SINGAPORE'S merchant fleet will grow to more than 12 million gross registered tonnes (grt) next year if the trend in registration continues, says the Marine Department. Since 1989, the total tonnage has been growing at an average of 10 per cent per year. This rate has been unsurpassed since the Singapore Registry of Ships (SRS) phased out its ''open'' system of registration in 1979, the department said. In February, the Singapore fleet hit the 11 million grt mark with the registration of the Kamnik, a general cargo ship of 11,905 grt owned by Genshipping Pacific Line. This came barely a year after the 10 million grt mark was breached when the 106,110 grt VLCC (very large crude carrier) Sentosa Pride joined the fleet in April last year. In June 1991, the fleet broke through the nine million grt mark following the registration of a tanker of 54,931 grt. Singapore's success in developing its merchant fleet to be ranked 12th in the world is due largely to promotional efforts for the SRS, and the conditions the port offers to owners. ''Singapore is a natural choice for shipping and shipping-related activities because it is located at the crossroads of the main shipping routes,'' the department said. ''Its well-developed infrastructure and facilities also enhance its development as an international maritime centre.''