Static and Fatique Tests on Chain Links and Chain Connecting Links
Authors: van Helvoirt L.C. 
Published: October 2008

The static strength and the fatigue strength of common 3-inch stud-link chain and of various types of 3-inch connecting links have been investigated and the factors influencing the fatigue resistance of a mooring line determined. Tests were carried out in the high-cycle fatigue range in an artificial sea-water environment, in order to include some influence of corrosion on fatigue behavior in the investigation. Static strain-gauge tests were carried out and the results compared with the results of the fatigue tests.

Various types of connecting links (i.e. 'Kenter"links, 'Baldt'-links and D-shackles) were tested and a comparison was made between 'common' chain and connecting links. The results of these tests have led to a relatively simple method of calculating fatigue damage and provided a basis for making recommendations for inclusion in design and -manufacturing specifications for mooring chain.

For cases where 'Kenter'-linkS are used as connecting elements, a simple in-situ inspection procedure can be used for the complete chain; a method is described for determining the remaining fatigue life of any particular mooring chain. This may considerably reduce the cost involved in premature chain replacement.

As the connecting elements have been found to be the weakest part in each mooring line, the use of continuous chain is recommended for permanent mooring systems. A limitation is imposed on chain length by handling and transport rather than by the chain manufacturing process.

Introduction

Permanently moored floating production systems will form an attractive possibility for the development of offshore oil and gas field, particularly in deep water. These systems can be anchored using standard anchor chains or wire rope. It is therefore of importance to improve our understanding of the long term behavior of anchor chain under cyclic loading. This was the incentive to initiate a test programmed in which a total of- twenty-eight 3-inch studded chain links of various manufacturers were subjected to testing. During these tests it was found that the connecting elements that were used to replace failed links formed the weakest part in the mooring arrangement under cyclic loading. Experience with the mooring of ships has also shown that the failure frequency of connecting links is, in general, significantly higher than the failure frequency of common links (Ref. 1). Therefore, in addition to the common 3-inch studded links, three types of connecting elements have been tested; Center-type detachable connecting links, Baldt-type detachable connecting links and D-shackles (Fig. 1).

All common links were of 'Oil Rig Quality' Although all studs ought to have been welded on one side only, the studs of two common links had been welded on both sides. Furthermore, in order to account for a possible influence of corrosion, all common links, four Center-type connecting links, two Bald type connecting links and one D-shackle were fatigue tested in a simulated marine environment. Both the static tests and the fatigue tests on three Countertypes and three Baldt-type connecting links were carried out in air.