About all those oil tankers off the coast of California … | Grist

The giant ships burn fuel to keep lights on, power equipment, and heat the large volumes of crude oil resting in their tanks.

I'm assuming that crude oil can't be heated in steel frac tanks, places other than salt caverns, and salt caverns.

While underground caverns may not seem like the best place to store an emergency oil supply, they're actually very secure. For one thing, since they're 2,000 to 4,000 feet (610 to 1,219 meters) underground, the extreme pressure prevents cracks from forming and leading to leaks [source: DOE]. Also, the natural temperature difference between the top and bottom of each cavern encourages the oil to circulate, which maintains its quality.


6 Answers 6


As Solar Mike's answer says, crude oil is viscous - too viscous to easily pump.

Crude oil has a "pour point:" the lowest temperature where it will flow under gravity.

Heating the crude oil keeps it above the pour point, so it can be pumped. With the large volume of oil in a tanker, it makes more sense to keep it fluid, rather than letting it cool down and then heat it (very slowly) back up). I found one article (PDF) that recommends tankers keep the oil at least 10°C above the pour point to promote circulation within the tank, which reduces sedimentation and wax formation.


There are so many misrepresentations in the references it is difficult to answer. Basically, crude oil in tankers is not heated. If viscosity is too high for effective pumping, the most likely action is to add a low viscosity oil. This will be done at the crude source.

A very simple distillation "topping plant", will separate light material from a crude to dilute the heavy crude. This was done at a few Venezuelan oil fields with notoriously high viscosity crude, but I don't know if they are now in production.

For older tankers with boilers (not diesels) the fuel for the boiler is heated to improve spray from the burners. For anyone actually interested in tankers, Wikipedia has a reasonable summary under "oil tankers". As far as heating is concerned, it is not mentioned and the diagrams show no steam piping in the holds; it is the familiar old "difficult to prove a negative".


Others here have covered why tankers get heated. A related topic is why salt caverns and other deep underground storage are not heated. It's not that they can't be heated - it's that they don't need to be heated.

It turns out that deep caverns are rather warm already... in most places (away from tectonic boundaries which aren't great for long-terms storage anyway), once you get about 10 meters below the surface the temperature is around 12-13°C year-round. Per Wikipedia, the temperature rises with depth (~25-30°C/km), so at 600m you're above 27°C, and by 1200m you're above 42°C, which should keep most crude oils rather warm (the gradient can vary, but the point is that it will tend to get warmer the deeper you go).

A few places (e.g. a PDF as noted in another answer) mention keeping crude oil in tankers at around 10°C higher than the pour point for that fraction. Fractions with high pour points can have pour points ranging from 13-52°C (per this site). So for all but the most difficult crude oil, a deep salt cavern alone should be sufficiently warm.

  • $\begingroup$ 12-13 °C year-round: That depends on where you are (it is the average mean temperature on the surface for a whole year). The temperature gradient is about 25–30 °C/km and only contributes about 0.3 °C for 10 meters (though there may be local effects, like water flow through the ground). Where I live it is 8 °C. Averaged over entire countries it varies from 28 °C in Burkina Faso to −5.35 °C in Canada) $\endgroup$ Commented May 17, 2020 at 20:06
  • $\begingroup$ I have to say I was surprised when I read up on permafrost today... I had no idea it could go as deep as 2000 ft! That said, for most places where one might want to store oil (geologically stable, not permafrost, etc.) my understanding is that once you're below about 500-1000 ft you can store most crude fractions. $\endgroup$ Commented May 19, 2020 at 6:00

They heat the oil to make it easier to pump for off-loading otherwise it is too viscous.

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    $\begingroup$ I have a vague memory of a crude tanker where the heaters failed in one tank, and the crude cooled and "solidified" The heaters were repaired but the crude could not be pumped because only the underside softened and the pumps cavitated. Could have been trying to off-load in a cold climate. The cure was for the vessel to return to warmer climates where the heat wasn't whipped away by the exterior air, heat the cargo, then return to offload. (might have been news, might have been a fictional story) $\endgroup$
    – Criggie
    Commented May 17, 2020 at 0:02
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    $\begingroup$ Can you add the missing punctuation? $\endgroup$ Commented May 17, 2020 at 19:56
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    $\begingroup$ @Criggie Oh, that's a thing. Railroads have been known to mishandle bulk cargo or just get caught by weather. I remember hearing about a railroad that got a coal train that had gotten drenched in freezing rain or something, and was a big ice cube with some coal in it. So they dragged it to the southern end of their system and let it sit for a few weeks, then dragged it back in better weather. $\endgroup$ Commented May 18, 2020 at 18:35

Re. 'maintains its quality' aka, the good stuff.

What is the most important source of natural gas? Coal, oil or other?

"Above the maximum temperature the oil is converted to natural gas through the process of thermal cracking." "with nothing acting as a capstone it [will lose, and will continually lose], all its gas."

If it's already 'cracked', it has to be transported in a gas tanker, instead of sludge that just sits there. And if there are any cracks (permeability of the container), the most volatile and profitable constituents are lost.

A barrel of crude oil can be converted into 47% gasoline, "20% is heating oil, 8% is jet fuel, 18% natural gas" and 7% : "the rest being coke, greases and asphalt". – Could I make money off of the negative oil price?

If it 'cracks' bad enough and leaks, you're left with the sludge (heating oil, coke, greases and asphalt). And the more it cracks into tiny gas molecules the more it's going to want to leak. And the more it leaks the less your profit margin.

Too cold and you can't pump it. Too hot and it cracks. It needs to be just right. Because if you're not shipping me (the good stuff) crude oil that I can turn into 47% gasoline (and all the rest, especially that 18% of natural gas), then I'm going to buy it from someone else.

From Ward's answer: promoting circulation inside the tank "reduces sedimentation and wax formation." - Sediments don't pump very easily and no one pays $4 a gallon for wax. I don't know how much it costs to turn wax back into the more profitable thing that it once was, or even if that can be done economically or at all, but I rest assured it costs more money than keeping it 'hot'.


Because the ocean can be very cold

As MartyMcGyver discusses, underground storage does not need to be heated. Tankers, however, are sitting in water, which (apart from the salt) has the highest thermal absorption of any substance; that is to say, the worst possible insulator.

The US Pacific coast is some of the coldest seawater in the continental U.S. (Alaska notwithstanding of course). Don't let the palm trees and 90 degree days fool you: due to ocean currents, it gets a lot of arctic water come down from Canada/Alaska. That's why L.A. surfers wear wetsuits and Florida surfers do not.

Different deal in the Gulf coast or Florida generally, where ocean temp is in the 70s F. I suspect even the port of Stockton CA (which is tidal) would have respectable water temps in the 110F summer heat.

  • $\begingroup$ I am currently in the SF Bay area and I can attest to the chilly water here. That said, the positive excursions of the Pacific's surface temperatures in recent years are notable. A few degrees might not seem like much but it indicates a stupendous amount of energy being absorbed. There's no small irony that this warming trend that makes shipping crude oil a little easier is accelerated by the self-same fossil fuels in the form of their combustion into CO2. $\endgroup$ Commented May 22, 2020 at 4:22

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