NEPEAN Engineering - Huge Stainless Steel Fabrication Project

Installation of Rake Arm
Installation of Rake Arm

When ASSDA Accredited Fabricator NEPEAN Engineering was awarded the tender for the manufacture of the stirring mechanisms for 10 thickeners for the Goro Nickel Mine in New Caledonia, they had no idea of the enormity of the venture.

But having now completed the two-year undertaking they reflect on what has been their biggest stainless steel project to date.

Although the nickel mine was a massive  development, NEPEAN Group Owner and  Managing Director David Fuller admits the  initial stages of commencement were a little  “stop-start”. 

“We won the tender 2 years before from GLV  Australia Pty Ltd (Dorr-Oliver Elmco) but the  project was cancelled,” he says. “We then  had to re-tender and were lucky enough to win  it again.”  Manufacturing finally commenced in August  2005 and was completed in March 2007. 

David says the project resulted in 410 tonne of  stainless steel product, including an additional  370 tonne of carbon steel. The contract export  value was $10 million. Varying grades were  used including 338 tonnes of 316L, 65tonne of  904L and 7 tonne of AL6XN.

The thicknesses  ranged from 1.6mm up to 80mm.  Six of the thickeners were 70 metres in  diameter and required the manufacture of  33 metre long raking arms. Because a highly  corrosion resistant material was needed  where the nickel extraction occurred, Nepean Engineering used 904L for its high nickel and chromium content.

The thick sections required  meant that 316L and 904L were used to  avoid sensitisation and the subsequent risk of  intergranular attack.  Super-austenitic grade AL6XN with 6%  molybdenum and high nitrogen, offered better  corrosion resistance and was used in one of the  smaller thickeners which extracts cobalt.

This  material was imported from America.  When manufacturing commenced Nepean  Engineering experienced quite a few  challenges as a large amount of material was  non-standard size. “316L angle was unavailable so all angles were  pressed from fl at plate,” David says. 

Pressing was performed across the grain. This  required joining 2 x 8 metre sheets using sub  arc welding so that the longer angles could run  across the sheet. The sheets were then cut to  fi t the plasma cutter, which could handle 6 x  17 metres. Some of the angles were formed  in Nepean’s 1000 tonne press and others were  subcontracted for specialist pressing. Special dies were made for Nepean’s press  to allow larger angle radius.

Furthermore,  pipe unavailability due to wall thickness  requirements meant up to 2 semi-circular half  sections of pipe had to be pressed then sub  arc welded together to achieve a die of the  required length and diameter. For quality purposes, all welding required  procedures. NEPEAN Engineering created a  procedures manual for approval by the client. 

Contamination from processing and handling  was an issue that required focus. Nepean Engineering built a new factory dedicated to  stainless steel with inserts at work stations, on forklift tines and on cranes plus separation  sheets on presses and rolls to avoid cross contamination.

All welds were pickled after  fabrication. However, it became evident that  not all contamination had been removed with  further contamination also occurring due to airborne grinding particles. In order to  provide a clean surface with a uniform overall  appearance, fl ap disc grinding and garnet  blasting was performed which removed any  contamination, excess fl ux, heat tint and  oxides. 

With such a large quantity of stainless  steel on site and with varying grades and  material thicknesses, clear identifi cation was  imperative. Traceability was adopted on all  parts and processes of the project with the  introduction of a colour coding system to  identify the different grades of stainless steel.  Material heat numbers were stamped on all  components. Maps and naming schedules  were used so that each component had a part  number and could be identifi ed on a drawing. 

A spreadsheet was produced to advise the  client of the heat number of the plate from which each part was cut. This then could be  traced to a material certifi cate to provide the  chemical and mechanical properties of that  particular plate or item.  Weld traceability was also adopted on all  parts of the project.

Again maps and naming schedules were used so that all welds could  be identified. Each welder was assigned an  identifi cation number, which was then traced  against the weld number and placed on a  spread sheet similar to the material traceability  spreadsheet.  Other parameters traced were the type of wire  used, wire batch numbers, fl ux type and batch  and welding procedures. 

Non destructive testing (NDT) was employed  with dye penetrant and ultra sonic tests on  the non-magnetic, austenitic stainless steel  components and magnetic particle and  ultrasonic on the carbon steel components.  David Fuller said “the job was a major  challenge but one that NEPEAN Engineering  rose to”. “The experience we have acquired,  along with the additional infrastructure built  puts us in good stead for future projects of this  magnitude.”   

> NEPEAN Engineering
Greg Winter, Marketing Sales Manager
23 Graham Hill Road
Narellan NSW 2567
Phone (02) 4646 1511