I guess you are putting a mixture of oil and water into the sludge/ waste oil tank and then heating to evaporate the water.
You will then be left with a concentrated mixture of oil and solid waste which you have to pump ashore.
If you heat oil too much it will carbonise and thicken the oil residue, it sounds as if this is what has happened to you.
Probably the best thing to do is put a load of MDO or even better MGO into the tank to thin it down and then pump it ashore.
Clean break will help emulsify oil with water but unless you have plenty of water in there it won't help shift the concentrated, carbonised, sludge that you have in the tank. If you keep on heating it without pumping it ashore once the water has boiled off you will have to go in the tank with pneumatic drills to get it out.
Good luck, and let us know how you get on.
It is always better to ask a stupid question than to do a stupid thing.