At first glance, pipeline giant ONEOK (NYSE:OKE) appears to offer income-seeking investors a superior opportunity when compared to fellow pipeline giant Williams Companies (NYSE:WMB). Not only does ONEOK have a higher yield -- 4.9% versus 4.5% for Williams -- but it expects to increase its payout at a 9% to 11% annual pace through 2021 while Williams' current forecast is that it will boost its dividend 10% to 15% in 2019. However, when we drill down a bit deeper, we find one metric that puts Williams Companies' dividend ahead of ONEOK's for investors seeking a lower-risk payout.

A look at ONEOK's dividend

ONEOK currently expects to generate between $1.675 billion to $1.805 billion in distributable cash flow this year, which is money it could pay out in dividends. That's enough cash to cover the company's high-yielding dividend with plenty of room to spare. So far this year, ONEOK has generated more than $240 million in excess cash after paying its dividend, even though it has increased the payout 11% from last year's rate. That equates to a dividend coverage ratio of about 1.38 times, which is very healthy for a pipeline company.

$100 bills with the word dividend on top.

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ONEOK is using its excess cash, plus money it raised earlier this year by selling stock, to bolster its balance sheet as well as finance high-return growth projects. Overall, the company has more than $4 billion of expansions under construction, which should come online through the end of 2020. ONEOK believes that these growth projects should provide enough cash flow to grow its dividend at a 9% to 11% annual pace through 2021 even as it maintains a greater than 1.2 times dividend coverage ratio and conservative leverage metrics, including a debt-to-EBITDA ratio of less than 4.0 times. That combination of a high yield, a high growth rate, and a strong financial profile puts ONEOK in a class of its own.

A look at Williams Companies' dividend

Williams Companies also generates gobs of cash, with the pipeline giant on track to produce $2.6 billion to $2.9 billion in distributable cash flow this year. At the midpoint, that's enough money to cover its high-yielding payout by an even more comfortable 1.6 times. That coverage ratio is worth pointing out since it implies that Williams has the potential to offer an even higher-yielding dividend. If the company were to match its coverage level to ONEOK's by increasing the percentage of cash flow it pays out, William's yield would increase to 5.1%.

Williams, meanwhile, expects to invest about $3.9 billion into expanding its portfolio of pipeline assets this year and another $2.6 billion in 2019. This investment should boost Williams' distributable cash flow to between $2.9 billion-$3.3 billion next year, which should support a 10% to 15% dividend increase even as the coverage ratio climbs to around 1.7 times. Furthermore, Williams expects its leverage ratio to fall from 5.0 times in 2018 to less than 4.75 times next year.

While Williams hasn't yet announced a dividend growth forecast beyond 2019, the company should have no problem increasing its payout at a fast pace in the coming years due to the volume of expansion projects it has in development. The company noted at its analyst day earlier this year that in addition to the $7.3 billion of projects already under construction that it had another $5 billion that it was close to sanctioning, which were part of the more than $20 billion of future opportunities it has identified. Because of that, the company believes it's well positioned to invest between $2.5 billion to $3 billion per year in expanding its pipeline portfolio. That investment rate should be enough to support double-digit annual dividend growth even as the company maintains a strong coverage ratio of more than 1.5 times and continues pushing down its leverage ratio.

Digging deeper uncovers a better option

ONEOK currently offers a slightly higher yield and greater visibility into future dividend growth than Williams, which is why many income investors would probably prefer to invest in that pipeline giant. However, digging deeper, we find that Williams gives investors a safer payout since it has a higher coverage ratio. As a result, the company can retain more cash flow to invest in expansion projects that will drive future growth. That more conservative coverage ratio and funding profile are things that risk-averse investors love to see in income stocks.