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.

The performance of pipeline transportation of oil & gas mixtures can be optimized by different ways. This paper introduces a synergetic statistical approach based on field data to analyzing the oscillations of pressure by determining simultaneously Hausdorff's Dimension "D", Hurst's Index "H" and the Entropies "E", as a useful tool for managing the multiphase pipeline transportation systems. Introduction Practically, in most cases, the pipeline transportation of oil & gas mixtures encounters the fluctuation in pressures and flow rates that can cause the complexities and instability in the oil and gas production and gathering systems. To improve the transportation conditions and to provide adequate safety for working pipelines the studies on implementation of a new approach to analyzing the dynamical system have been carried out. The results obtained in studies allow diagnosing the hydro-dynamical states of oil and gas flows in pipelines and its ordering. Consequently, it can help to transport the oil and gas mixture through existing pipelines at maximum flow rate but with minimum pressure losses. Background One of most important missions in pipeline transportation of oil and gas mixture is to improving hydraulic regimes based on forecasting the dynamical conditions. This may be achieved by regulating the flow regime or elimination of phase inhomogeneity of flows in the pipeline. The most effective way to settle the problems is to establish the principally new technologies allowing optimizing the solutions at minimum expenses for their practical realization. Transportation of production at maximum flow rate with minimum spending the energy through submarine pipelines represents the most important duty in operation of oil and gas pipeline network in offshore conditions for JV Vietsovpetro. Necessary condition for solving this dilemma is to operating the pipelines at lowest level of pulsation and minimum hydraulic losses. This may be executed by preliminary gas separation from oil or excluding most of resources influencing on stability of dynamical state of system. In conjunction with this, the execution of field experiments in working pipelines would require unproductive expenditures of finance as well as time. More and more attention is paid on development of method diagnosing the state of an object using data collected during routine operation. Accidental fluctuations, occurring in hydraulic systems, often have determined character. They happen due to own system and can serve as resource of information on internal characteristics. The existence of factors affecting on stability of working regime in pipeline, such as: gas flow rate ratio, flow rate, pipe size, depositions, leads to occurrence of oscillatory movement that heap on pulsations of pressure and flow rate. Meanwhile, they change the spectra and other characteristics of initial noises. So that the analysis of pulsations allowing finding out those changes, can lead to diagnosing the degree of inhomogeneity of being studied system. Because the oscillations of pressure and flow rate have more significant chaostic character, hence for their analysis the latest achievement in the theory of dynamical chaos is to be involved.

In production and transportation of waxy crude oil as well as in practical Rheological studies of non-Newtonian liquids, it is very important to find and characterize its thixotropic properties. There are some ways to study this phenomenon, but nowadays the existing equations still can not quantifiably describe the time-dependent behavior of thixotropic liquids with acceptable accuracy using in this case an exponential or hyperbolic functions. The authors have conducted the screening studies on rotational viscometer Rotovisco RV-20 with different kinds of waxy crude oils from JV "Vietsovpetro" fields offshore Vietnam. Based on the received experimental data and its analysis the authors have proposed a new equation showing a very good agreement between experimental and calculated data (the average deviation is less than 2%). The general time-dependence of shear stress at constant shear rate can be much more precisely expressed. A new idea "the measure of thixotropicity" of liquids at given shear rate and temperature is also proposed to characterize the properties of thixotropic liquids.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractTransport of oil and gas from producing wells to separation units in two-phase flow by one pipeline could significantly reduce the budget for building the product gathering system. However, practice and studies showed that it would be effective only if the gas flow rate ratio β≤ 0.7.JV Vietsovpetro crude oil, mainly being produced from the basement of southern part of the White Tiger field, is transported in form of two-phase flow from Satellite Platforms (SP) to Central Processing Platform (CPP) for gas separation through submarine pipeline (without thermal insulation). The crude has high wax content (up to 27% by weight), high pour point temperature 33-34 o C, and average GOR of 166 m 3 /m 3 @ 20 o C & 100 kPa(a). The transport and gas separation processes encountered many difficulties due to pressure and flow rate fluctuations. Increasing pressure in the pipelines to 2200-2500 kPa(g) improved the situation, but not enough. A consequence was also that the well flow rates were decreased. This paper describes the results from a full-scale field test on transportation of 5848 m 3 /day (@ 20 o C & 100 kPa(a)) well product from SP to CPP as an oil and gas mixture. Studies were carried out on subsea un-insulated pipeline with OD 0.324m and length around 2000m, with gas flow rate ratios, varying from 0.623 to 0.868. Developed on the above basis the technology of preliminary gas separation on the SP and then transport of the gas and oil by two separate pipelines has assisted JV Vietsovpetro in elimination of a/m difficulties. Application of this technology would gain significant benefits when most of the wells on the SP moved to gaslift production.

As in production and transport of waxy crude oil, and as in practical rheological studies of non-Newtonian liquids, it is very important to find and characterize its thixotropic properties. There are some ways to study this phenomenon, but still existing equations can not quantifiably describe the time dependent behavior of thixotropic liquids with acceptable accuracy, using in this case an exponential or hyperbolic functions.