Weld Inspection by ROV
Authors: Wyeth H.T. 
Published: October 2008

Accepted method of Non-Destructive Testing such as Magnetic Particle Inspection, Radiography and Ultrasonic technique have an extensive history as effective can be quantitively assessed. To a limited extent, this technology has been utilissed underwater by divers during inspection programmes to establish the condition of components within subsea structures and associated installations. This paper highlights recent developments which have allowed these three processes of Non-Destructive Testing to be carried out with greater accuracy and repeatability in the underwater environment using Remotely Operated Vehicles (ROV?s).

INTRODUCTION

The increased utilization of fixed petroleum production facilities offshore over the last thirty years has led to a recent increase in the structural integrity monitoring programmes required, as many of these facilities enter the latter part of their design life. Since statutory regulations require that saturation and surface orientated mixed gas diving techniques be utilized in water depths greater than 50 meters, there has been a concomitant increase in the development and utilization of Remotely Operated Vehicles to perform deep water cleaning and inspection tasks. However, the inspection tasks themselves have been generally limited to corrosion monitoring utilizing cathodic potential metering devices, visual assessment via video or photographic means, and limited Ultrasonic techniques to provide wall thickness measurements or a flooded member detection capability. This presentation traces the development and successful utilization of advanced NDT techniques to inspect circumferential butt welds in deep water, using an ROV system.

RESOURCE DEVELOPMENT

Current state-of-the-art ROV?s incorporate features which led themselves to an extension of capabilities. Figure 1 illustrates a 2.3 tonne, 56kW hydraulic vehicle which incorporates a 13:1 hydraulic intensifier capable of providing a 206 MPa water jet which enables weld cleaning to a Grade of Sa 3.0. The vehicle is fitted with dual, multi-function manipulators and has enhanced station-keeping capabilities provided by; an onboard gyro compass, depth and altimeter transducers and an accelerometer feedback system. This ROV also has the capability to lock onto underwater structures by way of an Articulated Attachment Device (AAD). The successful application of and NDT methodology may be measured by the accuracy and repeatability of the results obtained. To achieve acceptable results, an ROV system must be capable of:–Positioning an inspection package and maintaining station in currents up to 1.02 ms/s (2 knots).–Providing sufficient control and dexterity to enable an inspection package to be positioned with an accuracy of 100mm.–Incorporating sufficient dexterity of manipulative functions to enable Radiographic film coupons to be accurately placed on the pipe surface.–Incorporating sufficient dexterity of manipulative functions to allow a magnetic induction yoke to be accurately positioned adjacent to the weld to be inspected.–Providing sufficient hydraulic and electronic interfaces to enable tooling package functions to be operated via the ROV control systems.

Figure 2 illustrates the standard to which underwater welds can be cleaned as preparation for detailed NDT inspections. This cleaning technology, using ROV?s has been applied for several years and is an obvious prerequisite for the detailed examination of welds and the hear-affected zone.