They will be supported by partners during their operations, including Australia, Japan and Singapore.
The Royal Navy's Offshore Patrol Vessels (OPVs) are responsible for protecting the UK's interests both domestically and internationally.
There are currently two generations of River-class ships in the Royal Navy, the Batch 1s and the new Batch 2s.
As announced in the Integrated Review earlier this year, the UK is committed to a more persistent presence in the Indo-Pacific.
The region covers some of the world's most important sea lanes, stretching across India, China, Japan, the Philippines, Indonesia and Australia.
Cover image: HMS Spey leaving Falmouth Harbour with 'dazzle' paintwork (Picture: Bob Sharples Alamy).