Pipeline repair technology damage and repair assessment of pipelines with high residual stresses

Abstract

Today in the offshore industry, there are an increasing number of pipelines that require both maintenance and repair. A wide specter of research in pipeline repair technology is available. Damage to a pipeline could be a quite complex event to analyze, due to the many different combinations of internal pipe stresses and damage types. Standards, such as DNV and ASTM have experimental based assessment methods for evaluating many of these damage combinations, however, there are some of these methods that do not have a straight forward assessment method. In this thesis a discussion on how to assess and repair damages to pipelines with significant longitudinal stresses, in combination with an internal pressure and an external surface damage is conducted. The study proves that damages with these types of combined loadings are very depended on the depth of the damage itself as well as the significance of the axial stresses. With the help of simple FEA simulations combined with previous experimental studies, it was possible to make a proposal for a pre-accidental repair chart. The purpose for this type of chart is to be able to rapidly decide which repair method that should be used to repair a damaged pipeline, by only taking some simple assay measurements of the damaged area. With the help of a rapid decision making, the downtime cost due to a damaged pipeline could be significant reduced, as well as repair costs. Various types of clamps are a very common method to repair these types of damages. Clamps provide the ability to surround the damaged section of the pipe with an enclosed and pressure tight environment. Other repair methods such as composite systems or simple grinding of the damage could also be a solution where the stresses in the damaged section of the pipe are not too extensive. As an alternative to grinding of an external damage, an idea of a milling machine using a ball mill to remove damaged material in a gouge is proposed. The machine is based on a technology using a coating removal tool, and the method of grinding an external damage. A ball mill will be used to remove the damaged material in a gouge, corrosion or a scratch in order to either reduce the stress concentrations in the area, or to prepare the damage for further repair. This further repair could be to fill the milled slot with a new molten pipe material using underwater welding, or to prepare for clamp and composite repair.