Page 7 - Offshore Well Construction

CHAPTER 3
Casing Design
Petroleum Extension-The University of Texas at Austin

INTRODUCTION
Casing design is about achieving the total depth of the well safely, with the most cost effective number of casing or
liner strings.

Purpose of Installing Casing
In order to allow the drilling and completing of a well, it is necessary to line the drilled open hole with steel pipe or
casing. Once in place, this pipe is cemented, supporting the casing and sealing the annulus in order to
• strengthen the hole.
• isolate unstable, flowing, underbalanced, and overbalanced formations.
• prevent the contamination of freshwater reservoirs.
• provide a pressure-control system.
• confine and contain drilling, completion, produced fluids, and solids.
• act as a conduit for associated operations (drilling, wireline, completion, and further casing or tubing strings)
with known dimensions (IDs, etc.).
• support wellhead and additional casing strings.
• support the BOP and Christmas tree.

There are primarily six types of casing installed in an onshore or offshore well:
• Stove pipe, marine conductor, foundation pile
• Conductor string
• Surface casing
• Intermediate casing
• Production casing
• Liners

Stove Pipe, Marine Conductor, Foundation Pile
Stove pipe is used for onshore locations and is either driven or cemented into a predrilled hole. The pipe protects the
immediate soil at the base of the rig from erosion caused by the drilling fluid.

Marine conductor is a feature of offshore drilling operations where the BOP stack is above the water. It provides structural
strength and guides drilling and casing strings into the hole. It is usually driven or cemented in a predrilled hole. The string
helps isolate shallow unconsolidated formations and protects the base of the structure from erosion by the drilling fluid.

Foundation pile is usually jetted in or cemented into a predrilled hole from a floating drilling unit—where the BOP stack
is on the seafloor. Again, the string isolates unconsolidated formations and supports the guide base for the BOP stack,
Christmas tree, or flowbase, and guides drilling and casing strings into the hole.

Conductor String
This string is used to support unconsolidated formations, protect freshwater sands from contamination, and case off
any shallow gas deposits. The string is usually cemented to the surface onshore and to the seabed offshore. This is the
first string onto which the BOP is installed. If surface BOPs are used (i.e., jackups) the conductor string also supports
the wellhead, the Christmas tree, and subsequent casing strings.

3.1