Abstract: An enclosed offshore tank for storing crude oil in a flowable form where the problems associated with flow assurance are prevented. The enclosed offshore tank includes a floor, an insulated cover and perimeter wall secured to a floor forming the offshore tank. The enclosed offshore tank can include a means of floor heating, wherein the floor is provided with piping which receives heated fluid to heat the crude oil. The perimeter wall can have an inner wall and an outer wall forming an annulus, wherein a heated fluid from a heating source fills the annulus to heat the crude oil. A microwave heating system can include at least one microwave generator, at least one waveguide and at least one radiating element to indirectly heat crude oil in the offshore tank.