Structural Health Monitoring with Application to Offshore Structures

Srinivasan Chandrasekaran

• Publisher: WSPC
• Year: 2019
• ISBN:
  9789811201103
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Structural Health Monitoring (SHM) deals with assessment, evaluation and technical diagnosis of different structural systems of strategic importance. Extensive knowledge of SHM shall lead to a clear understanding of risk and reliability assessment of structures, which is currently mandatory for structures of strategic importance like bridges, offshore structures, etc.

This comprehensive compendium features explanations and salient illustrations of SHM with applications to civil engineering structures, in general and offshore structures, in particular. The book is unique with respect to its contents, experimental case studies in lab scale and text presentation style. A detailed subject matter of this nature is currently scarce in the literature market.

The must-have volume is a useful reference text for senior undergraduate and postgraduate students, professionals, academics and researchers in civil engineering, ocean engineering, mechanical engineering, and structural engineering.

Contents:

• Structural Health Monitoring: An Overview
• Structural Health Monitoring: Detailed Perspective
• Sensor Technologies
• Applications of Structural Health Monitoring
• References
• Tutorials, Keys and Test Papers

Readership: Researchers, academics, professionals, senior undergraduate and postgraduate students in civil engineering, ocean engineering, mechanical engineering and structural engineering.Structural Health Monitoring;Offshore Structures;Vibration-Based Monitoring;Wireless Sensors;Civil Engineering Structures;Experimental Studies;Deep-Water Platforms00

Chapter 1

Structural Health Monitoring: An Overview

This chapter deals with the necessity of health monitoring of structures, focussing on the various components involved in structural health monitoring (SHM). While implementation of the SHM scheme imposes a big challenge in real timescale, the details of various factors that influence the implementation process are also discussed. Several components involved in the SHM process are highlighted, while specific issues with respect to concrete structures are discussed in detail. This chapter also summarises several advantages of SHM along with the long-term and short-term benefits both from economic and safety perspectives.

1.1.Introduction

Civil engineering structures in general and offshore structures in particular attract huge capital investments apart from demanding a longer period of time for their construction, installation and commissioning. Continuous monitoring and proper maintenance of structures are vital requirements of the modern society. It is no longer optional for the owner to upkeep public buildings in terms of its functional maintenance, and legal mandates have also been imposed on the safety and security of public buildings; in this aspect, health monitoring of infrastructure is also supported and funded by government agencies. While advanced methods of analysis and design of structural systems, which are in compliance with the code guidelines, confirm least probability of design failure, construction difficulties pose various challenges, resulting in deviations to the design solutions causing uncertainties. In addition, ageing of structures poses additional challenges in terms of strength degradation and non-availability on demand.

In addition to helping engineers recognise poor structural conditions and other safety issues, advancements in SHM also help professionals determine potential risks of buildings caused due to ageing and other environmental factors. SHM is particularly useful in preventing water and flood damage caused by failed dams, dykes, pipelines, and other similar structures. SHM essentially examines the state of the present condition of structural systems to assess their functional fitness and performance levels. If the condition assessment envisages a poor performance level in comparison to that of the desired ones, repair processes are immediately initiated. It is therefore emphasised here that structural repair is invoked only when the structural system undergoes significant damage; but such practices are not acceptable in the case of structures of strategic importance, such as nuclear power plants, large reservoir dams, offshore and coastal structures. Hence, in order to plan for a schedule of preventive maintenance, both in terms of shutdown time of the facility and economic planning, SHM is used as one of the essential tools in reliability engineering. Further, SHM also enables the reduction of long- and short-term costs with respect to maintenance and repair of public building; this reaps the economic benefit of the construction industry to a great extent.

Large, complex and costly engineering structures are constructed to last for a longer life span. While the usual practice is to design them for maintainable conditions, their design life is usually extended even under non-maintainable conditions. The general scope of structural health monitoring (SHM) includes structural assessment, monitoring and control, which can be abbreviated as SAMCO; all the three components are vital in SHM. Structural assessment involves the assessment of actual conditions and load-carrying capacity of the structural systems. Diagnosis is a vital part of SHM, which involves integration of various sensors used for measurements, computational power and processing ability within the SHM system itself for effective outcome. Structural monitoring deals with the supervision of structures on a continuous basis using sensors or electronic gadgets. They are carried out in order to maintain the functional utility of the structure; to be very precise, it is done to ensure the availability of the system on demand. Thus, it includes both periodic and preventive maintenance. Structural control deals with the control of the dynamic response behaviour of structures under various environmental loads. This involves the establishment of control mechanisms so that responses are under the preferred limits even on unforeseen increase of load magnitudes. The above three vital components can be prioritised as follows:

•Assessment involves the preparation of existing conditions of structure in terms of its geometric fitness and load capacity and therefore deals with the examination of those conditions of the structure that one can take advantage of beyond its design life.

•Monitoring is more or less related to maintenance, ensuring the availability of the structural system on demand. Monitoring of structures does not necessarily mean knowing the status of structures in real time. Since all structures are designed with a margin of safety, which can be used to exploit its design life to at least a marginal extent, tolerance can be allowed in delaying maintenance. Monitoring, therefore, enables effective planning of maintenance.

•Control aims to reduce or mitigate undesirable (modes of) response even under unforeseen increase in magnitude of loads. This is very vital in the case of offshore compliant structures as their load resistance is often derived from FORM resistance and not from their strength. Excessive displacements in global degrees of freedom, as a rigid body motion, may become undesirable as this may even challenge their hydrodynamic stability. Hence, control of both rotational displacements in particular and translational displacements in general is very important for ensuring structural safety and safe operability.

Priority depends upon two factors: (i) type of the structure and (ii) economic considerations, which drive the whole concept of application of SHM in structural engineering. In view of a normal type of structure under given budgetary normal considerations, assessment of the condition of the structural system is very important. In order to develop an effective design, one can even prescribe control mechanisms; but, prior to that, maintaining the utility value of the structure to ensure its availability is also equally important.

SHM actually deals with the development and implementation of methods and techniques, which are useful for ensuring the availability of the structural system to perform its intended function on demand. Thus, the main objective is neither exercising a control algorithm nor the assessment of load-carrying capacity but ensuring the functional utility value even under critical environmental conditions. Continuous monitoring (or even periodic monitoring to a larger extent) ensures preventive maintenance and helps policy and planning guidelines to ensure the functional value of strategically important structures. For example, a periodic and preventive maintenance of a highway bridge or gas pipeline shall ensure uninterrupted service and minimum downtime, even in the case of any critical repair. Civil engineering professionals agree to the fact that maintenance of infrastructure facilities is vital in order to elevate the standard of the structural systems in terms of their serviceability, appearance and safety. As certain clauses of structures, such as industrial structures, highway and railway bridges, nuclear power plants, offshore structures, naval structures, etc. are vital for the economic growth of the country, ensuring their availability on demand is necessary for the safety and security of public life. They also influence the economic growth of the nation in the international market. Society essentially depends on these structures for various reasons, such as economic, environmental, life-quality updates, safety and employment perspectives. Most of such structures also reach critical age, which can result in strength degradation, degraded quality of appearance, decreased load-carrying capacity and reduction in the overall dependency. In order to ensure and continue a comfortable dependency on these structures, both periodic and preventive maintenance are important. There are three ways by which maintenance can be attempted: (i) periodic maintenance; (ii) preventive maintenance; and (iii) critical maintenance (maintenance on demand).

Critical maintenance is more alarming and dangerous, where a structure is maintained only after its critical age. In this case, recovery of strength of the structure is very difficult. For example, let us consider offshore structures used for oil and gas exploration and production. An offshore platform working continuously on oil and gas production results in an outcome or commercial benefit, which could be revenue, employment, constant research and development for further exploration and production. If, due to unavoidable reasons, structure needs to be shut down for maintenance, the shutdown period (known as downtime) will primarily lead to loss of revenue, which is not preferred. On the other hand, if a total shutdown could be avoided by way of preventive maintenance, it can lead to several advantages; economic benefit is the foremost one. When the structures reach critical age, there can be strength degradation due to material corrosion in sea. Thus, these structures will not be able to alleviate the encountered lateral loads successfully. It may also result in structural failure, which can cause disaster. One cannot afford to lose such novel, unique and high-investment structures. Preventive maintenance can avoid such catastrophic failures. To carry out preventive maintenance, one must assess the present condition, monitor the condition continuously and then plan the repair procedure even before the structure actually needs it. Instead of doing a periodic maintenance, strategic structures can demand a preventive maintenance. Preventive maintenance is possible only through detailed assessment, monitoring and planning of repair, which are all encompassed under the framework of structural health monitoring. Therefore, one of the most important deliverables and main outcomes of SHM is the avoidance of a premature failure or a breakdown of the facility.

Let us consider another example: a naval dockyard, which is essentially an open channel to house large vessels for their periodic maintenance. Periodic maintenance could be partial or complete weld upgrade, painting, treatment for biofouling, upgrade fault correction for electromechanical systems, etc. Navy operates on various kinds of strategic vessels like submarines, which need to be inspected for periodic maintenance or emergency fault correction; such operations are usually carried out in a dockyard. Dockyards are very few in number and quite expensive. If a dockyard is undergoing periodic maintenance, which demands the shutdown of operation during an emergency requirement of docking a naval vessel, functional assurance of the essential service becomes unguaranteed. Hence, the shutdown time caused by a periodic maintenance schedule on a dockyard deprives the basic utility value of the system itself; this can be avoided if the dockyard undergoes preventive maintenance. Maintenance should be carried out in a preplanned and preventive manner, so that the dockyard always remains functional even during critical environmental conditions. However, utility value can be slightly decreased in terms of its operational capacity, but a complete shutdown of the dockyard can be avoided. Hence, preventive maintenance of essential services is far more advantageous in comparison to periodic maintenance. SHM ensures constant maintenance costs and high degree of reliability of the service instead of high maintenance cost and low degree of reliability.

1.2.SHM Analogy

When a human being falls sick, he is physically examined by a medical doctor. Similarly, inspection demands a complete analysis of the structural condition, which can be done by structural monitoring. Further, in the case of a human being, health can be monitored on a continuous basis through several means under any situation as per the advice of the doctor. In a similar manner, struct...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Preface
• About the Author
• Contents
• List of Figures
• List of Tables
• 1. Structural Health Monitoring: An Overview
• 2. Structural Health Monitoring: Detailed Perspective
• 3. Sensor Technologies
• 4. Applications of Structural Health Monitoring
• References
• Tutorials, Keys and Test Papers
• Glossary
• Index