System for mooring a large vessel for production of hydrocarbons, such vessel for example being a FPSO vessel, the mooring system including at least two floating bodies of the hang-off type, moored by means of differentiated compliance spread anchoring system to the sea bed, allowing partly weathervaning of the large vessel; a riser system extending from the sea bed to each the floating bodies, and means for transfer of fluids from the two floating bodies to the large vessel, the floating bodies and the large vessel being provided with locking means, locking floating bodies to the large vessel at the bow and the stern of the vessel in a mechanically rigid manner, thus providing a integrated, rigidly fixed unit comprising the large vessel and the floating bodies.