Fatigue Life of TLP Flexelements
Authors: Gunderson R.H. , Stevenson A. , Harris J.A. , Gahagan P. , Chilton T.S. 
Published: October 2008

ABSTRACT
This paper presents a methodology for estimating the fatigue life of elastomer/steel flexelements. Flexelements are laminated composites usually consisting of between 10 and 25 elastomer layers, each bonded to a rigid laminar reinforcing shell. Such flexelements are used as TLP tendon flexjoints, and drilling riser ball joints. Test programs have been devised to qualify flexjoints for use in specific TLP structures, e.g. Hutton, Jolliet and Snorre. The present program has undertaken a systematic study of flexelement fatigue life characteristics.
The method for estimating the mechanical fatigue life of flexelements presented herein is conservative. Although the amount of test data is limited, the methodology was validated by the test results. The method is useful in preliminary design of components and, as the data base grows, is expected to circumvent the need for expensive and lengthy test programs.
The methodology consists of analyzing the flexelement under fatigue load conditions by means of fracture mechanics inconjunction with intrinsic material fatigue characteristics and a suitable failure criterion. Fatigue tests on elastomer samples and flexelement geometry are used as input to a computer program that produces an application specific fatigue life design curve. To validate the method, large scale flexjoint fatigue tests were performed under controlled conditions in a test rig capable of simultaneously applying cyclically varying axial load and angular rotation. After each test, the specimens were sectioned to allow all internal cracks to be measured and compared with analytical predictions. Cracks grow for most of the flexelement life in a stable manner at a rate consistent with the applied load. The criterion for failure, a cracked area in anyone layer greater than 50% of the totallayer area, is based on a significant departure of rotational stiffness measurements from this stable behavior.

INTRODUCTION
This paper presents a new procedure for determining the fatigue life of elastomeric flexelements. It is based on fracture mechanics techniques developed for elastomeric materials with failure criteria deduced from fatigue tests on actual flex elements. The procedure follows a format similar to the familiar SIN approach for steel and other metals.
Elastomeric flexelements have been used in a variety of structural applications including helicopter rotor bearings, drilling riser flexj oints, and TLP tendon flexjoints [1]. Small sized components have normally undergone exhaustive fatigue tests during the development phase. However, larger components have generally been proof-tested and subjected to very limited high load-low cycle tests as a part of their development phase and acceptance tests. The fatigue tests described herein were performed on full scale TLP tendon flexj oints. However, the procedure for determining the fatigue life of a tendon flexjoint is also applicable elsewhere.
The flexj oint test program was sponsored by Esso Exploration and Production, U.K. as part of the Esso Deepwater Integrated Production System (EDIPS) research program initiated in 1987. The flexjoints were designed, fabricated and tested by Metalastik Vibration Control Systems with Materials Engineering Research Laboratory acting as elastomeric consultants.