A riser forms the connection between the top side of an off-shore platform and a pipeline on the sea-bed. Such pipeline systems may be equipped with launch and receive facilities for cleaning and general intelligent pigging of the entire pipeline. The riser may be of different material or of heavier wall thickness, making self contained intelligent pigging of the riser difficult, less effective or cost prohibitive. Today, offshore operators are strongly committed to protect the environment and pigging and riser inspection will assist in determining the condition of the installation and the riser in particular. In this description a new approach for a detailed ultrasonic inspection of the risers only is explained, based on the application of a compact umbilical (cable, tether) operated tool. This requires that the riser is bled down and liquid tilled. The inspection tool provides on-line presentation of results for immediate evaluation. The system uses straight beam ultrasonic sensors for general corrosion survey in the pipe material and a novel ToFD (Time of Flight Diffraction) technique for additional inspection of the circumferential weld area.
For risers, which are usually encased or not accessible from the outside, inspection from the outside is impractical and often impossible. In this case, inspection from the inside is more practical and can provide full coverage over the entire length or height. Single side access from the platform must be arranged to utilize umbilical operated tools. This requires shutting down, depressurization and opening of the riser. Umbilical operated tools can now be used for ultrasonic corrosion detection and weld area inspection with the ToFD (Time of Flight Diffraction) technique.
PROBLEM DESCRIPTION
A typical example of a riser is illustrated in Figure 1, Any inspection tool would have to enter through the launch and/or receive trap, used for cleaning pigs, or a specially made entrance point. The risk of a free-swimming intelligent pig getting stuck in the riser or pipeline may be considered too great or cost prohibitive. Risers and pipeline systems are often old with unknown obstructions or features. Moreover, the prime area of interest for inspection is often the tidal zone only in the heavy wall thickness riser. Different riser configurations exist, being J-tubes on the inside of a platform leg or on the outside, or risers inside a pull tube. As the pull tube may be open at the bottom water is present between the pull tube and riser, this is an area that needs much attention as external corrosion may occur in this area.
APPROACH WITH UMBILICAL OPERATED TOOLS
Before discussing the special tools for riser inspection the experience with umbilical operated tools is presented: For the inspection of (off-)loading pipelines special cable-operated ultrasonic pigs [l] are available and thanks to ongoing developments more and more complex geometries can be inspected. Today, even distances up to 10 miles can be covered with an umbilical operated tool. As Figure 2 and 3 show, a motor driven crawler is used for diameters over 16. This self- propelled tool makes the operation independent of pumping facilities and other (expensive) assistance such as divers for offshore assistance. An array of ultrasonic probes is mounted on the front of the inspection tool.
To deploy the tool, the liquid filled pipeline must be interrupted and opened to insert the tool. Apart from power supply and hoisting equipment no other facilities are needed. The umbilical allows full control over the tool speed and performance and above all on-line presentation of the inspec
