In Statoil mooring chains have been inspected since the first floaters were installed in the 1990’s, and replacement of top chain started as early as year 2000. Background for replacement has been wear, surface corrosion, end of design fatigue life and check of condition for lifetime extension evaluation. For seabed chains in site inspection has been, and still are, difficult and expensive. High quality inspection data is limited. Technology development the last years has allowed for cost efficient seabed chain replacements, and presently even seabed chains have been retrieved to investigate integrity and remaining fatigue life. Statoil started full scale fatigue testing of chains in 2011, and the results from the first tests, presented in Fredheim et al (2013), indicates that the surface condition of the chain is vital for the fatigue capacity. Fatigue test show that chain fatigue capacity varies significantly. The surface conditions of the chains are site dependent. To ensure timely replacement, it is vital to know the remaining fatigue capacity of the chains. An extensive fatigue test program was thus initiated, and is still ongoing. As testing has proceeded, different degrading mechanisms and fatigue capacity dependencies have been revealed. Further testing is still required, and more tests are scheduled for the following years. The paper sums up testing so far (Dec 2016), and presents the most important findings and results.
When an aged mooring system seeks a life extension, it is necessary to assess the remaining fatigue life of the corroded mooring chain. This paper summarizes the results of fatigue tests performed on mooring chain samples retrieved from six different fields in West Africa and North Sea. The impacts of corrosion on fatigue life on the samples were researched. The tests were managed under a Joint Development Project, “Fatigue of Corroded Chains (FoCCs JDP)”. The objectives of the JDP are (1) to derive a methodology for assessing the remaining fatigue life of corroded chain, (2) to develop guidance for performing reliable FEA of chain links to assess remaining fatigue life, and (3) to provide more rational basis to improve industry guidance on mooring line replacement criteria for life extension. Fatigue test procedure was defined by the fifteen (15) participating members. The procedure specified the testing parameters, including mean tension, tension range, and test frequency. Six sets of fatigue tests have been completed in seawater with the number of cycles to failure recorded. These chain samples were retrieved from floating production and storage units, e.g. FPSOs and FSUs, that were still in service. Fatigue data obtained from the tests were plotted against the design SN curves and results from fatigue testing of new chain. It was found that most of these samples have limited amount of fatigue capacity remained. Most interesting finding is that the sharpness of the surface feature on the corroded chain link has a significant impact on the remaining fatigue life. Another interesting finding is that the surface feature created by corrosion can be quite distinct and unique depending on the geophysical locations where the sample came from. These findings and test results may serve as references for life extension assessment of an aged mooring system.
Fatigue of mooring chain is for many floating offshore installations a limiting factor in design. With aging installations and the need for field life extension beyond the original design life, questions on mooring chain endurance are raised. Current SN curves utilized in fatigue limit state (FLS) calculation are based on full scale testing of new chain, performed at a high mean load level (20% of the chains minimum breaking load (MBL)). The high mean load level in the tests do not correspond to the conditions for many chains in operation, as mean load in fatigue relevant seastates are often significantly less than mean load used in the new chain fatigue tests. Mooring chains in operation also experience different degree of corrosion, both general corrosion and pitting. Surface roughness and corrosion pits contribute to crack initiations, and thus reduce fatigue capacity. Fatigue tests with new chain condition cannot be assumed representative for corroded chains. As part of mooring integrity programs, Equinor has been replacing mooring chains since year 2000. To assess actual fatigue capacity, many chain segments have been full scale fatigue tested. First tests started in 2011, and the tests cover different degrees of corrosion. The tests have been performed at typical mean load levels relevant for operation of the installations, which for most cases are less than 20%MBL. From these tests it is observed that fatigue capacity in some cases are better than expected for new chain, even for chain segments with significant corrosion. Fatigue test results show a large effect of the mean load. For test cases with significant corrosion and high mean load (20%MBL), a significant reduction in fatigue capacity compared to new chains is found. This paper presents some of the fatigue test results on used chain, highlighting the effect of the mean load for the given chain conditions. Effect of corrosion at mean load of 20%MBL is also included. The paper discusses some of the underlaying causes for the mean load dependency.
The last years Statoil has replaced some of our seabed mooring chain segments. Some of these chains have corrosion pits caused by Microbiologically Influenced Corrosion (MIC). In 2016 and 2017 one full length of a seabed chain segment, including anchor, was retrieved from a SEMI at approximately 300m water depth in the North Sea. The chain has been 20 years on the seabed. The corrosion on the chain was carefully documented, and showed significant levels of MIC. The extent of the MIC showed a strong dependency on seabed contact and how well the chain was buried in the sediments. The observed MIC is caused by Sulphate Reducing Bacteria (SRB). After corrosion identification, the chain has also been subject to full scale fatigue testing. This paper presents the technical condition of the seabed mooring chain, describing the different levels of MIC, typical SRB corrosion attacks, and the results from the fatigue testing.
Fatigue capacity of mooring chains is one of the important parameters in design of mooring systems for floating offshore structures. Fatigue life is often a limiting factor. With life extension of existing offshore installations, the fatigue capacity and effects of corrosion become even more important, as there will be large costs for mooring line replacements if safe life extension can not be granted, and the effect of fatigue failure can be fatal. Estimation of the fatigue capacity of mooring chains is thus of high importance both for safe and cost-effective design of new mooring systems, and for the safe life extension of older mooring systems. The standards used for design of mooring systems outline a somewhat simplified approach for fatigue analysis, where load cycle range is the only parameter included in the analysis. The fatigue capacity curves used are based on full scale fatigue tests of new chains, where effects of heavily corroded surfaces are not considered. Further it is indirectly assumed that mean load does not have any effect on fatigue capacity. Work presented the last years has indicated a strong effect of both mean load and surface condition, where also formulas for fatigue capacity including these parameters have been developed and presented. The conclusions are based on a large set of full-scale fatigue tests of both new chains and used chains, where the used chains are tested at different mean loads and different levels of corrosion. Equinor has run a large number of used chain fatigue tests. For these tests, each set of tests is typically made from one chain length, with similar condition on all links, and usually run at one mean load only. There are test sets with some variation in either mean load or surface condition, which have added valuable data for the understanding and verification of the effect of these parameters. The effects are well documented, but due to small variation within each set there are uncertainties regarding the quantification of the effects. The latest full-scale fatigue test results, from a chain with significant corrosion pits, include a systematic approach to quantify the effect of mean load. For the chain tested, five tests have been run at low mean load, and five tests at high mean load. This paper presents the results from these fatigue tests. The results are discussed and compared with other fatigue test results on both new and used chain, and with the formulas for fatigue capacity accounting for mean load and surface corrosion.
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