WASHINGTON — The Treasury Department said Wednesday that it expects to start offering investors a new Treasury security with variable interest rates in January. It will be the first new Treasury security in 15 years.

Treasury officials said they hope to be able to tell investors the size of the initial offering in November, providing time for bond traders to adjust their computer systems for the new product.

Offering a variable interest rate carries some risk to the government, which would have to pay more if, as economists expect, rates begin to rise in coming years from the current super low levels. But the government is counting on attracting more investors who will be drawn by the prospect of potentially higher yields.

Treasury officials said they viewed the rate risk as minimal because the bills will basically substitute for short-term Treasury bills which have to be renewed every three months, a frequency that already carries a rate risk for the government.

A final rule describing the new floating rate notes was published Wednesday in the Federal Register. The floating rate notes are the first new product Treasury has offered investors since TIPS, Treasury inflation protected securities, were put on the market more than 15 years ago.

Matthew Rutherford, Treasury's assistant secretary for financial markets, said that Treasury is not currently contemplating any other new products such as a 50-year bond or a 100-year bond. Rutherford said he thinks Treasury's 30-year bond adequately served the needs of investors looking for a longer-term security.

Rutherford told reporters at a news conference that Treasury still believed it would be able to meet its borrowing needs and keep the government operating until sometime after Labor Day when Congress will return from its August recess.

He refused to be more specific about when Treasury will run out of bookkeeping maneuvers to avoid hitting the current $16.7 trillion debt limit but private economists have said Treasury may be able to avoid hitting the limit until late October or early November. Treasury has been using various bookkeeping maneuvers to avoid hitting the borrowing limit since May 17.

Treasury Secretary Jacob Lew is urging Congress to move quickly in September to take away the "cloud of uncertainty" about the nation's ability to pay its bills by increasing the borrowing limit and avoid a repeat of 2011 when a prolonged standoff over the issue between Republicans and the administration rattled financial markets.

Treasury announced Wednesday that as part of its regular quarterly refunding auctions next week it would sell $32 billion in 3-year notes, $24 billion in 10-year notes and $16 billion in 30-year bonds.

The money raised would be part of the $209 billion in borrowing it is projecting will be done in the July-September quarter. It has projected $235 billion in borrowing needs for the October-December quarter but that amount will depend on Congress increasing the debt limit.