William Furlow
Senior Editor
With so much interest in deep and ultra deepwater E&P activities, many don't realize there are tech-nological challenges being met in shallow water as well. In particular, the jackup rig, a long-time workhorse of shallow water, is evolving into a more versatile and stable production solution.
Doug Hines president of OSL Offshore Systems and Deck Machinery said his company is seeing a dramatic increase in the topsides loads jackups are being asked to carry. This goes for state-of-the-art newbuilds as well as upgrades of existing rigs. The common element is production facilities. With early production, the operating strategy behind the economics of many deepwater fields would appear that shallow water operators have taken this concept to heart as well. Now, instead of just moving in to drill, operators want a jackup rig that can both drill and produce wells in shallow water.
What this means for Hines, who designs and fabricates jacking systems, is dramatically increased topsides loads. The challenge is how to safely pick up and set down these loads, avoiding the increased risk of a punch-through and overcoming the technical limits of a system that was never envisioned to carry this kind of weight.
When it comes to design, everyone would like to start with a clean sheet of paper, but in reality this is rarely the case. There are literally hundreds of jackups at work in offshore basins across the globe. Hines and his people have to work within the constraints of space and existing capacity when retrofitting a new system onto an existing rig. This is a challenge since in many cases the design of the jackup legs' rack and pinion system is bumping up against the technical limits of the materials involved. That is to say, for the size and number of teeth in the gears, and the material the systems are made of, there is only so much additional capacity available. Add to this the very real risk that a heavier rig will punch through the seabed while jacking up, and it is important to rethink the whole design. This is what OSL did.
AC variable frequency drive
Hines said the key to increasing the jacking system's capacity is understanding how the weight of the hull is distributed over the pinions of the jacking system chord. In a conventional rack and pinion system, it is the lowest pinion making contact with the leg that takes the bulk of the load. Hines said the distribution of weight generally drops 20% from one pinion to the next. So for example, on a three-pinion set, 50% of the weight would rest on the lowest pinion, 30% on the next pinion up, followed by 20%, and then tapering off.
OSL's goal was to design a system that would evenly distribute the leg load across the various pinions. Rather than have this variation, he said, a design was brought forward with only 2% variation. This accomplished two things. The system's overall reliability was increased substantially, and the fatigue life of the specific teeth that where supporting most of the load also was extended.
To maintain constant speed, the new OSL electric drive system varies the torque. Also, gradually accelerating and decelerating the system does not overstress the pinions at startup. On a conventional system, there is a surge of current when the jacking system begins. This stresses and fatigues the pinions at the start of the process.
The new system releases the brake, but instead of going immediately into motion, the vessel is held in place so all pinions are balanced in load before the acceleration of the jacking process begins. One of the additional benefits of this electric system, Hines said, is that the motors are fluxed while the brake is still in place. To ensure the legs can handle the weight of the rig, the system reads the torque on each leg. The torque is gradually increased to the level that it can lift the rig. If, for example, one of the legs is about to punch through, the system increases the speed 300% on that leg, effectively extending it, until the torque is within 10% of what the original reading was before the punch-through occurred. Then, the system decelerates and holds a level position. This system can also compensate for a sinking leg during preload to maintain a set level position, a key capability for reacting quickly to adverse conditions. Once one of the legs makes contact with the seafloor, the torque is maintained in a dynamic state at that set position until the other legs do the same.
When all of the legs are stable, the preload operation begins in this dynamic state of operation.
This system can be installed in a new jackup or on the drive input of an existing jacking system as a retrofit. The variable frequency drive motor is adapted to the input on the gearbox of the existing drive at the proper horse power and torque capability.
Using expertise its parent company Oilgear developed in the hydraulics business, OSL designed a system that varies the load on the individual teeth so that the weight of the rig is equally distributed among the teeth that are in contact with the rack. At the same time, the system allows the legs to respond rapidly to changes in the load they are receiving. Another benefit of this design is that it can vary the load distributed to each pinion on a chord. In the case of a storm, for example, there is a control feature that allows the distribution of the load to be inverted. This means a set percentage of the load can be transferred to the upper pinions to compensate for the storm loads from wave action against the legs; this equalizes the storm load effects on the overall structure.
1,500 kips
Conventional rack and pinion jacking systems reach their limit at around 1,500 kips. The geometry of the rack and pinion designs is regulated by American Gear Manufacturer Association standards. The tooth configuration must conform to these referenced standards. From the standpoint of materials, 100 ksi material is the limit for this type of application. More exotic metals could be used, but are cost prohibitive. The only other way to increase capacity is to increase the width of the gear. Currently the limit for width on the contact area of tooth to rack is approximately 9-in. To construct a gear that is wider becomes prohibitively expensive. Another option is to increase the number of pinions in contact with the leg. Most jackup rigs operate with 36 pinion systems. Hines said expanded systems have from 54 to 72, but this can add as much as $200,000 per pinion to the cost of the system.
To dramatically increase the variable load, Hines said it is necessary to step back and re-examine how rigs are jacked. The designs now in place all use the "poke and stroke" cylinder operated method - an Oilgear and Bethlehem shipyard design system, in which there is a cylinder and a yoke that move the rig up or down the leg or the conventional rack and pinion type system.
A new approach
OSL designed a system that uses a set of three cylinders synchronized to operate as a continuous linear motor (CLM). These cylinders engage the leg 120 degrees out of phase. So, as the bottom cylinder is engaging the leg, the top cylinder is releasing its grip. As two of the cylinders do their work, the third is resetting to take on its share of the load. In this design, two of the three cylinders are always pushing against the legs, sharing the load. Hines emphasized that these cylinders provide continuous, smooth motion, moving the rig up and down at around one to two feet per minute.
The design is easily scalable, Hines said, to handle higher variable loads by increasing or decreasing the diameter of the cylinders, as well as the number per leg chord. Because these are not gear drives with single tooth engagement, the rod end rack engagement chuck of cylinders engage the rack in the leg with multiple teeth, equally distributing and spreading out the load on the gear teeth. Because the entire face of the chuck of two cylinders is engaged at all times, this design also distributes the load more evenly over a greater area of the rack. This improves the load distribution and offers greater wear characteristics on the rack.
Using this design, Hines said his company can build jacking systems that can handle loads several times greater than the largest jack-ups currently on the market.
