Developments in Fatigue Design Guidelines for Anchor Chains as Mooring Lines

Abstract

Abstract This paper discusses the specification and design of moorings using anchor chain, with particular emphasis on long term mooring. A brief review of historical field data of offshore platform chain mooring lines is first presented. Current design methods for determining fatigue life and extreme loads for chain moorings are then discussed and compared. With the numbers of floating production systems and other installations which use long term moorings growing rapidly, it is suggested that fatigue willbecome an increasingly important design parameter for all moorings. The majority of current Design Guidelines give only limited guidance on the design of chain moorings for long term position mooring. All test data on anchor chain fatigue life presently in the public domain is presented in both tabular and graphical formats. A detailed appraisal of this data is made. It is concluded that there are a number of areas which warrant further study in the development of design guidelines for chain moorings, particularly with respect to fatigue life. This is currently the subject of an extensive joint industry research and development project in Europe, and is discussed in detail in this paper. Introduction Anchor chains are a widely used means of catenary mooring for both Mobile Drilling Units (MDUS) and, more recently, Floating Production Systems (FPSS). In addition other offshore structures, such as tanker single point moorings and spar loading buoys are frequently moored by means of chains. Ships almost universally employ chain anchor lines. The design of anchor chain links, known as "common links", is over 100 years old and their dimensions follow a standard which is international (l), (2). Common links are made from a length of round steel bar, which is bent into a loop with straight sides, figure 1. The ends of the bar are flash butt welded, the join being positioned on one of the straight sides of the loop. A short length of rounded bar with shaped ends, known as a stud, is fitted between the straight sides of the loop. The primary purpose of the stud is not, as might be imagined, to increase the strength of the link, but to prevent the chain becoming tangled when slack. The stud is fixed in place by squeezing the sides of the loop inwards. For further security the end of the stud opposite to the flash weld may be welded to the loop. Chain for ships is supplied in three grades, referred to as Grades 1, 2 and 3. For offshore use, chain of greater strength than these ship grades is normally specified. The chain grades most commonly used for offshore applications are K3 and ORQ (Offshore Rig Quality), which are essentially equivalent grades. Recently, however, two chain grades of higher strength than K3 (ORQ) have begun to gain in popularity for offshore use. These are 0RQ+20% and K4, which are approximately 20% and 30% stronger, respectively, than K3. A third grade, K5, which is stronger again than K4, is also likely to find increasing favour in the future.