A New approach to positioning QC for Dynamically Positioned drilling rigs
Client: Global oil and gas operator
Location: West of shetland
Asset type: Drilling rig
Client Background: Global Oil and Gas Operator
Business challenge for the customer
Lloyd's Register (LR) was contracted by the client to provide Rig Positioning Quality Control (QC) services to support the accurate and efficient positioning of a DP3 the semisubmersible dynamically positioned (DP) drilling rig, West of Shetland. The tolerance for the well was +/-5m at the seabed, in water depths of 700m. The client sought to challenge standard approaches and achieve efficiencies where possible.
How Lloyd's Register helped
LR identified solutions to simplify the rig positioning operation and achieve tangible cost savings whilst ensuring the accuracy and reliability of the as-spud well location.
The rig was a latest generation DP3 semi-submersible, permanently installed with multiple high accuracy surface and underwater positioning systems, including full independence and redundancy. The traditional approach to DP rig positioning QC is to contract an independent positioning contractor with personnel and additional equipment mobilised for the duration of the rig move. We identified that if the rig's own positioning systems could be independently audited and verified as being t for purpose and meeting the well tolerance specification, then with appropriate oversight, and relevant quality control in place, the requirement for a survey contractor could be removed and cost savings realised by using the rigs own positioning system.
This was achieved by:
- A desk-top 'error budget' exercise to confirm that the drilling rig's positioning systems would achieve the project tolerance, if calibrated and operating within specification. These include 4 DGPS, dual HiPap500 USBL and multiple heading reference systems.
- Provision of a QC positioning specialist to oversee the performance an operation of the onboard systems by the DP operators during trails and operations.
- During pre-project DP trials, we completed a positioning audit and verification of the rig and oversaw a full calibration of both USBL systems relative to a deployed acoustic beacon was witnessed. The data and results were verified as being acceptable for achieving the project specification in 700m water.
- A 'black-box' V-NODE unit, containing DGPS, heading and AIS sensors was installed on the rig, allowing the data to be telemetered in real time via Iridium to the secure IRIS webserver. The position of rig and drill centre is then independently computed on project datum and displayed live on the web map, enabling additional onshore QC.
When the rig arrived on location the surface 'Location Arrival Fix' was computed on board from accumulations of position fixes for the drill centre, and observed with the APOS navigation system from all 4 DGPS systems.
'Onshore', further independent checks on the real-time data were performed, with the rig shape plotted live on the web map relative to the intended well location, providing a visual confirmation on project status and positioning reliability.
The well location on the seabed was initially marked using the rig's starboard USBL system to place 4 markers around the intended well position. The rig then moved its drill centre onto the 'surface location' and proceeded to deploy the drill-string, observed by the ROV within the marker array. The final well position computation and QC confirmed the as-built well was within tolerance and drilling ahead commenced.
- Mitigated the requirement and cost of mobilising a Survey Positioning Contractor for the duration of the rig move.
- Independent verification of the high accuracy positing system permanently on board.
- Only one Positioning Specialist was required on-board experienced in DP rig positioning systems and operations.
- Independent position QC undertaken onshore within IRIS provides additional and comprehensive quality assurance for positioning reliability.