A software developed for determining the remaining strength factor (RSF) on drilling risers in the presence of pitting corrosion is presented in this paper. The software follows the methodology provided on API 579-1/ASME FFS-1 Part 6 and is adapted specifically to drilling risers. It bypasses API 579 level 1 assessment approach, based on visual comparison against pitting chart images, by the more stringent methodology proposed by API 579 level 2 assessment (pit couples' interaction). Using 3D scanned internal surface images where pitting is present, pits are categorized and modeled in terms of maximum diameters, maximum depths and minimum distance between pits. Moreover, using image acquisition and optimization subroutines, pit couples were selected based on the shortest distance among pits. Minimum ligament distance for pit couple's selection and a significant pit population are considered conservative methods for estimating the RSF under pitting. This software is intended for evaluating RSF on widespread pitting surfaces; however, it could also be used for localized pitting, pitting within a locally thin area, and a locally thin area in a region of widespread pitting. The software is intended to be an aid for inspectors and NDE engineers during fit for service evaluation of drilling risers under pitting. When pitting corrosion is above the threshold limits established for a level 2 assessment, a level 3 assessment involving Finite Element Analysis (FEA) is still required.
Depending on the operating conditions, drilling risers can encounter several types of internal corrosion mechanism, among them pitting corrosion. In the occurrence of pitting corrosion, the capacity of the riser to hold its design strength might decrease. The ration between the reduced strength capacity as a result of wall thickness loss and the designed strength capacity is called remaining strength factor (RSF). ASME FFS 1/API 579 Part 6 is a recognized industry method for estimating pipes and pressure vessels' RSF under pitting. Depending on their severity, pits can be evaluated visually against pitting comparison charts (level 1 assessment), or by measuring pit dimensions and distance between pits (ligament interaction/pit couples-level 2 assessment) or using finite element analysis (FEA-level 3 assessment).
