The problem
The failures observed in liquid coatings when a pipeline undergoes hydrostatic pressure testing include fine surface crack lines, delamination of the coating from the pipeline surface and or deep cracks on the coating to steel. Holiday testing and visual inspections identify the cracks in the coating but the delamination from the steel substrates, with voids, can only be identified after destructive testing.
The test
As part of a project qualification test, Speciality Polymer Coatings conducted a series of tests on the effect of hydrostatic testing on its SP-2888 coating. The testing and results were supervised by an independent test laboratory.
For the test program, a 305 mm diameter pipe spool was hydro tested in accordance with AS 2885.5-2002 Pipeline and Gas Liquid Petroleum-Field pressure, testing to 110 per cent specific minimum yield strength with the view to examine the performance of the SP-2888 coating system.
The 305 mm spool pipe was blasted to class 2.5 with minimum 75 microns profile (peak to valley) and coated with SP-2888 coating to various thicknesses from 939 microns dry film thickness (DFT) to 1,734 microns DFT. Sections of the pipe were left uncoated for comparison purposes.
The pipe was then filled with water and entrapped air removed. It was then pressurised to the test pressure and the strength test continued for 24 hours and repeated.
The pressurising rate was kept at approximately 2-3 minutes from start to maximum pressure and depressurising at approximately 30 seconds. pH of the water was 8.37 and at a temperature of 21 degrees Celcius.
The results
After completion of the hydrostatic pressure testing, the coating was checked for adhesion, cracking, folding, delamination, air voids, stretching, and holidays. Holiday testing was done in accordance with American Standard Test Method (ASTM) G62-07 and knife adhesion testing in accordance with ASTM D6677-07.
No holidays were detected with excellent adhesion rating of ten (coating was extremely difficult to remove and there was no sign of disbondment or voids between the coating and the substrate). Additionally, no cracking folding, delamination or stretching was observed showing that SP-2888 is not adversely affected by hydrostatic pressure testing.
SP-2888 is an epoxy/urethane coating, with the hardness, CD and adhesion properties of an epoxy coating and with the flexibility typical of urethanes. It is an industry recognised pipeline coating with a worldwide track record of successful service rated for operation up to 80 degrees Celcius. It is used for coating of girth welds, valves and fittings, as well as a mainline rehabilitation coating. Due to its superior abrasion, impact and gouge resistance, it is ideally suited for horizontal directional drilling.
