Image source: Seadrill Partners.If you want to make a long-term investment in a company that operates in a poorly performing industry, you have to be prepared for some short-term turbulence. Seadrill Partners (NYSE: SDLP), which contracts out rigs for deep-sea drilling, is one such company. Over the past month, the stock has fallen nearly 40%, and double-digit percentage swings on any given day are not uncommon.