Cool Sorp tion's CLPA process for recovering crude vapor emissions.The technology for tackling the emission of volatile organic compounds (VOCs) during offshore oil loading operations is now becoming available in Norway in the form of offshore vapor recovery units (VRUs) from both Aker Maritime and Kvaerner.
Although committed to reducing its VOC emissions by 30% in the period 1989-99, the Norwegian government is in danger of having its green credentials questioned in this respect. VOC emissions in Norway have increased substantially during the 1990s as offshore production and loading have increased. The oil sector is now responsible for 56% of all VOC emissions in Norway.
VRUs could change this picture, suppliers say, though the payback period varies a lot depending on the circumstances, and under some circumstances they are unlikely to pay for themselves.
This is not good news for oil companies, some of which are looking rather unrealistically for systems with a two-year payback period.
But in the long term the use of such systems may be inevitable, both in Norway and elsewhere, given the likelihood that governments will eventually resort to legislation to tackle the problem of increasing VOC and other emissions
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Aker Maritime
Aker Maritime has teamed up with the Danish vapor recov ery specialist Cool Sorption, Europe's largest manufacturer of vapor recovery units (VRUs), in the process taking a 35% stake in the company.In 1995 the Danish company installed a unit - the world's largest - at Norsk Hydro's Sture terminal capable of recovering 90% or more of the non-methane hydrocarbon content of vapors released during tanker loading operations.
In 1996 only 40% of tankers loading at Sture had the equipment to connect to the VRU, but even so VOC emissions at the terminal in 1996 were reduced by 26% to 16,277 tons.
Almost 53,000 bbl of crude were recovered last year, and Hydro itself reckons the VRU will pay for itself within five years.
Aker Maritime and Cool Sorption recently performed a study for Hydro of a possible vapor recovery scheme for the Njord floating storage unit, and in mid year were doing the same for an unspecified Statoil shuttle tanker, reports Tomm Lund, Aker Engineering's project manager for field development and concept studies.
The Sture unit uses Cool Sorption's patented cold liquid absorption (CLA) process. This has now been further developed into the cold liquid pressure absorption process (CLPA), also patented, which is the technology being offered for offshore use.
In the CLPA process, the vapors are first compressed to 5-8 bar and then fed into a first-stage absorber. Here some of the hydrocarbons are absorbed in fresh crude which is routed back to the cargo tanks.
The remaining vapors pass to a second-stage absorber, where the remaining hydrocarbon content is absorbed in a flow of cold kerosene. Cooling is by means of direct seawater heat exchange, and the temperature of the liquids is kept above the level at which anti-hydrate chemical injection would be required.
The kerosene/hydrocarbon mixture is then separated in a splitter, the kerosene being returned to the absorption column, and the hydrocarbons to the vapor stream.
As currently configured, the CLPA process gives a high recovery rate for the more valuable fractions, up to 98% for pentane, butane and propane, and up to 90% for ethane, according to Anker Jacobsen, managing director of Cool Sorption.
An important feature of the CLPA process is that the gaseous components contained in the vapor are not returned to the crude. Other systems which do not incorporate this feature run the risk of creating a "champagne" effect in the crude, leaving it more unstable than it originally was, Lund says.
The CLPA process also functions normally even when the concentration of vapors is low. Other systems are liable to go into reverse in these circumstances, he says.
Work is still under way to optimize the dimensions of an offshore VRU, but for a 3,700 bbl/hour unit, Lund promises a compact piece of equipment with the footprint of a 20-ft container and weighing no more than 10 tons.
Kvaerner Process Systems
Following successful testing of a pilot VRU on the Tove Knutsen shuttle tanker, Kvaerner Process Systems (KPS) has a letter of intent from Statoil to provide a full-scale unit for a shuttle tanker.The cost of a full-scale unit will be in the range of NKr 30-40 million, according to Reidar Strande, vice president for sales/products.
Backing for the pilot project was provided by Statoil, Saga, Norske Conoco and SFT, the Norwegian state pollution authority.
The process used by KPS is not new, though its application in an offshore context faces problems not met onshore, such as the ship's motions.
The vapors are collected and compressed, and then fed into a cooled slipstream of crude taken from the main loading line. Operations in the absorption column take place at 5-10 bar.
The crude is routed via a pressure reduction device into the storage tank. Some gas overflows in the column, but this is mainly inert gas which is routed via a riser to the open air.
"It's a simple operating system, and installation of the equipment is also easy as there are only a few connections to the existing pipework to be made," Strande says.
The pilot plant for Tove Knutsen was installed in two 20-ft containers with a total weight of 20 tons.
During tests the plant achieved a non methane VOC recovery efficiency of 70-80% by weight. However, operations were interrupted when bad weather obliged the tanker to disconnect from the loading buoy, and Strande says that in the absence of these interruptions, it is calculated that an efficiency of 80-87% could have been achieved.
Fears that the returned VOCs would create a "champagne" effect in the crude - because the components would make it more unstable - proved unfounded, according to Strande.
Kvaerner's system is also well suited for use on FPSOs, Strande says. The unit tested on Tove Knutsen was in size applicable to a typical production ship.
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