Summary of the 4-Year Research Project: Anchor Chain Cables Offshore
Authors: Lereim J. 
Published: October 2008

The present paper summarizes the main aspects studied in the Veritas Joint Industry research project entitled "Anchor Chain Vables Offshore -Material Properties-Reliability". The research project included experimental and numerical assessments of the strength and reliability of Chain of Oil Rig Quality. The study covered fracture, fatigue, stress analyses, NDE-evaluation of fabrication defects, the reliability of applied NDE method and an overall fracture and reliability analysis.

INTRODUCTION

Significant and unacceptable failure rates were experienced on offshore mooring chains during the last decade and a great demand for reduction in failure rates arose. That situation lead to the generation and execution of the Veritas Joint Industry Research Project entitled "Anchor Chain Cables Offshore-Material Properties and Reliability." Most relevant and necessary aspects related to the reliability of offshore mooring chains of the grade ORQ were examined during the study. The program included the following main topics:Fracture Strength and ToughnessFatigue StrengthStress analyses and line tension analyses.Reliability of applied NDE methodSize distribution of defectsOverall Fracture and Reliability Analysis

The reliability analysis utilized the data obtained in the proceeding sub activities in order to derive the most important factors that influences the actual reliability level to be experienced in the mooring chains. On that basis the most rational actions for possible improvement in reliability level may be derived.

OPERATIONAL EXPERIENCE

In spite of the fact that rather moderate nominal load levels (line tension) appear in mooring lines, failure has occured at failure rates up to 10% and at load levels which are typical for normal operating conditions.

The operating requirements limit the normal operating load to be less than 1/5 of minimum guaranteed breaking load, while typical operational load level on a semisubmersible is in the order of 1/5-1/4 of minimum breaking load. Thus, failure at normal operating conditions should hardly ever appear. However, during handling and operation of the wind-lasses, sometimes shock loading may be generated due to climbing and slip in the windlass/ fairlead which may not be recorded by the conventional load cells. Therefore, local shock loading may appear on the recorded static load level during handling!. Further, dynamic amplification of the maximum load present may enhance the load level significantly, which however may not have been registered by the conventional load cells. The latter is indicated by recent non-linear dynamic analyses with values up to 70% higher than the corresponding predicted quasistatic values2. Still the actual load level will be well below the actual minimum guaranteed breaking load and failure is not expected. The local loading condition will however vary depending on the location along the mooring line. In the fairleads where the chain enters the offshore unit, the single links are subject to local bending where additional stresses appear through the bending, as well as through bending stresses due to friction between neighbouring links3