Platform Riser Repair and Protection
Authors: Macicek L.V. , Keprta D.F. 
Published: December 2007

Description of Problem

Atlantic Richfield Co. operates an oil and gas field (primarily gas) about 7 miles off the coast of Texas in the High Island area. The water depth is approximately 40 ft. Production is gathered from 18 individual caisson well structures and processed through two platform complexes. Submarine flowlines (3 1/2-in. diameter, 5,000 psi working pressure) transfer the gas (1,500 to 4,500 psi) and oil from the caisson well completions to the platform production facilities. The flowlines are buried to a depth of 6 ft and are protected by sacrificial aluminum anodes. There are 60 protected by sacrificial aluminum anodes. There are 60 risers connecting the flowlines from the wellheads to the platform production manifolds and all the risers were originally welded to the flowlines. In the initial construction of the field facilities, the first 40 risers were coated with an extruded plastic (polypropylene) having a thickness of 35 mils. The other 20 risers were protected by a 250-mil thickness of neoprene. Deterioration of one of the polypropylene-coated risers required a replacement of this well's riser and prompted a study of the cause of deterioration. prompted a study of the cause of deterioration. Laboratory analysis of the riser confirmed that the problem was a result of corrosion following the problem was a result of corrosion following the breakdown of the polypropylene coating and not the result of a metallurgical defect, Inspection of the remaining plastic-coated risers disclosed corrosion and coating plastic-coated risers disclosed corrosion and coating damage on several risers. The decision was made to replace these risers to insure that there would be no danger to the environment and to minimize the possibility of deferred production. The neoprene-coated possibility of deferred production. The neoprene-coated risers were in excellent condition and were only slightly corroded at the field joints above the neoprene.

Polypropylene Polypropylene At first, risers on the caisson well structures were not protected by framing or boat landings and were subjected to mechanical abuse. (Later installation of doughnut-type boat landings has alleviated much of the problem of mechanical damage.) The neoprene coating withstood the mechanical abuse much better than the polypropylene. Polypropylene is an excellent coating for use in a sea-water environment, but with exposure to the atmosphere, ultraviolet rays deteriorate the material and cause embrittlement. When the coating is extruded on the pipe, a layer of rubberblend adhesive is trapped in a fluid state between the pipe and the polypropylene shell. The fluid condition of the rubber polypropylene shell. The fluid condition of the rubber blend allows it to flow into and seal small cracks and chips in the polypropylene shell. Exposure to ultraviolet rays not only deteriorates the shell but reduces the fluidity of the rubber-blend adhesive. Handling, clamps, and contact with the structure cause mechanical abrasion and tearing of the embrittled coating. Corrosion of exposed metal surfaces causes the riser to deteriorate.

Neoprene

Neoprene, with its higher impact resistance and flexibility, is highly resistant to mechanical damage. It is much less susceptible to deterioration by exposure to ultraviolet radiation than polypropylene, and the bonding of the neoprene to the pipe inhibits the migration of corrosion when breaks occur. In the application of the neoprene, the pipe is first sandblasted to white metal and coated with an adhesive primer. The rubber blend is wrapped with an overlap primer. The rubber blend is wrapped with an overlap onto the pipe and covered with a layer of nylon tape. The coating is cured under pressure until the rubber bonds to the pipe. This method of shop application results in total pipe contact by the coating. Neoprenecoated risers have been used in the High Island field for 3 1/2 years and are still in excellent condition.

Epoxy

Epoxy is an effective and easily applied material for making field repairs and coating field joints. The original field joints in High Island were coated with either polypropylene tape or polyethylene shrink sleeves. Bonding to the pipe was poor and in many cases holidays occurred, with subsequent corrosion. In the riser replacements, epoxy was used for coating the field joints. Our experience has been with a two-compound epoxy that will cure in air or under water. The method of application was to coat the field joint with a 1/4-in. layer of the epoxy and then wrap with a strip of plastic screen. The screen embeds in the epoxy to prevent flow or sagging and assists in removing holidays. An alternative method, useful for applying the epoxy at or below the surface of the water, is to apply the epoxy to the screen and then wrap the screen around the pipe.

The Hydraulic Couple Connection

A technique for making underwater connections using a hydraulic couple has worked successfully for the High Island riser replacements. The couple (see Figs. 1 and 2) forms a seal around the pipe by rubber packers. The rubber packers and mechanical slips are packers. The rubber packers and mechanical slips are set hydraulically by injection of a fluid epoxy. When the epoxy cures to a solid state, the tool is permanently set.

Riser Replacement

At the time of the first riser problem it was not known whether the deterioration was a result of a defective pipe joint or of corrosion. Therefore, for security the pipe joint or of corrosion. Therefore, for security the hydraulic couple was installed below the deteriorated pipe joint, requiring the use of a crane boat to jet pipe joint, requiring the use of a crane boat to jet mud from around the flowline. Removal and inspection of the pipe joint confirmed that corrosion in the splash zone caused the problem. Below the water level the pipe had not deteriorated.

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