Set up two directional drilling stations about 8,400 ft across from eachother, start drilling and then meet in the middle to connect the two bores.Sounds simple enough but the key word there is “sounds.” Getting thisaccomplished takes an incredible amount of careful planning, teamwork anddrilling experience.
That’s what made the crossing under the Choctawhatchee Bay in Destin, Fla.,near the major seaside City of Fort Walton Beach, such a success: carefulplanning, teamwork and drilling experience.
Installation of a 23-mile, 10-in. high-pressure steel gas pipeline in Destin,required the use of horizontal directional drilling (HDD) in order to getapproximately 8,400 ft of the pipeline under the Choctawhatchee Bay.
The Choctawhatchee Bay is located just east of Pensacola Bay on the westernend of Florida’s panhandle. The bay is separated from the Gulf of Mexico alongmost of its length, but connects through the Pensacola and East passes, enteringthrough the south at Destin Pass. Where the HDD crossing would be done, the bayis about three miles from the Gulf of Mexico.
With the exception of the 8,400-ft crossing, the remainder of the pipelinewas constructed using more conventional methods. The HDD portion of the project,handled by Michels Directional Crossings, a division of Michels Corp.,Brownsville, Wis., required a challenging intersect under the bay. Michels hashandled a few of these in the past.
According to Michels officials, a new record length of 8,400 ft of 10-in.steel pipe was established without an intermediate exit as crews intersected oneend of the pilot bore from one drill rig location with another pilot bit from asecond drill rig location.
The intersect point took place beneath the ground surface, with one pilot bitmeeting the opposite precisely on the ends, eliminating redundant overlapdrilling. This has never been achieved in previous hole intersects performed byMichels, says Tim McGuire, vice president of Michels Directional Crossings.
“With this one, Jeff Mueller [Michels’ assistant HDD operations manager] wasable to hit them right on the ends so there was no overlap drilling required. Sofar there hasn’t been that many done this way. This was the first one that we’venever had to overlap,” McGuire said. “They were able to keep [drilling fluid]circulation throughout, which was phenomenal and critical.”
The purpose of the project was to construct and install 23 miles ofhigh-pressure steel gas pipeline from Santa Rosa Beach to just north of FreeportHigh School. Project owner Okaloosa Gas District had set a four-week period tocomplete the project.
The drilling under Choctawhatchee Bay took place right next to a highwaybridge that connects Fort Walton Beach to the mainland on Highway 293. For thisproject, Michels used two of its bigger drills. Crews set up a 1.2 million-lbrig — nicknamed Hercules — on the south side of the bay in order to drillapproximately 6,000 ft north and one of its 220,000-lb rigs, which was modifiedfor the project, on the north side to drill 2,400 ft south. The 12 ¼-in. pilotbore was to be drilled approximately 115 ft below the Choctawhatchee Bay. Groundconditions were silty sand with some clay and shells.
How the Intersect Works
To directionallydrill an intersect crossing, two pilot heads are drilled toward each othersimultaneously along a predetermined borepath. Once they get close to eachother, they are carefully coordinated back and forth by the drilling crews untilone of the pilot bits is in the hole of the other. For the Okaloosa project, ittook crews about a week to drill the pilot bore and complete the intersect.
Once the intersect was completed, crews mobilized for the pullback of the10-in. steel pipe. Reaming of the pilot hole was performed ahead of theinstallation to make it large enough to pull in the product pipe, Mueller said.Crews prepared the site the night before and laid the pipe out on the north sideof the bay. The pipe required just two welds and crews pulled about 300 ft ofthe steel pipe the night before to get it started. The next morning, crewscompleted the entire pullback in approximately 15 hours without anyproblems.
As McGuire noted, critical to completing the pilot bore and pullback was thatfull annular drill fluid circulation was maintained throughout the crossings. Hecredited the experienced crews for using careful drilling techniques combinedwith fluid monitoring and management for that achievement.
“If circulation wasn’t maintained, it would have raised the concern for aninadvertent fluid release into the bay and that would have shut the job down andwould have been quite costly [in terms of time and money],” McGuire said.
One problem that developed while completing the pilot bore from the south endafter the intersect was completed was that the drillstring got stuck, whichthreatened to impede the progress of the project. During pre-planning of theproject, this was a primary concern of the Michels’ team as a stuck drillstringunder the water could result in a loss of an entire string of drill pipe andthreaten the success of the crossing.
“[The drillstring got stuck] while they were trying to push out after theyhad made the intersect,” McGuire explained. The 220,000-lb rig was then used topush its drillstring up to the stuck drillstring and free it up. Once the stringwas free, a continuous drillstring was tripped into the entire length of thehole and used to complete the crossing.
Michels crews faced several challengesin completing the project, including heavy road traffic at the drilling sitesand working near water. According to Mueller, where the drilling was takingplace was a heavy traffic area at both ends.
“It is a major bridge crossing between Okaloosa Island and the mainland, soit’s very, very heavily traveled every day,” he said. “We had a lot of trafficcontrol and a very narrow setup area of about 35 ft. There is a lot of trafficat all hours of the day so there was a lot of pre-planning with the gas companyand the Florida [Department of Transportation] to actually get approval to setup where we were. There were also some fiber-optic cables in the right of way,which was also restricting us. We only had a 6-ft area that we could actually goin the ground.”
With traffic timed at about one car every five seconds, protection of theworkers was imperative. Sand-filled and jersey barriers were constructed toalert and prevent traffic from running into the work area; signage was alsoerected.
Because of the narrow 30- to 35-ft work area, Mueller said thatcrews built a riprap area along the shoreline to give them an additional 8 ft ofworkspace. Riprap is rock, concrete or other material used as a hard, artificialshoreline facing to reduce erosion. The riprap at the bay was made of largestone and crews added timber mats on top of it in order to work on it.
Working so close to the bay also required careful planning so as there was noactivity taking place in the water. The bay system is fringed by forestedwetlands and also characterized by marshes and patches of oyster beds. Thesystem supports the endangered Okaloosa darter. Numerous fish, birds andwildlife also make their home in the Choctawhatchee Bay.
“Everything needed secondary containments because we were like 30 ft from thewater,” Mueller said. “There were a lot of environmental restrictions withregard to hydrocarbons, fuels and oil that close to the ocean.”
Michels Directional Crossings completed the project in three weeks using atotal of 16 crewmembers. The pipe support contractor was Patterson & WilderConstruction Co. Inc., of Wilmington, N.C.
Sharon M. Bueno is managing editor of Trenchless Technology.