Metering of High-Vapor-Pressure Crude Oils: The Case for Mass Measurement in Shared Pipeline Systems
Authors: Gold R.C. , Simmons S.G. 
Published: December 2007

The paper discusses the relative merits of mass vs. volumetric measurement as the basis for equitable allocation of high-vapor-pressure (HVP) crude oils commingled in shared pipeline systems. Measurement uncertainties for both methods are compared. The problems of continuous density measurement and of automatic problems of continuous density measurement and of automatic sampling for composition analysis and water content determination are discussed.

Introduction

Capital and running cost economics make it commercially sensible to transport crude oil in shared pipelines. This practice is employed widely in North Sea operations. practice is employed widely in North Sea operations. where crude oil from several different fields or licensed areas may be gathered into a single pipeline for transfer to a land-based terminal. The compositions of the field crudes commingled in the pipeline may differ, with each containing varying proportions of reinjected-natural gas liquid (NGL) condensate and other light hydrocarbons. These light components are recovered from the cocktail of total received crude at the onshore receiving terminal, with the propane and butane re-exported as individual products, separate from the main bulk of the commingled products, separate from the main bulk of the commingled stabilized crude. Some dissolved gases also may be sold separately or used locally as fuel. The method used to measure each offshore subscription to the pipeline and the total cocktail received at the terminal must allow for accurate and equitable pro rate allocations to participants on the basis of their contributions to the total of each marketed component. Legislative requirements for the measurement of oil won and saved also must be satisfied. This paper discusses the alternative principles of measurement and examines the factors that principles of measurement and examines the factors that influence the measuring, uncertainties in each case. On the new BP Intl. Ltd. Magnus Platform, measurement is made of the total mass of live crude exported into the Ninian shared pipeline system. The Magnus metering system is described briefly to illustrate the application of a practical high-vapor-pressure (HVP) crude mass measurement system.

Measurement Principles

The allocation of a shared crude oil pipeline throughput may be based on two basic measurement principles. These are volume and mass.

Allocation by Volume

The arguments for using volume as a basis for allocation are primarily commercial rather than technical.Crude oil is sold by volume. It is the traditional basis for international oil trading. The standard international commercial unit is the U.S. barrel, although for European fiscal accounting, the unit is the cubic meter. Both are oil volumes measured at reference conditions of temperature and pressure (barrels at 60 deg.F, 14.7 psia [cubic meters at 15 deg.C, 101.325 kPa]). The trading basis for liquefied petroleum gas (LPG) is by weight.There is significantly less variation in the value of crude oil when the oil density changes if the value is defined in dollars per barrel rather than dollars per tonne. Generally. low-gravity (light) crudes are intrinsically more valuable than heavy crudes.Pipeline fill figures and storage tank capacities are essentially volumetric. For stock computations and system control, both pipeline and terminal operators require reports of movement in volumetric terms.Allocation by volume requires only volumemetering, with supplementary measurement of temperature and pressure; the techniques and equipment for these pressure; the techniques and equipment for these measurements are reliable and well established.

Volumetric allocation also avoids the need for on-line density measurement since, except for qualitative valuation, density is not a first-order factor in the calculations. The practical and commercial advantages of volumetric allocation are offset by a number of factors that may affect the accuracy of the measurement. These may arise particularly from such parameters as the shrinkage effect particularly from such parameters as the shrinkage effect (nonideal mixing) and volume variations with temperature and pressure. The uncertainties of these factors are particularly acute in the case of crude oil containing particularly acute in the case of crude oil containing significant proportions of light-end components. Such factors do not apply in the alternative case-allocation by mass.

Allocation by Mass

Allocation on the basis of mass is now the most widely practiced procedure in North Sea shared pipeline systems. The reasons are related largely to fundamental physical principles. However, the commercial/trading considerations also apply in the specific case of condensate because it is sold by weight. There are two principal arguments for allocation by mass rather than volume. 1. Mass is an inviolate property. Unlike volume, itremains unaffected by temperature and pressure.

JPT

P. 2002