Deep water platforms in the Louisiana Gulf Coast area can be cathodically protected with impressed current at a much lower present value total cost than can be obtained with galvanic anodes. Shell's experiences with impressed current systems on nine structures in 288 to 340 feet of water are documented in this paper. The following-are discussed:
problems encountered in obtaining complete protection with early installations,
means of solving these problems,
the resulting modified design which is being successfully employed, and
improved potential readings obtained with reference electrodes mounted on the platform.
An economic comparison of impressed current versus galvanic anode systems is also included.
The current trend toward drilling in deeper waters off the Louisiana Gulf Coast presents many interesting engineering challenges. One of these is to cathodically protect the immersed portion of the drilling and production platforms at the most economical cost. Until recently a majority of the structures placed in the Gulf of Mexico were routinely protected with sacrificial anodes aluminum, magnesium, or zinc. However, in deeper water and with larger platforms the current and sacrificial material requirements have increased considerably. To protect a structure in 300 feet of water requires up to 170,000 pounds of aluminum for a 20-year life system. This assumes high efficiency products described by Burgbacherl and others are employed. The weight requirements of less efficient materials are considerably greater. Such galvanic systems are costly. To achieve improved economics, Shell has been using impressed current systems on all deep water structures since 1967. To date, impressed current facilities have been installed on nine platforms in 288 to 340 feet of water. Others are currently being designed or fabricated including one for a 373-foot water depth structure. These facilities are described along with a discussion of problems encountered and comments on the economics of impressed current versus galvanic anode systems.
Shell's approach is to develop deep water leases with complete self-contained drilling and production platforms. 2 The basic deep water structure is an eight-pile (plus four to eight skirt piles), two-level platform. The structure measures 40 to 45 feet between piles with the lower deck approximately 50 feet above the mean water level. The lower level is skirted with 20-foot wing decks. The wells are located between the four center piles. From 18 to 32 wells can be drilled from each structure. Once the wells are completed, equipment is installed to produce the self-contained unit. Included is an electrical generating package with 100 percent standby capacity. Thus, power is available to operate impressed current rectifiers. Figure 1 shows a platform-with production facilities. The complex underwater geometry that must be protected from the corrosive action of the sea water can be seen on Figure 2.
Initial Impressed Current System Design The generally accepted Gulf Coast design criteria of five milliamps of current/square foot of surface area in the water and two milliamps of current/square foot of surface area in the mud were used in the first 20 year design life-deep water impressed current systems.
