XSENS WHITEPAPER – KNUTSEN OAS – Monitoring Shuttle Tanker Loading Processes Using XSENS Clamp-On Meters

Monitoring Shuttle Tanker Loading Processes Using XSENS Clamp-On Meters

Why is it so difficult to measure flow from offshore installations being loaded onto shuttle tankers, and how did XSENS manage to measure this accurately?

XSENS Clamp-On Flow Rate Meter installed at Knutsen OAS Shuttle Tanker

CUSTOMER PAIN

Hoses from offshore installations to shuttle tankers are submerged in seawater during loading operations. In 2007, an oil loading leakage incident at the Norwegian Statfjord field occurred. The oil leakage was undetected for several hours resulting in an estimated 21.750 barrel oil spill. As a result, a strict requirement was implemented by Oil Companies: Loading in the North Sea must be measured directly on the shuttle tanker to enable instant detection of leaks.

Knutsen OAS, a fully integrated shipping company, used traditional clamp-on flowmeters onboard to fulfil the requirement. However, they faced challenges with flow measurement from these traditional non-intrusive ultrasonic meters on some of their loading operations in the North Sea.

Reports from the incumbent supplier of the onboard flow meters concluded that signals disappeared due to high content of gas in the liquid flow during loading and that their flow meters could not perform measurements in such conditions. XSENS was then approached by Knutsen to use our advanced non-intrusive ultrasonic technology to solve this challenge.

XSENS takes on the challenge

With this background, XSENS was asked by Knutsen OAS to go offshore on their shuttle tanker “Frida Knutsen” and perform a portable-meter trial while the tanker was loading from Åsgard C and A.

The testing uncovered that there was too much attenuation using our Time of Flight (ToF) transducers. However, our Doppler transducers provided good measurements, so a combination of these two types would give us readings during the whole loading process.

The signal attenuation comes from gas flashed out from the oil due to pressure drop between the offshore installation and the shuttle tanker and also from disturbances by opening and closing valves to the storage tanks on the shuttle tankers. (Figure 1 below shows that when ToF signals disappear, the Doppler signals gives good measurement and vice versa).

XSENS - Combining ToF (Time of Flight) measurement and Doppler gives a robust measurement system in gassy oil applications.

On the vessel, several locations were tested (A,B and C) to see if one location had better results than the others. XSENS concluded that the degassing of oil occurs before the oil enters the bow of the shuttle tanker so the meter location in itself was not an issue. Due to easy access, location A was selected.

XSENS - Evaluation of measurement locations at Shuttle Tanker.

XSENS Permanent Solution

After a successful trial with our portable system, Knutsen OAS placed an order with XSENS for building permanent meters to be installed on their shuttle tankers Frida and Sindre.

After delivery of these Meters, we started to measure loading of oil on both shuttle tankers in October 2023. In all of these loadings, there is a reference measurement taken on the offshore installation where the originating oil is pumped from. There are also tank gauges on the shuttle tankers that can measure how much oil is loaded into the shuttle tanker. In the first months we saw that we had good signals from all loadings, but in some of the loadings the deviation between XSENS readings and fiscal readings from the offshore facilities showed some under readings from our meters. XSENS investigated this and found a way to compensate for under readings using existing signal responses from the pipe wall.

Each field has its own flowing signature depending on viscosity, pressure, difference between facility and shuttle tanker, temperature, composition and density of the oil, making these measurements quite challenging, but not impossible for XSENS Flow Solutions.

XSENS meters have now been in operation doing such loadings on more than 10 fields several times for over a period of almost a year. Results on the next page show how good we are measuring each of such loadings during low flow, no flow and high flow rates well within the requirements for such loading operations.

XSENS Flow Meter installed at KNUTSEN OAS Shuttle Tanker

Test results from multiple loadings

Below are some loading results from 2024 performed against fiscal metering systems.

XSENS - Flow measurement results Shuttle Tanker

Appreciation

We would like to express our gratitude to the Knutsen OAS team for allowing XSENS into this market and for having faith in us during challenges in the project. Together, we can now look back on this measurement challenge as a success story.

Statement from Manager at KNOT Management AS

“We have now used XSENS meters on two of our shuttle tankers for measuring flow during loading operations successfully since Sept 2023, and it has proven to be a very reliable flow meter”