What: Shares of Ensco PLC (NYSE:ESV) had another bad month in December, falling 10.1% as the oil market suffered mightily...again.

So what: Ensco is suffering from the same problem as many of its offshore drilling competitors: a lack of new offshore drilling contracts. Oil companies big and small are cutting back on capital spending, and as rigs come off contracts, there's simply no new work for them. So, the company faces a dire financial future unless oil prices turn around in the near future.

ESV Chart

ESV data by YCharts.

The problem is that instead of oil prices rising as U.S. oil production drops, they're moving even lower. There are a variety of reasons why, from OPEC's rising production to China's shaky economy, but the fundamental problem for Ensco is that $35/barrel oil will lead to major financial difficulty if it lasts too long.

Now what: The longer oil prices stay below $50, the worse the financial condition of offshore drillers gets. Ensco can survive through 2016 without an uptick in demand, but if this weak oil market lasts years, it's going to be a major problem for Ensco and everyone else who has invested billions of dollars in ultra-deepwater offshore drilling rigs.

I think there will be upside in stocks like Ensco eventually, but when that upside will come, or if it will be before drillers start to go bankrupt is anybody's guess, and that's what keeps me cautious in offshore drilling right now.