The tanker Stenberg, which is 144 meters long and 23 meters wide, was on its way to the Stenungsund port from Latvia when the drunk captain was discovered by a pilot from a Swedish pilot boat, due to bring the ship into its berth.
Once aboard, the pilot found that the captain was in an intoxicated state and unable to steer the vessel. The pilot decided to anchor the tanker and alert the Swedish coast guard who began an immediate investigation and got the drunk captain to provide a breath sample.
"The limit of what is allowed is 0.2 permillage (of alcohol). But he had more than 1.0 and is now suspected of being seriously drunk whilst at sea," Erik Bagge of the coast guard told the Göteborgs-Posten newspaper.
The TT news agency reported that that man's breath sample was 1.2 permillage and was so drunk he could not respond to the apprehending officers's attempt to question him.
"He was very drunk the whole day yesterday and we have yet to conduct a hearing with him," spokesman Kalle Isaksson told the TT news agency on Tuesday.
The drunk captain's cargo was potentially lethal as the massive tanker had 15,000 tonnes of naptha, an oil product that is highly flammable, which can cause devastation to nature and wildlife if it leaks.
It is standard procedure for a pilot boat to accompany a vessel the size of Stenberg into its berth. The boat runs parallel to the larger ship and a member of the pilot boat crew jumps aboard the vessel to aid its arrival into the dock.
On this occasion, the pilot raised the alarm and dropped the anchor before a senior officer later guided the vessel to the quay in Stenungsund.
Coast guards said the captain remained aboard the ship for several hours before he was taken ashore and arrested for intoxication at sea.
"There was no way to take him off the vessel earlier because he was so drunk," added Isaksson.