Water Pipeline for Historical Island Installed Beneath Boston Harbor
Construction crews are installing a water line beneath the ocean floor in Boston Harbor using horizontal directional drilling aided by an advanced navigation system.
D'Allessandro Corp. based in Avon, Massachusetts, has the contract to replace a 6-inch cast iron water main with an 8-inch, high-density polypropylene (HDPE) pipe to serve historic Spinnaker Island in Hull, Massachusetts. Subcontractor Henniker Directional Drilling LLC of Henniker, New Hampshire, is performing the horizontal drilling and installing about 1,200 feet of pipe.
Drilling is proceeding under 5 to 15 feet of water and 20 feet of ocean substrate, with a wireline guidance system utilizing magnetic fields to keep the borehole on course to extremely close tolerances. The ParaTrack wireline system, manufactured by Vector Magnetics LLC, is supplied and operated by Prime Horizontal LTD as a subcontractor to Henniker Directional Drilling.
A Military History
Located in the Hingham Bay area of Boston Harbor, Spinnaker Island, formerly known as Hog Island, is part of the town of Hull and is linked to the mainland by a quarter-mile long timber-trestle causeway. It is one of only two harbor islands that are not part of the 34-island Boston Harbor Islands National Recreation Area. In 1920, the Army built Fort Duvall on the island, and just before the outbreak of World War II installed a pair of 16-inch guns similar to those on Navy battleships to defend the southern approaches to the harbor. The land-based guns were fortified with massive reinforced concrete casemates - one of the casemates is still visible there. In the 1950s, the island became the site of fire-control and radar systems for a Nike missile launch site, located in what would become Webb Memorial State Park.
The 6-inch, unlined cast iron water service pipe serving the island was installed in 1941, but is too small to serve the more than 100 luxury condominiums occupying the island today. This pipe was installed only 5 or 6 feet beneath the ocean floor.
In 1987, the island was redeveloped as a condominium and marina complex and renamed Spinnaker Island by the new owner.
According to the current project's civil engineer, Steven Bernstein, P.E., Principal of the SLB Group LLC, the timber-trestle was designed by the Massachusetts Highway Department in 1940 to be as strong as a railroad bridge, and easily supported U.S. Army tanks heading to and from Fort Duval. But while the trestle was certainly capable of carrying a water main on the causeway, the owners decided against this since the pipe might become problematical for crews performing future bridge repair and maintenance. Instead, Bernstein recommended installing the pipeline using directional drilling beneath the seabed - a suggestion that was adopted.
D'Allessandro Corp.'s contract requires furnishing and installing a new 8-inch diameter HDPE water service under Allerton Harbor (Hull Bay) parallel to the Spinnaker Island Causeway. The project includes an above-ground backflow device and water meter located inside a cabinet connected to a concrete slab with foundations, an above-grade fence and gate, plus landscaping. Project specifications call for the pipe to be installed using horizontal directional drilling, with a small portion of the work being done by conventional trench excavation.
Horizontal directional drilling, or HDD, is a trenchless method of installing underground telecommunications and power cable conduits, water lines, sewer lines, and other utilities along a steered course using a surface-based drilling rig. This method is used when trenching or excavating is not practical, such as in crossing waterways, roadways, urban congested areas and environmentally-sensitive locations.
The HDD technique involves drilling a pilot hole over a designed course using a continuous string of hollow drill pipe, or stem, which transmits power from the surface-based drilling rig to the down-hole drill bit. When the drill stem reaches the desired exit point a back reamer is attached to the drill stem for enlarging the pilot hole during the pull-back, and a pre-assembled string of product pipe (such as HDPE pipe) is connected to the back reamer. As the drill stem is pulled back towards the drill rig, the hole is enlarged and the string of product pipe is installed.
Henniker used a mid-sized, 22-ton Vermeer 100x120 Series II Navigator Directional Drilling Machine powered by a 225HP John Deere engine to drill the pilot hole. These are very powerful machines, able to drill and pull back hundreds of feet of pipe product and drilling equipment.
Out of Sight Steering
Navigating an unseen drill head over an underground course is a complex operation requiring specialized equipment and sophisticated computer software. A good example is the drilling course designed for the Spinnaker Island pipeline:
According to the drill course design, pilot hole drilling was to begin on the Hull mainland side of the project, heading downward at a ground entry angle of 11.25 degrees for almost 110 feet, then continue downward in a relatively flat curve for a distance of about 195 feet to elevation -33.25, which is approximately 20 feet below the ocean floor. From this point, drilling was to level off and continue horizontally for roughly 630 feet. Then the course design changes to an upward flat curve that continues for about 220 feet before adjusting to a straight line at a 12.5 degree angle until the drill head exits the ground on Spinnaker Island.
To navigate this course, Prime Horizontal employed Vector Magnetics guidance tools and ParaTrack software. With this system, a device called a magnetic guidance (or steering) tool is placed inside a non-magnetic case and deployed inside a non-magnetic drill collar just behind the drill bit. The system measures three components of the earth's magnetic field, reduces the effects of interference from local magnetic fields produced by nearby pipelines, power lines, utility lines, railways or highways, and determines the attitude and direction of the drill head. This information is transmitted back up the drill pipe wire to a receiving unit used to navigate drilling operation over the pre-designed course.
For the Spinnaker Island project, each section of the drill stem was 20 feet long and contained a wire inside which was connected to the drilling head assembly. Inside this head assembly was the magnetic steering unit which monitored line and grade and conveyed this data over the wire to Michael Porter, Jr., Guidance Engineer for Prime Horizontal. Porter analyzed real-time data from the steering unit during drilling and advised Charlie Hunt, the drilling operator for Henniker Directional Drilling, when to adjust the drilling head to specific coordinates and direction to meet the design plans.
HDD is aided by the use of a drilling fluid, usually a thick slurry made of water and bentonite clay. This fluid is pumped to the drill bit to remove soil cuttings, prevent the hole from collapsing, cool the drill bit and act as a drilling lubricant. Referred to as drilling mud, the mixture of drilling fluid and cuttings is sent to a reclaimer machine that removes cuttings and maintains required drilling fluid viscosity.
The drilling contractor employed a Ditch Witch FM 50 Mud Mixer Machine to produce the bentonite/water drilling fluid, and an American Auger MCM-2000 Machine to remove cuttings and allow the recycling of drilling fluid.
Creating a Pipe String
Crews stockpiled 40-foot lengths of HDPE pipe on the causeway sidewalk and joined them together as needed with two McElroy fusing machines to create the continuous string of pipe for the water main. This specialized equipment melts two pieces of thermoplastic pipe together with heat and pressure. Pipe lengths are held in line axially to allow pipe-ends to be faced, establishing clean, parallel mating surfaces perpendicular to the centerline of each pipe. Then the two end surfaces are heated to a high temperature, forming the melt pattern, which penetrates the pipe. When enough pressure is applied, the molecules from each pipe-end mix. As the joint cools, the molecules return to their original form, the original interfaces are gone, and two pipes have become one monolithic pipe.
In this way, work crews created the 1,248-foot pipe string that was eventually pulled through the borehole. The Vermeer drilling machine pulled back approximately 11,000 pounds of 8-inch HDPE pipe plus some 1,300 feet of 3-1/2-inch diameter steel drill pipe, while overcoming soil resistance to back reamer cutting.
A Smooth Operation
According to Jeff Martin, Owner of Henniker Directional Drilling, the performance of Prime Horizontal's drill steering system was "pretty impressive."
"It was incredible. After drilling for more than 1,200 feet, the borehole was within 1 inch of the design course," Martin said.
Engineer Bernstein noted that directional drilling proceeded smoothly as it passed through what geologists call a drumlin - an elongated or oval glacial drift - consisting mostly of sand, gravel and marine clay.
And he added that the installed HDPE pipe easily passed the required pressure safety test. "After placing the end caps on the main, D'Alessandro filled the pipeline with water, which took about 1-1/2 hours. Then the pressure test took place, including pressurizing the pipe line to 200 psi and holding for 10 minutes, and at 150 psi for 2 hours, passing both requirements," he said.