U.S. environmental activists said they planned to protest on Tuesday against the launch of the second of two oil rigs central to Shell's plans to drill for oil in the Arctic.

The Washington state activists, who have staged frequent demonstrations over the last two months against Royal Dutch Shell's oil exploration in the Chukchi Sea off mainland Alaska, said they expected the rig to leave a Seattle-area port in the early morning and were planning a water-borne protest.

Shell did not immediately respond to requests for comment.

The company could begin drilling for oil in the Arctic off Alaska as early as the third week in July, when it expects sea ice to begin clearing.

The first drilling rig arrived in Dutch Harbor off mainland Alaska on Saturday morning and will remain there until ice begins clearing where the company plans to drill through late September.

The company was given a conditional green light by the U.S. Department of the Interior in May to return to the Arctic for the first time since its mishap-plagued 2012 drilling season.

Protesters around Washington have staged ongoing demonstrations over Shell's intention to drill for fossil fuel in the Arctic, one of the most environmentally sensitive regions in the world, saying a spill would be destructive to the ecosystem and extremely hard to clean up.

Shell maintains that it has a robust safety and clean-up plan should a spill occur.