. However, the probability of failure (PoF) evaluation in RBUI method is usually carried out in semi quantitative way by comparing failure parameters associated with the same damage mechanism between a group of platforms located in the same area. Therefore, RBUI will not be effective for platforms spread in distant areas where failure parameter associated with the same damage mechanism may not be the same. The existing standard, American Petroleum Institute, Recommended Practice for Structural Integrity Management of Fixed Offshore Structures (API RP-2SIM), is limited on the general instructions in determining the risk value of a platform, yet it does not provide a detail instruction on how determining the Probability of Failure (PoF) of platform. In this paper, the PoF is determined quantitatively by calculating structural reliability index based on structural collapse failure mode, thus the method in determining the inspection schedule is called Risk-Reliability Based Underwater Inspection (RReBUI). Models of 3-legs jacket fixed offshore platform in Java Sea and 4-legs jacket fixed offshore platform in Natuna Sea are used to study the implementation of RReBUI.

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.

Reassessment Issues in Life Cycle Structural Integrity Management of Fixed Steel Installations

2002 ◽  
Author(s):  
A. Stacey ◽  
M. Birkinshaw ◽  
J. V. Sharp
Keyword(s):  
Life Cycle ◽  
Structural Integrity ◽  
Offshore Structures ◽  
Iso Standard ◽  
Existing Structures ◽  
System Integrity ◽  
The Status ◽  
Important Trend ◽  
Integrity Management ◽  
Inspection Data

There is an increasing number of ageing installations in UK waters, many of which are being or will be operated beyond their original planned service life. This important trend, in combination with (a) the introduction of risk-based goal-setting regulations which require the maintenance of life cycle integrity as a key target, (b) the development of guidelines in the draft ISO standard for offshore structures, ISO 19902, and (c) significant technology advances in recent years (e.g. in loading, fatigue, fire and blast integrity and system integrity), makes reassessment an important consideration in the structural integrity management of offshore installations. The paper outlines procedures in place for reassessment, including those in the draft ISO standard, and reviews recent technical advances relevant to this area. The important role of inspection and maintenance for existing structures is assessed and related to both current practices and target requirements. The need for reliable and comprehensive inspection data is important for reassessment and the status of this is reviewed. An overall framework for reassessment is developed in the light of the above issues.

Long Term Analysis of Steep and Breaking Wave Properties by Event Matching

2018 ◽  
Author(s):  
Thomas B. Johannessen ◽  
Øystein Lande
Keyword(s):  
Wave Breaking ◽  
Offshore Structures ◽  
Return Periods ◽  
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 relevant 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. The present paper is concerned with the long term distribution of strongly nonlinear load effects and a methodology is proposed which incorporates CFD analysis in a long term Monte Carlo analysis of crest elevations and wave kinematics. Based on a long term time domain simulation of a linear surface elevation, a selection of events is run in CFD in order to obtain a database of linear and corresponding fully nonlinear wave fields with the possibility of wave breaking included. In the subsequent long term analysis, a large linear event is then replaced by the closest matching event in the database. A technique is developed to Froude scale the database results and shift the origin in time and plane so that the database of typically only 100 events give a close match to all the events in the simulation. The method is applied to the simple case of drag loading on a cylinder which is truncated above the still water level such that only the largest waves impact with the structure. It is observed that whereas the Event Matching method agree well with a second order model for return periods lower than 100 years, the loading on the cylinder is significantly larger for longer return periods. The deviation is caused by the increasing dominance of wave braking in the largest crest and illustrates the importance of incorporating wave breaking in the analysis of wave in deck loading problems.

Life Extension of a High Pressure Transmission Pipeline Using Structural Reliability Analysis

2002 ◽  
Author(s):  
Andrew Francis ◽  
Mike Gardiner ◽  
Marcus McCallum
Keyword(s):  
High Pressure ◽  
Reliability Analysis ◽  
Structural Reliability ◽  
Failure Modes ◽  
Structural Integrity ◽  
Life Extension ◽  
Structural Reliability Analysis ◽  
Natural Gas Pipeline ◽  
Design Life ◽  
Transmission Pipeline

Pipeline designers and operators recognize that the commercial viability of operating high-pressure gas pipelines decreases with time. This is because the structural integrity levels of the pipeline decrease, due to the action of deterioration processes such as corrosion and fatigue, until the level of mitigation required to ensure adequate safety levels becomes uneconomical. For this reason pipelines are assigned a nominal design life of typically 40 years. This paper describes the application of structural reliability analysis to a high-pressure natural gas pipeline having both onshore and offshore sections, in order to determine the extent to which the asset life could be increased beyond the design life without any significant reduction in reliability and hence safety levels. The approach adopted was to identify the credible failure modes that could affect each of the onshore and offshore sections and determine the probability of failure due to each failure mode taking account of the uncertainties in the parameters that affect each mode. Based on a detailed consideration of the results of the study it was concluded that the life of the asset considered here could be extended to 60 years without any significant reduction in safety levels. Moreover, it was concluded that if certain mitigating measures were to be implemented in the future then it would be possible to increase the asset life to significantly more than 60 years.

A Novel Approach to Improving the Structural Reliability of a Fixed Offshore Platform Subject to Wave-in-Deck Loading Using a Holistic Structural Integrity Management (SIM) TRIAD Approach

2020 ◽  
Author(s):  
Mehrdad Kimiaei ◽  
Jalal Mirzadeh ◽  
Partha Dev ◽  
Mike Efthymiou ◽  
Riaz Khan
Keyword(s):  
Structural Reliability ◽  
Structural Model ◽  
Structural Integrity ◽  
Reliability Assessment ◽  
Offshore Structures ◽  
Offshore Platform ◽  
Offshore Platforms ◽  
Effective Manner ◽  
Integrity Management ◽  
Fixed Offshore Platform

Abstract Fixed offshore platforms subject to wave-in-deck loading have historically encountered challenges in meeting target reliability levels. This has often resulted in costly subsea remediation, impacted platform occupancy levels or premature decommissioning of critical structural assets due to safety concerns. This paper addresses the long-standing industry challenge by presenting a novel structural reliability approach that involves converging the analytical behavior of a structure to its measured dynamic response for assessment. In this approach, called the Structural Integrity Management (SIM) TRIAD method, the platform model is calibrated based on the measured in-field platform natural frequencies using a structural health monitoring (SHM) system, so that the reliability assessment can be performed on a structural model whose stiffness is simulated as close to reality as possible. The methodology demonstrates the potential of unlocking structural capacity of offshore structures by removing conservatism normally associated with traditional reliability assessment methods, thus significantly improving the ability to achieve target structural reliability levels in a cost effective manner. The SIM TRIAD method has been implemented while assessing an existing fixed offshore platform subject to wave-in-deck loads, which is located in East Malaysian waters. It has enabled the facility operator to achieve acceptable target structural reliability and has assisted in developing an optimized risk-based inspection (RBI) plan for ensuring safe operations to end of asset field life. The methodology and findings of the assessment are presented in this paper to illustrate the benefits of the SIM TRIAD method.

Structural Life Expectancy of Marine Vessels: Ultimate Strength, Corrosion, Fatigue, Fracture, and Systems

2015 ◽  
Vol 1 (1) ◽  
Author(s):  
Bilal M. Ayyub ◽  
Karl A. Stambaugh ◽  
Timothy A. McAllister ◽  
Gilberto F. de Souza ◽  
David Webb
Keyword(s):  
Structural Reliability ◽  
Failure Modes ◽  
Structural Integrity ◽  
Structural Response ◽  
Probabilistic Methods ◽  
Life Assessment ◽  
Operational Environment ◽  
Stiffened Panels ◽  
Factors Affecting ◽  
Marine Vessels

This paper provides a methodology for the structural reliability analysis of marine vessels based on failure modes of their hull girders, stiffened panels including buckling, fatigue, and fracture and corresponding life predictions at the component and system levels. Factors affecting structural integrity such as operational environment and structural response entail uncertainties requiring the use of probabilistic methods to estimate reliabilities associated with various alternatives being considered for design, maintenance, and repair. Variability of corrosion experienced on marine vessels is a specific example of factors affecting structural integrity requiring probabilistic methods. The Structural Life Assessment of Ship Hulls (SLASH) methodology developed in this paper produces time-dependent reliability functions for hull girders, stiffened panels, fatigue details, and fracture at the component and system levels. The methodology was implemented as a web-enabled, cloud-computing-based tool with a database for managing vessels analyzed with associated stations, components, details, and results, and users. Innovative numerical and simulation methods were developed for reliability predictions with the use of conditional expectation. Examples are provided to illustrate the computations.

Sign in / Sign up

Export Citation Format

Share Document