The Influence of Proof Loading on the Fatigue Life of Anchor Chain

Abstract

Abstract This paper discusses the influence of proof loading on the fatigue life of anchor chain. Experimental fatigue data are presented for smaller scale open link and stud link chain. 10 mm grade 80 lifting chain was used for the fatigue tests, where 10 mm is the diameter of the bar stock forming the links. Stud link chain was fabricated by fillet welding studs into the open link chain. Lifting chain is geometrically similar to anchor chain, and this results in similar stress concentration factors at the location where fatigue cracks originate. All the fatigue tests were conducted in laboratory air. Proof loading was shown to substantially increase the fatigue life of both the stud link and the open link chain. Residual stresses imparted during the proof load operation can explain the increase in fatigue life. A simplified elastic- plastic, stress-strain algorithm demonstrated its ability to model the data trends. Introduction Historically, anchor chain failures caused by fatigue have been rare in the offshore oil industry. Berg and Taraldsenl analyzed 25 anchor chain failures which occurred in the North Sea from 1976 to 1980. None of the failures were caused by fatigue. Brittle fracture in the bend region of the link was the primary cause when the failure occurred in the free catenary portion of the mooring. Brittle fracture in the flash butt weld was the primary cause when a failure occurred at a fairlead during hauling in of a chain. Fatigue failures have been rare in the oil industry because most chain manufactured to date has been used for temporary mooring of ships and mobile offshore drilling units (MODU'S). For MODU service conditions, chain life is limited by interlink wear and corrosion, and not by fatigue. This conjecture is supported by Dowdy and Graham.2Dowdy and Graham performed dimensional checks and nondestructive testing of numerous individual chain links from a semisubmersible mooring system that had operated in the Gulf of Mexico for over 13 years. They found that the most severe chain wear occurred in sections located near the windless or fairlead during normal operation. In these regions, the chain diameter reduction was between 8% and ll%. In areas away from the fairlead and windless, the diameter reduction was between 2% and 5%. Magnetic particle examination showed no indication of fatigue cracks in common links, however, numerous fatigue cracks were found in the kenter connecting shackles. The chain which normally stays in the chain locker was found to be severely pitted. Although fatigue has not been an important factor for MODU moorings, fatigue is an important design concern for permanent floating production or storage system (FPS) oorings. There are several reasons for this. FPS moorings have long service life requirements, and a 20 year design life is not uncommon. At a characteristic wave period of 5 seconds, this represents a potential for 125million cycles of applied loading. It is difficult and expensive to periodically replace an FPS mooring becausethese units are often located in remote deep water areas.