The design is due to be finished by May 2019, with building works scheduled to start at the end of 2019, for a finish in June 2021.
Last year, Stornoway hosted a total of 43 cruise ships but only few of them were bigger boats.
Cruises ships more than 156 metres long are currently unable to berth in Stornoway's harbour and are forced to anchor and bring passengers ashore in smaller boats.
Under the new proposal, the port will be able to berth vessels up to 350 metres long. Stornoway Port Authority estimates that an additional 35-40 ships will be attracted per year, boosting visitor numbers and local economy.
Bidders for the new contract have to submit their applications by September 14, with the winner being awarded the next month.