Out-of-Plane Bending (OPB) Test of Large Diameter Mooring Chains

Chevron Corporation and Bluewater Energy Services (BES) performed a chain out-of-plane bending (OPB) test, called OPB MAX hereafter, at DNV GL’s laboratory in Høvik-Norway. The test was performed to study the OPB phenomenon for a chain diameter which was larger than the maximum diameter tested by the OPB JIP. The goal was to understand chain OPB physics for such a large diameter, measure interlink stiffness and maximum sliding moments and validate BES’ in-house finite element model. The current study is a collaboration between all involved parties and the results will be presented in three papers. The first paper summarizes the test setup and instrumentation. The second paper describes the test results, compares them with the OPB JIP estimations and tries to describe the chain OPB physics. The third and the last paper presents the FEA results performed by BES’ in-house finite element model. This paper is the first of the three and focuses on the test setup and instrumentation. The testing machine has been developed by DNV GL and is capable of applying tensions up to 350 t and interlink rotations in the range of ±3 degrees. Two 7-link chain specimens of R4 and R4s grades, both with the nominal diameter of 168 mm were tested at five tension levels from 150, to 350 t. Testing was performed in both wet and dry conditions. Twenty strain gauges were used to measure 3 OPB and 2 IPB moments at 5 mid-link positions. Twelve strain gauge rosettes were used on 3 links to evaluate SCF’s on the OPB hotspots. Seven inclinometers were used to monitor link rotations. DNV GL utilized a digital image processing tool to capture relative movements of chain links and developed a specific data processing tool to calculate the interlink stiffness, perform statistical analysis and provide several levels of data evaluation and comparison between the tests. The paper will provide a description of the test matrix and test objectives are given with the background of the previously performed OPB tests. Next a detailed description of the test rig is presented including the utilized instrumentation. Finally, an explanation of the implemented real-time test monitoring and the performed post-processing on the readings, in line with the test objectives is mentioned. The initial test results are briefly provided at the end.