zach mccann - sonomatic ltd

Copyright © Sonomatic Ltd 2016

A Brief History of Non Destructive Testing of

Austenitic Generator Retaining Rings

www.sonomatic.com

Revision: 000 – Mr. Peter Day, Dr. Peter Ford and Zach McCannDate: 23/11/2016


Sonomatic• A leading provider of advanced and conventional inspection and integrity

services to the Oil and Gas industry

• Sonomatic has over 35 years experience

• Development of inspection techniques

• Development of deployment solutions

• Field delivery, analysis and reporting

• Long and successful track record of inspection of safety and

business critical offshore structures

• Privately owned – US shareholders

committed to development

(reinvestment for growth)

• >140+ employees

• Turnover approx £23m

• Global presence, head office

Warrington


Contents

Brief overview retaining ring

metallurgy.

Overview of Applied NDT

Methods Circa 1970 – Present.

Surface Testing Techniques

Penetrant Testing

Eddy Current Testing

Ultrasonic Techniques

Pulse Echo

Time of Flight Diffraction

Creep Waves

Implications for Engineers


Metallurgy• In the 1960’s to mid 1980’s Retaining Rings were manufactured in

either ferritic steel or high strength austenitic steel such as 18%Mn

5%Cr. Austenitic steels were highly susceptible to stress corrosion

cracking if moisture was present.

• 18%Mn/5%Cr end rings were predominant up to the early 1990’s.

Since the late1980’s retaining rings have been manufactured in

18%Cr 18%Mn and 0.6% nitrogen high strength steel which is highly

resistant to SCC and have largely replaced the 18/5 material.

• The change in the material composition to 18-18 for the new

generation of the Retaining Rings was considered at the time a fix to

the 18-5 SCC degradation mechanism and by enlarge these rings

were also perceived to be a fit and forget solution.

• However after commissioning of 18-18 retaining rings there were

cases of cracking, fretting and corrosion reported.


• In 2014 an EPRI survey of 18-18 retaining ring damage was released and

noted that the incidence of arcing damage in retaining rings was 75% in

Australia and less than 5% in Europe and North America.

• This new damage mechanism was at the time difficult to detect especially

in the early stages and initially was only detected by visual and fluorescent

dye penetrant inspection during ring removal.

Metallurgy

• The Arcing of this material causes the

nitrogen to fuse with the chromium,

generating chromium nitrides and

therefore, reducing the ductility of the

material, allowing the initiation of

cracking.


• Retaining Rings are very highly stressed components and have been

known to fail catastrophically.

• The purpose of inspection is to help

identify what is often not detectable with

the human eye.

• Inspection of these rings is very

challenging and specialised requiring a

level of understanding of the damage

mechanisms to be able to adequately

search for and detect them.

Evolution of the Inspection Process


• 18%Mn/5%Cr retaining rings were predominant up to the early 1990’s.

These retaining rings are susceptible to stress corrosion cracking (SCC)

through operation, generated from a moist environment

• Therefore they required regular inspection. In the 1980’s there were 2

options available.

• First - Penetrant Inspection but this required ring removal.

• Second – Manual Ultrasonic inspection gave the option to inspect without

ring removal saving both time and money, this was often combined with

eddy current inspection at the OD of the rings.

Evolution of the Inspection Process 18-5


Penetrant Testing

Penetrant testing is a simple low tech process, but the

rings must be taken off the rotor. Visible red dye has low

sensitivity to stress corrosion cracks (SCC). Therefore

high sensitivity fluorescent penetrants must be used which

require well trained technicians.

• A. Sample before testing; B. Liquid penetrant applied; C. Surplus wiped off leaving penetrant in crack; D. Developer powder applied, dye soaks into powder; E. View coloured indications, or UV lamp shows up fluorescent indications.


Manual Pulse Echo Ultrasonic


Limitations of Manually Scanning End Rings

• Difficult to differentiate defect responses from substrate

responses such as the rotor teeth and end turn windings.

• Poor signal to noise ratio due to high attenuation.

• No permanent record for subsequent data interrogation or

archiving.

• Uncertainty of coverage.

Manual Pulse Echo Ultrasonic


• Following the successful introduction of digitised ultrasonics to

other areas of the industry including power generation, digitised

UT of retaining rings was introduced in the early 1990’s.

This was revolutionary technique called Time of Flight Diffraction,

utilising the Zipscan, based on a 286 processor and a massive

40Mb hard drive.


• This technology did not only allow for

greater probability of detection(POD)

but also allowed for better

measurement accuracy for input into

FEA assessments of the indications

detected.

• There was of course also the benefit of

repeatability, if required, through a

permanent digital record.


Inspection of Shrink Fit using TOFD


ToFD Scan of Shrink Fit Area


ToFD Image of Defect in End Ring


Example of ToFD Showing Response from Amortissuer and Near Bore

Defect


Limitations of TOFD

• Whilst TOFD was a big step forward it has limitations. TOFD is

best suited to detecting radial axial flaws. Circumferential flaws

at changes in section are unlikely to be detected.

• TOFD is best at detecting isolated defects. Where clusters of

SCC exist the diffracting responses get swamped, cancelling

each other out.

• For these reasons TOFD must be supplemented by pulse echo

focusing on the change in section areas, circumferentially.

Pulse echo is also preferred for defect characterisation.


• 18%Mn/5%Cr end rings were predominant in the early 1990’s. Since

the late1980’s retaining rings have been manufactured in 18%Cr

18%Mn and 0.6% nitrogen high strength steel which is highly resistant

to SCC and has largely replaced the 18-5 material. However it has

shown a susceptibility to arc damage.

• Unfortunately unless it is very severe, arc damage was not detectable

with the Ultrasonic techniques designed to detect SCC.

• To detect early stage arcing there was only one way and that was to

remove the rings and inspect visually, time consuming and expensive.



• In an effort to allow for the detection of early stage arcing with out

removing the retaining rings, numerous trials have been carried out on

locally obtained samples, using local knowledge, to develop an

inspection approach for what is predominately a problem in Australia and

New Zealand.

• Many different approaches were considered when looking for a solution

to detecting early stage arcing, with two identified as the primary

approach for this requirement. Combined creep wave and TOFD

inspection.

• Creep waves are a form of pulse echo which are highly sensitive to inner

surface breaking defects.

• Creep waves are affectively combined with a finely tuned TOFD setup for

confirmation and sizing.

• Detection capability of <1mm approximately 0.5mm.



Combined ToFD & Creep Wave Scan on 18%

Cr 18% Mn

Retaining Ring Shrink Fit

mm 4000 3950 3900 3850 3800 3750 3700 3650 3600 3550 3500 3450 3400 3350 3300 3250 3200 3150 3100 3050 3000 2950

mm

90

100

110

120


Further Development of Insitu

Scanning• Insitu scanning is possible but currently limited due to the airgap and

the types of probes required to detect early stage arc damage.

• There are developments and trials ongoing, for probes to improve the

POD of arcing damage using smaller probes required to access the

limited space in the airgap.

• Insitu inspections do run a higher risk

to the rotor, however solutions are

being worked on currently to

eliminate the risk and deliver the

same efficiencies and POD’s of the

full access inspection approach.


Implications for Engineers• Advances in NDT instrumentation and techniques can provide

high quality data. This can be stored, interrogated in great detail

and archived for future reference.

• The role of inspection is to give an understanding of the condition

of assets. It is the Metallurgist/Fracture Mechanics task to advise

the Engineer on what is or is not a critical defect.

• It is on the basis of the critical defect size that Ultrasonic

techniques are designed. If the critical defect size is a crack with

a radial height of <3.00mm - that is much more amenable to

detection than arc damage <1.00mm deep.

• The earlier these indications are detected the easier it is to work to

a planned repair process and no unplanned outages.


Any Questions?

zach mccann - sonomatic ltd

Engineering