Assessment on Semi-submersible Drilling Unit with Fatigue Cracks

Abstract

Description of the material: Semisubmersibles have been widely used in deepwater applications. During their service life, fatigue cracks may be initiated. The presence of such cracks at critical locations can compromise the safety of the whole structure. In such cases, the ultimate strength check alone cannot meet the structural integrity requirement. Therefore, there is a need to further analyze the risk of fracture failure caused by crack extension to prevent potential losses of life or property and damage to the environment. To that end, the Engineering Critical Assessment (ECA) approach has been accepted in several design codes. Based on the principles of fracture mechanics, this method uses the conservative Failure Assessment Diagrams (FADs) to judge the impact on safety of a given flaw under specified loading conditions. However, the method in existing codes is limited to well-defined and standard welded structures. Further study is needed when ECA is applied to semi-submersibles. Accordingly, the development of the FADs specifically for critical components of semisubmersibles and the corresponding assessment procedure would be necessary. In the paper, failure assessment has been conducted on a typical twin pontoon semisubmersible platform with surface cracks at hot spot locations based on the proposed FADs. Global FE analyses were performed under design loading conditions. A very detailed local FE model at the critical locations was established for the fracture crack driving force analysis. Application: The proposed procedure can be used to perform failure assessment on semi-submersibles with fatigue cracks at critical locations. The assessment result can be useful in making a rational repair or no-repair decision and improving the inspection strategy. Results, Observations, and Conclusions: Failure assessment has been conducted on a typical twin pontoon semisubmersible platform with surface cracks at hot spot locations. The generic level 2A curve in the BS7910 was compared with the constructed FADs to examine whether it is conservative or not. Significance of Subject Matter: Since the method in existing codes is limited, further study is needed when ECA is applied to semisubmersibles. The development of the FADs specifically for critical components of semisubmersibles and the corresponding assessment procedure would be necessary.