The extension of life, or the change of use of offshore structures requires re-assessment of their capacity to carry the specified loads, which may have changed from the original design premise. The paper presents the methodology of using non-linear finite element analysis to simulate the limit states of response of a large offshore structural component to the imposed loads. The work described here was initiated when linear analysis coupled with code check formulations showed that a significant degree of strengthening would be apparently required to enable the structure to function satisfactorily. However, structural non-linear simulation combined with a limit state assessment method proved conclusively the strengthening to be unnecessary, thus eliminating costly offshore fabrication and reducing shut-down time.
The first of the Brent concrete based Condeep platforms were designed in 1973 by Norwegian Contractors and when the Brent Field began production in 1976 it was estimated the field life would cease in 1988. Now, twenty years after initial production, the Brent topsides are being converted to low pressure gas production facilities for reservoir de-pressurisation - the first stage in the Brent Long Term Field Development. This development is the largest brownfield engineering project ever undertaken in the history of the North Sea at a cost of about $2.1 bn. The topside reconfiguration has required the re-assessment of the structural steel work to carry the process facility modifications, which include significant safety upgrades by relocation of the process equipment away from the living quarters. The re-assessment for structural strength made extensive use of the finite element (FE) method in a linear format coupled with strength checks using formulations from a relevant structural code (BS5400,1982). Essentially, this linear analysis forming the basis for the design encompasses the structural redundancies within the system but assesses the strength of the structure.