Dynamic flow measurement system for tanker offloading

November 19th, 2014, Published in Articles: EngineerIT


Shipping across Egypt instead of through the Suez canal is cheaper, quicker, and more flexible, but it requires custody transfer metering that is accurate with billing acceptable to owner, customer, and Sumed – the transport company. Ain Sukha offloading terminal for very large crude carriers (VLCC) is particularly problematic for dynamic flow measurement due to instability variance characteristics of the VLCC pumping during discharge operations, with the resulting possibility of introducing entrained air during discharging.

Moreover the station is situated at the entrance to the shore terminal at the end of an 8 km 1,2 m sub-sea pipeline. Sumed commissioned in cooperation with Saudi Aramco the design and pilot build of a state-of-the art metering system on import line #2. FEED and uncertainty studies were done by TÜV NEL, and the system designed and supplied by Krohne Oil & Gas.

Sumed’s primary function is to handle, store and transfer crude oil from different customers from the Red Sea region to the Mediterranean sea through a trans-Egypt pipeline. Any systematic measurement offset will directly affect the Sumed’s revenues and it may also affect the competitive edge Sumed enjoys versus alternative transport routed via the Suez Canal. Ain Sukhna at the Red Sea is a crude oil receiving station where crude oil is imported from an offshore single-point mooring to tank storage. From the tank storage, the product is transported via two 1,06 m pipelines to SidiKerir near Alexandria on the Mediterranean coast for export to tankers, thus saving cost and passage time through the Suez Canal.

In order to control the batch quantities and minimise any offset during the intake of crude oil, Sumed decided to install a real- time dynamic metering system in addition to the existing static measurement of their shore tanks.

The dedicated metering skid is located on land close to the shore tanks and approx. 8 km away from the buoy and comprises:

  • 5 x 40 cm Krohne Altsonic V ultrasonic metering streams in parallel
  • 1 x 20 cm Krohne Altsonic V ultrasonic stream for low flow
  • 1 x 91 cm direction ball prover
  • 1 x redundant supervisory system.

Beside the ultrasonic meters, the stream is fitted with a degasser system, upstream, downstream and prover interconnection valves for flow line-up, and flow balancing control valves. The bi-directional ball prover determines the required correction factors for the flowmeters.

Fig. 2: Map of Egypt showing the pipeline.

Fig. 1: Map of Egypt showing the pipeline.

A density meter (Solartron) in the fast loop at the metering skid outlet header monitors the product properties and is connected to the redundant stream flow computers (Summit 8800). A viscosity meter in the fast loop at the metering skid outlet header is used for the flowmeter body correction calculations. The redundant PLC performs the full line-up of the system and required process control. The existing sampler was connected to the metering system. The representative sample is analysed in the lab and subsequently used to determine the final batch loading figures.

A redundant human machine interfaced (HMI) supervisory computer system is used to monitor and control the entire loading sequence, scheduling of future vessel loadings, lab analysis inputting, and finally the generation of the measurement ticket.

Fig. 3: Skid mounted and awaiting commissioning.

Fig. 2: Skid mounted and awaiting commissioning.

Why select ultrasonic measurement?

This offloading facility has very specific properties, creating a challenge for the development of a dynamic metering system:

  • High required throughput (up to 12,000 m3/hr, ± 76 000 bbl/hr)
  • Large variety of product viscosity (between 3 and 25 cSt and sometimes up to 100 cSt). cST is measurement unit ofs kinematic viscosity. Its metric equivalent is mm ²/s = 1cST.
  • Low back-pressure (only the product level in the shore tanks creates back pressure)
  • Possible gas entrapment at start and end of loading sequence
  • Possible free water

In order to understand the full evaluation it is worthwhile explaining here the simple basics of ultrasonic flowmeter principles. Transit time meters work by transmitting ultrasonic pulses along a path diagonally across a pipe spool. A pulse “travelling with the flow” completes the path a little quicker than a pulse “travelling against the flow”.

The operating principle is illustrated in Fig. 3. The two ultrasonic transducers work as emitter and receiver, so we can send an ultrasonic beam in either direction. The difference in transit time of upstream and downstream pulses is used to measure the velocity of the fluid. Within the ultrasonic meter a number of paths (five paths for the Altosonic V) are located in a specific orientation to obtain the overall average velocity of the fluid in the pipe. Here the path through the centre line of the pipe is the most essential as it differentiates between turbulent and laminar flow, while the other four are used for a weighted contribution.

Fig. 4: Measurement principle ultrasonics.

Fig. 3: Measurement principle ultrasonics.

Fig.4: Proving/verification results vs. time.

The measurement principle (like all principles) has advantages and disadvantages as shown in Table 1. Clearly, the worst that can happen to a measurement system is damage to one or more of the flowmeters and secondly mis-measurement.For both reasons mechanical flowmeters (turbine, PD meters) are not the suitable choice, as they would be damaged by surges, cavitation, gas bubbles or particles and further, there will be a significant offset due to the large range of product viscosities. Based on the track record and measurement advantages, ultrasonic meters were selected for this application. However, in order to eliminate any possible installation effects on the measurement results, each flowmeters was calibrated in-situ against a certified bi- directional ball prover.

 Table 1: Factors determining the choice of ultrasonics
Advantages Disadvantages
Mixed-phase flow or over-range is completely harmless  Sensitive to imperfect flow profiles
No moving parts
Negligible pressure loss
Good linearity and repeatability
Provide a speed of sound measurement that can be used for product detection

Uncertainty analysis of dynamic measurement

Measurement is not an exact science and even when state of the art equipment is installed and industry best practice is followed there will always be an uncertainty associated with measurement systems. Having the ability to calculate this uncertainty brings major benefits in both improving confidence and providing evidence that the uncertainty is within specified allowed limits. Uncertainty analyses have become very important in the oil and gas industry where the value of the product has led to the need for highly accurate measurements (0,1% mis-measurement of a VLCC results in false billing of +/- € 160,000 per discharge at current market prices.

Uncertainty analyses performed by TUV Nel use statistical methods to calculate the uncertainty of a measurement system. Thetheory of uncertainty analysis is outlined in the guide to the expression of uncertainty of measurement (GUM) and includes the following steps:

  • The relationship between the measurement inputs and the final result is defined.
  • The measurement is split into a number of sources of input uncertainty, all of which contribute to the overall uncertainty.
  • The magnitude of uncertainty for each input uncertainty source is determined
  • The effect that each input uncertainty source has on the final result is determined by calculating sensitivity coefficients
  • The uncertainty sources are then combined together to calculate the overall uncertainty in the final result

The model created for the AinSukhna metering station consists of a series of spread-sheets, which calculate the uncertainty in mass and standard volume of oil transferred from tanker cargoes to the Sumed pipeline at Ain Sukha. In order to calculate this, each part of the measurement system has to be accounted for.  The uncertainty is firstly calculated for each of the input measurement devices, including temperature, pressure, density, basic sediment and water, prover, and flow rate measurements. The uncertainties of each of these measurements are then combined to calculate the overall uncertainty in mass or standard volume transferred.

Contact John Alexander, Krohne, Tel. 011 314-1391, salesza@krohne.com

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