Hydraulic Rig Technology and Operations

Les Skinner

• Publisher: Gulf Professional Publishing
• Year: 2018
• ISBN:
  9780128173534
• Topic: Sciences physiques
• Subtopic: Énergie

Hydraulic Rig Technology and Operations delivers the full spectrum of topics critical to running a hydraulic rig. Also referred to as a snubbing unit, this single product covers all the specific specialties and knowledge needed to keep production going, from their history, to components and equipment. Also included are the practical calculations, uses, drilling examples, and technology used today. Supported by definitions, seal materials and shapes, and Q&A sections within chapters, this book gives drilling engineers the answers they need to effectively run and manage hydraulic rigs from anywhere in the world.

• Presents the full range of hydraulic machinery in drilling engineering, including basic theory, calculations, definitions and name conventions
• Helps readers gain practical knowledge on day-to-day operations, troubleshooting, and decision-making through real-life examples
• Includes Q&A quizzes that help users test their knowledge

Chapter 1

History of Snubbing

Abstract

Snubbing is the process of pushing pipe into or pulling pipe from a well with significant pressure on the well such that the expulsion force acting on the pipe is less than the weight of the pipe already in the hole. Stripping is a similar process, with pressure on the surface, but the weight of the pipe string is greater than the expulsion force. Both of these imply that the well pressure is underbalanced (i.e., the formation pressure is greater than pressure inside the wellbore such that there is pressure at the surface during both operations).

Snubbing has a long history with patents issued starting in 1924. Some of these were for mechanical, cable-operated devices, while others used hydraulic pressure acting on large cylinders for hoisting or pushing pipe into wells under pressure. Advancements and technology improvements make this type of work highly versatile.

Keywords

Snubbing; Stripping; Underbalanced; Pipe-light; Pipe-heavy; Compression; Tension; Blowout preventer; Conventional snubbing unit; Hydraulic snubbing unit

Chapter Outline

• 1.1 Background and Definition
• 1.2 Early Development
  • 1.2.1 Townsend Patent—1924
  • 1.2.2 Otis Patent—1929
  • 1.2.3 Minor Patent—1929
• 1.3 Early Snubbing Experience
  • 1.3.1 Kettleman Hills Field—1931
  • 1.3.2 Managed Pressure Drilling—1933
  • 1.3.3 Pressured Completions—1929–1974
• 1.4 Recent Developments
  • 1.4.1 All Hydraulic Units
  • 1.4.2 Combination Hydraulic/Mechanical Units
  • 1.4.3 Snub Drilling Unit
  • 1.4.4 Miniaturized Rig-Assist Units
• Chapter 1 Quiz
• Bibliography

1.1 Background and Definition

What is snubbing? Oil field snubbing is the process of pushing pipe into or pulling pipe from a well with significant surface pressure when expulsive force from well pressure exceeds the weight of the pipe in the hole. Stripping is the process of lowering the pipe in the hole when the weight of the string exceeds the expulsive force exerted on the pipe by well pressure. Obviously, the well is underbalanced in both operations since bottom-hole pressure inside the wellbore is less than reservoir pressure. Part of that reservoir pressure is experienced at the surface, sometimes a significant part.

The reasons for naming this operation “snubbing” in the past is unclear, but it probably has to do with a similar term used in several other industries. Snubbing has traditionally meant “securing a load with several wraps of rope around a stationary anchor point.” In marine and railroad operations, vessels were traditionally secured with several line wraps around a bollard on a dock or in a yard. As the vessel moved slightly, tension could be increased by tugging on the rope. If positioning is required, the rope could be let out a bit while still maintaining control of the vessel.

Horses are often broken by securing a rope to their bridle and then making a few wraps around a “snubbing post” located in the center of a corral. The trainer changes the length of rope available to the horse by slacking off the rope or taking up line without completely releasing the animal. This prevents the horse from hurting itself while expending its energy against the snubbing post and not just the trainer.

Most likely, oil field snubbing was a combination of these two concepts. Snubbing has been used for securing loads on pipe in wells for decades. The author’s grandfather, a cable tool driller, explained one use for oil field snubbing. When early wells were drilled with percussion tools, the hole interior was often very rough since it had little or no fluid involved, and the bit was free to drift from side to side leaving ledges, doglegs, and undergauge (tight) hole sections. Casing in the late 1800s and early 1900s was thin-walled and relatively light. My grandfather often referred to surface casing as “stovepipe.”

Excessive pipe-on-wall friction meant that the pipe would not slide into the hole under its own weight. So, cable tool drillers would run a “catline” line through a pulley mounted on the rig floor, taking a few wraps on the casing and tug on the catline “snubbing” or “pulling” the pipe down into the well even though there was no pressure involved.

This technique was used by the author in 1976 to “snub” large-diameter casing into a storage well drilled in a salt dome near Andrews, Texas. It worked well for grandad, and it worked well for me.

Snubbing became the process of “pulling” pipe into the hole when pressure inside the well was trying to push it out of the hole. This is also known as being “pipe-light” meaning the pipe in the hole is not heavy enough to go down under its own weight. The well pressure exerts too much upward force to allow the pipe to go down the hole by weight alone. Obviously, this requires a pressure seal at the surface that is loose enough to allow vertical pipe movement (i.e., if there is no seal, there is no pressure inside the well).

Snubbing is often confused with another operation frequently performed on live wells, stripping. Stripping is used to run pipe into a well under pressure when there is sufficient pipe weight in the hole to overcome the expulsive force trying to push pipe back out of the well plus friction of all types. This means that the string is “pipe-heavy.” Stripping on a drilling rig is often performed, for example, to get a bit back on bottom after having taken a kick or influx from the formation into the wellbore. This allows the entire kick to be circulated out of the well safely from the bottom up.

There are three basic requirements for snubbing or stripping:

• • Both snubbing and stripping require a seal at the surface on the outside of the pipe to prevent pressure and fluids (often gas) from escaping upward around the pipe. Obviously, if there is no seal around the pipe, there is no upward expulsive force acting on the cross-sectional area of the pipe. Venting the pressure would make it far too dangerous to perform any work on a hydrocarbon-producing well in the first place.
• • Both operations also require a float or check valve inside the string below the surface to prevent fluid from flowing up the pipe and out into the air for the same reason.
• • Finally, there must be a means to handle the pipe to keep it from being pushed out of or falling into the hole. If the string is “pipe-heavy” only a load hoisting means is needed to lower the pipe under pressure. However, if the string is “pipe-light” some provision must be made to control the upward movement of the pipe as pressure tries to push it out of the hole.

When stripping is being performed, the pipe is in tension along its entire length. However, when snubbing is performed, the pipe above the seal at the surface is in compression both going into and coming out of the hole. This requires special equipment and skill to prevent catastrophic buckling and pipe failure depending on a number of factors including the pressure in the well, the pipe diameter, and its wall thickness.

Crewmembers are often exposed to very high volume hydrocarbon leaks when a seal fails during a snubbing operation. There is an inherent exposure risk associated with snubbing operations because crewmembers must be in close proximity to the upper snubbing BOP and the pipe string. Recently, some snubbing operations are being performed remotely to mitigate this risk with nobody positioned above the snubbing “stack.” In the past, however, crewmembers were very close to potential leak sources while running or pulling pipe under pressure prompting well control pione...