Guided wave ultrasonic testing is fast developing into an efficient screening tool to rapidly inspect long sections (50+ m) of offshore pipelines and risers for corrosion. The technique enables rapid inspection and screening of sections of lines from a single inspection location, like an offshore platform. Over the last year, the authors organization has applied ultrasonic guided waves to the inspection of a variety of offshore and onshore assets through the excitation of a particular wave propagation mode known as the torsional mode. Torsional guided waves were chosen due to its anticipated advantages in terms of an extended choice of parameter selection and also because of its insensitivity to the type of liquid hydrocarbon/crude oil filling the lines.
This paper details our experience in using ultrasonic guided waves with special emphasis on offshore installations like export and import risers. As part of the endeavor to increase the confidence in the technique, extensive verification was carried out on risers that could be removed from service. Additional verification was carried out on the in-house 75-meter test loop. From these studies it was possible to determine the optimum inspection parameters for the types of defects found in the lines. A novel method was developed in-house to approximate the local corrosion depth fi'om the parameters reported from guided ultrasound. Case studies from two crude oil import risers are presented to illustrate the experiences. The unique challenges of inspecting offshore risers and the testing considerations are discussed. The benefits and limitations of the technique are discussed, and typical guidelines for riser inspection using guided wave are suggested.
Several hundred miles of pipelines operate in deep water transporting gas and crude oil from production wells to platforms, platforms to platforms and to the shore. Risers are piping components that connect sub-sea import or export pipelines to installations on production platforms. These components bear the brunt of the degradation experienced by the line, and therefore its reliability is key to the safe and continued operation of the trunk line. Several corrosion control and repair techniques exist, but there are serious limitations to the ability to inspect the risers for any developing integrity threat. The need for a consistent and reliable inspection technique prompted Shell to evaluate torsional guided ultrasonic inspection, which is an emerging and promising technique for this purpose.
Degradation Mechanisms In Offshore Piping
Offshore transportation piping can be broadly classified into (a) sub-sea main lines (b) riser piping and (c) top side piping. Damage can be classified as (1) external damage (2) internal damage and (3) failure of components. The table below lists a few of the critical segments and the type of degradation. The most commonly seen modes of failure are also identified 1. The critical sections of offshore piping and the types of degradation are listed below.
Risers experience the most severe degradation among all the components shown here. Apart from internal and external corrosion, risers also are prone to mechanical damage during installation or during service (like propeller hits, boat impacts etc) that can cause severe localized corrosion. The vulcanized neoprene coating used to protect the splash zone is also seen to degrade on protracted exposure to UV rays, causing degradation and micro-cracking- these are potential locations for localized corrosion. Damage in the neoprene also promotes barnacle growth, setting up crevice corrosion cells. Risers are also damaged by extraneous factors like environmental events, the damage her
