Modern walking systems use various mechanisms to allow moving loads in any direction, spinning about a point, and steering. So-called ‘plank walkers’ from the early days of rig walking systems could not move in all directions. In the early days, there was either a walker that could move in any direction on the outside of a structure, or a walker with limited steering inside a structure.
Figure 1. An Entro industries Autowalk Pony Sub system, featuring automatic lifting, steering, and walking.
The Entro Kingpin Walking System is designed to overcome the limitations of earlier walkers by enabling a large foot to fit in a small space and move in any direction at any time. A roller traverses over the top of a steel ‘foot’ in a guide track. The guide track is connected to the foot by a center pivot pin, which allows the roller to traverse in any direction over the foot without rotating the foot itself. A mechanical linkage between the foot and the rig structure prevents the foot from rotating, which means that the foot can be long (to reduce ground bearing), but still allows walking in any direction while longer than the space between the main beams of a drilling substructure frame.
Automation: the next step in walking technology
The world is becoming increasingly automated, and walking systems are no exception. Drilling companies are always looking for ways to improve safety, to reduce rig-up and rig-downtime, and to extend the capabilities of their drilling rigs. New walking system technologies can help.
The walking cycle
In almost all walking systems, the walking cycle consists of four phases. In phase 1 (the lifting phase), the rig is lifted off of the rig mat onto rollers or skids. In phase 2 (the travelling phase), the rig is rolled or skidded over the walking feet. In phase 3 (the lowering phase), the rig is lowered back onto the rig mat and the feet are usually lifted off of the rig mat. In phase 4 (the resetting phase), the foot is pushed ahead of the rig.
Without automation, workers act as spotters to coordinate with the operator on every phase of the cycle:
- In the lifting phase, these spotters have to ensure that the feet are preloaded correctly and lifted evenly.
- In the travelling phase, the spotters have to watch for any equipment that may be caught or hanging up on other pieces of equipment.
- In the lowering phase, they have to ensure the rig is lowered evenly (which can be just as important as lifting evenly).
- In the resetting phase, the spotters have to make sure each foot is lifted high enough to clear the support surface.
In an automated system, every phase can be faster and safer. Since the rig is automatically lifted to the correct height, spotters do not get too focused on the feet and can watch out for unexpected snags during traveling. Automation allows the walking cycle to go from phase to phase instantly, instead of requiring the operator to wait for confirmation from each spotter that all the feet are ready to go. Lifting and lowering are consistent, meaning that no time is lost lifting a foot or the rig too high off the mats. With Entro Industries’ AutoWalk system, the operator can interrupt the cycle at any time to adjust the cycle and get back to walking without reprogramming the system.
Figure 2. A typical automatic walking machine has multiple walking feet (like the example shown above) connected to a single central control unit.
Automatic walking systems often use position and load sensors in the lift jacks to control the lifting process.
In a walking system without automated lift control, lifting a load requires expert coordination and planning. Because the operator cannot see the whole rig at once, spotters should watch each walking foot to make sure the feet are correctly positioned. To start a lift, each foot should be lowered onto the ground until the foot starts to bear significant load. The spotters must watch the foot and ensure that rig mats are firmly planted on the ground, that the walking feet are bearing on the rig mats, and that all backlash is taken out of any pin joints.
Typically, pressure compensated hydraulic control valves can match lifting speeds between each jack; however, this cannot account for differences in soil conditions, differences in rig matting, or simple mistakes like a jack not extending because a hydraulic quick-disconnect was not properly connected. As such, the spotters must pay careful attention to the entire lifting process to ensure correct preload, speed, and height, which can keep their attention directed towards operating the machine and away from keeping people safe.
With an automatic system, the spotters are better able to focus on keeping personnel in the area away from the rig. Typically, only one spotter and one operator are required to coordinate a safe rig move. The spotter stands on the opposite side of the rig to the operator and keeps personnel clear. Position and load sensors in the jacks can automate the preloading process. Some systems simply extend the jacks a fixed amount, or extend the jacks until a certain load is reached, to ensure proper preload. Entro Industries’ AutoLift system uses a machine-learning algorithm to monitor changes in load with respect to jack extension to determine when a jack is loaded. This system automatically compensates for changes in weight distribution on the rig and changes in soil compressibility, ensuring the rig is always lifted level.
Automatic lifting ensures that the rig is lifted to the same height on every walking step. This prevents overlifting (which can cause damage to the rig by lifting it unevenly), and underlifting (which can create a hazard if a high rig mat is snagged by the rig).
This is part one of a two-part article. Part two will be available shortly.
Written by Nathan Klammer and Jason Ross, Entro Industries.
Read the article online at: https://www.oilfieldtechnology.com/special-reports/19062019/the-next-step-part-one/
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