Evalution and Selection of Steel for Welded Offshore Drilling and production Structures
Authors: Peterson M.L. 
Published: October 2008

Analysis of several Gulf of Mexico offshore structures has shown that laminated steel, poor quality welding, and notch brittle steel were possible causes of failure. Researchers at Continental Oil Company have developed a steel specification for structural steel to be used in critical locations of "warm water" structures that will eliminate many structural steel problems. This specification describes a carbon manganese fully killed steel. The result is a weldable lamination free steel with typical yield strength of 42-54,000 psi, tensile strength of 64–76,000 psi, and Charpy V-notch toughness of 30-60 ft-lb at O°F.

INTRODUCTION

OFFSHORE TEMPLATE TYPE STRUCTURES

Conventional Gulf of Mexico offshore template type drilling and production platforms are unique structures consisting of three major parts--the piling, the jacket, and the deck section. The jacket or template as it is sometimes called is a space framed structure which acts as a guide for pile driving, serves as bracing to transmit horizontal loads into the piling at the mud line and serves to prevent the piling from buckling from the compression loading. The jacket is attached to the piling at a single point. This connection is usually just above the water level; i.e., the jacket, which is that part of the structure located under the water, is actually hung from the piling and this part of the structure is really a "hanging tower." The deck section of the structure located above the jacket-to-pile connection is a conventional column supported structure (Figure 1).

These structures may function as a platform for drilling, production, storage or as living quarters and must withstand very severe environmental forces. Platforms are designed under several different loading conditions and combinations of loadings that result from the various operating conditions. Each individual structural member is designed for the load or combination of loads that produce maximum stress for that member. Axial compression loads or combined axial compression and bending usually control the member sizes. For this reason higher tensile strength steels offer little advantage over the lower strength structural steels.

OFFSHORE STRUCTURE DESIGN CONSIDERATIONS

The conventional approach for the design of an engineering structure is to size the members based upon the working stress or some fractional factor of the yield strength or ultimate tensile strength. This approach is inadequate in situations where low stress brittle fractures occur. Brittle fractures are characterized by little or no plastic deformation at the failure and are promoted by:A triaxial state of stressLow service temperatureA high strain rate or rapid loading

A notch or stress concentration in a welded joint can amplify the rate of loading. A notch may also create a triaxial stress condition. Structural designers must keep in mind that the limiting load or allowable stress depends upon these considerations and that steel may not react in its normally ductile state. It may fail in its brittle state at a much lower stress.

The fact that steel can undergo brittle fracture must be recognized and accepted. This is not to be considered an indictment of the use of structural steel as brittle fracture can be prevented.