The Cognac Fatigue Experiment
Authors: Marshall P.W. , Larrabee A.D. , Burk J.D. , Egan G.R. 
Published: October 2008

The Cognac platform was installed in 1978 in 1020 feet of water. It carried an instrumentation system which measured oceanographic parameters, deck motions, and strains in select braces. It also carried an array of precracked and as-welded fatigue specimens attached to one of the instrumented braces at a depth of 170 feet. The specimens are subject to the same sequence of loads and cathodically protected seawater environment as the structure. They are mounted so as to amplify the stresses in the parent brace, accelerating the rate of fatigue damage in order to produce useful information within the term of the experiment, which runs to the year 1998. The first two sets of specimens were removed in 1979 and 1981. This paper will describe the experiment and report the results obtained to date, including:comparison of measured seas with those used in the original design analysiscomparison of measured stresses with those obtained from the original design analysis (Ref. Kinra and Marshall 1979).using measured stresses, comparison of actual crack growth with that predicted by various proposed da/dN curvessimilarly, comparing failure/non-failure of the as-welded specimens with predictions based on various S-N curves.

INTRODUCTION

When the Cognac platform was installed in 1020 feet (306 m) of water in 1978, it carried an instrumentation and monitoring system designed primarily to provide feedback to the designers on the validity of their design assumptions. Structural integrity monitoring for damage detection was ruled infeasible by the high degree of redundancy (6 to 20 parallel load paths at each level) and complexity (15 levels in series contributing to measured topside motions).

The instrumentation system consists of:Directional wind, wave, and currentTopside accelerations and deflections (instrument chutes permit occasional below water measurements to get mode shapes)Strain measurements in a few selected bracesOn-board minicomputers for initial data management and a recorder for raw time series storage.

The data obtained have permitted comparison between computed and measured member forces and structure deflections in large waves from two hurricanes using .various coefficients with both traditional and advanced wave force computation methods. Parallel analytical efforts have explored the modeling assumptions needed to match measured natural periods, mode shapes, and apparent damping. However, these points are not the subject of this paper.

As an adjunct to the instrumentations system, Cognac also carried a number of fatigue gages on one of the instrumented braces at 167 ft. (50m) below sea level. These consist of four precracked fracture mechanics specimens and two as-welded bending specimens as shown in Figure 1. They are subject to the same seawater environment and cathodic protection as the platform brace. They are also subject to the same sequence of slow, non-stationary, wide band, random load cycles except that the stresses are amplified so as to produce measurable fatigue damage in the fatigue gages in a small fraction of the expected fatigue life of nearby tubular joints in the structure.