Chain Out of Plane Bending (OPB) Joint Industry Project (JIP) Summary and Main Results

Abstract

In 2002, several mooring chains of a deepwater offloading buoy failed prematurely within a very small time frame. These chains were designed according to conventional offshore fatigue assessment using API recommendations. With this first deepwater buoy application, a new mooring chain fatigue mechanism was discovered. High pretension levels combined with significant mooring chain motions caused interlink rotations that generated significant Out of Plane Bending (OPB) fatigue loading. Traditionally, interlink rotations are relatively harmless and generates low bending stresses in the chain links. The intimate mating contact that occurs during the proof loading and the high pretension of the more contemporary mooring designs have been identified as aggravating factors for this phenomenon. A Joint Industry Project (JIP), gathering 28 different companies, was started in 2007 to better understand the OPB mooring chain fatigue mechanism and propose some mooring chain fatigue design recommendations. This paper summarizes the various test programs that were implemented within the more than 6 years long project, including full scale fatigue tests on chains, a quasi static OPB stiffness measurement campaign, and tests on small samples addressing the environmental parameter influence on fatigue initiation and crack propagation stages. The main output from the FEA scope of work, performed to support the experimental tests, will also be described. Finally, the paper will address the major step that has been achieved regarding implementation of a standard practice in offshore industry using a multiaxial fatigue criterion to address OPB hotspots.

Multiaxial Fatigue Analysis of Mooring Chain Links Under Tension Loading: Influence of Mean Load and Simplified Assessment

“…The Dang Van fatigue criterion was used for deriving the multiaxial safety factor. The main conclusions were published in different papers [15] [18].…”

“…Although the mooring system had been designed according to industry standards, it failed due to unknown loading mode, OPB [14]. This incident launched a Joint Industry Project (JIP) gathering 28 different companies with the objectives of understanding the mechanism of OPB and establishing recommendations for fatigue life design [15].…”

“…The main strategy for minimizing OPB is to improve the articulation of the fairlead or chainhawse [15]. For example, Jean et al [14] proposed to implement double articulations and low lever arms between the first chain link and articulation axes for reducing the bending moment, consequently reducing the bending stresses and improving the fatigue performance.…”

“…Nevertheless, the Dang Van fatigue criterion can be also be implemented for finite fatigue life [78] and [79]. It has been adopted for predicting fatigue behaviour in different industrial applications, ranging from cardiovascular stents [80] to mooring chains [15]. Several methods can be found in the literature based on the multiscale approach for finite fatigue life.…”

“…Some of these factors have already been studied (e.g. Joint Industry Projects on Out-of-Plane Bending [15], [18] and corrosion [143] As already noted in Chapter 2, chains can experience undesirable loading modes, such as OPB.…”