DUBAI, Jan 13 (Reuters) - A group of institutional investors that last year took a stake in Saudi Aramco"s oil pipelines network sold $2.5 billion in dual-tranche amortising bonds on Thursday, significantly below the amount sought, a bank document showed. Amid turbulent debt markets, final spreads were unchanged from initial guidance, with the deal drawing around $5 billion in orders, according to the document. A separate bank document had shown that the investors, via EIG Pearl Holdings, were seeking to raise $3.5-4.4 billion. "Investment grade deals are getting difficult in light of market conditions," a banker away from the deal said. "Even the Coca Cola bond deal struggled," he added, referring to Coca-Cola Icecek, the soft drink"s Turkish bottler, which sold $500 million in sustainability-linked bonds on Thursday, according to fixed income news service IFR. Global debt markets have been rattled by the U.S. Federal Reserve"s indications of a faster run of interest rate hikes and stimulus withdrawal, with the resulting rise in borrowing costs leaving investors more reluctant to lend to companies until the picture is clearer. Inflation-adjusted yields have risen more than nominal yields this year Inflation-adjusted yields have risen more than nominal yields this year EIG Pearl Holdings sold $1.25 billion in a tranche maturing in 14-1/2 to 15 years at 185 basis points (bps) over U.S. Treasuries (USTs) and $1.25 billion in paper maturing in 24-1/2 to 25 years at 235 bps over USTs. The tranches are expected to have a weighted average life of 10.2 to 10.7 years and between 23-1/2 and 24 years, respectively. Final maturities and the amortisation schedule are expected later on Thursday, the document said. In June, a consortium led by U.S.-based EIG Global Energy Partners bought 49% of the Aramco Oil Pipelines Company from Saudi Aramco (2222.SE), which retains a 51% stake. As part of the deal, Aramco agreed a 25-year lease and leaseback arrangement with the pipelines group. The bonds are being issued through EIG Pearl Holdings, in which investors led by EIG control an 89.45% stake, with the remainder held by Abu Dhabi sovereign wealth fund Mubadala Investment Company. The EIG-led group includes 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 bonds will partly refinance a $10.8 billion loan that backed the pipelines deal. Sources said previously that the loan would be refinanced across two or three bond deals and that the first bond issue would likely raise around $4 billion. read more Citi (C.N) and JPMorgan (JPM.N) are coordinating the deal, which involves 17 other banks. read more