Composites Used For Repair by Statoil

    Statoil has used composites to repair steel topside components for more than 10 years on the Statfjord and Gullfaks platforms, using wet layup methods designed for each application, says Melve. “We repaired four water lines at an onshore oil storage facility between 1983 and 1987,” he says. “After nine years, only one pipe leaked.” Statoil originally employed a laminating procedure with alternating layers of chopped strand mat and heavy woven glass fabric wet out with vinyl ester resin, a technique based on standard methods for joining two sections of composite pipe. The company produced an internal specification document that addressed repair procedures depending on the severity of the pipe defects, in collaboration with Norwegian composites fabricator Odda Plast (Odda, Norway).

    While the procedure is being used successfully for a wide range of repairs, Statoil initiated an internal program in the mid-1990s to verify the laminate repair procedures and improve them where possible. Using a 6 inch/150 mm diameter, X50 steel pipe with a 0.3 inch/7 mm wall thickness, designed to hold 150 lb/18.9 bar of pressure, laminates of various types and thicknesses were applied and experimentally tested using strain gauges.

    Test results were compared to finite element modeling. Results showed that surface preparation is critical to successful bonding, and that stiffer laminates reduced local stresses. However, if the steel pipe was completely corroded away, no laminate was strong enough to carry the pressure. “If the steel pipe was at least partially intact, some of the axial loads are still carried by the steel, reducing the peel forces on the laminate,” says Melve.

    New techniques emerged from the study, says Melve. “We found that application of a ductile ‘primer,’ consisting of C-glass veil material wet out with Vantico’s (Duxford, Cambridge, U.K.) Araldite LY3505 epoxy resin, smooths out the local high stress peaks at the steel/laminate interface,” he explains. Better results were achieved using knitted fabrics supplied by Devold AMT AS (Langevag, Norway) as compared to the chopped strand mat and glass fabric.