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Service Rig Stability
By Reid Penner
As we mentioned before, the 4th edition of API 4F has increased the wind loading requirements for service rigs. As well, with road bans becoming more and more costly for rig operators, there is an increased need for lighter rigs to meet reduced road weights. While seemingly unrelated at first glance, these two requirements are inescapably linked, and have resulted in a Designer’s Paradox for mobile rigs.
There is a direct relationship between wind acting on tall structures and the weight of the structure. This affects service rig stability and is especially problematic in mobile rig design. Quite often, design parameters are requiring higher wind loading but lighter road weights. While wind loading is the driving force behind service rig stability, it is affected by other factors, including tipping line angle and center of gravity (CoG) location.
The required rear load beam size is directly related to the overturning moment of the wind, divided by the weight of the rig. If the wind load increases on the rig, the rear load beam has to get larger if the weight is to remain the same. Conversely, if you decrease the weight of the rig, the load beam again has to get larger if the wind loading is to stay the same. Due to road weight limits, this usually means that the only solution we have to deal with large wind loads, is to increase the rear load beam width.
Because the mast is the largest and highest structure on the rig, it is the greatest contributor to wind loading. This also means that its shape is a huge factor in determining which wind direction has the highest load. Since most masts are wider than they are deep, it usually works out that the worst wind direction is around 35° from the side of the mast. In one of our rod rig projects, we were able to shift the worst-case wind loading to the front side of the mast by going to a triangular mast design. This resulted in much less side loading, allowing for a smaller rear load beam on the rod rig.
Since mobile rigs rarely have the mast centered in the chassis of the truck (like a crane), the wind has a much different effect on stability at different angles. Another contributing factor is that mobile rigs rarely have a square base. Due to the large load beam at the rear bumper, the “stability footprint” is triangular in most cases. This generally means that the most-likely direction a rig will tip over is at an angle to the side, around the “tipping line”.
The location of the center of gravity of the rig is also very important in rig stability, especially since rigs usually have a triangular stability footprint. This usually means that a rearward center of gravity is better for side stability, as it is closer to a larger area of the footprint. However, moving it back also decreases stability in the rearward direction. This is especially undesirable, because a loss in rearward stability results in less available hook load on the blocks; move the center of gravity too far back, and the rig will pull itself over.
Couple all these factors together, and a very complex problem emerges. Firstly, legal road weight limits push the overall weight of the rig down, increasing the required load beam size. Hook load requirements also push the center of gravity forward in the rig. Due to the triangular shape of the stability footprint, this results in a smaller distance to the tipping line, and a requirement for a larger rear load beam. Add in the fact that the worst-case wind loading occurs at an angle roughly the same as the tipping line, and the load beam is required yet again to get bigger.
With more stringent standards to meet for wind loading of service rigs for safety concerns, and with a higher demand for lighter service rigs able to operate longer in the year, the only apparent method for reaching both goals is to increase the load beams on these lighter rigs.
That is, unless an innovating way of stabilizing the rig can be found…