Michael Walsh (GATE LLC) | Grant T. Gibson (GATE LLC) | Michael H. Dupre(Shell International E&P Inc.) | Paul E. Partridge (Partech)
There is limited data on the performance of coatings, materials and cathodic protection systems installed subsea. Most systems remain submerged for their useful life and are often abandoned on the seabed. The recovery of the hardware provided an opportunity to autopsy the hardware for the purposes of validating cathodic protection designs and coating systems; specifically coating performance, anode consumption, and internal wall thickness allowances.
There were four types of coating on the hardware: Two-part epoxy paint, fusion bonded epoxy pipe coating, heat shrink sleeve field joint coating and thin film fluorocarbon coating for friction reduction. Apart from some higher interfacial porosity, the fusion bonded epoxy coating was comparable to new coating. The paint coating exhibited poor adhesion and blistering in some areas but was well bonded in others. The heat shrink sleeve was in good condition with no signs of water ingress to the weld between the sled piping and flowline. Although adhesion of the thin film fluorocarbon coating was good, it exhibited advanced signs of breakdown with a number of small blisters.
Using data developed by Shell Exploration and Production Company for cathodic protection and the simple yoke structure on the hardware, a coating breakdown factor of 5.4% was calculated. This is lower than the as-designed case of 10.4%.
Cathodic protection design for hardware is part of a corrosion protection system which should be addressed during component design and fabrication. The verification of electrical continuity is an important step in the fabrication of the equipment that has implications on the corrosion protection and integrity of the equipment.
Tahoe I was a single satellite gas well in 1,500 FSW (feet of seawater) (460 m) that produced back to host platform approximately 12 miles away in 280 FSW. The dual flowline system, installed in 1993, was one of the first deepwater Gulf of Mexico subsea tie-backs. Each flowline consisted of a flexible jumper which started at the well and was 3,000 ft. in length. The jumper connected to a 4-inch steel flowline that extended the remaining 11.5 miles to the platform.
Soon after first production, one of the flexible pipe segments failed, providing the opportunity for the first deepwater diverless repair. The Tahoe flowline flooded with seawater remained out-of-service for more than 30 months. The flowline was repaired and returned to service in July of 1996 (Figure 1).
The repair was completed in four steps:
Step 1 ? Cutting the pipe
A diverless cut of the steel portion of the flowline adjacent to the transition fitting between the flexible jumper and the steel line was made using a Wachs saw. The saw was attached to a Remote Operated Vehicle (ROV), which utilized the ROV hydraulics to operate the saw.
Step 2 ? Installation of the Termination Sled
Recovery of the steel flowline segment was completed with a pipe lay barge. The termination sled was welded to the flowline and then the flowline and sled were returned to the seabed. The flowline termination sled included an upward facing male hub for the collet connection system. The female portion of the connector would be welded to the flexible pipe side of the flowline.
Step 3 ? Recovery of the Flexible Pipe
The flexible pipe was lifted onto the deck and the damaged portion was repaired. A female connector was attached to the flexible pipe for mating to the termination sled.
Step 4 ? Reconnection
The Collet Connector was stabbed over the upward oriented Male Hub Connector. The diverless Collet Connector was then actuated by an ROV, completing the conne
Document ID: NACE-06106
Publisher: NACE International
Source: CORROSION 2006, 12-16 March, San Diego, California
Publication Date: 2006