No 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 generate low bending stresses in the chain links. The intimate mating contact that occurs due to the plastic deformation 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 26 different companies, was started in 2007 to better understand the Out of Plane Bending (OPB) mooring chain fatigue mechanism and to propose mooring chain fatigue design recommendations. This paper summarizes the computational Finite Element Analysis (FEA) scope of work that provided the understanding and validation of the OPB mechanism through correlation with the test program results on chains. In addition, a multiaxial assessment of the fatigue stresses is studied and the main results are presented in this paper.

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.

Mooring chains are critical components of off-shore installations. The fatigue assessment of these components often requires complex calculations to determine the loadings in the mooring chains. Traditionally the loadings can be converted into fatigue lives using S-N curves such as the DnV Posmoor curve, or the API RP2 SK curve. Deep water SPARs undergoing vortex-induced-motion (VIM) in loop current conditions may be subject to higher mean/cyclic loadings with considerably lower fatigue life estimates — compared with earlier installations in which fatigue life estimates were so large that fine tuning fatigue prediction methodologies was only of academic interest. In this case a more accurate evaluation of the fatigue performance of mooring chains is needed. In this study the stress concentration factors (SCFs) of a studless 5.25” (133mm) mooring chain were examined in a seven-pocket fairlead. The chain-fairlead system analyzed in this study had a very tight fit (i.e., was not designed for passing connector links), and the results of this study will, in general, not be applicable to other chain-fairlead combinations without additional study. The computed SCFs of the stud-less link interacting with the fairlead pocket were compared to the corresponding SCFs in a chain link away from the fairlead. The study shows that the maximum SCF ratio is 1.15, significantly less than the upper bound 2.5 value recommended by Det Norske Veritas in OS-E301 in lieu of detailed analyses. This has a significant impact, nearly an order of magnitude, on fatigue life prediction of the chain, justifying the analytical effort. The study also found that the SCF of the chain link in the fairlead is a function of the geometry of the chain and the fairlead. As a result some guidance is provided in this paper with respect to the implications of minimum SCFs on other link and fairlead geometries. This study combines computational efforts from NEL and ChevronTexaco in a two-pronged approach where: 1) NEL provided calculations addressing parametric variations of the chain link angles of the mooring line leaving the fairlead and the chain tension levels, and 2) ChevronTexaco validated simplified modelling assumptions done by NEL to make the parametric problem tractable.

Several mooring chains of an off-loading buoy failed after only 8 months of service. These chains were designed according to conventional fatigue assessment using API RP 2SK T-N curves to a fatigue life of 20 years with a factor of safety equal to 3 on life. Of particular interest is that the mooring chain failure underwent significant mooring chain motions that caused interlink rotations. Although traditionally neglected, these interlink rotations, when combined with significant chain tensions can cause bending stresses in the chain links. In this paper we identify a mechanism, here identified as Out-of-Plane Bending (OPB) that explains the extensive fatigue damage causing the mooring chains of the off-loading buoy to fail. A previous paper [4] presented experimental results of applying inter-link rotation to a pre-tensioned chain. Various pretension levels were used, with instrumentation to extract link angles and chain link stresses. In this paper, the physics of the OPB mechanism is examined through finite element models of the 124mm chain link tests. The various modes of interlink rotation are examined. The proof loading procedure that the chain undergoes at manufacture is identified as a likely cause for creating a tightly mated surface that is conducive to activating the OPB mechanism. To comply with Single Buoy Moorings (SBM) requirements addressing publication of internal research, many of the graphs included in this paper have had the stress values removed from the y-axis. However, with SBM’s management approval, some numerical references to stress amplitudes remain in the text. Overall, this limitation does not detract from the study, trends are evident and relevant comparisons can be made.