When utilizing a tripod as an overhead anchor you must keep the force from your system to generate inside the footprint of the tripod (see Figure 4.) When a pulling force is applied outside the footprint you run the risk of pulling the tripod over. You may be saying "the feet of the tripod should be secured to eliminate any vertical or horizontal movement!" and your right. However by pulling and creating a force load outside the footprint your going to introduce unwanted stress to the legs of the tripod. By using some simple rigging solutions, such as a Halligan or tieing off to the manhole steps, (see Figures 5 and 6) you can always keep your forces where their supposed to be.
Now let's focus on the rigging operation below the anchor. Overall this rigging operation should be kept simple, efficient and effective.(SEE.) Let's ask ourselves "what do we need?" The answer is simple. We need a means to get the rescuers up and down. We also need a belay system to protect the rescuers and the victim in the event there is a main line failure.
The up and down or vertical movement can be done two ways. Our choice of haul system's will be 4:1 mechanical advantage system (MAS) or a 5:1 MAS (see Figure 7). Utilizing a MAS to lower your rescuers can become very beneficial in the event a problem arises and a rapid exit of the confined space is necessary. Keep in mind however whatever the depth of your confined space is multiply that by five. That will give you enough rope if need be to haul from the outside of the tank, bottom location. a confined space that is 15 feet in depth will need ideally 75 feet of rope.
Another means of vertical movement is lowering the rescuer by means of a break bar rack. The only drawback to this is it only provides one direction of travel, down. If we decide to use this method we must have a haul system built and ready to be "piggybacked" onto the lowering line allowing us to bring the rescuer(s) back to the surface (see Figure 8.) I like to place this line (vertical movement) in the center of the rigging plate and expand left and right as we progress.
Preparing Belay Systems
The next portion of the rigging operation that we will cover is the belay system. Our rigging will be set up to supply four rescuers with belays. We do this because, in the event we need to perform a rapid intervention operation, there is no lost time rigging for our back-up team. Remember two in/two out.
Something I highly recommend and see infrequently is to utilize a separate anchor attachment point for your belays. Why? The answer is simple. If you rigged both your belays on your tripod and it failed or tipped over due to a sudden impact load all your belays would be lost. By utilizing separate anchor attachment points we can keep the belays totally independent and still effective in the event there is a failure or toppling of the tripod. It takes a little more work during initial rigging but it's well worth it.
These belay lines will be attached to the rescuers dorsal or back attachment point located on their harness and will remain there throughout the operation. The only rope that gets removed is the lowering line which is connected to the front connection point of the rescuers harness. Furthermore the only reason we remove that rope is if a second rescuer needs to be lowered into the space. If the size of the space will not support two rescuers, there is no need to remove any line.
Now that we've covered a basic overhead rigging concept let's discuss placement of the systems. There is no need to overcrowd the entrance to the confined space with an over abundance of ropes and equipment. If you're utilizing a 4:1 MAS as your raise and lower system that should be the only rigging system where the control point is located directly over the entrance point of your confined space. Your belay systems should be redirected using directional pulleys to a control point away from the entrance point. If your means of lowering the rescuer is a break bar rack that too should then have a control point away from the entrance. By bringing your systems back away from the entrance point it allows you to better organize your rope operation and eliminates system confusion.
Now that we've tackled the rigging portion let's quickly touch on the placement of supplied air lines and any communication cables. A great spot for these items is on the open connection points left on your tripod. You can utilize kootney (knot passing pulleys) to guide the air and communication lines in and out of the confined space. A nice trick to keep these items away from the hole is to lightly secure them to one of the tripod legs by means of a prusick cord or piece of one-inch webbing.