A view of the Oslo business district at dusk. Norway topped the rankings for the second year running in the RSF's 2018 world press freedom index. Morten Falch Sortland | Moment | Getty Images

When it comes to sovereign wealth funds — state-owned investment vehicles — those countries that can afford to squirrel away surplus revenues do. As of September 2017, sovereign wealth funds around the world had amassed $7.4 trillion in assets under management, according to the Sovereign Wealth Fund Institute (SWFI), up from $3.4 trillion almost a decade ago. Funds invest in a range of financial assets, from stocks and bonds to real estate and precious metals, but all have an objective to maximize the long-term return from their investments. Of total assets held by SWFs around the world, $4.2 trillion's worth were held by oil and gas-related funds. The most prominent examples of these belong to oil-rich countries like Norway, Kuwait and Saudi Arabia whose wealth has been amassed from energy exports. These types of funds are designed to act as a buffer to oil price volatility, as currently seen in markets. There are non-commodity based SWFs too, however, which currently hold a total $3.2 trillion of assets. China's SWF is a good example of a non-commodity based fund, with $813.8 billion worth of assets amassed since 2007. CNBC looks at the world's largest sovereign wealth funds in the world, ranked by total assets under management as of September 2017 according to the SWFI. —By CNBC's Holly Ellyatt and Katrina Bishop.

10. China’s National Social Security Fund

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While most countries have only one sovereign wealth fund, China has four. One of them is the National Social Security Fund, which was set up as one solution to the country's rapidly-ageing population. It has $295 billion of assets under management, as of September 2017. According to the council which oversees the fund, it serves to "supplement and adjust the social security spending during the peak time period of the aging of population." China is battling with the costs of an elderly population as the effects of its one-child policy, introduced in the 1970s, hit home.

9. Qatar Investment Authority

The Doha Skyline at the Sheraton Grand Hotel on November 17, 2016 in Doha, Qatar. Photo by Mark Runnacles | Getty Images

The Qatar Investment Authority was founded in 2005 as a vehicle to invest revenue from budget surpluses. Qatar is one of the largest exporters of liquefied natural gas -- it has the world's third largest gas reserves after Iran and Russia -- which gives the Gulf state a significant budget surplus and revenues that don't look likely to dry up soon. It also has the world's highest oil and gas wealth per capita, according to the Qatar National Bank, which also estimated in April 2017 that the state's gas reserves would last for another 134 years at the current rate of production. The sovereign wealth fund has $320 billion of assets and says it has "built a major global portfolio that now spans a broad range of asset classes and regions."

8. Government of Singapore Investment Corporation

This photograph taken on March 4, 2016 shows the skyline of the Singapore financial business district at night. Roslan Rahman | AFP | Getty Images

The Government of Singapore Investment Corporation — or GIC as it is now known — is one of the older non-commodity funds on this list, established in 1981. It has $359 billion of assets ranging from equities to real estate and natural resources. Reserves grew rapidly as the fund made the most of a high savings rate in the country in the 1970s, according to the SWFI. The GIC's website says the fund was set up to "preserve and enhance Singapore's foreign reserves for the future," and has investments in over 40 countries.

7. SAFE Investment Company

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Established in 1997, the SAFE Investment Company is the Hong Kong subsidiary of China's State Administration of Foreign Exchange (SAFE). Its main purpose is to manage China's massive foreign currency reserves. It currently has an estimated $441 billion under management. The fund is known for its investments in foreign-listed companies. Over the years it has held stakes in banks in Australia and New Zealand, according to the Financial Times, and European companies including chipmaker ARM Holdings, Telecom Italia and Fiat Chrysler.

6. Hong Kong Exchange Fund

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Another Chinese sovereign wealth fund is the Hong Kong Exchange Fund - or to give it its full name, the "Hong Kong Monetary Authority Investment Portfolio", which is managed by the city-state's monetary authority, the Hong Kong Monetary Authority. Established in 1935, it is used to support the Hong Kong dollar and has $456.6 billion under management. "The Exchange Fund's primary objective…is to affect, either directly or indirectly, the exchange value of the currency of Hong Kong," the fund says on its website. " The Fund may also be used to maintain the stability and integrity of Hong Kong's monetary and financial systems to help maintain Hong Kong as an international financial centre." The fund invests primarily in the bond and equity markets of OECD (Organisation for Economic Co-operation and Development) countries, according to the SWFI.

5. SAMA Foreign Holdings

A view The King Abdullah Financial District (KAFD) on April 13, 2016 in Riyadh, Saudi Arabia. David Degner | Getty Images

Oil-rich Saudi Arabia invests its surplus petro-dollars in a fund called the SAMA Foreign Holdings, a "massive pool of foreign currency reserves," according to the SWFI, which is controlled by the country's central bank. Most of the wealth held in the fund is derived from Saudi Arabia's oil revenues - the Gulf state is the largest crude oil exporter in the world - but it also manages certain Saudi public pensions. The country has another SWF called the Public Investment Fund. It is notoriously secretive about its holdings and investment strategies, but has $514 billion of assets under management, according to the SWFI, and is invested in cash deposits, fixed income and equities. SAMA's net foreign assets (the value of overseas assets it holds minus the value of domestic assets owned by foreigners) are down from a record high of $737 billion in August 2014, when the government started drawing down assets to pay its bills amid falling oil prices and to plug a budget deficit, Reuters has reported.

4. Kuwait Investment Authority

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The Kuwait Investment Authority (KIA) is the oldest sovereign wealth fund in the world, established in 1953 to invest the state's surplus oil revenues. The fund has $524 billion of assets under management, an amount which has also decreased since the dramatic fall in oil prices since 2014. The KIA says its long-term objective is to provide "an alternative to oil reserves, which would enable Kuwait's future generations to face the uncertainties ahead with greater confidence." Keen to stress its independence from the state, the fund says its investment decisions are based "exclusively on commercial considerations, not on the political or foreign policy interests of the State of Kuwait, and will continue to do so in the future."

3. China Investment Corporation

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The China Investment Corporation (CIC) is seen as the country's flagship sovereign wealth fund and is responsible for managing part of China's foreign exchange reserves. The CIC has assets of $813.8 billion under management and has a diverse range of global investments. In the U.K. alone, it has stakes in London Heathrow Airport and utility firm Thames Water, as well as owning the London headquarters of Deutsche Bank. In January 2017, it bought a 45 percent stake in 1221 Avenue of the Americas, aka, New York's iconic Rockefeller Center and bought a 49 percent stake in 1 New York Plaza in 2016 for $700 million, according to reports. The non-commodity fund was established in 2007 as "a vehicle to diversify China's foreign exchange holdings and seek maximum returns for its shareholder within acceptable risk tolerance," according to its website. However, there are reports that it was founded following a bureaucratic spat between the Chinese ministry of finance and China's central bank.

2. Abu Dhabi Investment Authority

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The Abu Dhabi Investment Authority (ADIA) was established in 1976 and its main source of funding comes from oil exports. It is the largest sovereign wealth fund in the Middle East, with assets worth $828 billion. Like its Gulf neighbor Saudi Arabia, the ADIA has a stake in a U.K. airport, London Gatwick, and a range of other international investments, including in Norwegian gas company Gassled. The fund said its portfolio is spread across 24 asset classes, and in dollar terms, its 20-year and 30-year annualized rates of return were 7.4 percent and 8.4 percent respectively, as of the end of 2014.

1. Norway’s Government Pension Fund Global

A view of the Oslo business district at dusk. Norway topped the rankings for the second year running in the RSF's 2018 world press freedom index. Morten Falch Sortland | Moment | Getty Images