Investigation Of Ranger I Collaspe Metallurgical And Fracture Analysis
Authors: Jerner R.R. , Harcourt John A. , Powell Charles W. 
Published: October 2008

The May 10, 1979, collapse of Ranger I, south of Galveston, Texas, occurred as a result of initiation and growth of a fatigue crack in the aft leg/ stiffener fillet weld runout. The 23 inch fatigue crack apparently developed and grew over the structure's lifetime. After the fatigue crack became critical and complete leg separation occurred, the leg moved laterally and dropped to the mat surface. The leg apparently remained in this general position for a period of time while settling into an attitude approximately 9° to 11° off vertical. Applied loading became critical and the aft leg collapsed at the 44 foot level. This in turn allowed the aft end of the upper hull to fall. Hull side failure of the aft leg together with bending and separation of the forward legs followed.

INTRODUCTION

The authors were requested by the United States Coast Guard Marine Board of Investigation to travel to the site of the collapse of Ranger I in the Gulf of Mexico approximately twelve miles south of Galveston, Texas. The purpose of this trip was to provide on-site advice and consultation concerning the recovery, significance, and preservation of fractured members which might have contributed to structural collapse. They were also requested to conduct detailed metallurgical studies of pertinent fractures and to provide a report of factual findings. Ranger I, a self-elevating mobile drilling unit, was located in about sixty feet of water when the mishap occurred on May10, 1979.

INVESTIGATION

Wreckage Recovery and Macroscopic Examination

A view of the lower or mat section wreckage as initially observed from the recovery vessel is shown in Figure 1. The starboard forward leg failed in bending at the 38 foot level. The port forward leg separated at the 68 foot level. Figure 2 is a top schematic plan of the mat, leg nomenclature, and the orientation referenced herein. The aft leg was completely separated and hairpinned over the mat structure. The lower portion of the aft leg, its fractured end partially buried in bottom mud, was inside the "moon pool". The upper aft leg portion, originally near the hull, was buried outside the mat structure. A diagram showing the approximate leg location and attitude is shown in Figure 3. The final mat attitude, with respect to the ocean floor, can be pictured by reference to Figure 1.

Forward Starboard Leg Recovery

The two forward legs were retrieved prior to attempting aft leg recovery. The forward starboard leg was secured with a lifting line to the barge derrick and separated from the mat by circumferentially cutting at approximately the 24 foot level. The freed leg was then lifted to the barge for close inspection. The fracture surfaces were photographed, macroscopically examined, washed with clear water, and oiled to inhibit corrosive action. The upper leg separation, at the 89 to 90 foot level, occurred predominately in shear. The fracture surface associated with the collapse, which occurred at the 38 foot level, was macroscopically examined. Tensile overload fractures extended from 270° through 0° to 90°. The fracture probably initiated in the upper corner of the 270° pinhole and propagated above the pinhole to approximately the 30° position. Another fracture initiated at the 90° pinhole and propagated toward the 0° pinhole but veered downward at termination. The leg also experienced clockwise torsional deformation. It was determined that this leg failure occurred subsequent to the aft leg collapse.