Minimal downtime gear-driven drawworks attracts North Sea interest

The Wirth GH 2500 EG gear-driven drawworks system has a compact footplate of only 2.5-3.0 meters with DC, AC, and hydraulic drive options available.

Chain driven drawworks have dominated the drilling market for more than 40 years. The simple, solid and inexpensive design has remained virtually unchanged since inception. The very simplicity of chain drawworks - with the chain acting as a driving element bridging large distances and allowing compact construction through the parallel arrangement of the driving shafts - has been a key element in its continued success. In addition, the chain drawworks accommodates slight misalignments and, due to its standardization, is an inexpensive component.

The latter two points, however, can also pose certain disadvantages. By increasing the performance transmitted, the width of the chain and thus the insensitivity to misalignment increases. Despite the robust design of chain drawworks, chains inevitably have to be changed out due to the length required during the course of operations. This ultimately makes the cost advantage of chain less important during its life cycle.

As an alternative, the German equipment manufacturer Wirth has developed a series of gear-driven drawworks, the first of which has been in constant use for 27 years without failure to the toothed-wheel gearing system. On chain-driven drawworks, speeds are limited to the speed of the chain - typically 1,100-1,200 rpm. By using a gear shift system, higher speeds can be achieved and are reinforced by the use of a frequency-controlled AC or DC motor up to a maximum driving speed of 2,000 rpm.

In addition, variable speeds are possible with direct selection of any gear speed. According to drilling equipment general manager, Wilfried Kroppen, speeds of up to 1,600 rpm can be achieved for a short period of around 30-40 seconds using a DC motor, "If you are pulling out of the hole, however, it might only take 20 seconds to retrieve the crown block."

"With a chain-lift version," Kroppen continues, "you lift up, open the clutch, then free fall, and regain movement in a few seconds. With this system, there is no free fall. From setting slips to pulling then up, one complete round trip can be achieved in around 59 seconds. This system regains time by lowering the empty crown block, then re-accelerating immediately."

Initial operator concerns were expressed about how the speed of operation could be handled using two instead of three motors. In this system, this is achieved by changing the internal ratio of the compound gear so the operator is always in control.

Netherlands project

A gear-driven system was installed for the NAM-KTB project onshore in the Netherlands in 1990 and has since completed 500 round-trips at a depth of over 9,000 meters with less than 0.2% downtime. During one year of operation, the drawworks was run constantly at full speed up and down 24 hours a day. "The operator took oil samples every week to check the cleanliness of the oil, but even at the end, no oil change was necessary," says Kroppen. "The system uses a special filter where oil is pumped through the filter before it gets to the drawworks, which keeps the oil very clean."

The success of this installation has now led to several applications offshore, most notably on the Brent, Oseberg South, Visunds and Jotun Fields and on theScarabeo 7 and Shelf V drilling rigs. Kroppen points out that there has been little need to adapt the basic Wirth system for individual projects.

"Apart from minor difference such as control systems, lifting capacity or drive motors, each project has been virtually identical," says Kroppen. "The only major difference is that we have installed three DC motors on the Scarabeo 7 rig while the others have been fitted with two motors."

With commissioning now underway on the Jotun Field and the Oseberg South system due for delivery in September, Wirth has recently received a further order for the Saipem 10000 drillship which is destined for operation in deepwater areas. The order includes two 4,500 HP drawworks each driven by three AC motors, four 2,200 HP pumps and two 601/2-in. hydraulically driven rotary tables.

Typical configuration

The Wirth GH 2500 EG drawworks is based around a drum base frame which accommodates the drum support, drum shaft, brake cylinder fixture, eddy-current brake and compound gearing. The frame itself has built-in fastening points. Two 1,000 kW DC motors drive the gears at a rate of 1,000 rpm. The power transmission is accomplished through rotary flexible clutches onto the two three-stage gear shifts each incorporating three hydraulically-operated disk clutches.

The initial speed of the clutch is altered by using a spring assembly. The output shafts of the two gearings are connected to the gearing shafts of the compound gearing by curved tooth couplings. A centrally arranged unit in the drive carriage controls the oil lubrication and shifting pressure oil supply of the gear shifts.

The power transmission from the compound-gearing output shaft onto the drawworks drum is affected positively. The drum of the drawworks has been provided with a Le-Bus system and for normal operations it is used with only three rope layers. The drawworks are also equipped with a disk brake and a positive-locking toothed clutch for the eddy-current brake which is mounted at the end of the second drum shaft.

As an option, the drawworks can be equipped with an autodriller, which allows operation of the sensitivity-controlled drilling tool in a speed range from 0-67 meters per hour for lowering operations. The autodriller is of dual design and is mounted on the drive carriage at both gear shifts.

Use of the autodriller also enables the operator to lift a string of up to 250 tons in order to draw the bit from the bottom or to pull the string very slowly, for example in, emergency operations due to failure of the DC motors. The power transmission via the autodriller is effected electrically by a frequency-controlled three-phase current drive. Both the drawworks and the autodriller are controlled and supervised by remote operation from the driller stand.

Weight savings

Wirth has also been making triplex mud pumps used in conjunction with its drawworks systems for three decades. Originally around 6 tons heavier than conventional systems, the Wirth pumps supplied up to 2,000 HP. Kroppen concedes that the pumps were over designed, but points out that finite element analysis was not common practice at the time. "We noticed the move towards lightweight equipment," says Kroppen. "As a result, we did a lot of re-engineering on the heaviest items of our system to save weight without compromising on safety and speed."

As an example, the gearing frame of the mud pumps were originally built to normal engineering calculations making them 30-40% heavier than they needed to be. By applying the techniques of finite element analysis, however, Wirth was able to identify areas where 12 mm steel plate could replace 18 mm plate. "Saving kilos in several areas eventually led to saving tons," says Kroppen.

Today, Wirth's originally over-designed pumps are more suited to the shift towards using 7,500 psi, 2,000 HP pumps. To achieve the same output, a 1,600 HP pump needs to add extra heavy equipment. "The push for higher output pumps came from the operators as they are more suitable for higher pressure wells," says Kroppen.

The Wirth drawworks system can achieve 2,000 HP with two motors and two gearboxes, or 4,500 HP with three motors. AC motors can be supplied if specified by customers. Wirth generally uses a 2,800 kg GE design, although an ABB version - which weighs twice as much - can be supplied with minor modifications made to the footprint. The biggest lifting capacity is currently around 4,500 tons, but the company also makes versions for lifting 1,000 tons and 3,000 tons. For floating structures, the drawworks are modified to include items such as semi-automatic heave compensators.

In addition to re-designing the gearing frame, Wirth included other improvements to the drawworks, such as a hydraulic system for changing liners and valves and producing fluid component ends, piping and coolers in stainless steel - although titanium was specified for the Visund project. "The drawworks is a fully integrated system, making it safer and faster," continued Kroppen.

The concept of total integration enhances space saving, availability, and reliability. The compact construction means that the system only takes up a width of 2.5 meters with a reduced performance weight and there is less likelihood of equipment failure. To bring costs down, the firm pre-produces items which involve long lead times.

Gearboxes, for example, are built in a series of 6-8 units and a reasonable number of mud pumps are kept in stock. As a result, the company often only has to make up the frame and assemble the complete package reducing lead-times typically by around three months, delivering systems in 6-7 months.

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