The Irish Sea Pioneer: A New Generation Of Giant Liftboat
Bollinger-built unit incorporates high tech to help cut costs The first of a new class of giant liftboats will soon enter the work fleet in Liverpool Bay, England. Built in the U.S., the Irish Sea Pioneer was jointly designed by Bollinger Shipyards and Halliburton Energy Services, Inc. to service the oil drill platforms dotting the U.K.'s sector of the North Sea. The self-elevating vessel consists of a 180-ft. by 92-ft. (54.8-m by 28-m) ship hull with a 9-ft. (2.74-m) draft. An electrically- driven jacking system raises and lowers four, 240-ft. (73.2-m) legs to the sea floor, then raises the vessel to the desired working height. Although small OSVs have successfully tended oil drill platforms in the calm, shallow waters of the Gulf of Mexico, the Irish Sea Pioneer leads the field in terms of seahardiness, size, and scope of operation.
High Tech Solutions Considerable technological upgrades and innovations were necessary for a lift vessel to function safely year-round in the hostile, open-ocean environment of England's coastal waters. Where the tripod leg arrangement on conventional vessels has been primarily a means of stabilizing ships against minimal tidal motion, the Irish Sea's four triangular lattice/strut legs, measuring 13 ft. (3.96 m) per side, provide stability in tidal currents up to 2.4 knots, in 120-ft. (36.6-m) waters, and provide a maximum working platform height of 200 ft. (60.9 m) above the sea floor. The vessel is built to survive 70-knot winds and 40-ft. (12.2- m) seas.
Unlike previous models, the leg housings on the Irish Sea are built into the ship's hull for safety and stability. Most previously manufactured lift vessels utilized a tubular leg. The change to the truss/lattice style was necessary to reduce weight and wave drag, and required a radical change in jacking gear. The boat will provide living and working quarters for roving platform maintenance crews, effectively replacing the separate and expensive live-aboard crews currently crewing the rigs. These reduced housing needs will streamline the platform's profile as viewed from shore, and improve the economic viability of offshore oil production. In addition to its maintenance and monitoring functions, the Irish Sea will also provide support facilities during construction of new platforms and in emergency situations. The vessel will follow a regular maintenance route returning to home port in Liverpool at week's end.
Cruising at six knots in a 20-knot wind, travel time between rigs will rarely exceed one day. Approach paths and optimum docking locations are determined with a Laser Positioning System consisting of a laser ranging head, permanent optical reflectors on each platform, and a small computer in the bridge. System accuracy is estimated to within 8-in. at a distance of 1.25 miles. The rack and pinion jacking system lowers the legs at the rate of 12 ft. per minute. Full motor torque is applied to each leg as it reaches the sea floor, firmly grounding the spud-can feet. The inverted, cone-shaped bases measure 18 ft. (5.5 m) across, and are designed to minimize scouring and sliding in the hard sand and clay of Liverpool Bay. According to Klaus Schoener, marketing manager of Marine Systems, Siemens Energy and Automation Services, the most difficult moment of the docking procedure occurs as the legs hit solid ground while the vessel is still floating at sea level. In the few moments between set-down and when the vessel and crew are safely suspended above wave height, an intermediary, "soft torque" system keeps the legs anchored, yet allows the vessel to ride with the sea flow.
Siemens, supplier of the drives and control system, worked closely with LeTourneau, Inc., (the leg and jacking gear manufacturer) to achieve this unique function. Once all four legs are down, the hull begins climbing the legs at a rate of 6 ft. per minute.
Self-leveling sensors automatically control the speed and keep the deck flat. The legs can lock into any position both statically and dynamically. Likewise, the hull can lock into any position along the leg. Preloading occurs as soon as the hull is above wave height. Two diagonally opposing legs are raised slightly and the full weight of the vessel tests and grounds the remaining legs. The procedure is repeated for the opposite legs. With the legs securely set, the vessel resumes climbing to its working height. Once the vessel is in working position, it plugs an umbilical cable into the platform's service center assuming direct control of all functions including the fire/water system, an important and necessary safety function while the vessel and crew are in close proximity to the drill rig. The crew reviews and monitors all functions, performing routine maintenance and repairs before unplugging and continuing on its scheduled rounds. Both approach and deployment are accomplished without disrupting platform production activities. Electric Drives Replace Hydraulics Among its many innovative features, the new vessel has all electric drives, unlike the more traditional hydraulic systems utilized on smaller workboats. The decision to employ electric lift drives came after engineering analyses revealed a need for more than 20 tons of hydraulic fluid to operate a comparable hydraulic system, a quantity unacceptable in the environmentally- sensitive coastal waters.
The electrical system proved to be less expensive than a hydraulic system of equal performance. The 92 percent overall efficiency rating of the electrical also meant a reduction in actual installed power alloted for jacking. Finally, the built-in redundancy of the electrical system guarantees uninterrupted performance in spite of multiples. The variable speed electro- mechanical jacking system includes the legs, jacking gear, and control system as an integrated package supplied by Siemens and LeTourneau. The package consists of 48 drive/motor combinations with 12 drives per leg, four on each side of the leg. The motors are connected via a rack and pinion system, and are equipped with failsafe disc brakes. According to Mike Oser, engineer for Halliburton, over 500,000 ft. of electrical cable were installed on the Irish Sea.
Propulsion & Control The propulsion system consists of four Aquamaster Z-drive units located near the corners of the vessel. The aft thrusters each deliver approximately 1,500 hp; the forward thrusters deliver 1,200 hp.
An integrated approach shares power between the thrusters and the jacking system. While underway, full power is given to the thrusters. During positioning, the thrusters and jacking system run simultaneously. The jacking system, which is only lowering the legs at this point, requires only five to 10 percent of available jacking power. Sufficient power remains to operate the thrusters at full power. Once all four legs are securely anchored in position, power is diverted from the thrusters to the jacking system to raise the hull out of the water.
In addition to the space savings, economic benefits, and system redundancy for crew safety, the allelectric system offers a measure of environmental safety not possible with hydraulic units. With the rigs positioned off of some of England's most valuable and fragile coastline, a spill of hydraulic fluid in the volumes needed to service a vessel of this size would spell disaster for the local tourism and fishing industries, as well as Britain's highly-prized marine and nature reserves.
Deck Layout The superstructure is located far forward to maximize the separation between the wellhead and the crew's quarters. The vessel provides a helideck, firefighting equipment, operations control and utilities support, as well as two heavylift cranes, and deck space for maintenance equipment. It features comfortable living quarters for a crew of 42 — 10 vessel operators and a maximum of 32 maintenance specialists.
The unique leg structures and the jacking/braking system were built at LeTourneau's facility in Longview, Texas. Siemens'plant in Alpharetta, Ga., built the variablespeed electric drive units. The company also supplied the control panels, brake resistors, level sensors, switchover circuits and phase-shifting transformers. The majority of the construction work took place at Bollinger's shipyard in Louisiana, with final outfitting accomplished in Corpus Christi, Texas. The Irish Sea Pioneer is classed and certified by the American Bureau of Shipping (ABS). It was shipped to the U.K. in early October for final commissioning. The vessel will be operated by Seaforth Maritime, Liverpool Bay, England.