US investment houses Goldman Sachs and Merrill Lynch dealt oilfield support services group Petrofac a double-blow on Friday with some bearish comment. Merrill Lynch has downgraded Petrofac to 'underperform' from 'neutral', with a price target of 930p.The US broker believes that the recent improvement in contracting activity has been at the cost of reduced margins.Goldman Sachs goes a little easier on the stock, downgrading it to 'neutral' from 'buy' after the share price's recent good run.