Chains in Mooring Systems
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Mooring systems of floating structures consist of long lengths of chain, rope or wire, or a combination of these elements. As part of a station-keeping system, the mooring lines have to keep the movements of the structure to a minimum. The mooring lines have to withstand the loads acting on the moored structure in addition to loads acting directly on the mooring components. If a mooring line fails, the floating structure can lose station and cause severe damage to structures and environment as well as economic losses and loss of lives. Awareness of corrosion, wear, fatigue and relevant loading conditions during design will improve the design and extend the service life of the structural components. The overall goal of this study is to find out how mooring chains work as structural components. The theory part of the report includes a study of offshore loading conditions, different types of mooring systems, causes of mooring line failure, failure detection of mooring lines and fatigue. However, there is a special focus on mooring chains. Offshore standards and recommended practices provide common chain link designs and minimum mechanical properties of links, but in order to study chain links as structural components, the stresses and strains are of importance. Normal stresses in chain links are calculated analytical using classic beam theory and curved beam theory. In addition, three-dimensional elastoplastic finite element models are applied for a more detailed investigation on the stress distribution in chain links. The presented analyses are limited to chains subjected to pure tension, although torsion and bending due to interlink friction may occur. Both stud links and studless links are analyzed in the computer program Abaqus 6.12. Mooring components as chain links enter in operation with a residual stress field created by the required proof test. However, traditional design of mooring chains does not consider the presence of residual stresses [1-3]. This study shows that residual stresses play an important role. When residual stresses are added to the operational stresses, the resulting maximum tensile stress is 3.65 and 3.30 times the nominal stress for stud links and studless links respectively. The maximum tensile stress is located at the link surface at the crown section. Such tensile stress concentrations at the surface of a material are unfavorable due to fatigue crack propagation.Both whole links and worn links are modeled with Abaqus 6.12, using solid elements. The worn links have a reduction of cross-sectional diameter of 2.6 % to 13.2 %. Wear will reduce the cross-sectional area and cause some sharp edges, but at the same time increase the contact area. The positive effects of wear seem to surpass the negative effects of wear when the wear is moderate.