During the periodic inspection of the Alvheim subsea system 2013 a number of cracks were observed at the Mid Water Arch (MWA) tether anchoring arrangement. The MWA and associated anchor block are critical design elements. Detailed investigations were initiated in order to determine future development of the cracks and their severity.
The application of advanced non-linear finite element analysis as part of the inspection and maintenance strategy resulted in significant cost savings compared to a solution based on immediate mitigation action.
This paper describes the background for occurrence of these cracks and the analyses used to determine their development:
• The cracks are located in non-loadbearing locking brackets. The function of the brackets is primarily to secure the pins connecting the top part of the tether hinge to the anchor block.
• During construction the locking brackets were welded to the pin and to the tether hinge. This way the non-structural element became part of the load bearing system resulting in very high stresses in the bracket and subsequent crack development. It could not immediately be excluded that the cracks observed could initiate further cracking into main bearing parts of the hinge.
• FE modeling using Abaqus [1] was used to analyze the criticality of the situation. Non-linear material properties and removal of elements were applied in order to simulate crack initiation and crack growth.
The system was analyzed by modelling the load paths from initial assembly on land, installation loads and finally the loads during operation. Removal of elements was introduced to replicate the crack growth pattern observed on ROV still photos from periodic surveys 2012 and 2013.
The analysis documented the principle mechanism behind the crack development and further demonstrated that the risk of failure of any of the load bearing elements was negligible.
The results of the analysis provided the necessary documentation for the appropriate precautions and at the same time plan for execution of mitigation measures which would have minimal economic impact.
