The Colville River cathodic protection (CP) system is unique in many ways. The cased pipelines are isolated from the 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 riverbanks 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. Conventional reference electrodes were unsuitable for monitoring the CP system, and custom steel reference electrodes were installed. The 100 mV Polarization criterion will be used to judge the performance to the CP system.
Location
Phillips Alaska, Inc. has constructed the Alpine Development to produce oil located beneath the Colville River delta on the North Slope of Alaska. The development is approximately 34 miles (55 km) west of the established Kuparuk oilfield. Above ground insulated pipelines will connect Alpine 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. Horizontal directional drilling (HDD) was used to cross the river.
The Colville River is the largest river on Alaska's North Slope and drains approximately 20,700 square miles (53,600 km'). 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
Pipeline System Description
There are four crossings, consisting of a 14 inch (350 ram) crude oil pipeline, 12 inch (325 mm) seawater pipeline, 2 inch (60 mm) arctic heating fuel (AHF) pipeline, and an 8 inch (220 mm) impressed current anode, which were completed in 2000. The crude oil, seawater and AHF pipelines are housed within 20 inch (500 mm), 18 inch (450 mm), and 8 inch (220 mm) casings, respectively, providing secondary containment and leak detection under the environmentally sensitive Colville River.
The three pipeline casings (Oil, Water, AHF) 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. 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. Power will be provided by a remote generator, which is fueled initially by propane and later by natural gas produced at Alpine.
REFERENCE ELECTRODES
Conventional Challenges
Monitoring this cathodic protection system poses certain physical, environmental, and economic challenges. The conventional pipe-to-soil potential measurement of buried pipelines consists of placing a reference electrode at grade. For this system, conventional measurements are impractical because the pipelines are buried nominally 85 feet (26 m) below grade and approximately 1,200 feet (366 m) is beneath the largest river along the North Slope, with the remaining length crossing through permafrost. Additionally, access to the site would be both time consuming and expensive requiring helicopter in the summer or tracked vehicles in th
