Cost-Efficient Structural Upgrade and Life Extension of Ekofisk Platforms with Use of Modern Reassessment Techniques

摘要

The application of subsidence monitoring and mitigationrnefforts combined with the conversion to remote operation haverncontributed to safely prolonging the operating lifetime of thernbest producing platforms in the Ekofisk field in the NorwegianrnNorth Sea.rnThis is the first paper in a series of seven (7) papers, whichrnsummarize the main efforts done to mitigate the effects ofrnsubsidence. The paper presents the background for thernreassessment work, the tasks involved and summarize thernmain results thereof.rnThe paper series address the update of environmental criteriarnand the practical use of modern analysis techniques in thernstructural reassessment of offshore platforms, with thernobjective to keep the operation costs low whilst documentingrnsufficient safety for the remaining service life. For thisrnpurpose, the assessment of four jacket structures at the EkofiskrnField is presented.rnBased on the reassessment procedures presented here, thernplatforms were strengthened by increasing the capacity of therncomponents governing structural collapse by use of highrnstrength steel fibre reinforced grout or installation of newrnbraces. The grout was injected by the use of a ROV. Therntheoretical capacity of these components has been validatedrnthrough laboratory tests, whilst the capacity of the jacketrnstructure was determined by use of nonlinear finite elementrnand pushover analyses.rnThe platforms have been instrumented to verify thernprocedure for calculation of wave loads on the jacket. A ROVrnwas used to retrieve the instrumentation devices.rnWave loads on the deck structures have been calculatedrnusing a computer program based on the theory of change inrnfluid momentum. This program has been calibrated andrnverified through results from model tests. Another method forrnthe computation of wave-in-deck loads is based on thernVolume of Fluid (VOF) method which is used to solve thernincompressible Navier-Stokes (or Euler) equations. This latterrnmethod is also used in verification of the wave-in-deck loadrnestimates.rnThe following elements and their interrelation are the key tornthe success of the work to extend the lifetime of the Ekofiskrnplatforms; subsidence monitoring, remote platform operation,rnextreme wave warning procedure, update of environmentalrncriteria, wave-in-deck load prediction, reliability analyses,rnadvanced finite element analyses, measurements ofrnenvironmental data, wave kinematics and platform responsernand finally strengthening of the platforms.rnAs the safety level is consistently documented within thernframework of modern reliability theory, this method ensuresrnthat the minimum mitigation needed is identified. Thernalternatives to this approach if not used are; extensivernstrengthening, jacking of deck, restrictions in the operation ofrnthe platform or decommissioning of the platform.