The system, comprising of 14 NASNet sleds and 22 Mobile Transceivers (MTRx), was delivered to Subsea 7 in early July 2007, and subsequently mobilised to Subsea 7's umbilical lay and construction vessel MV Toisa Perseus, for operations on the Agbami field situated 350km southeast of the city of Lagos. Subsea operations ranged from water depths of 1,280 to 1,650 msw.
On this project, the underwater acoustic positioning system provided total coverage for an area of approximately 50km²; this is believed to be the first time anyone in the world has achieved such wide field coverage with so few transponders.
The workscope was completed between September and December 2007 during which positioning was provided for:
• Installation of 12 suction piles to accommodate manifolds
• Installation and testing of 12 manifolds:
- 6 Production manifolds
- 2 Gas Injection manifolds
- 4 Water Injection manifolds
• Installation of 30 no. UTA foundation structures for umbilical ends
• Installation and testing of 15 infield umbilicals (total 26.2km), wet end to wet end
• Installation of hydraulic flying leads
• Pre-installation and connection of electrical flying leads.
Managing Director Mark Patterson said: "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. 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 by the Toisa Perseus on site in the Agbami field, with a network of 13 NASNet stations. The network was calibrated using a classic box-in method whereby the vessel navigated a circle around the station at a radius of approximately 0.7x water depth.
Both work class ROVs on the Toisa Perseus (Hercules 3 and 4) were fitted with dual ROV receivers enabling rapid (typically 1 per second) position updates across the entire field in one complete referenced network.
The 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 Subsea 7 and Nautronix survey teams to monitor pitch and roll via the 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.
Sam Hanton, Senior Project Surveyor, Subsea 7 said "We definitely benefited from the increased accuracy on umbilical lay which would not have been covered using Long Base Line technology."
Further operational benefits of NASNet were highlighted 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.
Mr Hanton added: "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."
The offshore phase of the operation onboard the Toisa Perseus was supported by Nautronix personnel throughout, both offshore and back in base. The Nautronix offshore crew developed an effective working relationship with the Subsea 7 vessel and survey team.
During the operations some issues were observed and many lessons have been learned, which will allow Nautronix to improve the system and provide an even better service to our customers going forward with the next deployment of NASNet.
Mr Hanton commented: "There is no doubt that the experience gained with the system will enable further improvements and efficiencies to be applied through stages of planning, installation and operation on future projects using the system."
Martin Day, Survey Manager, Subsea 7 added: "NASNet was selected by Subsea 7 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. "
"I would also like to highlight the quality of both our personnel and the Nautronix team who delivered the success of this project."
These sentiments have also been echoed by Geoffroy Lacoin, Offshore Installation Technip "The first Phase of Technip Agbami Installation campaign was successfully achieved using the NASNet system, leading to the entire satisfaction of Nautronix customers. Technip thanks Nautronix personnel for their excellent teamwork with Subsea 7 during both the engineering and the offshore phases and is now looking forward to use Nautronix knowledge gained during the Agbami operations on their vessels for future projects"
Nautronix hope that this is just the start of an exciting chapter in NASNet history. The latest system has been deployed for Acergy on another Chevron field, Frade, offshore Brazil.
With further projects in discussion, Nautronix are excited about the future of the system.