Drilling under the ocean: contractor, engineer team up with Air Force, Navy to install nuclear...

The smaller directional drilling machines widely used on utility construction projects were designed specifically for utility construction--making trenchless installations of cable, pipe, conduit and duct.

But as horizontal directional drilling (HDD) units came into wide use, their versatility

and capabilities have attracted the attention of project engineers and planners in industries other than those related to utilities. Today, HDD equipment is performing a variety of unusual and interesting installations for projects in a variety of industries.

Many involve water, including installation of intake pipes into lakes and rivers and outflow lines into various bodies of water. The most complex of these are to install shore landing systems to shield and protect undersea cables from powerful ocean forces.

While a shore landing is far from routine, a number have been successfully completed for telecommunications projects and at Air Force stations that monitor nuclear explosions.

An example of the latter application is a project to install critical elements of a near-shore cable protection system for a United States Air Force Hydroacoustic Data Acquisition System (HDAS) monitoring station on Wake Island, part of a worldwide network to detect foreign nuclear explosions in violation of nuclear test ban treaties.

The United States National Data Center, Patrick Air Force Base, FL, is responsible for monitoring compliance with nuclear test ban treaties and uses a variety of techniques to accomplish this task, including technologies to monitor air space, underground and oceans.

Key ingredients

A critical element of the detection system is HDAS, comprised of underwater sensors linked to a shore facility by undersea cable.

For the Wake Island project, a 40,000-pound pullback HDD unit was employed to drill three pilot holes beneath the sea floor and beyond the surf zone. At exit points, Navy divers attached three-inch HDPE conduit to the drill string which was pulled through the pilot hole to the drill unit positioned on the island's beach.

Cables with sensors designed to detect underground explosions will be installed inside the conduits and then connected to instruments in a terminal building on shore. Information collected will be transmitted via satellite to the National Data Center at Patrick Air Force Base and the Comprehensive Nuclear-Test-Ban Treaty Organization's International Monitoring System Division in Vienna, Austria.

The Naval Facilities Engineering Service Center (NFESC), Port Hueneme, CA, together with Sound & Sea Technology, Ventura, CA, provided technical support for the Wake Island project. Directional drilling work was completed by Island Mechanical Corp., Kapolei, HI, a company experienced in making shore landings with HDD equipment, including previous HDAS projects.

Senior Project Engineer Stan Black, Sound & Sea Technology, provided details of the Wake Island project for this report.

Black explained that over-the-shore cable shore landings of placing cable on the surface of the ocean floor approaching land have required long-term inspection, maintenance and repair with cables subject to corrosion, wear and abrasion from wave forces, and fishing and anchoring activities.

Wrote Black in his final report on the project: "A preferred method to substantially reduce the maintenance and repairs required for undersea cable shore landings is to install the cables in bore holes that are drilled under the high wave energy zone. While the initial costs of a directional bore installation are usually higher than those for surface-laid cable on the sea floor, the life cycle costs for a shore bypass installation are typically significantly lower. In addition, HDD shore landings often reduce or eliminate environmental issues associated with conventional surface-laid and trenched landings."

Black said NFESC and Sound & Sea Technology (SST) have successfully planned and installed several cable shore landings, including an HDD HDAS shore bypass conduit system at Ascension Island in the South Pacific Ocean and a fiber-optic cable communication system shore bypass at San Nicholas Island, CA.

Extensive prep, logistics

Preparations for the project included environmental compliance documentation and terrestrial and sea floor surveys to provide the data needed to plan installation of the shore bypass conduits. Analysis of the near-shore survey results and discussions with the cable and sensor installation contractor determined where the shore bypass conduit exits should be located. Approximate water depth at those points was 55 feet.

Logistics of working on Wake Island was a major challenge, and getting necessary equipment to the site involved meticulous planning. The Wake Atoll (Wake, Peale and Wilkes islands) is in the mid-Pacific Ocean, 2,460 miles west of Honolulu and 1,590 miles east of Guam, the two nearest ports for staging equipment. Supplies are shipped by infrequent ocean-going barges and by air.

To accomplish the shore landing required directional drilling and diving equipment, consumables and general construction equipment. For general construction support, trucks, cranes, backhoes and forklifts were already on the island.

Following an analysis of transportation options, the decision was made to ship drilling-related consumables and support equipment ahead by barge and leased drilling equipment and personnel by chartered U.S. Air Force C17 aircraft.

Selection of the location to launch the bores and paths of pilot bores was critical. Many utility construction personnel experienced in directional drilling likely will be surprised at the thoroughness of planning for this and other remote horizontal drilling installations.

"A thorough review of available literature on Wake Island and data available from previous projects was completed prior to selection of the best exit points for the bores and set-up location for the drill unit," said Black. "Preliminary results of bathymetric multi-beam sonar with back scatter analysis and Remotely Operated Vehicle (ROV) visual surveys were used to plan the routes for the bores, the bore exit points, and the near-shore cable routes from the bore exits to deep water. The ROV is a small 'eyeball' with a camera and propeller connected to the surface by an umbilical which allows observation of the sea floor without the need for divers."

Launch site for the bores was on the southeast shore of Wilkes Island, adjacent to the existing building pad. While no cores were taken for this project, Black said that information from previous vertical borings for seismic instrumentation on the island demonstrated that it is primarily coral down to depths of over 325 feet with volcanic formations beyond 300 feet.

The project report states that samples of the coral and rock from the beach area were recovered from the site during a pre-survey visit and tested to determine hardness and abrasiveness. These samples showed solidified coral with 4,000 to 6,000 psi compressive strength, classified as "medium strength" coral for drill bit selection.

"Experience with similar types of formations suggested that drilling operations would progress at approximately 175 feet per day with minimal fluid returns," said Black.

Preliminary routes of the bores overlaid on bathymetry obtained during the near shore high resolution multi-beam survey indicated a suitable unobstructed sea floor cable route to deep water with depth at the planned bore exits to 55 feet.

Equipment selection

A Ditch Witch JT4020 All Terrain (AT) drill unit was used to make the installations. Drilling support equipment included a MM5 mud mixing system, a 1,000 gallon fluid storage tank and 500-gallon Ditch Witch vacuum excavation system to keep the site free of drilling fluids escaping the bore hole.

Black said three primary factors determined selection of this model drill unit: its size permitted transportation by air to the remote site; its capability to drill lengths of 1,000 feet in coral and volcanic formations; and low mud flow requirements which minimized the amount of fresh water that would be needed during drilling operations. Black said the machine's dual pipe design with the drill bit turned by a pipe inside the drill pipe allows the machine to drill in hard formations without the use of a mud motor which would require significantly higher volumes of drilling fluids.

A Subsite electronic walk-over tracking system, specially modified for the project, was used to track the drill during both on and offshore segments of each installation.

Drilling operations were conducted by an experienced four-person Island Mechanical crew who specialized in offshore work and drilling through coral formations supported by personnel from Sound & Sea Technology, Underwater Construction Team TWO and NFESC.

The three bores were launched from the same entrance pit with the drill unit moved slightly and rotated a few degrees to separate the drill path for each bore. To achieve the necessary undersea depth beyond the shore, the pit was approximately 180 feet from the shore line. Sequence of the bores was the east (left bore looking down and seaward at the site) first, center bore second and west (right) bore third. Bore one was 883 feet, bore two 885 feet and bore three 926 feet.

Executing the plan

Steps in drilling pilot holes and pulling back conduits were the same for each installation:

* Navy divers inspected the target pilot bore exit point to confirm it was free of live coral and marked its location with a surface buoy;

* A line floated on the water surface by peanut floats at 20-foot intervals was brought from the exit buoy to shore;

* Divers set a line on the sea floor along the projected drill path to assist them in establishing the actual drill head position relative to the planned bore path;

* The pilot hole was drilled while tracking drill head position every 30 feet; a remote tracking antenna was used by divers to make locates during offshore segments of the drilling operation;

* When the drill head exited the ocean floor, divers marked the exit location, removed the drill bit and downhole beacon transmitter, and installed a pulling swivel and backreamer in preparation for conduit pull back;

* Three-inch HDPE conduit was staged on shore, and a tag line was installed inside the conduit with the assistance of suction from the vacuum excavation unit;

* Using an inflatable boat, conduit was pulled off the shore. Before the shore end of the conduit passed the shore line, the conduit was pumped full of sea water by the vacuum unit. The boat then pulled the conduit to sea, positioning it over the exit point of the bore and divers brought the end of the conduit down to the exit point and attached it to the pulling swivel; and

* The drill string and conduit were then pulled through the pilot hole.

Clearly, accurately tracking the drill head and making necessary adjustments to keep it on course was critical and complicated by having to track much of the bore path under the salt water of the ocean. Tracking underwater segments with electronic equipment designed for land use proved ineffective on a similar HDAS installation at Ascension Island in 2003.

Explained Black: "To accommodate tracking over water, the instrument manufacturer, Ditch Witch, developed a remote submersible antenna that is cabled to the terrestrial receiver with a 100-foot long umbilical. The remote antenna is located at the bottom of a 3-foot-long pipe which was placed directly on the sea floor, while the terrestrial receiver remains on the surface support boat. By placing the antenna on the sea floor, the signal attenuation due to sea water conductivity is minimized."

Knowing the intended exit point and using the previously installed float line and lead line on the sea floor, divers had a visual reference to determine any deviation of the drill position from the planned path. Once the beacon was located, divers would relay the deviation from the planned path to the surface where it was then passed by radio to the drillers on shore, along with standard locating information.

"Because a diver voice communication system was not available," Black said, "hand-pull line signals were established to direct the diver's movement of the remote antenna on the sea floor. The pull-signals were passed from the surface to the diver via the 100-foot long electrical umbilical. For diver convenience, signals were recorded on a pointer mounted on the tracker receiver."

Timely completion

In spite of the complexity of the Wake Island project, the three installations were completed without significant problems. From the beginning of site preparation to pulling-in the conduit on the third bore took only 13 calendar days. Pilot holes of bores one and three took two days, with three days required for bore two.

More time was required for that bore after the drill head exited prematurely, in part due to an inaccurate depth location. Ten drill rods were pulled back, the tracker recalibrated, and the bore path was corrected and drilling proceeded to the planned exit point.

Black said on all three bores, drill operators could not change vertical inclination of the drill head just before exiting, apparently because there was not sufficient bearing strength in the coral below the sand to support deflection of the drill. However, the exit point was achieved by allowing the slight up angle of the drill path to intersect the down slope of the sea floor.

The Sound & Sea Technology final report summarizes results of the project and offers conclusions on how future operations might be improved:

* The three bores were successfully drilled and lined with HDPE conduits. All three exits had a relatively clear path for the cables to be laid over the sea floor to the deep water shelf break:

* Coral formations along all three bore paths were mostly soft with occasional harder layers. Drill spoil returns were minimal and much less than anticipated (only three of the 12 pallets of fluid additive transported to the site were used. During planning it was expected that as much as 6,000 gallons per day of fresh water would be required. Instead only 1,000 to 1,500 gallons per day were used;

* Drilling rates increased as work progressed. Drilling operations began at a rate of about 215 feet-per-day. During the last full drilling day, a rate of 435 feet-per-day was achieved;

* The drill rate was primarily a function of the underwater drill tracking. As the divers became more proficient and the process was refined, the drilling rate increased dramatically;

* The underwater remote beacon locating receiver worked well. Four receivers were taken on the job. Two of the units were damaged under water and one unit had a factory defect. Having enough spares is essential on remote site operations;

* The use of hand pull signals for directing the diver worked well in the Wake Island environment. For other site conditions with higher ocean currents and wave action, diver voice communications with topside probably would be required; and

* Drilling operations on very-remote sites can only be successful with detailed planning. Adequate spares and consumables are required. For the Wake HDD project, it was not possible to determine how much drill mud would be used--it was better to have too much and ship the excess back than to stop operations and wait two-to-four weeks for a new supply.

The Wake Island installation is not yet complete, Black concludes.

"Sensor and cable installations originally planned for October were delayed," he said. "In late August, the island was hit dead center by a Category V super typhoon with winds in excess of 150 mph. Travel to the island remains restricted. After island clean up is completed, the sea floor sensors will be installed and the cables will be routed through the conduits to the terminal building."

FOR MORE INFO:

Engineering Services: Sound & Sea Technology Inc., (805) 642-1440, soundandsea.com

Utility Contractor: Island Mechanical Corp., (808) 682-5353