A climate activist who chained herself to a Shell vessel last year to protest oil drilling in the Arctic argued this week that she was compelled to act to prevent environmental catastrophe.

Chiara D'Angelo, 21, who climbed the vessel's anchor chain with a sign that read, "Save the Arctic" and locked her harness to the side of the ship for three days and nights in Bellingham, Washington in May 2015, faces a possible $20,000 fine for crossing the so-called "safety zone" around the ship.

But in a hearing with the Coast Guard on Monday, D'Angelo and her attorney invoked what's known as the "necessity defense," arguing that her actions were far less dangerous than the risks posed by allowing the ship to depart for the Arctic's Chukchi Sea, where the oil giant was poised to undertake "one of the riskiest offshore drilling operations of all time," as D'Angelo told the Bellingham Herald.

Shell had planned to explore for oil in the remote and vulnerable northern waters despite warnings from the Bureau of Ocean Energy Management that the operation posed a 75 percent chance of a spill, risking the food supply for native Alaskans, who rely on fishing and marine hunting for subsistence.

D'Angelo, who was supported while on the chain by fellow activists who took turns keeping her company and delivering food and other supplies, told the Herald, "I acted to prevent harm."

"There was a 75 percent chance of a major oil spill in Inupiat harvesting territory," she said. "If you have this disaster there, you take out their food source."

Shell's Arctic plans were also the catalyst for large-scale protests by land, air, and sea last year. The energy giant called off its Arctic crusade "for the foreseeable future" in September 2015, stating that there were not enough indications of oil and gas in the region to justify continued drilling.

In January, five climate activists charged for blocking an oil train used the necessity defense in a groundbreaking case, marking the first time a U.S. judge allowed the argument to be used in a climate trial.

The defendants were eventually found not guilty of obstruction, avoiding jail time. Members of the jury clarified during the proceedings that they understood that the activists were trying to raise public awareness about critical issues.

One juror reportedly told them, "Thanks for the education."