Anchor Chain - New Data, New Design Lines and Practical Details
Authors: Bolt H.M. , Billington C.J. , Humphries M.J. 
Published: October 2008

Fatigue test data for K3, K4 and K5 anchor chain (54-100mm) generated in the BOMEL JIP are presented. The fatigue design curve derived for a mooring line is given and compared with the provisions of API RP 2FP1. The influence of combined tension/bending at the fabiead is quantified and comparison is made between Kenter and common link fatiguelives.

INTRODUCTION

This paper is the first publication of the findings from BOMEL'S Joint Industry Project (JIP) for the development of design guidelines for anchor chain. The work was initiated in 1989 at a time when there were no fatigue requirements in force, Furthermore, over recent years, a number of failures of mooring systems had occurred (1), This situation was particulate unsatisfactory given the drive for longer term andmore extensive use of floating systems. To ensure that the economic benefit and technical advantage for deep water could be exploited, the industry took a dual approach to assuring adequate fatigue performance and reliability of anchor chain.

Many of the chain failures that occurred before about 1985 were attributed either to poor material properties or to poor manufacturing processes. New specifications and effective quality assurance systems were therefore established and, as a result, the reliability of mooring chain increased markedly (2).

Nevertheless, steel chain links subject to cyclic loads remain susceptible to fatigue and work was put in hand to develop adesign methodology consistent with other components of offshore oil and gas production systems.

The BOMEL JIP focused directly on this second aspect, recognizing the need for fatigue data on which rational guidelines could be based, The motivation for the project wasto deliver useful guidance to practicing engineers and therefore the scope extended beyond pure tension fatigue of common links to consideration of in-service problems associated with combined tension and bending at the fairlead, the performanceof Kenter joining shackles and the efficacy of inspection.

JIP SCOPE OF WORK

The full scope of work encompassed the following activities:Review of the methods and processes employed in the manufacture of anchor chain, paying particular regard to those aspects which could affect fatigue behaviour.A comprehensive literature search for information related to chain failures, fatigue in chains etc.Review of design codes and existing regulations to determine the extent of current guidance on chain fatigue.Tension fatigue experiments in air on various sizes andgrades of chain, at various mean stress levels and with various stress ranges, in order to generate a fatigue S-N curve.Tension-bending fatigue experiments in air using various stress ranges in order to evaluate the significance of the bending moments introduced into chain links by thepresence of a fairlead.Appraisal of the inspection, maintenance and repair procedures currently in use and assessment of their effectiveness.

The final project report which is now released from the ounds of confidentiality, gives the reader insight into the manufacturing processes of chain, an overview of chain failures gleaned both from research and in-service.