How can the overall oil recovery efficiency be increased? Erik Gloppen reports on the challenge of oil recovery operations
The challenge with oil recovery operations is to have an efficient system, including the oil recovery operation in any sea state, discharge of emulsion in the vessel, heating of recovered material and effective STS discharge in open water or in harbour.
There is a lot of focus in cold climate, due to temp decrease mean increase of viscosity. However, sunny areas should not be forgotten as areas with challenges of oil with an increased viscosity. The sun and temp vaporise the oil, leaving only the heavy fractions left, while the light fractions easily vaporise.
To be able to have an efficient oil recovery operation neither weather conditions, daylight, visibility nor working conditions on deck should jeopardise your response. The recovery vessel should be prepared for 24 hours operation in respect of instrumentation to track the oil without air support.
When you have been able to track and contain the oil, the skimmer system must be able to recover the oil. That means that the oil recovery equipment must work, no matter of sea state as long as the oil is floating on the sea-surface. When the oil skimming commences, the skimmer system must be able to give the operator online information on his performance in respect of what he is pumping, the oil/water fraction and also the thickness of the oil slick he is working on. Based upon this information he can optimise his performance accordingly. Tests like NOFO ‘oil on water’ has proven the importance of recovering as pure emulsions as possible, verified by SINTEF.
FRAMO see that the operators focus more and more on safety for the hand on crew onboard the oil recovery vessels. This is of course essential, and also based upon the experience from among the Prestige spill. The crew must be protected as much as possible, as oil recovery operation may last for weeks and even months.
In such cases the working conditions are important. The best location for the operator is to be on the bridge together with the captain. Then the dialogue can be close and eventual misunderstandings can easily be avoided. When the equipment is in the sea, the only thing the operator should do is to fine adjust parameters like pump speed and other functionalities like thrusters, shovels etc. This is something he can do on his radio remote panel, based upon the readings he gain on the same panel from the phase-meter, flow meter and oil thickness meter. Maybe having a cup of coffee instead of standing on deck whether it is warm due to sunlight or cold climate freezing his fingers off.
Since the deck will get very slippery, either from oil washing in over the deck or oil dripping from the equipment. The working conditions will then be the same feeling as the Disney classic ‘Bambi-on the ice’-movie. To avoid these problems the oil recovery equipment area should be a closed area with grating on the deck. Eventual oil attached with the recovery equipment can be washed off and the oil water mixture is drained straight into the oil recovery tank.
There should be as little piping and valves between the oil recovery equipment and the tanks. While the oil recovery operation takes place there are a lot of flow enhancing techniques available to be able to get the oil into the oil recovery tanks. That is they are solved using water lubrication and/or heat.
In addition the skimmer should be a positive displacement skimmer with systems that force the oil it into the pump well and then pump it with a positive displacement pump.
During skimming the oil recovery tanks should be prepared for heating capability. The more viscous the oil is the more difficult the heating is. Forced circulation in heat exchangers is the most efficient method to introduce heat into the recovered emulsion. Many vessels utilise the mud tanks for oil recovery purposes. The combination of coils in mud tanks is something that should be avoided, so external heat exchangers is preferred. FRAMO manufacture something like 1800 deck mounted heaters for tankers from 360-1200 kW each. Based upon this experience we have the following proposal:
Two alternatives for the pumps, but both ideas are to keep the suction line as short as possible.
* Alternative 1 is to utilise the mud pump installed close to the tank, and to have trace heating around the suction line between the tank and the pump. The pump will take suction from one corner in the tank with a suction arrangement. Upstream the pump there is a temp sensor that gives signal to a temp regulated valve. The temp sensor has a set value (like 40 degree C), and give signal to a throttle valve, which diversify return to suction side, and to the opposite side of the tank. The effect is a hot surface on the suction line, thus relatively viscous oil will flow in the centre of the suction line towards the pump. Prior to pumping the heat exchanger and trace heated suction line is started some time before the initial suction is started. There will then be hot surfaces and the initial pressure drops are dramatically decreased. The volume of the tank will then gradually increase temperature as the return of the pump is dumped back in two locations in the tank.
* Alternative 2 is based upon a standard submerged cargo pump with end suction. To ensure properly flow and relatively low viscosity towards the pump in the initial phase, heating coils in the tanks are recommended around the pump. The pump can be installed in a recess, together with a heat exchanger. The warm emulsion is dumped back close to the suction of the pump and after the temp sensor has reached its set point, the valve throttle and more oil is dumped in the opposite side of the tank.
After a while (say 12 hours for 1000 m3), the temp of the tank has reached 60 degree C and the oil can be effectively discharged by a STS offshore to a receiving tanker. Such STS operations has been tested and verified by NOFO in up to 6 meters waves.
Focus should be on the complete system, from the oil spill is in the water until it is efficient offloaded from the recovery vessel.
* The Oil Recover System must be able to recover and handle oil in all weather conditions as long as the oil is on the sea surface.
* The system must be able to handle all type of viscosity.
* Remote operation must be possible to avoid the need to have people on deck.
* ll equipment must be approved for Gas Zone II operation.
Mr. Erik Gloppen is Manager Oil Spill Control Department, Frank Mohn Flatøy AS. For more information, visit www.framo.no"