Concrete Repairs With Corrosion Protection: An Economical Alternative To Platform Replacement

ABSTRACT The combination of structural repairs and galvanic (sacrificial) cathodic protection has extended the service life of an offshore reinforced concrete platform by 10 to 15 years. The 18-year-old structure had suffered severe corrosion-related deterioration because of shortcomings of the original design and exposure to both seawater and high-chloride brine leakage from the waterflood injection facilities. Several innovative repair techniques were used. The underwater sections of the columns were repaired using preplaced aggregate concrete. Embedded zinc anodes were used to protect the above and underwater portions of the reinforcement in the columns. The deck was repaired using cast-in-place concrete or shotcrete, and its underside was sprayed with metallized zinc for cathodic protection of the reinforcement. Other reinforced concrete platforms in the Gulf of Mexico have been protected with impressed current cathodic protection1. However, we believe this is the first application of galvanic cathodic protection on an offshore reinforced concrete platform in the Gulf of Mexico. This project proved that properly planned and executed concrete repairs can offer a much faster and economical alternative compared to replacement, even for severely deteriorated structures. DESCRIPTION AND FUNCTION OF THE WATERFLOOD PLATFORM The waterflood platform at Main Pass 69, about 20 miles east of Venice, Louisiana, is a concrete barge which was installed in 1972 for Shell Oil Company. The purpose of this facility is to support waterflood equipment which filters and reinjects water into underlying oil-bearing formations to maintain reservoir pressure and help sweep residual oil out of the ground. An overall view of the concrete barge is shown in Figure 1. The barge has a 45 ft. by 37 ft. deck area which is supported by 24 straight and 8 battered 16 in. diameter columns on a hollow concrete base of 45 ft. by 45 ft. by 12 ft. The vertical clearance between the two decks is 19 ft. 8 in., which was designed to provide 2.7 ft. of air gap during hurricanes. The dead weight of the structure excluding buoyancy was estimated to be 710 kips. The operational weight of the equipment on the deck was estimated to be 210 kips. The barge was constructed onshore using lightweight cast-in-place concrete, floated to the location and set on the bottom in about 2 ft. of water by flooding the base section. Draft of the barge and deck during tow was estimated to be 6.25 ft. A channel to the site as well as a 65 ft. square area (16 ft. below the mudline) at the site were dredged for the installation. (The dredged area at the site was backfilled with clam shells before lowering the barge.) The final elevation of the barge bottom was 8 ft. below mudline. Four 24 in. diameter 50 ft. long corner pin piles were driven to help secure the barge in place.