What is the purpose of a sand line in drilling?

The purpose of a sand line in drilling is indeed to move heavy objects and retrieve tools from the borehole. The sand line is a type of wire rope or cable that is used in conjunction with a hoist system on the drilling rig. It allows for the efficient management of heavy equipment and tools, such as drill bits and casing, throughout the drilling process. The ability to lift and lower these items is crucial for drilling operations, as it enables the crew to respond promptly to various operational needs, including making repairs, changing drill bits, or pulling equipment out of the borehole when necessary.

The context of the other options further clarifies why they do not align with the function of a sand line. Providing support for the entire rig structure would pertain more to the structural components of the rig rather than the sand line's specific function. Measuring the depth of the well relates to other tools and techniques (like depth gauges or measuring tapes) rather than a sand line. Circulating drilling fluid involves systems such as mud pumps and hoses, which are specially designed for moving fluid, rather than the sand line's primary role. Hence, the sand line's function is distinctly tied to moving heavy objects and tools in drilling operations.