Acoustic Emission - New Inspection Technique
Authors: Dunegan Harold L. 
Published: October 2008

This report shows that high amplitude acoustic emission signals are present from corrosion products accumulated on crack surfaces of a steel similar- to that used for offshore platforms. It is postulated that these signals, as well as those present during crack extension due to fatigue can be utilized to locate and evaluate fatigue cracks growing on an offshore platform.

Critical issues for successful continuous monitoring such as signal amplitude, separating valid signals from noise and operator involvement are given. Solutions of the critical issues involve the use of 1) frequency filtering, 2) spatial filtering, 3) parametric filtering, and 4) amplitude distribution analysis. An example of a new method of data logging using a computer-inter faced acoustic emission system which gives an operator a quick survey of the relative activity of all nodes' on a typical platform is presented. It is shown that acoustic emission techniques can provide practical alternatives to present methods being used for inspection of deep water offshore structures undergoing structural degradation due to fatigue crack growth.

INTRODUCTION

Acoustic Emission is defined as that class of phenomena whereby transient elastic waves are generated by rapid release of energy from a localized source or sources within a material. Acoustic emission signals have been observed from such phenomena as Martensitic phase transformations, breakaway of dislocations from material undergoing plastic strain, deformation twinning, and the initiation and growth of cracks due to fatigue, stress corrosion, hydrogen embrittlement, etc. Thus, acoustic emission testing differs from most other forms of nondestructive testing in that an energy reservoir in the form of strain energy must be present, and a propagating flaw in a structure then transduces minute amounts of strain energy to a transient elastic wave which will propagate through the structure at the speed of sound in the structure. The spectral content of this signal is very broad and can be detected up to frequencies as high as 30MHz. Most practical applications involve monitoring frequencies from 30kHz to 1MHz. This is high enough to eliminate most low frequency noise present in a test, yet low enough that the signals will propagate large distances in most metals with only a small amount of attenuation.

Acoustic emission techniques have evolved from the laboratory and are being applied in a fairly routine manner for the inspection of pressure vessels during proof testing.1,2,3 A manufacturer of blowout preventors4 utilizes acoustic emission monitoring of each unit during proof testing as a final quality assurance test prior to shipment. Others are utilizing the technique for inspecting underground pipelines, weld monitoring6, and for the inspection of aircraft components7. A comprehensive bibliography of acoustic emission publications for the years 1970-72 can be found in Reference 8.

GENERAL

Up to this time, acoustic emission techniques have not been used for monitoring of offshore structures, even though in many ways they are a better subject for application of the technique than proof testing of pressure vessels.