ABSTRACT

The proposed paper describes a method producing a three-dimensional acoustical image of the internal face of the riser. Inspection is carried out by a diver operating only at the moment from a lock-out submersible. An array of ultrasonic transducers is applied on the outside surface of the riser, and generates a sequence of focused Shear wave beams, which are reflected back to the transducer array by the defects on the other face. The point by point electronic scanning allows to construct a 3-D image of a 150 mm by 100 mm section of the riser. Each image includes approximately 5000 acoustical points, each one corresponding to a 20 mm by 5 mm by I mm riser element. Depth extension of defects in the metal is given to - 1 mm. All data are recorded on digital tape and processed by computer at the surface after each dive to produce detailed inspection documents. Approximate time for inspection of a I square meter riser surface (100 images), in operational conditions, is 3 hours.

Extensive laboratory tests are reported showing the capabilities of the method in evaluating the internal corrosion and especially the corrosion due to pitting. At the issue of these tests, the system is found to provide a detailed representation of the corrosion or erosion with satisfactory depth resolution. Several parameters are extracted from the data to characterize the corrosion status and to allow evaluation of the safety of the riser.

INTRODUCTION

Monitoring and maintenance of marine risers are essential for operators of offshore oil and gas production platforms, as any damage to the risers can result in a partial or total shut down of the production, with significant economical consequences. On the other hand, marine risers are specially exposed: highly corrosive fluids, thermal gradients, collisions with service ships, constraints due to connecting sealines instabilities, etc...

The significant damage can manifest itself either as:

  • A geometrical deformation, easily identified by a visual survey

  • Some external corrosion evaluated through visual surveys and potential measurements

  • Internal corrosion or erosion frequently localized (pitting).

  • Fatigue cracks.

These two later events are generally evaluated through techniques derived from on land surveys, poorly adapted to the needs of saturation diving :

  • Internal corrosion is evaluated either with instrumented pigs, requiring a shut down of the production and providing a very coarse information unable to identify pitting, or, with manual ultrasonic tests which are time consuming and inappropriate to provide a complete survey of the suspected areas.

  • Fatigue cracks, are visualized through magnetic particles inspections which are costly, due to the high quality cleaning required, and provide only a description of the surface outlet of the cracks.

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