The efficient maintenance of offshore structural integrity using reliability analysis

Efficient maintenance of offshore structures is one of the priority development areas of the offshore industry world-wide. The maintenance of the installations in a hostile environment such as the North Sea poses a particular problem. Owing to the large number of wave-induced stress cycles experienced by the structures, the integrity is subject to progressive degradation under fatigue crack growth. If cracks are discovered in service, the operators of the structures will need to decide on the appropriate course of action. The priorities for actions will also need to be determined for cracks of various sizes found in different locations of the structures. In the past, the above decisions have depended very much on engineering judgement. However, the latest developments in reliability fracture mechanics (RFM) analysis have for the first time provided an objective criterion for this type of decision making. Moreover, this criterion is consistent with the established structural design criteria based on reliability analysis of strength—load interaction. Therefore, it has become a real possibility to incorporate structural integrity considerations by design. The reliability-fracture-mechanics-based fatigue analysis includes studies on the uncertainties of load history, materials crack growth behaviour, fracture mechanics modelling, non-destructive inspection and other environmental factors. This paper will review the development in this methodology. In order to integrate the above analysis into the overall design process, it is useful to carry out reliability analysis for the complete structural system. The current state of the art of structural system reliability analysis is therefore briefly reviewed. The necessary development in order to accommodate fatigue reliability degradation is then discussed.     

On the use of risk acceptance criteria in the offshore oil and gas industry

Risk acceptance criteria, as upper limits of acceptable risks, have been used for offshore activities on the Norwegian Continental Shelf for more than 20 years. The common thinking has been that risk analyses and assessments cannot be conducted in a meaningful way without the use of such criteria. The ALARP principle also applies, but the risk acceptance criteria have played a more active role in the assessment processes than seen for example in the UK. Recently there has, however, been a discussion about the suitability of risk acceptance criteria to assess and control risks. The purpose of this paper is to contribute to this discussion by presenting and discussing a risk analysis regime that is not based on the use of risk acceptance criteria at all. We believe that we can do better if cost-effectiveness (in a wide sense) is the ruling thinking rather than adoption of pre-defined risk acceptance limits. This means a closer resemblance with the ALARP principle as adopted in the UK and other countries, but is not a direct application of this practice. Also the building blocks of the common way of applying the ALARP principle are reviewed. The Norwegian offshore oil and gas industry is the starting point, but the discussion is to large extent general.     

The use of fracture mechanics in failure analysis in the offshore diamond mining industry

This paper considers two major failures of offshore diamond mining equipment, which should not have occurred had both fracture mechanics aspects and materials behaviour been more fully understood. The two case studies include (a) failure of a wire rope swivel, and (b) failure of a load cell (used for monitoring rope load). In case (a) the swivel shank had failed in reverse bending fatigue, although it was thought initially to experience only tensile stresses. The linkage system to the haul rope, through a form of clevis plates, led to high bending fatigue stresses, and the failure was exacerbated by inappropriate remedial changes, including poor choice of material, as well as stress concentrating effects induced by a shrink fit stainless steel collar. In case (b), complete collapse of a gantry, sheave wheel system and partial A frame support resulted from the prior brittle failure of the load cell, and this could be attributed to incorrect heat treatment of the load cell material. In both cases quantitative use is made of fracture mechanics in the failure assessment.     

On the use of the hybrid causal logic method in offshore risk analysis

In the Norwegian offshore oil and gas industry risk analyses have been used to provide decision support for more than 20 years. The focus has traditionally been on the planning phase, but during the last years a need for better risk analysis methods for the operational phase has been identified. Such methods should take human and organizational factors into consideration in a more explicit way than the traditional risk analysis methods do. Recently, a framework, called hybrid causal logic (HCL), has been developed based on traditional risk analysis tools combined with Bayesian belief networks (BBNs), using the aviation industry as a case. This paper reviews this framework and discusses its applicability for the offshore industry, and the relationship to existing research projects, such as the barrier and operational risk analysis project (BORA). The paper also addresses specific features of the framework and suggests a new approach for the probability assignment process. This approach simplifies the assignment process considerably without loosing the flexibility that is needed to properly reflect the phenomena being studied.     

Nonlinear wave theory in reliability analysis of offshore structures

The reliability analysis of offshore structures under wave and wind actions is considered using second order random wave theory. To represent non-Gaussian properties of the resulting wave kinematics, the Hermite moment transformation is used. Further, the so-called sample-specific linearization method developed already (to be used in conjunction with the directional simulation method and the linear wave theory) will be extended to take into account both (1) non-Gaussianity of wave/wind load due to nonlinear load processes and also (2) the non-Gaussianity of wave kinematics due to the nonlinear wave theory. This allows an out-crossing approach to be used to assess the structural probability of failure and the involving out-crossing rate (which is not generally available for non-Gaussian processes) is required to be estimated. Using the proposed procedure, simple structures are analyzed in one- and multi-dimensional cases and the results for structural probability of failure are compared with those obtained using simple linear wave theory. Outcomes show that the use of nonlinear wave theory may affect the results considerably.     

Uncertainty modeling in the fatigue reliability calculation of offshore structures

This paper presents a procedure of modeling uncertainties in the spectral fatigue analysis of offshore structures with reference to the reliability assessment. Uncertainties of the fatigue damage are generally embedded in response characteristics of the stress process and the damage-model used. Besides commonly accepted uncertainties in offshore structural analysis, which are associated with the modeling of structures and the random wave environment, there are also uncertainties arising from joint flexibilities that occur during the response, the wave–current and water–structure interactions. Uncertainties in joint flexibilities are associated with degradation of member connectivities during a response process. Uncertainties introduced by the wave–current interaction are related to the modeling of a random sea state, applied wave loads and water–structure interaction effects in general. The water–structure interaction, which is an important phenomenon to be considered in the analysis of dynamic-sensitive structures, introduces some added hydrodynamic damping. The associated uncertainties are reflected in the response analysis via the damping term. Therefore, in a quasi-static response analysis, these uncertainties disappear. In the spectral fatigue damage, in addition to the uncertainties of stress statistical characteristics there are some other uncertainties associating with the damage-model used. These uncertainties are related to experimentally determined fatigue data and configurations of selected joints at which damages are likely to occur due to high stress concentrations. This paper presents these uncertainty issues with emphasis on the application of a reliability assessment. However, some other uncertainties arise from approximations inherent in the model. They are assumed to be either comparatively negligible or can be considered within the current uncertainty models so that they are not treated further in this paper. In the calculation of the fatigue damage, a non-narrow banded stress process is used.     

An integrated reliability-based design optimization of offshore towers

After recognizing the uncertainty in the parameters such as material, loading, geometry and so on in contrast with the conventional optimization, the reliability-based design optimization (RBDO) concept has become more meaningful to perform an economical design implementation, which includes a reliability analysis and an optimization algorithm. RBDO procedures include structural analysis, reliability analysis and sensitivity analysis both for optimization and for reliability. The efficiency of the RBDO system depends on the mentioned numerical algorithms. In this work, an integrated algorithms system is proposed to implement the RBDO of the offshore towers, which are subjected to the extreme wave loading. The numerical strategies interacting with each other to fulfill the RBDO of towers are as follows: (a) a structural analysis program, SAPOS, (b) an optimization program, SQP and (c) a reliability analysis program based on FORM. A demonstration of an example tripod tower under the reliability constraints based on limit states of the critical stress, buckling and the natural frequency is presented.     

Ice protection of offshore platforms

Climate change-induced reduction in the extent and duration of sea ice cover, as well as an increase in energy demands, has caused renewed interest in exploring and drilling for oil in Arctic waters. Superstructure icing from sea spray and atmospheric icing in the Arctic may impact offshore platform operations. Though icing has not caused the loss of an offshore platform, it can reduce safety, operational tempo, and productivity. Historically, many ice protection technologies were tested on offshore platforms with little success. However, new technologies and modern versions of old technologies used successfully in aviation, the electric power industry, and ground transportation systems, may be adapted to an offshore environment. This paper provides a framework for assessing the relative threat of ice accumulation types, such as superstructure ice, glaze, rime, frost, and snow, to the safety of platform functions. A review of ice protection strategies for functional platform areas is also provided.     

Framework for implementation of maintenance management in distribution network service providers

Distribution network service providers (DNSP) are companies dealing with network infrastructure, such as distribution of gas, water, electricity or telecommunications, and they require the development of special maintenance management (MM) capabilities in order to satisfy the needs of their customers. In this sector, maintenance management information systems are essential to ensure control, gain knowledge and improve decision making. The aim of this paper is the study of specific characteristics of maintenance in these types of companies. We will investigate existing standards and best management practices with the scope of defining a suitable ad-hoc framework for implementation of maintenance management. The conclusion of the work supports the proposition of a framework consisting on a processes framework based on a structure of systems, integrated for continuous improvement of maintenance activities. The paper offers a very practical approach to the problem, as a result of more of 10 years of professional experience within this sector, and specially focused to network maintenance.     

On risk-based operation and maintenance of offshore wind turbine components

Operation and maintenance are significant contributors to the cost of energy for offshore wind turbines. Optimal planning could rationally be based on Bayesian pre-posterior decision theory, and all costs through the lifetime of the structures should be included. This paper contains a study of a generic case where the costs are evaluated for a single wind turbine with a single component. Costs due to inspections, repairs, and lost production are included in the model. The costs are compared for two distinct maintenance strategies, namely with and without inclusion of periodic imperfect inspections. Finally the influence of different important parameters, e.g. failure rate, reliability of inspections, inspection interval, and decision rule for repairs, is evaluated.     

Improvement of digital geographic data quality

The article focuses on the study of characteristics of digital geographical data and the influence of these characteristics on the quality of the basis being ready for the decision-making processes. Characteristics of digital geographical data are mainly described from the point of view of their technical parameters and keeping the technological indicators while obtaining them. Our system brings a user's view into the assessment of characteristics. We define user's requirements on data and suggest the system of their evaluation. The assessment of data characteristics comes out of the standard ISO 19113 and the theory of value analysis. Both systems are joined into a complex system for data evaluation. Technical characteristics of data are evaluated mainly by the level of accomplishment of qualitative indicators, e.g. meeting all requirements of horizontal and vertical mean square error, or by completeness of filling of all expected information. Meeting all users’ requirements is usually expressed by the level of user's satisfaction with the particular product. This level is set based on the survey among users. Simultaneously, the weight of the individual evaluating criteria is set according to specific types of solved tasks. The system of data evaluation is supplemented with a system of calculation of costs that are needed for obtaining the data. It is possible, however, to work either with the complete database, or to count also the influences of particular groups of data (e.g. communications, residential buildings, etc.) on the complete quality of the final product. The system of costs calculation and evaluation of data quality then enables the optimisation of the use of funds or disposable time for obtaining quality data. The use of the complete system is presented in a pilot project in which a model of terrain passability by a military heavy vehicle is solved.     

An innovative method for offshore wind farm site selection based on the interval number with probability distribution

There is insufficient research relating to offshore wind farm site selection in China. The current methods for site selection have some defects. First, information loss is caused by two aspects: the implicit assumption that the probability distribution on the interval number is uniform; and ignoring the value of decision makers’ (DMs’) common opinion on the criteria information evaluation. Secondly, the difference in DMs’ utility function has failed to receive attention. An innovative method is proposed in this article to solve these drawbacks. First, a new form of interval number and its weighted operator are proposed to reflect the uncertainty and reduce information loss. Secondly, a new stochastic dominance degree is proposed to quantify the interval number with a probability distribution. Thirdly, a two-stage method integrating the weighted operator with stochastic dominance degree is proposed to evaluate the alternatives. Finally, a case from China proves the effectiveness of this method.     

Bilevel joint optimisation for product family architecting considering make-or-buy decisions

Product family architecting (PFA) aims at identification of common modules and selective modules to enable product family configuration for mass customisation. Due to nowadays manufacturers moving more towards assembly-to-order production throughout a distributed supply chain, the common practice of outsourcing of certain modules entails make-or-buy (MOB) decisions that must be taken into account in PFA. While the PFA and MOB decisions are enacted for different concerns of the manufacturer and the suppliers, it is important to deal with joint optimisation of the PFA and MOB problems. The prevailing decision models for joint optimisation are mainly originated from an ‘all-in-one’ approach that assumes both PFA and MOB decisions can be integrated into one single-level optimisation problem. Such an assumption neglects the complex trade-offs underlying two different decision-making problems and fails to reveal the inherent coupling of PFA and MOB decisions. This paper proposes to formulate joint optimisation of the PFA and MOB problems as a Stackelberg game, in which a bilevel decision mechanism model is deployed to reveal the inherent coupling and hierarchical relationships between PFA and MOB decisions. A nonlinear bilevel optimisation model is developed with the PFA problem acting as the leader and each MOB problem performing as a follower. A nested genetic algorithm is developed to solve the bilevel optimisation model. A case study of power transformer PFA subject to MOB considerations is presented to illustrate the feasibility and effectiveness of bilevel joint optimisation.     

中医院技术标准效益评价指标体系探讨

本文采用循证科学的原理和方法,对我国现有的关于中医院综合效益评价的文献进行循证分析,对评价指标的使用情况进行描述性研究,并按技术标准的作用维度进行筛选和归类,以期为建立一套既系统、科学,又具较强适用性的中医院技术标准效益评价指标体系的构建提供理论参考,使技术标准效益评价工作规范、统一。     

多联机季节运行性能评价指标的差异性分析

分别对4套多联机系统进行35个工况的制冷与制热性能试验,获得多联机的全工况运行性能;在我国相关标准给定的典型气象参数和建筑负荷模型条件下,应用现有标准给定的制冷与制热季节性能指标对这些多联机进行评价,并与全工况季节运行性能进行对比,分析各种季节性能评价指标数据的差异,以期为我国多联机产品标准的修订提供参考。     

A synergetic approach for assessing and improving equipment performance in offshore industry based on dependability

The objective of this paper is to present a framework for assessing and improving offshore equipment performance based on dependability. The main idea is to employ principle component analysis (PCA) and importance analysis (IA) to provide insight on the equipment performance. The validity of the model is verified and validated by data envelopment analysis (DEA). Furthermore, a non-parametric correlation method, namely, Spearman correlation experiment shows a high level of correlation between the findings of PCA and DEA. The equipment of offshore industries is considered according to OREDA classification. The approach identifies the critical equipment, which could initiate the major hazards in the system. At first PCA is used for assessing the performance of the equipment and ranking them. IA is then performed for the worst equipment which could have most impact on the overall system effectiveness to classify their components based on the component criticality measures (CCM). The analysis of the classified components can ferret out the leading causes and common-cause events to pave a way toward decreasing failure interdependency and magnitude of incidents which ultimately maximize overall operational effectiveness.     

以可靠性为中心的维修的经济效益评估模型

以可靠性为中心的维修(Reliability—centered Maintenance,RCM)的经济效益评估模型是国内外首次系统、全面、简单、有效评估核电站维修优化成果的方法。本文从RCM经济效益评估内容和分类着手,详细介绍了分析后取消项目、增加项目显性及隐性经济效益评估模型,并结合大亚湾核电站现状进行了实证应用与分析。     

Conjunct roles of client and vendor control in offshore-outsourced project performance

This study explores the conjunct roles of a series of formal and informal control mechanisms exerted, respectively, by client and vendor in offshore-outsourced project performance. Using a sample of 203 offshore projects executed by vendors in China, the results indicate that client process control enhances (or complements) the effect of vendor outcome control, yet impairs (or substitutes) the effect of vendor process control. Conversely, client outcome control enhances (or complements) the effect of vendor process control, yet impairs (or substitutes) the effect of vendor outcome control. Further, for the two informal control mechanisms, the results indicate that client relational control enhances (or complements) the effects of both vendor process and outcome control, whereas vendor clan control only enhances (or complements) the effect of client outcome control on offshore-outsourced project performance. These findings not only contribute new insights for the organisational control and the outsourcing literature, but also provide managerial guidance for client and vendor managers on how to exert and fine-tune their control mechanisms to promote project performance.     

标准质量与实施效果评价方法及应用研究

标准在提高产品质量、服务质量以及提升核心竞争力等方面发挥了越来越重要的作用。标准质量和应用实施的效益是影响标准作用发挥的重要因素。本文主要对标准质量与实施效果评价的内容与过程、社会效益和经济效益的指标体系进行了分析,提出了量化综合评价的指标体系,并对量化综合评价的方法和实施过程进行了初步探讨。