Andreas Widlund, who says he became frustrated witnessing the hunt for Arctic oil, joined Greenpeace soon after quitting the oil industry and is among a team of six taking part in what Greenpeace has dubbed 'The Crossing'.
Widlund's biography on the campaign group's site says he "felt he could not continue to work actively for a business that drives the climate crisis forward".
Originally from Umeå in northern Sweden, he has joined activists from Germany, the USA, Austria and New Zealand for the trip.
On Tuesday, using inflatable boats and climbing gear, the campaigners managed to clamber on board the Polar Pioneer oil rig run by Shell which is heading towards Alaska to drill for oil.
They then put up a banner in protest of Arctic offshore drilling, but promised not to interfere with the ship's navigation.
In a post on its website, Greenpeace described the crew as "determined to shine a white hot light on Shell’s reckless hunt for extreme Arctic oil. With them in spirit are millions of people from around the world who have joined the call for a global sanctuary in the Arctic".
"We don’t know how this journey will end or whether we'll succeed, but we know that we are not sailing towards this challenge alone," the statement added.
Widlund later tweeted: "I'm just an ordinary guy and I think that if you had the opportunity to do what I do you'd have the courage and strength to do it."
The youngest member of the group, Zoe Buckley Lennox, 21, from Australia posted several further updates on Wednesday. Describing the atmosphere, she said: "The wind is howling and the sound of flapping tarps fills the air". Other posts were about the crew's lack of access to toilets and coffee.
— Zoe Buckley Lennox (@zoevirginia) April 8, 2015
— Andreas Widlund (@WidlundAndreas) April 8, 2015
A spokesperson for the US government told Swedish newspaper Dagens Nyheter on Tuesday that the activists were exposing themselves and the crew to "great risks" and Shell's US spokeswoman Kelly op de Weegh sent an email to the UK's Guardian newspaper stating that Greenpeace was deploying "illegal tactics".
“We respect their views and value the dialogue,” she wrote.
“We will not, however, condone the illegal tactics employed by Greenpeace. Nor will we allow these stunts to distract from preparations underway to execute a safe and responsible exploration programme.”
After Shell formally lodged an injunction designed to get the crew removed from the ship, Greenpeace USA's executive director Annie Leonard said: “This injunction is Shell’s latest attempt to keep people from standing up for the Arctic. Shell thinks it can do whatever it wants, but there’s one thing the company still clearly fears — ordinary people standing up to save the Arctic."
“Shell wants activists off its rig. We want Shell out of the Arctic."
The Polar Pioneer set off from Malaysia last month. It is one of two rigs Shell is hoping to use for exploratory drilling later this year, although the company is yet to secure the permits it needs to do so.
The Greenpeace activists say they are prepared to stay on board for "days or weeks" in order to raise awareness of their campaign.
They argue that Shell and other oil giants have failed to demonstrate that they could clear up a major oil spill in icy waters.