Ships and Offshore Structures XIX

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This represents a significant improvement in integrity management IM approach over traditional methods where there is little, if any, interaction between the design and in-service phases of the life of the asset. Further, targeted inspection and data collection of asset health for critical areas within the hull leads to overall risk reduction.

Introduction As analysis techniques become more sophisticated, ships and offshore structures have become more complex and innovative. As a result, their designs, with certain unique aspects in their configuration, do not necessarily mimic those of their predecessors. In particular, naval vessel design emphasizes minimum structural weight in order to maximize payload. Compounding this problem is the fact that many organizations differentiate between the Capital Expenditure and the Operational Expenditure segments.

This can mean that design features which improve a vessel s operability, inspectability and maintainability are rejected at the procurement stage in the interest of decreased initial costs and, as a result, the traditional, prescriptive inspection and maintenance methods are no longer the most effective. It is imperative that those elements of the vessel s structure or machinery systems more prone to deterioration and damage be identified early in the design process.

While there remains a strong need for traditional rule-based and prescriptive approaches, marine assets are becoming more complex, have a higher degree of novelty, and many aspects of their designs are falling outside or ahead of the development of traditional Class Rules. Risk and reliability-based technology is finding increased application because of the greater use of performance-based criteria and because of the current demands from clients for more flexibility in the way classification services are provided. Ever-expanding technologies often require the abandonment of trusted methods, the stretching of boundaries and the adoption of new, unfamiliar procedures.

Risk and reliability-based design, operation and integrity management programs are all becoming more commonplace in this environment. In addition, the operator s control of the integrity management of assets must now reach far beyond the minimum. Society expects due diligence and proactive management from vessel operators.

Today s organizations must also adapt to constant advances in technology while burdened with the mandate to do more with less as budgets grow leaner. The leading edge operators will consider operability and maintainability from the initial concept and detailed design how best can it be designed for optimum inspectability and maintainability?

Traditional practice as exemplified by prescriptive Rules and standard methods lacks the flexibility to respond to these demands. Risk and reliability-based methodologies allow systematic and rational ways for dealing with variations and deviations from the standard approach. These more advanced methods of maintenance and inspection strategy development follow along an evolutionary continuum that other industries are also following see Figure 1 at the end of the chapter.

This chapter will describe an approach to developing both Risk-Based Inspection and Reliability-Centered Maintenance plans for vessel structures and machinery that have the potential to result in significantly improved asset integrity management and possible cost savings.

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The methods presented herein aim to support optimized life-cycle integrity management of ships and offshore units by utilizing risk and reliability-based methods. Risk-Based Approaches The marine and offshore industries are drawing upon the lead set by other industries nuclear, aircraft, etc. Risk-based methodologies for inspection plan optimization originated in the nuclear industry in the s and over the years have migrated into other industries such as the downstream petrochemical industry in the s and s. These approaches are now moving into the upstream sector of the oil and gas industry and, to a lesser extent, the shipping industry.

Of particular interest has been the application of risk-based inspection RBI and reliability-centered maintenance RCM techniques in which experience-based data related to various degradation mechanisms are applied to set inspection and maintenance frequencies and scopes.

The implementation of these risk and reliability-based techniques into the development of a plan provides an alternative to prescriptive time-based inspection and maintenance planning. Figure 1 provides a schematic on the evolution of inspection and maintenance plan strategies. The compliance-based strategy Phase 1 , also referred to as rule-based, is generally representative of the traditional class or regulatory requirements. Inspection plans derived from such an approach have generally been developed based on years of experience and tend to provide a broader brush inspection plan and a minimum standard.

These approaches are based on the experience obtained from inspections performed on unrestricted trading vessels. The condition or performance-based methods Phases 2 and 3 represent the next logical step in the evolution from traditional methods. Degradation models and input from subsequent inspections are used to forecast the condition of the.

When the condition is predicted to reach a predefined threshold, inspections are conducted. This method relies heavily on the likelihood of structural degradation but does not explicitly include the associated consequences, which is a key aspect of the next evolutionary step, the risk-based approach Phase 4. The risk-based methods include aspects of the condition-based methods using trending techniques to estimate likelihood, but also factor in an estimation of the consequences of the structure s degradation and potential failure, enabling the program resources to be optimized and focused toward inspecting those items which have a greater overall risk weight.

Ships and Offshore Structures XIX

Once those items are identified, optimum methods of inspection or maintenance are then selected. Figure 1. Evolution of Inspection and Maintenance Strategies 3. Slowly, such concepts are also seeing application via marine operators who see a way to tailor their inspection programs for a specific ship type and also to have a mechanism to adapt the program as the vessels age.

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Operators feel there are significant benefits in developing RBI plans that are tailored to their asset in regard to both design and operation. By taking this approach, the inspections are more targeted and the operational constraints better managed, resulting in a more optimized inspection program while maintaining the same level of safety.

Figure 2 demonstrates the basic purpose of RBI to allocate resources in accordance with risk. In other words, the goal of a risk-based inspection study is to allocate the resources. Figure 2. RBI Concepts Both the traditional approach to hull integrity largely class-based and an enhanced risk-based approach lead to the ultimate goal of confirming the hull structure is properly maintained in accordance with industry and statutory standards. For the risk-based approach, a major contributor to the development of such a plan is the foundation of experience from the Class Rules which begins with the historical experience of the class society.

The main drivers which have sparked the industry s interest in RBI for hull structures are the potential benefits of implementing such a plan. There are significant benefits in developing a plan that is tailored to a specific class or type of vessel rather than following a rule-based approach. The following provides a list of some key benefits from a risk-based inspection plan. Asset Specific Plan The plan is tailored for the particular design and operational variables such that resources are focused on the highest risk components.

The advantage of this is more focused inspections which target the critical components within the structure. The plan can also incorporate overall business requirements, such as required asset utilization or compartment downtime limitations. Demonstrable Basis for the Inspection Plan An RBI plan provides a rational basis for the extent and methods of inspections based on combining structural analysis and structural reliability results. This allows additional flexibility for inspection planning and execution, and provides a better understanding of what items are critical and when they become critical.

Formal Approach of Collecting Information and Assessing Inspection Results In order for an RBI plan to be executed and updated, data must be captured in a format that can be organized and assessed to verify the condition of the hull.

Often with. Generally, this is the extent of the data s usefulness. For RBI, the data collected from the inspections is used to validate and update the degradation models and determine if adjustments in future inspections are warranted as a result, some form of electronic integrity data management tool is typically required to store and trend data.

Potentially More Cost-Effective An RBI plan may provide justification to extend inspection frequencies, which may reduce the number of inspections and associated costs. However, this may not always be the case. In some cases, inspection intervals for compartments may be reduced and inspection scopes may be more rigorous, offsetting any cost savings that may be obtained in other compartments. Regardless of the cost, an RBI plan provides a means for risk reduction and a rational basis for the intervals and inspection scopes which are optimized based on the asset s service conditions RBI Methodology Structural reliability-based methods can assist in providing a framework for quantifying loading and degradation mechanisms such as fatigue and corrosion through a systematic consideration of the probabilistic uncertainty in each degradation mechanism.

Further, such methodologies allow for these mechanisms to be quantified in a time-dependent manner.

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By applying structural reliability analysis and risk assessment techniques to inspection planning, the operator is given a tool by which he can justify the allocation of resources to those structural components with a higher risk profile and, at the same time, potentially relax inspection activities for low risk components to optimize and target inspection efforts. The overall approach to the development of the RBI plan involves the use of structural reliability methods which are then applied to determine the inspection intervals based on environmental loading as applied to strength considerations of the hull girder, stiffened and un-stiffened plate panel strength, and finally to welded connection fatigue life.

By tracing the time-varying reliability index for these structural components, the risk-based inspection intervals can be determined. This methodology has been applied to several floating production units operating throughout the world. Figure 3 shows the typical flow chart for this hull structure RBI methodology. The various boxes represent specific information or analysis necessary to develop the inspection plan. The plan development not only includes structural analysis results but also historical data, tank service condition data, condition summary, qualitative risk assessment, leak potentials and information on all other external structures that may affect the hull inspection.

The process starts with an initial structural analysis of the hull, consisting of both strength and fatigue assessments. The analyses provide global stress and fatigue results in the as-gauged condition as well as local models of various critical areas to further refine the assessment. The results of these assessments allow the identification of specific critical areas of the structure that are more prone to high stress or fatigue damage so that they can then be targeted in the inspection program.

He has also authored four textbooks: Seismic Design Aids for Nonlinear Analysis of Reinforced Concrete Structures, Analysis and Design of Offshore Structures with Illustrated Examples, Advanced Theory on Offshore Plant FEED Engineering, and Dynamics of Offshore Structures in press , which are popular among graduate students of civil and ocean engineering and are also recommended as reference material for classroom studies as well as research in many universities in India and abroad.

Exploration processes need to become cost-effective, which is possible only by employing innovative design strategies to alleviate the environmental loads that are encountered at greater water depths. Because innova- tive platform geometries are envisaged, a detailed understanding of their response behavior under various environmental loads is inevitable. The chapters in this book are organized such that the reader gets an overall idea of various types of offshore platforms. Chapter 1 provides details of different structural forms, function, and limitations of offshore platforms including the newly developed FSRUs, offshore triceratops, and environmental loads acting on marine structures.

Closed form equa- tions for computing such loads are given for ready reference. Chapter 2 discusses the ultimate load design with illustrations on plastic design, shear center, and theories of failure, leading to a detailed understanding of plastic design.

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Chapter 3 gives details on fluid—structure interaction, and shows details of numerical models carried out in the research perspective. The hydrodynamic response of offshore structures with perforated members is one of the recent research applications found to be among the effective manners of retrofitting offshore structures. The results of recent research, validated by experimental and numerical studies, are also presented, and innovative ideas that lead to the retrofit and renovation of marine structures are introduced.

Chapter 4 is focused on an introduction to reliability as applied to marine struc- tures. Different levels of reliability, along with problem formation, are explained in a convenient style.

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