A simple derrick by itself is not of much use because if it is to discharge cargo it is to be plumbed over the hatch and then overside, to avoid this tedious action a system with two derricks called the union purchase system is used.
Here the runner wires of both the derricks are coupled together to swivels to a union hook and worked in conjunction with each other. The inshore derrick is plumbed over the quayside and the other is plumbed over the hatch. The preventer and side guys are rigged so as not to obstruct the movement of the cargo between the hatch and the quay. The side guys can also be replaced by a schooner guy which remains clear of the cargo working area.
When working the union purchase system the following precautions should be taken: