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Mistras completes 15-year structural monitoring project on Buchan-Alpha production platform

06 November 2017

MISTRAS Group announced its subsidiary in the United Kingdom recently completed a 15-year structural integrity monitoring project of the floating, semi-submersible Buchan-Alpha production platform, which was located in the North Sea’s Buchan oilfield, 120 miles northeast of Aberdeen, Scotland.

The Buchan oilfield was discovered in 1974 by Transworld Petroleum with Texaco as the license holder. The field was at first considered to be a difficult field to exploit with complex geology and a high overpressure.  BP was brought in to develop the field and as a short life was expected, the Drillmaster, a semi-submersible “pentagon” design drill rig, was converted into a production platform for the purpose, and re-named Buchan-Alpha. Upgrades to the structure were made as a result of the structural failure and capsize of the Alexander Kielland rig in March 1980, a sister rig of the same design, which had resulted in more than 120 fatalities. 

Oil production commenced from Buchan in 1981 via tanker loading buoy and in 1983 production peaked at 1.9 million barrels. In 1986, a gas lift compression system was installed along with an export pipeline to Forties Charlie through which the first export was commenced in 1987.

In 1996, the field together with others was sold to Talisman Energy (now renamed Repsol Sinopec Resources Ltd) who specialized in extraction from mature fields.

To prevent a repeat of the Alexander Kielland disaster, in combination with the upgrades to the hull structure, a rigorous hull inspection and monitoring strategy was put in place to provide continued assurance of the hull integrity.

In consultation with Talisman (now Repsol Sinopec) and Atkins, a tender was raised to install acoustic emission (AE) equipment to continuously monitor eight highly-stressed areas of weld, four areas on each of two nodes. The need was to identify and locate the onset and development of any fatigue cracking that occurred externally in the weld of this non-redundant structure, particularly during winter months when diving was not possible due to the weather. Sensors mounted on the inside of the structure would detect and locate the initiation and growth of any external (or internal) cracks. The monitoring equipment specification also called for strain and transient strain monitoring on several subsea braces, in order to obtain data on the actual stressing of the structure in service. Until then stressing had been estimated using wave buoy and other data, the new information was expected to improve the understanding of the loading actually experienced, and thereby also allow an accurate fatigue model to be developed.

In 2001 the tender for the structural monitoring system was awarded to Physical Acoustics Limited, later to change its name to MISTRAS Group, after its founder’s birthplace in Greece. Following a successful performance demonstration of the AE monitoring equipment on a large-scale fatigue test in the University of Cardiff engineering laboratory, installation of the equipment offshore took place during December-January 2002, going live at the end of January 2002. 

The equipment has operated continuously since then, providing real-time, secure web reporting for the client. MISTRAS managed the system remotely via network, checking the system daily and providing monthly reporting. 

Strain measurement used gauges on the top, bottom, and sides of the main braces that were sampled simultaneously 30 times-per-second whenever strain exceeded a set level. In addition, all gauges were sampled simultaneously every second on a continuous basis, and instantaneously sampled whenever an emission was detected by an AE sensor. The strain data was used to inform and validate the predictive fatigue models and fracture mechanics analysis which are fundamental to the basis of the hull inspection regime.

In 2004, a source of acoustic emission was picked up that was classified as having a high probability of being an actively-growing crack due to its high correlation with instantaneous strain. The system indicated that the AE source was located well away from the highly-stressed critical welds on the edge of the monitored area, strictly this was outside the scope of the monitoring system. The indication was nevertheless investigated and the inspection team found a crack growing from a “mouse-hole” in the internal bracing structure. Following structural analysis, the crack was ground out, and that source of emission has not been observed since. 

In March 2012, an additional 60 AE sensors were installed to extend monitoring to cover two 2.6m diameter x 21m (8.5ft x 69ft) sub-sea braces. The monitored area on each included seven circumferential welds. MISTRAS’ “Sensor Highway” systems used to support these AE sensors and additional strain gauges were of a new, compact, low-power design, which could be located in the braces and networked to topsides, reducing the cables needed from 100 to four, greatly simplifying and reducing the cost of the installation. 

In 2015, platform operating policy was changed, meaning that ahead of significant storm events the platform would be down-manned and powered down. As a result of this, the original system monitoring the eight highly-stressed areas, which was in the old mud pump room with cables run through watertight glands to each of the 40 sensors, was also replaced with low-power Sensor Highway systems at each node. NiMH-battery backup power packs were installed for these and the other systems in the lower horizontal braces, allowing them all to run for up to five days without power. This maintained monitoring during the most severe period of storm loading following each down-manning, allowing any storm damage that had taken place to be identified and located.

Buchan-Alpha production finally ceased in May 2017. On August 8th, the 44-year-old, 12,000-ton vessel, after spending 36 years as a floating semi-submersible production platform, finally left its moorings to be towed to the Veolia Dales Voe facility in Shetland for dismantling. 

MISTRAS’ structural monitoring solutions incorporating acoustic emission crack detection played an important role in supporting safe operations on Buchan-Alpha far beyond its original planned design life.

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