RSS Archer submarine, Wikimedia Commons
The maritime city-state of Singapore operates a technologically advanced naval force consisting of four Challenger-class (Sjöormen class) submarines purchased from Sweden in 1995 (first boat), and 1997 (remaining three boats). Additionally, the Republic of Singapore Navy (RSN) signed a deal with Sweden in 2005 for two Archer-class submarines (Västergotland-class), which entered into service in 2011 and 2012, respectively.  The RSN comprises one fleet; three commands (coastal operations, logistics, and training); a diving unit; and two bases (Changi and Tuas). 
The four Challenger-class submarines entered into service in the late 1990s, and are equipped with modern wire guided torpedoes. Because of its teardrop shaped single hull and X-rudder configuration, the Challenger-class has effective underwater maneuverability, albeit with a limited diving depth of only 150 meters (compared to 300 meters for Archer-class submarines).  One of the reasons Singapore opted for Swedish boats is to benefit from the latter's extensive experience of operating in littoral waters, an imperative that also drove Singapore's decision to purchase two additional submarines from Sweden in 2005.
Singapore's Submarine Tables
The two Archer-class submarines replaced two older Challenger-class vessels, and are equipped with a Stirling Mk 3 Air Independent Propulsion (AIP) system. The procurement of these vessels has provided the RSN Submarine Squadron with enhanced stealth and endurance capabilities. The Archer-class submarines are single-hulled, with reduced noise and magnetic signatures. Their expected service life is 15-20 years.  As these boats were manufactured to operate primarily in the cold waters of the Atlantic, they required 'tropicalization,' i.e., the ability to operate in the warmer more saline waters of Southeast Asia. The tropicalization program included installing corrosion resistant piping and valves, a marine growth protection system, and additional cooling systems.  The modernization package for Archer-class submarines also includes an advanced sonar system (believed to be sourced from Thales); new combat data systems (DCNS SUBTICS); new flank array sonar (developed by Singapore's Defence Science and Technology Agency and ST Electronics); a WASS Black Shark heavyweight torpedo; and installation of a pressurized diver's lock-out to facilitate special forces operations. 
The Republic of Singapore's 193km coastline is located on one of the world's most crucial sea-lanes. The narrow Singapore Strait (only 2km wide near Sentosa Island, south of the main island), together with the Strait of Malacca, offer the most direct connection between the Pacific and Indian Oceans. According to the U.S. Energy Information Administration, in 2011 the world's second highest volume of barrels per day-15.2 million bbl/d of petroleum products-passed through the Strait of Malacca.  In 2012, Singapore handled 31.26 million TEUs of containers. It is connected to 600 ports worldwide, and was the world's busiest transshipment hub, with "about one-seventh of the world's total container transshipment throughput, and 5% of global container throughput."  Therefore, the RSN's primary mission is to secure Singapore's sea defense and protect its lines of communication.
Singapore's relations with its two key neighbors-Indonesia and Malaysia-have been characterized by mutual animosity and suspicion. However, current relations are stable and there is little danger of a military conflict. Furthermore, Singapore and Indonesia signed a Defense Cooperation Agreement in 2008 that includes plans for joint military exercises. In 2012, the two countries also signed a submarine rescue pact.  Relations with Malaysia have a unique historical and geographical context, as Singapore seceded from Malaysia in 1965, and relations were tense for a number of years thereafter. The islands of Pedra Branca (Pulau Batu Puteh in Malaysian), Middle Rocks, and South Ledge, which are located at the eastern entrance of the Singapore Strait, have been a further issue of contention. In 2008, the International Court of Justice declared that Pedra Branca belongs to Singapore and Middle Rocks belongs to Malaysia.  Over time, however, economic interdependence has eased some strains in the relationship. Singapore relies on Malaysia for basic resources such as drinking water, and Malaysia seeks to profit from Singapore's economic success, and particularly its financial institutions and technological prowess.
Currently, Singapore's main security concerns are piracy and terrorism.  The Southeast Asian region is subject to a high number of piracy attacks, with the waters around the Malacca and Singapore Straits accounting for a significant number of them.  The security of the straits is also endangered by a range of terrorist groups, and especially Jemaah Islamiyah. This group, allegedly affiliated with Al-Qaeda, aims to establish a pan-Islamic entity encompassing Indonesia, Malaysia, Singapore, the Philippines and southern Thailand. In 2005, the Singaporean Defense Minister cautioned that with the hardening of land and aviation targets, maritime targets have become 'lucrative.' 
In order to counter the threats of piracy and maritime terrorism, Singapore's Ministry of Defence has taken a number of measures at the national and international levels. For example, the Singapore Armed Forces (SAF) participates in the highest number of military exercises and forums of any ASEAN country. In the early 1980s, the Singapore Navy took part in only six foreign exercises, but by 2008 this number had risen to more than forty.  In the area of submarine warfare the RSN has participated in exercises with Australia, Indonesia and India (the RSN also participates in the Five Power Defense Arrangement naval exercises, which include an anti-submarine warfare component) , and the RSN maintains an informal working relationship with the U.S. Navy's NAVSEA-SUBMEPP. 
 Kate Tringham, "RSN takes delivery of second Archer-class submarine," Jane's Navy International, 3 January 2013, www.janes.com.
 "The Republic of Singapore Navy –Mission, Organizational structure and Naval Bases," Naval Forces, Special Issue, 2007.
 "Challenger class," Jane's Underwater Warfare Systems online edition, 28 September 2009, https://juws.janes.com.
 "Singapore" Jane's World Navies, 15 February 2013.
 For more information on tropicalization program undertaken for RSN submarines, refer to: "Factsheet - Submarine Tropicalisation Programme," Singapore Ministry of Defence, May 2001, www.mindef.gov.sg.
 "Singapore Submarine Forces," Jane's Underwater Warfare Systems, 25 June 2009.
 U.S. Energy Information Administration, "World Oil Transit Chokepoints," 22 August 2012, www.eia.gov.
 "About Us," Singapore PSAI, www.singaporepsa.com.
 Koh Swee Lean Collin, "Indonesia-Singapore Submarine Rescue Pact: Promoting Southeast Asian Naval Cooperation," RSIS Commentaries, S. Rajaratnam School of International Studies, 2012, www.rsis.edu.sg.
 "ICJ Awards Pedra Branca's Sovereignty to Singapore," Channel News Asia, 23 May 2008, www.channelnewsasia.com; "Singapore Defence & Security Report Q2 2010," BMI's Industry Report & Forecasts Series, p. 52.
 For more information on maritime security issues in Southeast Asia, refer to: "Maritime Bulletin," Maritime Security Programme, Singapore: S. Rajaratnam School of International Studies, www.rsis.edu.sg; Joshua Ho, "The Security of Sea Lanes in Southeast Asia," Military Technology, Vol. 29, Number 5, May 2005; J Ashley Roach, "Enhancing Maritime Security in the Straits of Malacca & Singapore," Journal of International Affairs, Vol. 59, number 1, Fall 2005.
 "Piracy and Armed Robbery Against Ships," Report for 1 January-30 September 2012, ICC International Maritime Bureau, October 2012, p. 5.
 Teo Chee Hean, "Emerging Security Trends: Challenges for Singapore and SE Asia," Military Technology, Vol. 28, number 2, February 2004.
 Koh Swee Lean Collin, "Seeking Balance: Force Project, Confidence Building, and the Republic of Singapore Navy," Naval War College Review, Winter 2012, Vol. 65 No. 1. p. 87.
 "Naval Diplomacy - Broadening Regional Co-Operation and Beyond," Naval Forces, Vol. 28, 2007, pp. 30-33.
 The U.S. Navy's SUBMEPP (Submarine Maintenance Engineering, Planning and Procurement Activity) specializes in engineering and management issues related to submarines, including submarine repair activities. For additional information, refer to: www.submepp.navy.mil.