Advanced Fracture Control Procedures for Deepwater Platforms and Compliant Towers
Authors: Marshall P.W. , Post J.W. , Frischmuth R.E. , Grover J.L. 
Published: October 2008

This paper describes an integrated approach to fracture control problems - applied first as a necessary adjunct to the authors' own engineering practice, then in developing the US National Codes for tubular structures, AWS D1.1 and API RP 2A, as well as related standards for materials, welding, and testing.

INTRODUCTION

A well engineered structure requires that a number of factors be in reasonable balance. Factors to be considered in relation to economics and risk in the design and steel selection for tubular connections include:static strength,fatigue resistancefracture toughness,homogeneity and resistance to lamellar tearing, andweld ability.

Many of these same factors arise again in setting up QC/QA programs during construction, including such issues as weld profile control and allowable flaw size to be applied during nondestructive testing. Human factors and organizational issues must also be addressed, such as personnel qualifications and lines of communication/approval.

CONVENTIONAL FRACTURE CONTROL

Most fracture control problems in offshore platforms occur in the tubular connections, or nodes. Use of the "hot spot" stress, as would be measured by a perpendicular strain gage adjacent to the toe of the weld in the region of localized plastic deformation, serves to bring fatigue and fracture problem for many different node geometries into a common focus (refs. 1, 2).

Use of elastic hot spot stress as an Indicator of ultimate strength is only approximate. Local yielding, mobilization of plastic section modulus, trail stresses, and load redistribution via plastic deformation occur as tubular connections reach their practical ultimate capacity. These phenomena, which occur in the presence of weld-toe notches, place severe demands on the materials being used, particularly in the chord or main member of a tubular connection. Although elastic-plastic analysis methods are now becoming more widely available, design practice typically uses well-calibrated empirical formulas for ultimate strength (e.g., punching shear in AWS D1.1).

Once the designer has taken care of the most fundamental requirement -- strength -- then it is time to consider such problems as fatigue and brittle fracture. Design for fatigue routinely uses the hot spot approach, with parametric stress concentration factors (SCF, e.g. ref. 3), and allowable hot spot stresses derived from prior generic fatigue analyses. EXHIBIT 1 shows the original American fatigue design curve based on hot spot stress, which was used to derive allowable one-in-a-lifetime allowable hot spot stress range of 80-ksi for structures exposed to a typical Gulf of Mexico wave climate. Lower allowable are used in more severe environments (ref. 4, 6).

Gulf of Mexico lowest anticipated service temperatures are 14 degrees-F air, 40-60 degrees-F water. Typical steel for "joint cans" (heavy wall chord section) is API Spec 2H, which calls for drop-weight nil-ductility transition at minus 40 degrees (ref. 6) .- class A fracture toughness.