Working offshore Egypt at West Delta Deep Marine, Technip completed 21 jumper metrology operations using Lodestar GyroCompatts from Sonardyne International. Using the latest Wideband long base line (LBL) acoustic positioning hardware, Technip reported that it was able to complete each metrology operation in an average of 13 hours, with one operation conducted in only eight hours.
Subsea metrology requires accurate, precise and robust acoustic measurements so that a spool or jumper can be successfully fabricated and installed. Lodestar GyroCompatts, integrating the features of Sonardyne’s most advanced LBL transponder with a Lodestar attitude and heading reference system (AHRS) in a single ROV-deployable subsea housing, are said to provide high update rate wireless attitude, heading, heave, surge, sway, pressure, sound velocity and acoustic positioning of any subsea object.
‘One of the main challenges of acoustic metrology is to ensure that the measured wideband acoustic range can be translated accurately down from the transponder to the hub level,’ said Edd Moller, Sonardyne survey support group manager. ‘This is traditionally done using inclinometer Compatts, which are then QC’d by rotating the Compatts in the hubs to determine misclosure in both the stab to hub mating and also inclinometer and alignment errors. Thus the main purpose of this project was to simultaneously use a GyroCompatt in each hub with precision stabs and receptacles. Acoustic ranges could then be made while rotating the GyroCompatts and then using the attitude data recorded by the GyroCompatt, the projections of the hub position could be deduced. This meant Technip could compare the calculated Hub-to-Hub distances during the different rotations in order to justify removing the requirement for this alignment QC procedure altogether.’
According to Technip project surveyor Vincent Latron, the results showed that the difference between the rotational alignments from the first four metrologies gave a maximum horizontal hub-to-hub distance error of 15mm.
‘The overall average error was only 6mm which was well within the Sonardyne position measurement quoted accuracy of 50mm and the required metrology accuracy of 75mm,’ Latron noted.