* Signs MOUs with Si**pec, Saudi Aramco and JX Nippon Oil & Energy * Indonesia has struggled to attract investment in refineries * Plans would double crude processing, triple gasoline output (Updates throughout) By Fergus Jensen JAKARTA, Dec 10 (Reuters) - Indonesia's state energy firm Pertamina signed on Wednesday preliminary agreements with global energy firms to upgrade five oil refineries in a new push by the former OPEC member to modernise its creaking energy infrastructure. ...