The Problem in Transportation of High Waxy Crude Oils Through Submarine Pipelines at JV Vietsovpetro Oil Fields, Offshore Vietnam
Authors: Thuc P.D. , Bich H.V. , Son T.C. , Hoe L.D. , Vygovskoy V.P. 
Published: July 2010

The transportation of crude oil with high wax content through a submarine pipeline built without thermal insulation can cause serious problems. The heat loss due to the sharp temperature gradient between the crude and the environment results in the crystallization of wax in the crude, with a subsequent wax deposition on the walls of the pipeline. This can result in:reduction of the actual pipeline diameter resulting in higher pressure drops; and,the formation of gelled interlocking structures of wax crystals in the pipeline, leading to shutdowns.

The safe and continuous operation of the pipeline has been dependent on the improvement in the flowability characteristics of the Vietsovpetro crude oils. Using pour point depressant chemicals can drastically reduce the potential risk for wax deposition and gelling. A study on the rheological properties of the crude oil enables an evaluation of different chemical types necessary to treat the crude. The pressure drop required to start pumping crude oil through newly constructed submarine pipelines or to restart the flow after an emergency production shutdown has to be determined. A pipeline model used to predict the restart pressures and flow characteristics has played a very important role in the technological concept and design of the product gathering system at Vietsovpetro oil fields.

Introduction

Currently, JV Vietsovpetro is operating three oil fields: Bach Ho (White Tiger); Rong (Dragon); and, Dai Hung (Big Bear). Crude oil is produced from fixed platforms, satellite platforms and MOU Daihung 1. After gas separation, the degassed oil is pumped to FSO for storage and subsequent export to shuttle tankers.

Submarine pipelines carrying crude oil produced offshore encounters many more problems than land pipelines. The Vietsovpetro pipelines are generally laid on the seabed without thermal insulation. The temperature of the water at the seabed is in the range of 25°C to 28°C, with an occasional low temperature of 21.6 ° C. The transportation of a waxy crude having a pour point just 6 - 10 ° C higher than the seabed temperature can be associated with these problems:High cooling rates of the crude oil throughout the year;Wax deposition on the pipeline walls reducing effective diameters and increasing pressure drops(1,2,3); and,Buildup of a gelled interlocking structure of wax crystals at low temperatures and flow rates which may cause production shutdown.

The pumping of waxy crude through a pipeline without thermal insulation can be facilitated using the following methods:Pumping crude oil conditioned by proper heat treatment. This involves pre-heating the waxy crude oil and heat tracing the pipeline to maintain the flow temperature above the pour point temperature of crude;Transporting crude oil mixed with water, i.e., in a two-phase flow transportation system;Pumping crude oil diluted with solvents or less waxy crude oil; and,Pumping crude oil treated with flow improvers.

Conditioning by proper heat treatment can improve the fluidity of some crude oils, but this would require a huge heat generating plant on one of the offshore platforms in the field.