Today, provided Shell does not strike to reclaim its gigantic oil storage buoy, they will be doing the same thing . . . and again tomorrow.
The fortification work keeps them busy, along with listening to radio traffic, answering endless press queries on the satellite telephone, cooking, washing up, trying to keep warm, and speculating about how and when this most unusual of "actions'' will end.
And, of course, recycling. Waste paper, plastic and cans are being collected separately and they have begun making compost from food scraps.
Shell has a warrant to evict one of nearly 20 occupants, the only one they have a name for - Jon Castle, Greenpeace's captain on the Brent Spar. A judge at the Edinburgh Court of Session has also ordered him to disclose the names of his squatter crew.
But Mr Castle, an imperturbable, slow-speaking Guernseyman, will hide himself away in a barricaded room if Shell manages to land a boarding party. "I won't be violent but I won't go easily,'' he says.
Other occupants also plan a non-violent retreat into redoubts in the middle decks of the 460ft tall structure 120 miles north-east of Shetland.
The Greenpeace crew had convinced themselves that Shell and the Sheriff's officers were coming this weekend. The oil company got its court order on Friday and on Saturday its contractors had been due to begin removing equipment from the Brent Spar prior to sinking it in deep water several hundred miles away in the north-east Atlantic.
Shell has the means to effect a boarding in the shape of the Stadive - a gigantic, self-propelling maintenance rig. It towers over the Brent Spar (which itself projects over 100ft above the waves), has cranes to drop boarding parties and is 20 miles away. But yesterday dawned calm and bright with not a sign of action. Just the usual view of more than 30 oil platforms.
Then at 11.10am, a helicopter came, circled in close and hovered for 10 minutes, clearly on a reconnaissance. It flew off; the waiting resumed.
Greenpeace can wait. Its team has food and fresh water and is regularly resupplied with fresh crew and supplies by a chartered trawler plying from Shetland. Only four have been on board for the duration of the occupation which began 22 days ago.
The Brent Spar, which has been empty for nearly four years, is a little cold and damp but there are plenty of beds, chairs, hot food and light - supplied by two diesel generators winched on board. The crew are mostly tall, bearded and from eight nations, with only one woman among them.
They could do with a hot bath but, nonetheless, conditions on board are hugely preferable to those experienced on the 12-hour journey to the Brent Spar and the sickening, six-hour wait until sea conditions are good enough to attempt the transfer to the rig.
Then, wearing a bright orange survival suit, you clamber down a rope ladder into a tiny dinghy to be whisked across two miles of 8ft waves by two highly competent Norwegian ladies. They wear bobble hats and are known as the inflatable dolls.
You are now in the shadow of the Brent Spar, which towers above looking rusty and forlorn and reeking of oil. A large hook is attached to the harness round your chest and you are winched nearly 100ft up to its decks.
Her last crew, before Greenpeace, left all sorts of things on board - from bedding and baked beans to life jackets, survival suits and a huge, new electric motor still in its crate. Shell says all of this will be removed before the Spar is towed away and sunk in 6,000ft of water. For 15 years it acted as a gigantic petroleum station, filling tankers from the Brent Field.
Now it is to become the first truly large North Sea oil structure to be disposed of by sinking - if Greenpeace can be evicted. Captain Castle says dumping would be an environmental crime. "It's a symbol for a wasteful, materialistic way of life,'' he says.
Shell argues that the environmental damage from the oily sludge, mildly radioactive salts and small quantities of toxic metals will be trivial and confined to a small area of the deep sea bed. To break up the Brent Spar on shore would cost pounds 34m extra.