Whilst it is known that mean stress has an effect on the fatigue endurance of steel components, this effect is not considered when designing mooring system components. The S-N and T-N fatigue design curves for mooring chain in the standards are based on tests carried out at a single mean load, which is 20% of the chain minimum breaking load (MBL), and these curves are used to compute the damage of all load cycles regardless of their mean value. Lately it has been found that the effect of the mean load can be larger than probably expected, and that mooring chains exhibit a significant increase of fatigue capacity when cyclically loaded at reduced mean load. In the majority of the floating units, the pre-tension of the moorings without environmental loads is below or well below 15% of the chain MBL, and most, if not all, of the in-service damage is produced at mean loads below 20% MBL. This in practice results in additional conservatism to the fatigue life computed using the existing design curves. Some deepwater units, subjected to high pre-tension level, can experience some or relevant damage occurring at mean loads above 20% MBL, which would be underestimated with the present design approach. The paper provides an insight on the effect of the mean load on the fatigue endurance of mooring chains and quantifies this dependency based on a large number of fatigue tests carried out on different chain diameters between 70 and 171 mm, different grades, and different mean loads ranging between 7% and 20% of the MBL of the tested chains. The Smith-Watson-Topper (SWT) mean stress correction model is used to transform the stress state of the tested chains into different stress states associated to different mean loads. Then regression analyses are performed and correction functions derived for the design curves of both S-N and T-N approaches to account for the mean load while keeping the same confidence of the existing curves.

With more than 50.000 tons in service to date, the Oil&Gas Industry has the need to understand the tension fatigue performance of grade R5 chains in straight tension, and corroborate the validity of the existing design methods. The chain fatigue design curves in API and DNV are based on fatigue tests obtained in the nineties and early two thousands. However the tests were performed on lower grades such as ORQ, R3 and R4, and small chains, 76 mm diameter being the largest studless chain tested. The industry has moved towards the use of large studless chains, especially in permanent units, where chain diameters above 150 mm are not unusual. This paper gathers information from a full scale fatigue test program on grade R4 and R5 studless chains, performed in seawater and with diameters between 70 mm and 171 mm. The chains being tested are actual production chains supplied for different drilling units and large permanently moored production floating units. The paper analyses the data and determines tension-tension fatigue curves based on API and DNV methods for computation of cumulative fatigue damage, regardless of other damaging mechanisms. Improved fatigue capacity is obtained with respect to the above recommended design methods.

With a majority of the reported chain failures related to fatigue, this phenomenon is one of the main topics to be studied as part of Mooring Integrity Management. Present fatigue design is mainly based on fatigue curves for chains under tension-tension loads in seawater. However, the applicability of these curves for different loading modes and specific environments remains unclear. This paper studies the fatigue behavior of the material used on chains as it builds the baseline for the performance of these mooring components. It includes uniaxial fatigue tests that were undertaken on R4 and R5 steel grades obtained from actual chains after all their manufacturing steps. Samples were not only tested in air and in synthetic seawater but different corrosion related parameters were also studied: frequency, temperature and cathodic protection. From the results of these tests, separated SN curves were obtained. Subsequently, these curves were analyzed and compared against present recommended design curves for material. Fractographic examination was undertaken to assess the effect of corrosion and cathodic protection and comparison between material and component response was also addressed. Results showed the strong synergy between corrosion and fatigue. Also, the improvement from fatigue design curves to actual response of the materials was quantified.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe extensive use of chain as an element in multi-component moorings by the offshore industry has led to a requirement to be able to predict the torsional response of chain under a variety of service applications. Chain exhibits some interesting behaviour in that when straight and subject to an axial load, it does not twist or generate any torque. However, if chain is twisted while carrying axial load, or subjected to axial load when in a pre-twisted condition, it behaves in a highly nonlinear manner, with the magnitude of the torque dependent upon the level of twist and the axial load. Consequences of this behaviour can include handling difficulties or even a loss of integrity in the mooring system, and care must be taken to avoid problems for both the chain and any components to which it is connected.Clearly an understanding of the way in which chains may behave and interact with other mooring components (such as wire rope, which also exhibits coupling between axial load and generated torque) when they are in service is essential. Even with knowledge of the potential problems, there will always be occasions where, despite the utmost care, twist occurs. Thus it is important to be able to determine the effects, but the sizes of chain which are in use in offshore moorings (typical bar diameters are 75 mm and greater) are too large to allow easy testing at meaningful loads and there is virtually no predictive information on the torsional response of chain available in the literature.This paper addresses the issues and considerations relevant to torque in large mooring chains. The authors introduce a frictionless theory which predicts the torques and endshortening in the chain as non-dimensionalised functions of the angle of twist. The theoretical model is compared with finite element studies that include friction.Experimental data is also presented in both "constant twist" and "constant load" forms for stud-less and stud-link chains of 41 and 56 mm bar diameter. Non-dimensionalised design curves are then given for typical stud-less and stud-link chain geometries, curves which will allow a designer to estimate the torque and end-shortening for any bar diameter, axial load and twist angle combination.

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