Deepwater Flowline Dewater/Dry with High Pressure Nitrogen
Location
Gulf of Mexico
Specifications
2 Flowlines:
6” x ~4 miles
6” x ~7.5 miles
Max water depth – 4,258 ft
Single riser - combined via a subsea wye at the base of the riser
Dewater and chemically dry from subsea to platform (utilizing a coiled tubing downline)
Provide topsides pig receiver
Challenges
Limited deck space with coiled tubing spread
Lines combined into a single riser
Controlling pig speed - gas expansion due to dewatering from subsea up the riser
Total volume of MEG required was less than the volume of the coiled tubing
No ROV on the receiving end for subsea to subsea work
Solutions
Due to the requirement of dewatering from subsea to topsides, the work would be performed via a coiled tubing downline. A three pig train containing the required slugs of MEG would be propelled with high pressure nitrogen. The project was completed successfully without incident.
Plan of Execution
Pre-Engineering and Planning
Design topsides receiver.
Run calculations for pressure and flow requirements of both the nitrogen system and the MEG pumping system.
Create flow diagrams for nitrogen system.
Create a detailed workplan for the execution of the project.
Run modeling on the pigs traveling up the riser to facilitate a plan to control pig speed.
Partnered with a global nitrogen subcontractor for the nitrogen generation spread.
Project Execution
Rigged up and function test nitrogen and MEG pumping spreads on the vessel at the dock.
Once on location, deployed coiled tubing and displaced coiled tubing with MEG.
Launched out pigs with MEG.
Propelled pigs with high pressure nitrogen.
Controlled pig speeds up the riser utilizing a schedule based on pressure responses at the receiver.
Technical Achievements & Benefits
Designed nitrogen injection system to effectively dewater and dry both flowlines.
Partnered with Viking Engineering to model the travel of the pigs up the riser in order to plan a safe and controlled manner to receive them.
