No abstract

The process of implementing a damage identification strategy for aerospace, civil, mechanical, and offshore infrastructure is referred to as structural health monitoring (SHM). As a subject, it has been developed and refined over the past three decades and the research in the area has accelerated in multiple subtopics in recent years. This article provides a brief review of damage detection strategies and sensor optimization. Damage detection strategies include global methods or localized methods, where the former uses changes in vibration responses of a structure before and after damage occurs to detect the damage, identify its location, and evaluate its severity, if possible. The latter uses localized geometrical changes or material changes to identify the damage. The review focuses on academia and industry experiences of modal‐based damage detection methodologies, where changes in modal frequency, mode shapes, modal shape curvatures, and modal strain energy are used as damage indicators. In a structural health monitoring system, sensor optimization is in the central position connecting data acquisition, signal processing, data analysis and structural health evaluation output. Sensor optimization aims to minimize the number of sensors to be used and to identify the optimal locations to deploy them. Many optimization techniques have been applied in sensor optimization, and the application of those methodologies is reviewed in this article.

An experimental study was conducted to investigate the ability of ribbon fairings to suppress fl ow-induced vibrations on a long fl exible horizontal cylinder. The test matrix included towing the cylinder at various speeds, towing the cylinder in regular waves, and investigating the infl uence of partial coverage on the response behavior. The test cylinder was 29 m long with a length to diameter (L/D) ratio of ~760. Interior to the tensioned cylinder model were six sets of unequally spaced biaxial accelerometers in a lightly pressurized environment keeping the interior dry. A string potentiometer was externally attached at the center of the model to provide a reference for later displacement estimates based on integration of the acceleration data. The time domain decomposition method (TDD) was used to recover mode shapes, damping characteristics, and modal contribution factors. For the uniform current cases, the fi ndings illustrate that ribbon fairings are effective and provide increased damping when compared with bare cylinders. Partial coverage demonstrates that localized suppression becomes increasingly less effective as the percentage coverage is reduced. The introduction of regular waves to the towed cylinder cases illustrates the ineffectiveness of ribbon fairing to suppress the orbital motions induced by the waves, which is preferable to the amplifi cation typically observed for airfoil fairings. Keywords: Combined current and regular waves, horizontal fl exible towed cylinder, ribbon fairings, time domain decomposition, uniform currents, vortex-induced vibrations.

Cables and various subsea product fl ow-lines are often subjected to fl ow-induced vibrations as a consequence of wave or wave and current fl ow excitation. An experimental study that explores the comparative suppression effectiveness of airfoil and ribbon fairings is presented. In the experimental investigation, an instrumented tensioned 29 m horizontal composite cylinder with a length to diameter ratio of 760 is subjected to a range of uniform current and combined current and regular wave conditions. In the analysis of the cross-fl ow response amplitude ratios, modal parameters, such as mode shape, damping ratio were extracted. The analytical procedures were performed using a modifi ed time domain decomposition technique. The analysis illustrated the amplifi ed response behavior for bare cylinders, and illustrated that full coverage by airfoil and ribbon fairings could effectively suppress fl ow-induced vibrations in current only conditions, but both fairings were ineffective when regular wave conditions were superimposed on the uniform current conditions, even though the airfoil provided larger damping than ribbon fairings. Under combined wave and current conditions, the response behavior of either suppression device was found to be insensitive to coverage densities.

There have been many interesting technical ideas and parallels in thinking that have resulted from the technical discussions that I have had with Professor Jerome J. Connor since 1986. This brief article provides a context and reflects upon his mentorship, which helped shape the thinking of a then young professor from another University, and on the impact that Professor Connor has had on his students and others beyond the halls of MIT's Building 1. The optimization topic presented here provides an example of that influence. Extracting the desired response information from an instrumented slender structure, while minimizing the number of sensors, is a challenging problem requiring well-defined objectives that can be used in an optimization process. In this study, a methodology that builds upon a Genetic Algorithm optimization procedure is used to investigate sensor placement needed to recover specific vibration modes. Data recorded from an experiment investigating the flow-induced vibration of a smooth horizontally towed cylinder is used to explore the optimization process and subtleties associated with its application subject to single or multiple objectives and gaps in sensor data due to several possible constraints. The use of the Paterno Front Method and the difficulty in accurately capturing higher modes are addressed.