ABSTRACT

Severe environmental conditions - high tides, rapid currents, and low temperatures - have made cathodic protection of the submerged steel part of offshore platforms in Middle Ground Shoal field, Cook Inlet, Alaska, difficult. Better understanding of the problems associated with obtaining protection has resulted in remote anode sled installations which provide satisfactory protection potentials (>-800 mv Ag-AgCl half cell). The primary parameter, determining the necessary level of protective current per unit area of exposed surface, is the rate of oxygen arrival at the metal-seawater interface.

INTRODUCTION

The first operating platform (Platform "A") was installed by Shell Oil Company in a joint operation with Atlantic-Richfield and Standard in the Middle Ground Shoal field, Cook Inlet, Alaska in October, 1964. Since then another platform (Platform "C") has been installed (Figure 1). These two structures are subjected to corrosion rates which are significantly higher than those usually observed in sea water.

If conditions were similar to the offshore operations along the Gulf Coast, corrosion alleviation would be fairly clear cut since a great deal of experience has been gained in this area. Unfortunately, the tides in Cook Inlet are among the highest in the world. During the year, diurnal tides range from a low of minus seven feet to a high of plus thirty feet. Slack tide, occurring four times daily, averages about thirty minutes, and then is not slack from surface to mud line. The tides create currents of up to seven knots. Weather records indicate that the ambient temperature during the winter could drop to -38°F, resulting in the water temperature decreasing to 30°F. The Inlet water, besides having a substantial dissolved solids content (:>20,000 ppm), contains glacial silts in amount up to 700 ppm.

The severe tides, the low winter temperature, the rapid currents, and the silt content of the water have made corrosion protection difficult. The difficulties experienced in attempting to protect cathodically several structures in Cook Inlet have been described recently by Benedict, Bertners, and Blount1. Shell's experience in developing cathodic protection systems for Cook Inlet is described in this paper.

PRELIMINARY FIELD CORROSION EVALUATIONS

Since the rate of corrosion of exposed steel surfaces to be expected in Cook Inlet was not known, preliminary investigations were made. Corrosion coupons were exposed with short term corrosion rates of 25 to 35 mils per year being obtained. Since normally short term corrosion rates tend to be high, these rates were thought to be greater than would be encounted for long term exposure. This thinking was influenced by published data which indicated an expected corrosion rate of 2 to 8 MPY for long term exposure of steel in sea water.2 There was, however, the question of the effect of water velocity on corrosion. LaQue3 reported tests which indicated that corrosion of steel by sea water increases as velocity increases. These tests showed corrosion rates of 5 MPY at a water velocity of 20 feet per second.

Because ice on the inlet prevented corrosion coupon exposures in the winter (Figure 2), ultrasonic thickness measurements were made on the legs of: Platform "A" at three intervals during 1965-1966.

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