Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of offshore driller Seadrill (NYSE:SDRL) and two of its subsidiaries -- Seadrill Partners (NYSE:SDLP) and North Atlantic Drilling (NYSE:NADL) -- all plummeted this past month. Surprisingly, Seadrill's 15% plunge was the smallest of the three.

SDRL Chart

SDRL data by YCharts.

So what: Not much happened at the parent company itself this past month except for continued fears of lower oil prices and the excess number of offshore rigs on the market today. The real events happened at the subsidiary companies, which dragged down Seadrill with them. To start, North Atlantic Drilling CEO Alf Ragnar Lovdal said the company's massive contracts with Russian oil giant Rosneft are growing suspect because of the sanctions imposed on Russia in response to its continued support of separatist rebels in Ukraine. The first rig that was supposed to start operations in July as part of that contract has already been terminated, costing North Atlantic more than $1 billion in backlog.

Similarly, the contract of a Seadrill Partners rig operated by BP was also terminated. While the deal has a pretty minor impact on cash flows and earning -- -about $160 million in cash flow, according to management -- this was yet another sign that the rig market remains weak.

Since Seadrill has an ownership stake in these two companies, as well as partial ownership of some of the rigs in question, it was inevitably going to take a hit from these events as well.

Now what: This shouldn't come as a surprise to those who have been following the rig market of late -- these sorts of things are going to happen. Cheap oil is going to curb producers' appetite for offshore exploration. That being said, Seadrill and its subsidiaries have some of the most modern fleets of offshore rigs out there. Once the oversupply of rigs is corrected these three should be in pretty good shape. It's just going to be a rough ride before things turn around.