Challenging the Hydro-Acoustics
Authors: Rygh J-E. , Skogvang Arnijot 
Published: October 2008

ABSTRACT
The Snorre Tension Leg Platform (TLP) is a buoyant Production platform that will be permanently moored in position over a well template by 16 tethers attached to four concrete suction anchors.
Complex hydro-acoustic systems have been developed, some presently being constructed. Others have already been used during installation of the Concrete Foundation Templates (CFTS).
It is the plan to perform simultaneous positioning of four tethers and 6 ROVS during installation of the platform. The platforms Acoustic Positioning& Control System (APCS)shall in addition to the Positioning tasks also provide means of communication with sensor packages on the CFTS. It shall also activate and deactivate submarine navigation beacons. The system (APCS) also includes a small hydro-acoustically operated valve control system. These controllers are battery-operated, self-contained hydraulic controllers with acoustic telemetric function. The APCS also provides position control for two permanent ROVS and divers.
Further, as part of this equipment, a tether-tip Position predictor has been delivered.

INTROOUCTION
Saga Petroleum as is the operator of the Snorre field on blocks 34/4 and 34/7 located in the Norwegian sector of the North Sea, in 310-350m water depth. The Snorre TLP, the main installation on this field, will be permanently moored in position by 16 tethers, above a well template and four concrete suction anchors.
A remote subsea production facility will also be Installed on the field, For this field development, Complex hydro-acoustic systems have been developed, Some presently being constructed. Others have already Been used during installation the concrete Foundation templates (CFTS).
The Acoustic Positioning& Control System (APCS) shall provide means of communication with sensor packages on the CFTS, and a means of activating and deactivating submarine navigation beacons. The Acoustic Positioning and Control System (APCS) also includes a small hydro-acoustically operated have control system. This controller is a self contained hydraulic controller with acoustic telemetric function. The APCS shall also provide position control for two permanent ROVS and divers. During tether installation, four tethers will be deployed simultaneously, one to each CFT.
For this operation the main function of the hydro-acoustic equipment is to measure the tether tip (anchor1atch) position and orientation relative to the CFT.
Further, as part of this equipment, a tether-tip position predictor has been delivered. This unit uses Kalman filters to compute the predicted tether Positions to enable safe stabbing of the anchor latches in their receptacles.

HYDRO-ACOUSTIC CHALLENGES

Positioning Tasks
During the project the need for highly accurate Subsea positioning systems was identified. First, the four concrete foundation anchors had to be positioned. Further, the positions of 16 tethers will be measured with an accuracy of+/- 20 cm.
It is the plan to perform simultaneous positioning of four tethers and 6 ROVS during installation.
To make this possible with acceptable position update rate, a system maximizing the utilization of the information in the acoustic position pulses is desirable. During evaluation of available systems it was concluded that a super short baseline system would give the best solution since it uses both phase and time information of the acoustic pulses.