[2] In 1995, the RSN acquired a refurbished Sjöormen-class submarine from the Swedish Navy, followed by another three in 1997.

[3] It is believed that the Challenger class were purchased to develop the required submarine operations expertise before selecting a modern class of submarines to replace them, as all the boats were over 50 years old.

[4] The Challenger-class submarines had undergone the Riken modernisation programme tailored to the specific operational conditions of the Singapore Navy.

As the submarines were designed by the Swedish for operations in the Baltic Sea, various modifications were required to suit them to tropical waters.

A comprehensive tropicalisation programme was carried out for all four submarines, which involved installing air conditioning, marine growth protection systems and corrosion-resistant piping.