Tests support steam injection method as unassailable technology in oil spill recovery.
The rise in offshore oil exploration and production has stimulated demand for offshore support vessel oil spill response capacity, but not all of the technology available is fit for purpose, a leading supplier in the field suggests.
When offshore support vessels with oil spill response capability store oil recovered from the sea in tanks onboard, the oil needs to stay viscous so that tanks can be emptied when the vessel reaches the shore. The tried and tested solution here has been the use of steam-injection techniques, where oil is heated using probes. Such a solution is offered by Norway’s Parat Halvorsen.
Steam-injection has a proven track record in providing the heat energy output required by NOFO (Norwegian Clean Seas Association for Operating Companies) standards to keep the oil-water emulsion recovered from the sea sufficiently viscous that it can be safely and efficiently discharged.
Demand for new offshore support vessels has been one of the shipping and offshore market’s few bright spots in recent years. Not unusually, new and established owners have sought cost advantages where possible, and one example can be found in the emergence of lower cost hot water coil-based solutions to maintain oil viscosity.
Parat argues that, while lower upfront costs may appear persuasive, hot water coils do not satisfy the requirements as set out in the latest NOFO standard. To prove its point the company invited representatives of shipyards, shipowners, consultants and the Norwegian Coastal Administration to its facilities at Flekkefjord a series of tests earlier this year that brought the two approaches head to head.
Kim Kristensen, Director, Marine and Offshore, Parat Halvorsen, says that the tests focused on whether heat transfer from a low temperature heating coil into a cold crude oil emulsion would be sufficient to be compliant with NOFO. Up to that point, he explains, the technology’s performance had only been tested in water.
“Heat transfer depends on both the viscosity and conductivity of the fluid concerned,” says Mr Kristensen. “Whereas water has a low viscosity and high levels of heat conductivity, an oil/water emulsion is highly viscous but with low levels of conductivity. They represent very different operating conditions.”
To verify whether alternative hot water-based solutions could deliver under situations where hydrocarbons are present, Parat installed a compact heating coil and a steam injection nozzle in its test tank. The tests measured performance of both solutions in water and in heavy oil.
The tests ran over two days, with water tests on the first day and heavy oil on the second, using a 200kW rated heating coil. The first internal test of the coil in water gave a maximum output measured at only 45kW. The coil was then spun tightly, which reduced the heat surface significantly. Before further testing, the coil was modified with 4mm spaces between each winding, increasing the overall length of the coil by 90mm. This raised the coil’s output in water up to 70kW.
Subsequent tests were carried out inside a production hall with an ambient temperature of 15˚C. The effectiveness of the coil was measured in water using a Kamstrup energy meter. The results from the test, show that the coil began with an output of 70kW, but this decreased over time. At the end of the test the average transfer effect was 68kW and the temperature of the water rose by around 26˚C at the various measuring points, with even distribution generally good throughout the tank.
When the test was carried out in oil the net day the average effect was just 7kW, approximately 10 per cent of that with the coil in water. Mr Kristensen says: “That shows us the effect measured in water must be reduced by a factor of 10 to give us a correct reflection of the coil’s capacity in oil. Using a hot water coil of this magnitude for ORO tank heating will not work because the transferred effect in oil is not even close to satisfying NOFO’s requirements.”
The temperature measurements in the oil were taken from the centre of the coil and 50mm and 250mm above it. These measurements all had similar temperatures, which indicate that resistance against heat transfer is found at the surface of the coil. “There is no evidence that a heat pocket is formed around the coil, and so circulation of the oil inside the tank will not influence heat transfers much,” Mr Kristensen points out.
“We have shown empirically that steam injection is the one viable solution proven to keep heavy oil viscous enough for easy loading and offloading. The results from the tests we carried out demonstrate without any doubt that using a heating coil will not work."
A tank heating system must satisfy various criteria if a vessel is to be approved by NOFO. All of the vessel’s Oil Recovery Operation (ORO) tanks must be equipped with a permanent system for heating the recovered oil and emulsion and this system must make it possible to raise the temperature by 15˚C for a volume of 1000m3 within a period of 12 hours.
The ambient design conditions set are for a sea water temperature of 5˚C and an air temperature of 0˚C. The NOFO standard says that the technology must be capable of providing full heat capacity for three tanks simultaneously and the calculations set a specific heating capacity of 3.44 kj/kg/˚C, which is the typical value for a 50-50 mix of oil and water.
No measurements were carried out in a mix of hydrocarbon and water. However this is not considered significant. Mr Kristensen explains: “Since the resistance of the transferred heat is found near the coil’s surface we can assume that when the coil comes into contact with a hydrocarbon, heat transfer will be much lower, even in an oil/water mixture.
“The results from the tests clearly showed that using a heating coil is not a viable option, with heat transfer in heavy oil using the hot water coil just 10 per cent of that achieved by the same coil in water,” he adds. “When we started the steam injection system, live temperature logging recorded the way oil was evenly heated in a matter of minutes.”
Before and after
Parat says it can now more confidently claim that steam injection has clear advantages compared to the hot water coil. When a hot water coil is used to heat emulsions, the tests showed heat transfer is inadequate, due to the low conductivity of the emulsion, while circulation locally around the surface of the heater is poor due to the high viscosity, which also reduces heat transfer. By contrast with steam injection heat transfer is very efficient and creates good circulation throughout the entire tank.
According to Mr Kristensen: “Before we carried out these tests we believed that a compact hot water heating coil would not be sufficient to heat up a tank filled with crude oil emulsion in accordance with the NOFO requirements. We also believed that using pumps to circulate the emulsion in the tanks would not create sufficient velocity at the heating surface of the coil to achieve efficient heat transfer. The tests were important as they gave us - and the industry - the empirical proof needed to back up these theoretical conclusions.”
He adds that a critical part of the Parat solution is the ORO multi nozzle arrangement, which heats the whole tank from one insertion point, while steam is supplied via the Parat MEL electrical boiler (approved for marine use by class societies including Germanischer Lloyd, Bureau Veritas, Det Norske Veritas and Lloyd’s Register). The company has also patented part of the hot water circulation loop used in normal operations, interconnecting heat recovery and heat consumers to ensure continuous operation. If an oil spill incident should occur, the vessel operator can bypass the boiler in the hot water loop and re-mobilise the boiler to generate steam for the ORO tank heating system.
Parat Halvorsen AS is based in Flekkefjord, Norway. www.parat.no