Wax Properties and Behavior in Offshore Production and Transportation Systems
Authors: Carnahan Norman F. 
Published: October 2008

Transportation of waxy crude oils presents special design and operation problems. Rheology and physical properties of waxy crude oils is not sufficiently defined by the pour point. One must also know the apparent viscosity and the static yield stress, under conditions that replicate those expected to be encountered in normal and upset operations. This work investigates the general nature of waxy crude oils and some of the special design problems they introduce for transportation systems.

Engineering design of waxy crude oil pipelines often involves an economic cost/benefit balance between a heavy-duty brute force mechanical system which will handle all flowing, shutdown and restart situations and a mechanical system which will handle normal situations (i.e., the most likely shutdown and restart situations) and which relies upon well-planned operating procedures to minimize risk due to extremely unlikely shutdown and restart situations. In the case of waxy oils, design also must take into account the need for control of routine deposition of waxy material on interior surfaces of the pipeline.

Design of transportation systems must consider the entire range of fluid properties and rheological behavior of high pour point waxy crude oils. Responsible design procedures must attempt to judiciously quantify and plan for the risks involved, using all physical property, phase behavior, and rheological information obtained from representative samples of the oil(s) to be transported.

Introduction

Operation and design of pipeline systems for handling problematic fluids remain one of the difficult design challenges in fluid transportation. Waxy crude oils, asphaltic crude oils, slurries, and emulsions exhibit gelling or thickening, during low flow or shutdown conditions. These fluids demand careful analysis of fluid rheology and phase behavior, together with an evaluation of practical design and operation alternatives.

Early design philosophies were heavily influenced by the operator's fear of an immovable ‘wax candle’, due to prolonged shutdown and cooling of waxy crude in a pipeline. Many waxy crude pipelines were located in parts of the world where political and atmospheric conditions might result in emergency shutdown of a pipeline for long periods of time. It was a reasonable fear. Therefore, owners and operators required designers to do their best to handle problems of restart. That era engendered design philosophies that focused mainly on restartability problems, in addition to the equally important planning of operations and preventive maintenance.

Modern design philosophies for waxy crude transportation systems pay due attention to both problems, restartability and routine operations, in which wax buildup and removal must be carefully monitored and controlled.

Practical design of pipeline facilities to transport waxy crude oils requires the corresponding development of a well-defined operating plan, including guidelines for normal and upset operations. Without such a well-defined operating plan, mechanical design would require that some pipelines have the design pressure rating of a cannon. The more practical alternative is to design for the reasonable operations, including most likely upset conditions, and to address extremely unlikely problems in the form of an operating plan for special conditions.