Agbami accolades for NASNet
  from: Offshore Engineer
  Monday, April 21, 2008

To the obvious delight of developer Nautronix, the NASNet underwater acoustic positioning system recently saw its first field-wide deployment – for Chevron’s high-profile Agbami development offshore Nigeria – courtesy of contractor Subsea 7.

The system, comprising 14 NASNet sleds and 22 mobile transceivers (MTRx), was delivered to Subsea 7 in early July last year, and subsequently mobilised to the contractor’s umbilical lay and construction vessel Toisa Perseus, for Agbami operations. Subsea operations on the field, operated by Chevron affiliate Star Deep Water Petroleum and located 70km offshore the central area of the Niger Delta, ranged from water depths of 1280m to 1650m.

The underwater acoustic positioning system, based on Nautronix’s Acoustic Digital Spread Spectrum (ADS2) technology, provided total coverage for an area of approximately 50km².

According to Nautronix, this is the first time that such wide field coverage has been achieved with so few transponders.

The workscope was completed between September and December last year, during which positioning was provided for installation of 12 suction piles to accommodate manifolds; installation and testing of 12 manifolds (six for production, two for gas injection and four for water injection); installation of 30 UTA foundation structures for umbilical ends; installation and testing of 15 infield umbilicals (totalling 26.2km), wet end to wet end; installation of hydraulic flying leads, and pre-installation and connection of electrical flying leads.

Nautronix managing director Mark Patterson comments: ‘This is a true milestone in the NASNet story. Subsea 7 has been a great supporter of NASNet since the vision was released in 2001 (OE September 2001). Their attendance at trials and customer feedback over the years has been incredibly valuable to the development of the system, so I’m delighted that they fully experienced its commercial benefits and operational capability on such an important project.’

After system mobilisation, the subsea stations were deployed on location by Toisa Perseus with a network of 13 NASNet stations, calibrated using a classic box-in method whereby the vessel navigated a circle around the station at a radius of approximately 0.7 x water depth.

Both workclass ROVs on the Toisa Perseus (Hercules 3 and 4) were fitted with dual ROV receivers enabling rapid position updates (typically one per second) across the field in one complete referenced network. Patterson says accurate installation of the suction piles and manifolds was achieved using NASNet MTRx systems which, as well as providing real-time position updates of the items throughout the water column, allowed the survey teams to monitor pitch and roll via internally integrated attitude sensors.

In order to monitor system performance, the location of three MTRx stations deployed across the field was regularly checked, especially during structure installations. The performance of the system throughout the operations met all expectations, reports Patterson.

Sam Hanton, Subsea 7’s senior project surveyor, says: ‘We definitely benefited from the increased accuracy on umbilical lay which would not have been covered using Long Base Line technology.’

Further operational benefits were in evidence when the scope of work was altered to accommodate drilling operations in the field.With the NASNet subsea reference network in place and covering the entire field, there was no need to redeploy in an alternative location saving considerable time on the project with minimal disruption.

‘The NASNet system has demonstrated that it has the potential to provide clear benefits to a project of this nature for Subsea 7 and its clients,’ says Hanton.

Martin Day, Subsea 7 survey manager, adds: ‘NASNet was selected to provide seamless field wide positioning tolerance of ±1m in water depths to support multiple structure and umbilical installations in water depths to 1650m. I am pleased to confirm that this objective was successfully achieved on the Agbami project. I am confident that the experiences gained and lessons learnt by both Subsea 7 and Nautronix on this first field development project for NASNet will be incorporated to provide further improvements and developments into the system and its operation.’

Nautronix will be hoping this is just the start of an exciting new chapter for NASNet. The latest system has been deployed for Acergy on another Chevron field, Frade, offshore Brazil, and further projects are under discussion.OE

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