Reliability-Based Design and Assessment for Lifetime Extension of Ageing Offshore Structures

2016 ◽  
Author(s):  
E. Mat Soom ◽  
M. K. Abu Husain ◽  
N. I. Mohd Zaki ◽  
N. U. Azman ◽  
G. Najafian
Keyword(s):  
Service Life ◽  
Offshore Structures ◽  
Risk Level ◽  
Type I ◽  
Offshore Structure ◽  
Design Assessment ◽  
Reliability Based Design ◽  
Remaining Service Life ◽  
The North ◽  
Cost Efficient

The methodology for Reliability-Based Design and Assessment (RBDA) of an ageing fixed steel offshore structure was established to support detailed re-assessment applied to the management of the structure’s safety, integrity analysis and reliability by evaluating the loading acting on the structure. It is a tool for the high-end analysis of the structure for risk-based design assessment and has been succesfully implemented in the North Sea under Shell operating company. The main purposes of RBDA are to manage a structure’s risk level over its remaining service life and to initiate the cost-efficient inspection or mitigation actions (if required). This method consists of Type I and II uncertainties used to determine the probability of failure for the structure over its remaining service life. However, limited work has been done so far on its application at many different regions, particularly in the South East Asia. Therefore, this paper investigates the robustness of the RBDA methodology applied to fixed offshore structures at shallow waters of Malaysia by considering the native environmental criteria, local authorities’ obligation and company requirements. It is shown that this procedure can efficiently assist in understanding the structure’s failure mechanism and correctly define the relevant type of mitigations required.

Offshore structures – why all offshore facilities should have a demanning requirement

2019 ◽  
Vol 59 (2) ◽  
pp. 789 ◽  
Author(s):  
Matt Keys
Keyword(s):  
Offshore Structures ◽  
Structure Design ◽  
Risk Level ◽  
Design Codes ◽  
Offshore Structure ◽  
Sea State ◽  
The North ◽  
Offshore Structure Design ◽  
Cost Implications ◽  
Level Of Risk

Most offshore structure design codes focus on setting appropriate safety factors to achieve an acceptable annual level of risk. Recent work by Atkins SNC-Lavalin, together with a large number of operators in Australian waters and the North Sea, has discovered that a large number of aging assets are implementing a demanning requirement to limit the risk of platform collapse to personnel, due to changes in loading or degradation of the structure. This work has shown there are two risk scenarios that should drive this requirement. The first scenario which is intended by the codes in limiting the overall annual risk. The second is to limit the collapse risk associated with a known forecast storm, as the level of risk from helicopter demanning is much lower. For all the older offshore fixed and permanently mooring floating structures assessed for a risk level considered acceptable for a forecast storm, this risk level would govern the sea-state demanning criteria. For recently installed facilities that are compliant with current standards, the findings were the same: that all facilities should have a demanning requirement. The level of this demanning sea-state limit has been shown to be lower than expected and is likely to occur only once in the asset’s life; therefore, the cost implications of implementing demanning procedures are minor. This paper presents the basis and range of findings for calculating the risks associated with an annual occurrence and an ‘in a forecast storm’ risk. Further, this paper proposes acceptable demanning limits for facilities designed to current and historical design codes.

scholarly journals Comparative Study of Structural Reliability Assessment Methods for Fixed Offshore Structures

2021 ◽  
Author(s):  
E. Mat Soom ◽  
M.K. Abu Husain ◽  
N.I. Mohd Zaki ◽  
N.A. Mukhlas ◽  
S.Z.A. Syed Ahmad ◽  
...  
Keyword(s):  
Structural Reliability ◽  
Oil And Gas ◽  
Offshore Structures ◽  
Mitigation Strategies ◽  
Life Extension ◽  
Risk Level ◽  
Long Distance ◽  
Design Assessment ◽  
Integrity Assessment ◽  
Structural Deterioration

The oil and gas sector has recognised structural integrity assessment of ageing platform for prospective life extension as a rising concern, particularly in encountering the randomness of the harsh ocean environments. This condition leads to uncertainty in wave-in-deck load estimates and a high load level being imposed on offshore structures. This emphasises the necessity of enhanced reliability, as failure might result in inaccessibility because of the uncertainties related to long-distance services, such as accuracy of predictions of loads and responses. Even though the established guidelines present a fundamental assessment, additionally, comprehensive rules are required. This paper performed a reliability analysis incorporating practical approaches that can more accurately represent time-dependent structural deterioration. The following two procedures have been adopted by a majority of significant oil and gas operators to monitor the safety and integrity of these structures: a) Ultimate Strength Assessment (USA) method and b) Reliability Design Assessment (ReDA) method. A comparison of these two reliability approaches was performed on selected ageing jacket structures in the region of the Malaysian sea. The comparative findings, namely, reserve strength ratio (RSR) at various years of the return period (RP) and ratio value for risk of failure regarding the probability of failure (POF), provided a check and balance in strengthening confidence in the results. The findings showed that the structural components might safely survive either using the USA and ReDA method in such conditions, as the reliability indexes were determined to be satisfactory compared to allowable values from ISO 19902 design specifications. Therefore, these evaluations were determined to control the risk level of the structure during the remaining of its lifetime and undertake cost-effective inspections or mitigation strategies when necessary.

Optimality and Acceptance Criteria in Offshore Design

2002 ◽  
Author(s):  
Oliver Ku¨bler ◽  
Michael Havbro Faber
Keyword(s):  
Optimal Design ◽  
Service Life ◽  
Net Present Value ◽  
Steel Structures ◽  
Offshore Structures ◽  
Theoretical Problem ◽  
Offshore Structure ◽  
Construction Costs ◽  
Suggested Approach ◽  
Wide Range

The optimal design of offshore structures is formulated as a decision theoretical problem. The objective is to maximize the net present value of the service life benefit. The general optimization problem is simplified by taking into account the cost impacts of only one possible reconstruction of the structure. The analytical solution to this problem has been derived for the case, where failure events follow a stationary Poisson process. The service life benefit is formulated in terms of the production profile, the design and construction costs, the costs of failure and the costs of reconstruction. In order to assess the effect of potential loss of lives, the costs of fatalities are included applying the concept of the Imp lied Costs of Averting a Fatality (ICAF). The suggested approach to optimal design, which can be applied for any type of offshore structure, is exemplified considering the special case of steel structures. Here it is standard to represent the ultimate structural capacity in terms of the Reserve Strength Ratio (RSR). For the purpose of illustration, the relation between material usage and RSR valid for monopod structures is applied. Optimal RSR’s and annual failure rates are assessed for both manned and un-manned structures covering a wide range of different realistic ratios between the potential income and costs of construction, failure and re construction costs.

scholarly journals LIFETIME EXTENSION OF AGEING OFFSHORE STRUCTURES BY GLOBAL ULTIMATE STRENGTH ASSESSMENT (GUSA)

2018 ◽  
Vol 30 (1) ◽  
Author(s):  
Ezanizam Mat Soom ◽  
Mohd Khairi Abu Husain ◽  
Noor Irza Mohd Zaki ◽  
M Nasrul Kamal M Nor ◽  
G. Najafian
Keyword(s):  
Oil And Gas ◽  
Continuous Production ◽  
Offshore Structures ◽  
Life Extension ◽  
Risk Level ◽  
Strength Assessment ◽  
Remaining Service Life ◽  
Oil And Gas Fields ◽  
Gas Fields ◽  
Extension Evaluation

Malaysia is the second largest oil and gas producer in Southeast Asia. Majority ofjacket platforms in Malaysia have exceeded their design life with various types of underwaterstructure irregularities. Therefore, it is essential to address the reliability of the jacket platformsin Malaysia due to ageing, increasing environmental loading and demand to prolong theproduction for a further 25 years. The main purpose of this analyses is to determine thestructure’s risk level over its remaining service life which is a vital information in managingageing facilities to cater for the demand of continuous production. Global Ultimate StrengthAssessment (GUSA) methodology was used to support detailed reassessment applied inmanaging safety, integrity analyses and reliability by evaluating the existing platform’s loading.It is a tool for high-end analysis of structures for Risk-based Assessment (RBA). In this paper,the reassessment of an ageing platform over 30-year-old, still in production is presented todemonstrate GUSA capability to perform the platform’s life extension evaluation. The outcomefrom these analyses can effectively assist in understanding the structure platform’s failuremechanism and correctly identify mitigation actions required. As part of the analyses, non-linearanalysis and probabilistic model as practiced in the industry were used in order to get ReserveStrength Ratio (RSR) and Annual Probability of Failure (POF) results. The accuracy andcomprehensiveness of this method will assist the industry, especially oil and gas fields’ operators,in decision-making, specifically in identifying problem-oriented-solutions as part of theirbusiness risk management in managing ageing facilities.

A Possible Basis for Nondestructive Characterization of Ropes of Nylon 6

1997 ◽  
Vol 503 ◽  
Author(s):  
H. Jiang ◽  
M. K. Davis ◽  
R. K. Eby ◽  
P. Arsenovic
Keyword(s):  
Tensile Strength ◽  
Service Life ◽  
Fiber Diameter ◽  
Structural Parameters ◽  
Crystal Form ◽  
Nylon 6 ◽  
Remaining Service Life ◽  
Nondestructive Characterization ◽  
Fractional Content

ABSTRACTPhysical properties and structural parameters have been measured for ropes of nylon 6 as a function of the number of use operations. The fractional content of the α crystal form, sound velocity, birefringence, tensile strength and length all increase systematically and significantly with increasing the number of use operations. The fractional content of the γ crystal form and fiber diameter decrease with use. These trends indicate that the measurement of such properties and structural parameters, especially the length, provide a possible basis for establishing a reliable, rapid, and convenient nondestructive characterization method to predict the remaining service life of nylon 6 ropes.

Downtime Analysis Techniques for Complex Offshore and Dredging Operations

2004 ◽  
Author(s):  
Remmelt J. van der Wal ◽  
Gerrit de Boer
Keyword(s):  
Wind Waves ◽  
Weather Forecast ◽  
Offshore Structures ◽  
Operational Mode ◽  
Offshore Structure ◽  
Hydrodynamic Models ◽  
Time Step ◽  
Made In

Offshore operations in open seas may be seriously affected by the weather. This can lead to a downtime during these operations. The question whether an offshore structure or dredger is able to operate in wind, waves and current is defined as “workability”. In recent decades improvements have been made in the hydrodynamic modelling of offshore structures and dredgers. However, the coupling of these hydrodynamic models with methods to analyse the actual workability for a given offshore operation is less developed. The present paper focuses on techniques to determine the workability (or downtime) in an accurate manner. Two different methods of determining the downtime are described in the paper. The first method is widely used in the industry: prediction of downtime on basis of wave scatter diagrams. The second method is less common but results in a much more reliable downtime estimate: determination of the ‘job duration’ on basis of scenario simulations. The analysis using wave scatter diagrams is simple: the downtime is expressed as a percentage of the time (occurrences) that a certain operation can not be carried out. This method can also be used for a combination of operations however using this approach does not take into account critical events. This can lead to a significant underprediction of the downtime. For the determination of the downtime on basis of scenario simulations long term seastate time records are used. By checking for each subsequent time step which operational mode is applicable and if this mode can be carried out the workability is determined. Past events and weather forecast are taken into account. The two different methods are compared and discussed for a simplified offloading operation from a Catenary Anchor Leg Mooring (CALM) buoy. The differences between the methods will be presented and recommendations for further applications are given.

Remaining service life diagnostic technology of insulators for power distribution equipment

2009 ◽  
Vol 167 (2) ◽  
pp. 1-9
Author(s):  
Shinsuke Miki ◽  
Hiroshi Okazawa ◽  
Taketoshi Hasegawa ◽  
Sei Tsunoda ◽  
Hiroshi Inujima
Keyword(s):  
Service Life ◽  
Power Distribution ◽  
Remaining Service Life ◽  
Diagnostic Technology
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