Bomba, John G., R J Brown and Associates (Far East) Pte Ltd

Abstract

Pipeline transport of waxy crude oils can be performed effectively, both from technical and performed effectively, both from technical and economic standpoints, through adequate system designs. Such pipeline designs must consider safety aspects, system integrity, ease of installation, and potential operating and maintenance problems. potential operating and maintenance problems. General topics which must be reviewed will include features of the waxy crudes, fluid heating and temperature loss control potential, factors influencing and methods of controlling wax accretions, and problems involved with shutdown and startup. A general review of pump effectiveness for waxy crude transport is provided. A cost comparison of pipeline transport with other methods of waxy crude movement, such as barge tow or tanker handling, is presented. This paper is essentially a review of waxy crude oils and their features related to offshore transport. It will serve as an information source for engineers who are inexperienced in dealing with waxy crudes and as a guideline and source of ideas for more experienced engineers that must design transport facilities. Effectively designed pipeline systems can be established to handle waxy rude oil transport. In most cases, pipelines are a cost effective method of transport when compared to other alternates such as barge or tanker shipment.

Introduction

There is nothing really new about transportation of waxy crudes by pipeline, even offshore. The industry has been doing it for years, sometimes successfully, sometimes not so, sometimes with drastic or catastrophic consequences. While this paper will probably not contain anything new, the contents are directed toward eliminating catastrophic failures and avoiding, in so far as is economically possible for a particular set of project circumstances, not so successful operations. The design process for an offshore pipeline is not much different from one onshore. The ocean is an infinite heat sink - this must be taken into account. Insulated pipelines usually require addition of weight to provide stability against hydrodynamic forces. Expansion problems are usually more severe offshore. Start-up problems exist. Unscheduled cool downs can be extremely serious. Produced water and gas can cause problems. No one Produced water and gas can cause problems. No one knows very much about two phase flow of oil-water emulsions and liberated gas. (This does occur!) With all these potential problems, how does one design an offshore waxy crude pipeline system? There are several ways, with the extremes being literally "Brute Force and Awkwardness" and "like a Nuclear Reactor Cooling System", and a middle-of-the-road, sound engineering approach, which this paper will try to present.

DEFINITION OF A WAXY CRUDE

A waxy crude is defined as a crude oil having a high pour point and low API gravity, and often containing paraffin wax (alkanes). A waxy crude is distinguished by the fact that it exhibits non-Newtonian viscosity behaviour at temperatures below about 20F above the pour point. This means that the effective viscosity below this value is not a function of temperature alone, but is also a function of the effective rate of shear in the pipeline. Shear stress and rate of shear must be pipeline. Shear stress and rate of shear must be determined to predict the pressure required to deliver a specified production volume. A waxy crude may exhibit Bingham plastic characteristics after gelling. The behaviour of this type of waxy crude varies from that of Newtonian fluids only in that a specified yield stress must be exceeded to initiate flow. The viscosity of a Bingham plastic is rate of shear and time dependent. For a laminar flow condition, the effective viscosity of the fluid depends on the uniform rate of shear, which is a function of the flow rate and pipe inside diameter. When the flow is turbulent, pipe inside diameter. When the flow is turbulent, there is a thin boundary layer adjacent to the pipe wall which is subjected to a rate of shear pipe wall which is subjected to a rate of shear under laminar conditions. Oil in thins boundary layer becomes rapidly mixed with the bulk of the oil in the centre of the pipe. It is this high rate of shear in the boundary layer which is the appropriate rate of shear for determining effective viscosity. Generally, this is obtained by multiplying the uniform rate of shear (laminar flow condition) by a correct ion factor depending only on the Reynolds number. If a waxy crude is allowed to cool below its pour point under static conditions in a pipeline (no flow point under static conditions in a pipeline (no flow in the pipeline), the wax will crystallize, causing the entire mass of crude oil to gel. Because of this gelling effect upon cool-down, the initial concept is to operate the pipeline at temperatures above the crude's pour point. However, there is no particular problem in transporting the waxy crude particular problem in transporting the waxy crude below its pour point, provided the fluid is kept in motion. More pressure is required for the fluid in non-Newtonian range, although there is no sudden change in fluid characteristics at the pour point. When a waxy crude pipeline operating below the crude's pour point is shut down for any reason, the resulting gel led state will require, upon restart, substantially more pressure to put it in motion. However, this additional restart pressure will be substantially less than if the pipeline was operating above the crude's pour point, shut down and then allowed to cool down statically. The most important criterion in designing pipelines transporting waxy crude is whether the line can be restarted easily after a shut down.

PRACTICAL ASPECTS OF PIPELINE TRANSPORT PRACTICAL ASPECTS OF PIPELINE TRANSPORT

We have identified the basic physical problems and some of the rheologic characteristics of waxy crudes which affect pipelining. The obvious solutions to these problems are:

  • Do not let wax deposit out

  • Keep the crude oil hot

  • Keep it moving

Factors Affecting Wax Deposition
Temperature

The primary factor affecting wax deposition in the pipelines is temperature. As the temperature of the pipelines is temperature. As the temperature of the produced fluid falls below the melting point of the produced fluid falls below the melting point of the wax, it tends to solidify and separate from the liquid phase. This is followed by the wax crystals bonding together to form larger masses (cohesion), and subsequent sticking of these masses to the equipment/pipelines (adhesion). The hardest waxes, those with the highest melting point, will deposit first at higher temperatures, probably in the production string of the well. The softer waxes production string of the well. The softer waxes will come out in the cooler portions of the surface equipment and transporting pipelines.

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