Residual Strength Assessment and Repair of Damaged Offshore Tubulars
Authors: Ricles J.M. , Bruin W.M. , Sooi T.K. , Hebor M.F. , Schonwetter P.C. 
Published: October 2008

The results of several current research programs related to the behavior of dented and corrosion-damaged tubular brace members and their repair is presented. Investigations were conducted involving large-scale testing and analytical studies. The analytical studies involved assessing the ability of several different methods of analysis, including simple engineering calculations and more sophisticated nonlinear finite element analysis, to predict damaged and repaired member behavior. The results of the test programs indicated that a significant amount in member deterioration could occur due to either dent or corrosion damage. The extent of the ability of a repair to reinstate a damaged member to its original design strength was found to depend on the extent of the damage. The accuracy and reliability of the analytical methods to predict damaged member behavior are illustrated.

INTRODUCTION

Presently, there are over 3500 major offshore fixed platforms located in U.S. waters. These structures exist in a hazardous environment, being constantly susceptible to collision with marine vessels and dropped objects which can cause darnage consisting of dent damage of depth d and out-of-straightness ?p (see Fig. 1). Surveys have reported that a significant number of fixed offshore platforms in the U.S. have dent damage.

The marine environment also exposes these platforms to corrosion, requiring the use of counteractive corrosion measures, such as cathodic protection systems and protective coatings. Despite these measures, there are numerous cases reported of platform members which have suffered corrosion damage1, 2. Shown in Fig. 2 is a schematic of a corroded brace member which had been removed from a Gulf of Mexico platform. A survey of this member by Ostapenko et al, indicated that the member had significant corrosion, consisting of some uniform, but mostly patch-type corrosion. The latter type of corrosion causes a non-uniform reduction of the cross-section's wall thickness. Testing of this, as well as another similar member, resulted in a capacity reduction of 35% and 5070, respectively, compared to a corresponding non-corroded tubular. The loss in capacity is attributed to a premature local buckle which formed in the most severely corroded area of the two specimens.

Improvements in oil recovery methods and economical justifications have resulted in many offshore platforms remaining in service beyond their original design lives. The effects of corrosion and dent damage on the safety of these as well as newer structures has become a growing concern. Knowledge of the residual strength and repairability of dent-damaged and corroded members is highly relevant to the safety, as well as the repairability and requalification of all platforms.

The strength of members with dents, both with and without out-of-straightness, has been a subject of extensive research since the late 1970's411. Most of the previous experimentalwork has involved small-scale specimens, where the diameter D is less than 76 mm and the dent-depth dd less than 0.2D. Even fewer studies have been conducted on the behavior of repaired bracing members with dent damage12-15. Virtually no prior research, except for that by Ostapenko etl.3 has been conducted on the subject of the affect of corrosion on structural performance.