The Colville River cathodic protection (CP) system is unique in many ways. The cased pipelines are isolated from the mainline CP system and are directionally drilled 85 feet (26 m) below the river surface. Both thawed soils under the active channel and frozen soils under the floodplain and river banks characterize the crossing. These conditions make for differential thermal and soil resistivity states over time. The CP system is designed to protect the casings from external corrosion, and must function in a very dynamic and aggressive environment. High temperature and high chloride content characterize the operating environment for the pipelines. The design of this system utilized laboratory testing of soils, a Finite Element Analysis (FEA) modeling effort, and a unique anode and reference electrode design. The anode is constructed out of an 8 inch (220 mm) seamless steel pipe that has a series of internal redundant positive connections as well as five (5) reference electrodes used for monitoring.
ARCO Alaska Inc. is constructing the Alpine Development to produce oil that is located beneath the Colville River delta on the North Slope of Alaska. The development is approximately 34 miles (55 km) west of the established North Slope Kuparuk oilfield. Kuparuk oil is transported to the 48 inch (1,200 mm) Trans-Alaska Pipeline that connects the North Slope to the Valdez, Alaska marine terminal. Above ground insulated pipelines will connect the Alpine Development to the Kuparuk pipeline system. To exit the Colville River delta the pipelines must cross the 3,600 ft (1,100 m) wide east channel of the Colville River. Several studies of alternative crossing techniques pointed to horizontal directional drilling (HDD) as a cost effective and environmentally sound means of constructing this pipeline crossing. The crossing is located 8 miles (13 km) southeast from the Alpine Development that is approximately centered within the Colville River delta (70°14'41 '' N, 150051'28" W). The Colville River is the largest river on Alaska's North Slope and drains approximately 20,700 square miles (53,600 km2). At the crossing the river is 3,600 feet (1,100 m) wide between high banks. The length of HDD bore is approximately 4,300 ft (1,310 m). Two-2,000 ft (600 m) sections characterize the river; the deeper "active" channel to the east and a floodplain to the west. Figure 1 illustrates a profile section of the river crossing, facing north or down river.
FIGURE 1 - Colville River Crossing, Profile Section
The four crossings, consisting of a 14 inch (350 mm) crude oil pipeline, 12 inch (325 mm) seawater pipeline, 8 inch (220 mm) utility casing, and an 8 inch (220 mm) impressed current anode, have been installed over two winter seasons. The crude oil and seawater pipelines are housed within 20 inch (500 ram) and 18 inch (450 mm) casings, respectively, providing secondary containment and leak detection under the environmentally sensitive Colville River. The three pipeline casings (Oil, Water, Utility) are coated with a dual layer fusion bonded epoxy and cathodically protected with a continuous parallel anode driven by impressed-current rectifiers. The anode is 8.625 inch (220 mm) diameter, 0.5 inch (13 mm) wall, steel pipe running parallel to, and positioned in order to optimize current distribution to the three casings. The anode is made of API Grade 5LX-65 seamless pipe. Current from the anode will be impressed onto the three pipeline casings from full-wave rectifiers located on the west bank of the Colville River.
Monitoring of the CP
system will utilize reference electrodes placed along, and isolated from, the anode as well as a re