Repair Scenarios Research Articles

Expanding the applicability of ISA TR84.02 in the field

The ANSI/ISA S84.01 standard was released in 1996. The companion technical report TR84.02 is in the process of being completed. The latter document is intended to provide the methodology to implement the safety performance requirements of the standard for the safety system. In this document, three techniques are presented; these being Simplified Equations, Fault Tree Analysis and Markov Modeling. Of the three, only the Simplified Equations approach would reasonably be utilized in the field by plant personnel. The Simplified Equations provided as part of TR84.02 comprehend common cause failures, systematic failures, and second failure prior to repair scenarios. They do not however comprehend the use of redundant field devices which are dissimilar, and as such have different failure rates. This situation is quite common in practice, and simple to manage using enhanced equations for the computation of PFDavg. A set of these equations for typical redundant architectures in the field, and several examples of their applications in safety loop analysis are derived and presented in this paper.

Design guidelines for dealing with breakdowns and repairs in collaborative work settings

We present the results of an investigation into collaborative work within UK breast screening centres. In particular, we focus on the ways in which the organization of breast screening work and the design of artefacts such as report forms afford the detection of breakdowns—i.e. situations calling for departures from the “standard” procedures—and their safe and efficient repair. We use this to devise a framework for classifying different breakdown–repair scenarios. These scenarios are distinctive in their repair costs and impacts and can be used to highlight the implications of design decisions in collaborative work settings. Finally, we show how, via a set of derived guidelines, the results of this analysis were applied to the design of artefacts, including a user interface and computer-generated documents, for a computer-aided image analysis system intended for application in the UK breast screening programme.

The challenge of numerical modeling of strains and stresses in concrete repairs

Designers presently have no quantitative rules to evaluate which of several proposed repair designs is the most appropriate in a given situation. A numerical model that would allow us to compare, on a quantitative basis and with relatively little work, different repair scenarios would be very helpful. This paper describes the problems that need to be solved in order to develop a realistic numerical model of the mechanical behavior of thin concrete repairs. The “ critical thickness” concept is defined as the superficial zone where there are steep relative humidity and temperature gradients due to daily and seasonal weather changes. Cracking of the repair layer is maximum within this “ critical thickness” because this zone is subjected to significant strain cycles.

Weld Joint Design for EVA Repair of On-Orbit Fluid Systems

Welding in space will be an important process for the permanent repair of both low- and high-pressure fluid line assemblies used on large spacecraft. Space station fluid systems are candidates for in-space repair. They provide a variety of tubing materials, sizes, pressures, and working fluids as well as including ancillary systems that demonstrate typical constraints to the repair process for a more realistic overall scenario. Damage to on-board fluid systems could occur as a product of faulty or fatigued joints, micrometeoroid impacts, collision with other objects, and over-pressure strain or burst. Incorporating terrestrial experience with automatic pipe-welding tools and that of both the U.S. and former Soviet space programs in space welding, hypothetical repair scenarios were analyzed using Space Station Freedom as a representative baseline. Weld joint design criteria are then derived. A new weld joint for in-space repair, which meets these system-derived criteria, is presented and a stress analysis is performed.

An application of a bathtub failure model to imperfectly repaired systems data

AbstractIn this paper we use an exponential power distribution to develop a bathtub failure model for repairable systems. These results are compared with those using the more familiar Weibull distribution. The analysis is extended to three types of repair scenarios: good‐as‐new repairs (in which the repair following a failure completely refreshes the failure intensity), bad‐as‐old repairs (in which the repair has no effect on the failure intensity) and imperfect repairs (in which the failure intensity is only partially reset). We apply this analysis to hydro‐electric turbine data and find that an exponential power distribution produces a well‐defined bathtub failure model when used with an imperfect repair assumption. We furthermore find that this bathtub model produces an improved fit over the equivalent Weibull model.