March 16 (Reuters) - U.S. energy companies added oil rigs for the seventh time in the last eight weeks on expectations crude prices in 2018 will top 2017, prompting more drillers to boost spending plans for this year. Drillers added four oil rigs in the week to March 16, bringing the total count to 800, General Electric Co's Baker Hughes energy services firm said in its closely followed report on Friday. The U.S. rig count, an early indicator of future o