NORTH KINGSTOWN, R.I. — Before you can build a wind farm in the ocean, you have to understand what’s on and underneath the ocean floor.
The differences between sand, silt, rocks and clay will go a long way to determining what kind of foundations can be used to hold towering wind turbines above the water’s surface and how those foundations will be anchored to the bottom.
“That data is crucial to how we build a wind farm,” said Paul Murphy, vice president of operations and engineering at Deepwater Wind.
Deepwater is set to embark on a study that could last a month or more to determine the underwater geology of 256 square miles of Rhode Island Sound about 18 miles southeast of Block Island.
There, in waters that it’s leasing from the federal government, the Providence-based company plans to install dozens and dozens of wind turbines over the next decade to supply power to New York, Rhode Island and Connecticut.
A liftboat brought to Rhode Island from Louisiana and retrofitted with a drilling rig at the Quonset Business Park was set to depart Sunday for Deepwater’s lease area, where it will take core samples from deep within the ocean bottom.
Once it’s in place, the specialty ship named “Supporter” will lower three tubular legs to the seabed about 120 feet below and then raise itself up about 30 feet above the water to create a stable base for drilling to proceed.
The results of the survey will be used to supplement the construction and operations plans that Deepwater must submit for approval to the federal Bureau of Ocean Energy Management, the arm of the U.S. Department of the Interior that oversees all offshore energy.
Deepwater filed the plan earlier this summer for the South Fork Wind Farm — the 90-megawatt project that would supply power to Long Island and constitute the first round of development in the federal lease area.
The plan for the Revolution Wind Farm — the 400-megawatt project selected by the Raimondo administration in May to supply power to Rhode Island — will follow at an undetermined date.
Plans for a 200-megawatt project chosen by Connecticut in June are at an earlier stage of development.
Deepwater is one of three companies that are planning wind farms in the same general swathe of waters between Block Island and Martha’s Vineyard. Vineyard Wind and Bay State Wind, both companies with foreign investors and offices in New Bedford, are the others.
Supporter, the distinctive ship with 200-foot long legs owned by Dubai-based Falcon Global, was brought up from the Gulf of Mexico because there are no ships like it in the Northeast. With offshore wind development only just beginning in southern New England — Deepwater completed the first project off Block Island in 2016 — an offshore construction industry in the region is still absent and companies must rely on ships built for oil and gas exploration in the Gulf.
The 28 people on the boat — its crew, drillers, engineers and marine mammal observers — will work in shifts around the clock so that core samples can be taken from up to 10 locations around the ocean.
The geology off the Rhode Island coast is complex, said Diane Baxter, associate principal with GZA GeoEnvironmental, of Norwood, Massachusetts. When glaciers receded during the last ice age thousands of years ago, they left behind rocks and mixtures of sands and soils.
To gain a better understanding of the consistency of the ocean bottom, she’ll direct a team that will drill 200 feet into the sediments, taking 10-foot long samples of about three inches in diameter along the way.
“You don’t know what you’ll find until you get there,” said Baxter.
One of the key qualities of the sediments that Baxter and the other engineers are interested in is their stiffness. Deepwater will feed the data they collect into a computer model that will look at how different foundation types — including latticework jackets like those at the Block Island Wind Farm or monopiles that are common in Europe — would fare over time under different wind and wave conditions.
Deepwater has yet to configure the wind farm or even decide on the types of foundations it will use.
“We’re going to let the data make the decision,” said Murphy.