By Shivam Patel

NEW DELHI (Reuters) - German shipbuilder ThyssenKrupp and its Indian partner have cleared field trials for building six advanced conventional submarines for the Indian Navy, according to an exchange filing, emerging as the sole contender for the $5 billion project.

Their potential rival, Spanish state-held shipbuilder Navantia, which partnered with India's Larsen & Toubro (L&T), could not meet the navy's requirements in trials held in 2024 for testing key technologies, an Indian defence source said.

The project is crucial to India's effort to modernise its military and boost its naval capabilities in the face of China's growing presence in the Indian Ocean region.

ThyssenKrupp's Indian partner, state-owned Mazagon Dock Shipbuilders Ltd, said in an exchange filing on Thursday that its field trials for the project were successful and the Indian defence ministry had invited the company for commercial negotiations next week.

The defence ministry, L&T and its submarine partner did not respond to requests for comment.

A key requirement for the project was air-independent propulsion (AIP) technology, which would allow the diesel-electric attack submarines to stay underwater for over two weeks. A conventional submarine without AIP technology would have to surface every few days to charge its batteries.

Currently, the 17 conventional submarines operated by the Indian Navy do not have AIP technology, which India's neighbours China and Pakistan have, according to nonprofit Nuclear Threat Initiative.

Analysts say the project for the six new submarines has been delayed by more than a decade, with the first of them now expected three to five years after a contract is agreed.

About half of India's conventional submarines have undergone multiple upgrades and retrofits over the past few years and are nearing the end of their productive lives.

($1 = 86.5025 Indian rupees)

(Reporting by Shivam Patel in New Delhi; Editing by Mark Potter)