DUBAI, Jan 10 (Reuters) - A group of institutional investors that last year took a stake in Saudi Aramco"s oil pipelines network has hired Citi (C.N) and JPMorgan (JPM.N) to help refinance a loan that backed the deal, bank documents and an investor presentation showed on Monday. A consortium led by U.S.-based EIG Global Energy Partners in June bought 49% of the Aramco Oil Pipelines Company set up by Saudi Aramco, which holds the remaining 51% stake. As part of the deal, Aramco struck a 25-year lease and leaseback arrangement with the pipelines group. The banks will advise issuer EIG Pearl Holdings - in which the investors led by EIG control a near 90% stake and Abu Dhabi sovereign wealth fund Mubadala Investment Company the remainder - on the sale of dual-tranche U.S. dollar-denominated amortising bonds, the documents showed. The bond offering will partly refinance the $10.8 billion term debt facility that backed the deal. Sources have said the loan would be refinanced across two or three bond deals, with the first bond issue likely to be at least $4 billion. Investors in the stake include China"s state-owned Silk Road Fund, Saudi Arabia"s Hassana, the investment arm of the kingdom"s largest pension fund, and Korea"s Samsung Asset Management, the investor presentation showed. Citi and JPMorgan began investor calls for the debt sale on Monday, the documents showed. The offering will include a tranche with a final maturity of between 14-1/2 and 15 years and a weighted average life of 10.2 to 10.7 years, and a tranche with a final maturity of between 24-1/2 and 25 years with a weighted average life of between 23-1/2 and 24 years, the bank documents said. A group of 18 U.S., European, Asian and Middle Eastern banks are also involved in the deal, including First Abu Dhabi Bank (FAB.AD), HSBC (HSBA.L) and Mizuho (8411.T).