four boats must be modernized by 2019. With the conservation program, boats can be kept operational until mid-2025. The cost of upgrading the four submarines is estimated to be 94 million euros. The first boat to be upgraded was , followed by , and . The upgrade of Zeeleeuw took longer than expected. Initially the upgrade was expected to have been completed by 2015, but it took until 2016 before the submarine was ready for service again. Currently, Bruinvis is being upgraded at the Den Helder naval base. The Dutch navy has also begun to prepare for the replacement of the Walrus-class submarines. Replacement In November 2014, the Dutch Minister of Defence announced plans to replace the Walrus-class submarines in 2025. By 2017, there was still no political agreement on the quantity or type of new submarines to be ordered, nor the tasks they were expected to perform. However, it seems certain that they will be replaced, since the alleged Russian threat was regarded an incentive to invest in a new class. The Minister of Defence, however, delayed the replacement by two years, until 2027. Roughly, there are two groups in the Dutch parliament – one in favor of replacing the Walrus-class by an equally capable class of large, expeditionary, diesel-electric submarine, and the other in favor of choosing a cheaper solution of smaller diesel-electrics, similar to Swedish and German submarines. It is unknown where the new boats will be built; since the Dutch RDM shipyard (the only Dutch yard capable of building submarines) is no longer in operation. The Defensienota (Defense policy for the coming years) of March 2018 revealed that the Dutch government is still planning to replace the Walrus-class submarines, with an allocated budget of more than 2.5 billion euros for the new submarines. Additional information on how to proceed with the replacement was expected at the end of 2018, when the Dutch Minister of Defence, Ank Bijleveld, was to send a so-called B-letter to the Dutch parliament. Minister Bijleveld also underlined in an interview that the new submarines should have the same niche capabilities as the current Walrus-class submarines: the ability to operate and gather intelligence in both shallow water close to the coast and in deep water in the ocean. In mid-2021 it was indicated that a revised plan envisaged taking a replacement decision in 2022 and to have the first vessel in service by