Structural Design, Fabrication, And Installation Of Offshore Conductor Pipe
Authors: Lang G.R. , Wood B.J. 
Published: October 2008

Conductor pipe or drive pipe, including platform conductors, Jackup-supported conductors and free-standing conductors, is an area that can offer significant cost savings. This paper contains a comprehensive discussion of various facets of conductor pipe including structural design, as well as fabrication and installation considerations. The API LRFD interaction equations for combined axial and bending loadings are revised to include the effects of infernal and external Conductor loads. This approach will result in a more rational and cost effective conductor pipe design. Other structural design considerations are discussed including selection of environmental criteria, vortex shedding, fatigue, SCFS, S-N curves, slenderness ratio limitations and deepwater applications. An equation for conductor penetration is presented which is based on loss of circulation of drilling fluids. Recommendations are also made concerning fabrication and installation issues including Iifting, driveability, welding, inspection, mechanical connectors and practical considerations. Finally a recommendation is made on drive shoe bevel shapes based on field studies of actual installations.

INTRODUCTION

In an offshore well, conductor pipe, also known as drive pipe is the external pipe which provides structural support for the internal well tubing and casing strings. The objective of conductor pipe design is to provide the most economical structural member that will safely support the well during all phases of its life, including installation, drilling and production. Each phase may have different structural requirements that govern its design. The selection of the structural member can have significant cost ramifications not only in weight savings on an individual conductor, but also in the weight of a drilling/production platform. The difference in conductor sizes, i.e., between 30" and 26" diameters, could heavily influence the jacket design by increasing or decreasing environmental loadings.

DEFINITIONS

There are several different offshore applications for conductor pipe with each having specific requirements for its design. These applications are best categorized by three basic types: platform conductor, jackup-supported conductor (marine riser) and free-standing conductor (self-supporting caisson). They are defined for the purposes of this paper as follows:Platform ConductorA platform conductor is installed through the guides of an existing platform. Lateral support is provided to the conductor by guides at the platform's horizontal elevations. They typically have diameters of 26"-30" with uniform wall thicknesses ranging from ½" - 1" or more. Typical loading and moment diagrams are shown in Figure 1.Jackup-Supported Conductor

A jackup-supported conductor, or marine riser, is a temporary conductor used only during drilling operations (usually about three to four months) which is installed by a jackup drilling rig. The conductor in this case has lateral support provided by multiple cables at the drilling rig. The cables should be taut to insure no deflections of the conductor are allowed at the rig.

Internal strings are supported by a mudline suspension system and are usually not grouted to the surface. Upon completion of drilling operations the conductor is either cut off slightly above the mudline to allow for future operations or plugged and abandoned (P&A).