North Sea jacket structures commonly support risers that carry product from the well to a topside processing plant.
The risers are supported using clamps attached to the jacket structure, but are often subject to dynamic loads that can lead to fatigue cracking. Data and evidence gathered in the field demonstrate that fatigue cracks most commonly initiate at the riser welds. In order to keep the product at an appropriate temperature and to protect the riser surface, a coating is applied to the riser. The coating can make it more difficult to inspect the riser, thus impacting the integrity assessment of the structure.
A recent application of the above explained conditions was recently encountered. In this application, the risers were 12 and 16 inches in diameter (304 and 406 mm) and were coated with a 0.2 to 0.5 in (6 to 12mm) neoprene coating.
The risers were subject to cyclic loading due to swell conditions typical in the North Sea, and the integrity of the welds near to the sea surface was in question as a result. Furthermore, the exact position of the welds was unknown due to the thickness of the coating obscuring any surface profile. The NDT challenge was to locate the welds without removing the coating, and then inspect the welds to determine if cracking was present. If cracking was discovered, an estimate of the size of crack was needed in order to assess the structural integrity of the riser. The critical welds were within 80 ft (25m) of the sea surface. Finally, all this information and
data had to be collected during the UK winter period, adding significant operational difficulty due to weather.
The solution to this challenging problem was provided by Eddyfi Technologies, via the TSC product line, with the development of an ACFM® array probe capable of inspection of welds under coatings up to 1.5 in (40mm) thick. The probe is capable of detecting a 0.8 in (20mm) long defect, 0.08 in (2mm) deep through up to 0.5 in (12mm) of coating. The High Lift-Off Probe Array inspects a 2.7 in (70 mm) wide band and is ideal for deployment using remotely operated vehicles (ROV) where placement accuracy is limited.
Two deployment tools were developed to allow the ROV to successfully detect and inspect the welds using ACFM®. The tools used permanent magnets to attach the probe to the coated riser surface. One was suited to axial scanning (to determine the position of the weld) while the other was designed for circumferential scanning to fully inspect the circumferential welds. The tools were simple in operation and robust, both of which are necessary to allow subsea inspection in adverse conditions.
The ACFM® probes were deployed successfully over a period of 5 days. All welds were located and inspected without removing the neoprene coating. Significant cost saving was achieved by avoiding the need to remove and reinstate the coating, which would lead to potential down-time on the asset, and by using ROV deployment techniques, thus avoiding costly and potentially unsafe diver-based operations.
Eddyfi Technologies, through its TSC product line, supplies a range of NDT equipment for both topside and subsea applications. Their specialist engineered solutions department provides subsea inspection solutions for challenging applications including node weld inspection, pipe welds and load bearing points. The ACFM® technique is particularly suited to subsea inspection due to insensitivity to lift-off, leading to more efficient and less costly inspections since marine growth and coating removals are not necessary.