Drilling At The Limit—Can Your Top Drive Handle It?

Abstract

Summary As deepwater drilling has advanced, the weight expected to be borne by a top drive has increased to levels at or near the load rating of the machine. The single load rating value normally associated with a top drive model (e.g., 1,000 tons) is a nominal figure that may not apply to every operating situation. A typical top drive has two main load paths: hoisting and drilling. Hoisting load passes through the elevators and link hanger, while drilling load passes through the saver sub, internal blowout preventers (IBOPs), and main shaft bottom end connection. Limitations inherent to rotary shouldered connections can lead to diminished drilling load capacity with respect to hoisting. Further complicating matters is the fact that the drilling load path is governed by disparate American Petroleum Institute (API) specifications: 7 and 8C (API SPEC 7 1997; API SPEC 8C 2003). These two specifications provide different calculation guidelines with regard to safety factors and are subject to a certain amount of interpretation. The end result is confusion about how much load a top drive can realistically handle. A 1,000-ton top drive may not be officially rated to hoist 1,000 tons through its IBOPs. Additionally, API sets forth a different load rating for the swivel bearing, so load rating for rotation differs from overall top drive load rating. This paper discusses the technical aspects of top drive load rating and clarifies the usable load capacity of a top drive in real-world conditions.