Redesigned jack up elevating system expands harsh-environment operating range

Dean E. Gaddy
Drilling Editor

Floating down the Mississippi River, the Rowan Gorilla V is being towed from LeTourneau Inc.'s Vicksburg shipyard to Rowan's Sabine Pass, Tex., shore base where the remainder of its 574 ft legs and drilling derrick are currently being installed. The world's largest jack up should begin work in the U.K. sector of the North Sea this November for Amoco Exploration Co. (Fig. 1). Photo by David Nance, courtesy of Rowan Companies Inc.

Redesigned jacking gears and triangular-shaped legs will allow a next-generation jack up to expand its water-depth rating in the North Sea, creating the possibility of bottom-supported drilling east and north of the Shetland Islands.

The Gorilla V ( Fig. 1 [36,128 bytes]), owned and built by Rowan Companies Inc., is expected to be finished in late October. The drilling rig includes many design enhancements over the Gorilla IV, a jack up which began work 11 years ago.

According to Danny McNease, senior vice-president of drilling operations for Rowan Companies Inc., the rig has triangular-shaped legs instead of the previous square-truss type. The triangular shape will provide a reduced drag coefficient, which will result in "less wind, wave, and current forces flowing up against the rig, enhancing its ability to drill in deeper water."

Elevating system

The Gorilla V incorporates a radical new design in the way the bull gears, pinions, and gear box have been constructed ( Fig. 2 [301,767 bytes]). McNease said prior elevating systems produced physical separation forces between the leg rack teeth and the elevating gear teeth ( Fig. 3 [185,680 bytes]), inducing horizontal push forces against the leg center and transferring the jacking forces around the leg.

"We have now redesigned the elevating system so that there are opposing forces applied against the jacking system and not the leg," McNease said. Thus, the horizontal separation forces are eliminated.

Instead, these forces are directed inward, with the opposed pinion leg racks effectively reducing the elevating-system pinion loading. "Not only does this give you a greater water-depth capability, 400 ft instead of 328 ft (Fig. 4 [163,064 bytes]), but we can now drill as far north as 61°. In addition, in a more-benign environment such as the Gulf of Mexico, the unit can drill in water as deep as 550 ft," McNease said.

Engineers have increased the normal elevating capacity by 267% per pinion to 1 million lb and the holding capacity 180% to 1.8 million lb per pinion above previously designed elevating systems.1 In addition, the intermittent elevating capacity (instantaneous lifting ability) has been increased by 347% per pinion to 1.3 million lb.

While increasing the elevating capacity, the number of elevating units has been reduced from 96 to 72. This has decreased the hull weight and increased the variable deck load, allowing the legs to support twice the elevated weight as before. Each leg can be independently operated through a remotely controlled system at a jacking speed of 1.5 fpm.

To buttress the jack up legs against the harsh environment of the North Atlantic and North Sea, additional steel thickness with a higher yield strength has been added to the leg chords and tubular bracing.

Nevertheless, total weight per foot of leg has been reduced. This is because a triangular design requires less surface area than a square-truss design. This allowed Rowan to install additional leg lengths at comparable weights to the previous square-truss design.

Cantilever design

The Gorilla V is being constructed under the rules of the American Bureau of Shipping (ABS) and Det Norske Veritas (DNV). Under the dual classification, the jack up will be allowed to operate in U.K., Norwegian, Dutch, Danish, Canadian, and U.S. territorial waters.

The jack up's hull dimensions are as follows:

Length-306 ft
Width-300 ft
Depth-36 ft.

According to Robert Rimlinger, project manager for Rowan, the drilling package, mounted on the aft portion of the jack up, incorporates a cantilever design. This feature allows the well center to slide from location-to-location, both longitudinally and transversely, encompassing a drilling envelope of 115 x 40 ft (Fig. 5).

Thus, if a 9 ft well-development pattern was required, a total of 36 wells could be drilled from one base location, providing it with the additional flexibility to perform extended-reach batch and template work.

Extension of the cantilever beams allows drilling operations 75 ft aft of the transom (back end of hull), compared to 52 ft on the Gorilla IV.

Design criteria

The rig's hook load is rated for 2 million lb suspended from a 2.5 million lb static-load derrick. The derrick is 170-ft tall with a 40 x 40-ft base. It is equipped with a 4,000-hp, 2 million-lb capacity-rated drawworks driven by three 1,100-hp electric motors grooved for 13/4-in. drill line.

The jack up's two-speed swivel utilizes two 1,100-hp electric motors, providing a torue rating of 94,000 ft-lb.

The prime movers include five turbocharged, after-cooled diesel engines rated for a total of 16,975 continuous hp. Each engine operates at 900 rpm driving a 2,400-kw, 600-v, ac generator. The jack up is equipped with a 49.5-in. independent rotary table driven by a 1,100-hp electric motor that is rated at 2 million lb static load.

The Gorilla V is also equipped with four 7,500-psi mud pumps, each driven by two 1,100-hp electric motors. Each mud pump has two independent, 5-in. ID mud lines feeding a 5-in. ID dual standpipe on the drill floor. Each mud pump is super-charged with a 100-hp centrifugal pump.

The mud system has a capacity of 5,200 bbl containing 12 mud pits. The mud pits are arranged into two separate systems, each containing six pits of different capacities. The mud pump and mud-mixing suction piping and valves allow utilization of independent mud systems.

McNease said the Gorilla V has the horsepower to drill deep horizontal wells starting from a 20,000-ft kickoff point and can drill in excess of 30,000 ft.

Combination drilling, production

All of the Gorillas have been designed for simultaneous drilling-and-production operations (OGJ, Mar. 10, 1997, p. 59). Since 1992, the Gorilla III has been serving as a drilling and production unit for the Cohasset project, located offshore Nova Scotia about 138 miles southeast of Halifax.

Production is obtained from the adjoining Panuke and Cohasset fields while a third field, the Balmoral, is produced through an extended reach well drilled 3.5 km from the Cohasset jacket.

Pan Canadian is the operator, with a 50% working interest. Nova Scotia Resources holds the other 50%. When the fields are abandoned in mid-1999, the Gorilla III will jack down and simply move to another project. Thus, the operator and government regulatory authority will have no concerns of decommissioning to worry about.

The Gorilla II is currently working for Mobil Oil Canada on the Venture platform drilling natural gas wells in the Sable Island area. The Gorilla IV is drilling development wells under contract to Phillips Petroleum Co. in the Joanne field in the J Block of the North Sea.

Speculative building

Rowan is one of the few companies willing to risk large capital expenditures on building without first securing long-term contracts. However, according to Paul Kelly, senior vice-president of special projects, Rowan builds and supplies rigs for niche markets such as harsh-environment areas where it feels it will have no problem in finding work.

The total number of hostile-environment rigs is currently between 13 and 16, depending on the criteria used in the classification. Of this number, Rowan will have four units once the Gorilla V is finished and two more, Gorillas VI and VII, will soon be added to Rowan's fleet.

Work began on the Gorilla VI before Gorilla V left Vicksburg. This jack up is expected to be completed the second quarter 2000. Kelly said the design is similar to the Gorilla V, but there will be "fine tuning" based on what was learned from the Gorilla V.

The Gorilla VI is not presently under contract but Kelly said there are several opportunities that are being explored. The Gorilla VII is expected to be completed second quarter 2001.

It also has no contract as of yet, but Rowan is confident about the future. "Once operators realize the diversity of the new Gorilla's expanded geographical range and its dual capabilities for drilling and production, the new Gorilla class of rigs will be in high demand," Kelly said.

Reference

Rimlinger, R., "Mobile Offshore Jacku-Up Units: The Next Generation," IADC/SPE paper 39372, presented at the IACD/SPE Drilling Conference, Mar. 3-6, 1998.