Abstract

Here generalized analysis is given for transport of highly paraffin and highly congealing oil taking into account layout, technology of transport and construction features of subsea oil pipeline.

In accordance with results of physical experiments for active oil pipelines the generalized mathematical model was made to explain thermohydrodynamic regime of telescopic construction oil pipe operation for paraffin and highly congealing oil transport.

Thermohydrodynamic and mass exchange process in subsea pipelines are explained by differential equation system:

  1. flow

  2. continuity

  3. energy

  4. heavy hydrocarbon fraction concentration forming wall depositions

  5. adsorbtion kinetic and hydrodynamic desorbtion of wall oil depositions

Based on mathematical model of existing subsea oil pipeline, new computer method was created for analysis of thermohydrodynamic and technological process while transporting highly paraffin and highly congealing oils. There was developed respective software package allowing to study both steadied and unsteady transport process in wide range of thermobaryc and technological parameters change characterizing oil pipeline operation.

Proposed method of analysis is intended for designing of new pipelines and modification of old subsea oil pipelines and change of transport technology in active pipelined, i.e. optimization of transport parameters and determination of oil pipeline safe operation limit eliminating critical transport regime-telescopic pipeline "freezing".

Digital experiments have shown that in certain technological transport condition the critical operation regime starts proceeding from intensive heavy hydrocarbon depositions (in solid phase) at inside pipeline surface. It is accompanied by spontaneous (uncontrollable) decrease (due to hydraulic resistance) of pipeline throughput. This process (if timely special measures are not taken) will result in full transport stoppage and subsequent pipeline "freezing".

Based on analysis of different technological parameters influence on thermohydrodynamic pipeline operation regime, there were considered and proposed method of thermal, hydrodynamic and chemical influence on wall depositions which allow to eliminate critical pipeline operation regime at different flow rates.

Here the results are given for digital simulation of pipeline operation in form of diagrams.

Introduction

World pipeline experience shows that intensive heat exchange between transported oil and environment in subsea pipelines leads to sharp change of thermohydrodynamic process (characteristics) in flow along the pipeline [1–5,11]. Oil temperature drop in flow direction cause change of its rheologic properties and is accompanied by phase transition as well as formation of oil wall depositions at pipeline internal surface. The above factors at certain technological conditions appear to cause gradual spontaneous reduction (due to flow hydraulic resistance) of pipeline flow rate. Ant if timely special measures are not taken it can lead to full stop of pumping with subsequent pipeline congealing (freezing). The above specific peculiarities of pipeline operation raise the problem of special technology development for high-paraffin and high-congealing oil transportation that should exclude possibility of this critical process occurrence during transportation.

Higher paraffin plays significant role in oil recovery and transport process. Here and after paraffin means alkane C17H36 and higher solidifying at normal temperature. Content of individual higher alkanes in oils, e.g. White Tiger and Dragon oil-fields, ranges from 1.3 to 4.6%. Its density in solid phase is 865.0 – 940.0 g/cm3 and in melt phase - 777 – 790 g/cm3. Depending on alkanes temperature and concentration, alkanes in oil can be in dissolved or crystallized state. Normal alkanes have low boiling temperature and molecular mass. They can form large crystals.

Isoparaffin and cycloparaffin have higher molecular mass as compared to normal ones. At certain conditions they can form monocrystals.

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