Field-Tested Model for Freeze Protection of Arctic Multiphase Pipelines

Abstract

Summary Freeze protection of water-bearing pipelines is an essential part of Arctic operations. Methods used to protect pipelines at the Prudhoe Bay field are discussed, including the development of an algorithm to predict the cooling times of multiphase flowlines and a field-tested program used to verify performance of the algorithm. Introduction The Prudhoe Bay oil field, located about 250 miles north of the Arctic Circle, is exposed to average winter temperatures of -35 degree Fahrenheit and below. Protecting outdoor pipelines from freeze damage is an integral part of field operation. Depending on the size, service, and location of an individual flowline, one or more methods can be used to protect it from freeze damage. The field is laid out with centralized drilling and separation facilities, similar to large offshore fields. Wells are drilled from centralized well pads, with 10 to 50 production and injection wells tied to a common manifold. Commingled flow from the wellpad manifolds is routed to one of six central separation centers where crude is stabilized for shipment to the Trans-Alaska pipeline system. Produced water and pipeline system. Produced water and seawater, gas-lift gas, and miscible injectant are treated at the central plants and shipped to the well pads in cross-country flowlines. Injection fluids are routed to individual injection wells from the well-pad manifold. Pipelines between the wells and well-pad Pipelines between the wells and well-pad manifolds are called "well lines," and pipelines between central facilities and pipelines between central facilities and well pads are called "flowlines." Freeze Protection Methods When a pipeline is scheduled to be taken out of flowing service, or a production upset occurs, a reaction time is calculated. The reaction time is defined as the time required for ice to begin to form in the line minus the time required to protect the line. For example, in a water line to be protected by pigging, the reaction time is the estimated pigging, the reaction time is the estimated freeze time at current conditions minus the time required to displace water from the line with a gas-driven pig. Consideration of reaction times ensures adequate actions will be taken before the onset of freezing. Various protection methods were developed and protection methods were developed and tested as the field was developed and new facilities were added. Fluid Displacement Methods. Displacement is a common means of protecting lines and can be completed by several different methods. Displacement of well lines with a heated water/methanol solution is used for production-well lines that are not level. The production-well lines that are not level. The solution is pumped into the line at the manifold building and the producing fluids are displaced to the wellhead and about 2,000 ft downhole to protect the wellbore from permafrost. Methanol displacement is the most common means of protecting shutin multiphase production and injection-well water lines.