Split phase sampling of high vapour pressure spiked crude oils
Authors: Dixon L.A. 
Published: December 2007

Introduction

Many oil companies who are operators of offshore fields are nowadays undertaking joint pipeline ventures to transport their crude production to shore. This has obvious financial advantages to the participating concerns in spreading the high costs of long undersea pipelines such as those existing or being built in the northern water of the North Sea. However, it does imply the need for accurate measurement and evaluation of the various inputs from the individual fields not only in terms of volume and mass but also of composition. This is the situation with a particular pipeline scheme where crude oil from several North Sea fields will be transported to shore through a common pipeline. In order to ensure that each of the participant companies receives its entitlement to an equitable apportionment of the mixed product arriving onshore, it is necessary to sample and assess the input crudes accurately and representatively. Commercial equipment is available for taking average samples from a flowing stream over a given time period. In the case of this scheme, however, the problem of representative sampling is made somewhat more severe by the fact that the input crudes will be spiked with light hydrocarbons, mainly propane and butanes, as a means of conveying these other valuable products to shore after separation on the platforms. The resultant spiked crudes may have vapour pressures up to about 100 p.s.i. at 100 degrees F and while existing as single-phase systems at pipeline pressure, they would be unstable at atmospheric pressure. All metered input streams must be sampled and analysed daily for their composition in terms of C4 and lower hydrocarbons individually, and C5 and higher hydrocarbons. In addition it is required that the quality of the C5+ fraction be assessed. The minimum volume needed to ensure that the samplers used will take representative samples is about 10 litres for each of the five input streams sampled over each 24-hour period. The fairly large numbers of samples involved mean the period. The fairly large numbers of samples involved mean the necessity to transfer them quickly to shore, preferably by helicopter, to avoid storage and administrative problems, also that large backlogs in the analytical processing are avoided. Due allowance needs to be made for weather delays which could be up to two weeks at some times of year, so that an ample supply of sample containers is required, about 150–200.

Sampling methods

Consideration of methods of sampling and handling these high vapour pressure crudes has suggested three possible alternatives:The sample could be simply injected into an empty container where it will flash into separate liquid and gas phases. Even at the end of the 24-hours sampling period, the fluid is likely to remain in two phases at the prevailing pressure. The container must then be taken ashore and the sample must be homogenised before further handling and analysis as a single phase. This would not be simple to perform and could be subject to major errors. Certainly it would require skilled operators to carry out the processing. The containers themselves would be large and would processing. The containers themselves would be large and would need a working pressure rating preferably higher than the feed-line pressure. Thus they would be relatively expensive and such pressure. Thus they would be relatively expensive and such containers of the size envisaged with their pressurized samples would not be acceptable as helicopter cargo.Alternatively, the spiked crude could be injected into a container equipped with a movable diaphragm or piston pressurized to line pressure. This would permit the sample to be collected as a single phase and perhaps a smaller aliquot taken at the same pressure for sending to shore. The containers, although may be pressure for sending to shore. The containers, although may be smaller, would still need a high working pressure rating and skilled operators would be needed to handle them. Again they would not be acceptable as helicopter cargo, they would still be costly and the difficulties involved in handling and analysing high pressure samples remain.The third alternative is to allow for the natural tendency of the spiked crude to flash into separate liquid and gas phases. Both phases would be collected in interconnected receivers phases. Both phases would be collected in interconnected receivers at low pressures and aliquots would be taken for analytical purposes. On-line sampling equipment is needed for each metered purposes. On-line sampling equipment is needed for each metered stream but this equipment would be simple to operate. The collected low pressure samples could be handled in conventional, inexpensive containers, cans for the liquid phase and small cylinders for the gas, which would be acceptable for helicopter transport. Processing in the shore laboratory would also be simplified and the liquid phase would be directly usable for oil quality evaluation. A further advantage is that the light hydrocarbon gases would now be largely concentrated in the gas phase where accurate determination would be more readily achieved. phase where accurate determination would be more readily achieved. Combination of the liquid and gas phase compositions in the flash ratio measured in the sampling equipment would then give the composition of the original spiked crude.