Using Testing Methods to Evaluate Performance and Integrity of Offshore Mechanical Systems
Authors: Bedoya Julian J. , Alexander Christopher Richard 
Published: October 2011

As engineering offshore applications become more critical and make increased use of material capacities, the ability to perform full-scale qualification tests has become extremely valuable. Types of testing can include proof of concepts tests, testing at design conditions, to destructive testing. In all instances, testing provides either increased confidence in theoretical calculations or numerical modeling, or opportunities to make design improvements and possibly save costs.

This paper emphasizes discussions on how testing can increase the reliability, integrity and performance of offshore systems. Test cases are discussed in this paper that include qualification of pipe through limit state experimental burst testing per API RP 1111, pressure cycle fatigue testing of a damaged offshore pipeline, and testing of an offshore carbon-fiber reinforced drilling riser.

Introduction

Numerical methods and analytical assessment techniques are invaluable tools that permit engineers make significant contributions to projects by saving time and money in the design, analysis, fabrication and testing stages.

A large portion of the value added by numerical modeling is the ability to parameterize the performance of a component relative to a large number of different designs. As a result, numerical modeling can streamline the design to fabrication process, with an increased level of confidence in the performance. However, numerical modeling is not a substitute for testing since the results of the former are subject to interpretation of the induced stresses, of the assumed material properties, and the correct definition of the boundary value problem. Appropriately designed and executed experimental techniques provide important information and insight that cannot be obtained by any other means. In many cases, testing represents the last stage before a design is launched, scrubbed, or refined. This is particularly important and useful when the limits of a material are sought out, as these determine factors of safety with respect to catastrophic failure, expected life of components and the performance of damaged components.

Factors of safety are governed by two main drivers: first and foremost is safety, and second is economic viability. Fryer and Harvey [1] provide an excellent discussion on this topic. The main take-aways are that as our knowledge of the material properties and performance of the design increase, our latitude for error decreases as we are permitted to choose lower factors of safety. Furthermore, the cost of acquiring this "missing" knowledge on performance can be very expensive, and the position of the "economic fulcrum" varies with each application. In some cases, it is more economical to pay for extra material and a larger factor of safety, rather than the engineering time to acquire more knowledge. There are other cases in which the material costs become prohibitive, impractical, or a "loss of function" results; in such cases the cost for the increase in knowledge is essential [1]. Offshore operations and equipment are frequently in the latter category where integrity and reliability of an application are essential. This paper provides brief overviews of different, real-world examples of when testing is especially appropriate, with a specific emphasis on offshore applications.