How to avoid four common conductor guide centralizer mistakes
2. ONLY CENTRALIZING THE ‘LEVEL’ OF THE JACKET AND CONDUCTOR WHERE MOVEMENT IS OBSERVED OR CONSIDERED LIKELY
It is important to consider the entire conductor centralization system when planning for centralizers. Often excessive movement will be seen at cellar deck level – however, once the entire system is analysed, this cellar deck movement can often be the result of poor centralization at the splash zone or other lower guide levels.
Making changes without a proper understanding of the system can also mean you are simply moving the issue to another part of the conductor.
3. MAKING CONDUCTOR GUIDES TOO SMALL
There is the view that sizing the guides to minimize its dimensions using the maximum OD of the running tool as a reference is the best practice. If this is the approach taken, once all tolerances and running clearances are accounted for, the maximum clearance between the conductor’s OD and the guide’s ID can easily exceed the allowable recommended by riser analysis.
This means centralizers will still be required to reduce the clearance between the guide and conductor but the space available might not be sufficient to fit an optimal design.
The result of this is that an operator might be forced to go for a retrofit centralizer solution when pre-installed units could have been used. If this is the case there are operational cost implications as well as additional health and safety considerations (rope access operations, divers, non-routine ROV ops).
4. THINKING THAT GOOD CENTRALIZATION EQUALS ZERO-GAP BETWEEN CONDUCTOR AND GUIDE
Centralization looks to reduce the clearance between the conductor OD and guides ID to an optimum value range (based on fabrication tolerances). The riser analysis results should provide a guide of what the maximum allowable clearances should be at each level but at the same time consider good practices of minimum clearances required just for running the conductor through the guides.
This alone will mean that a clearance will always be present in fixed OD designs.
When the requirements are more stringent, an adjustable blade centralizer design might be required. Even with an adjustable centralizer, a minimum gap of a few millimetres is always recommended to avoid the transfer of loads to guides and decks.
5. THINKING ABOUT CENTRALIZERS FOR YOUR PLATFORM?
Conductor guide centralizers provide service at the critical interface between the platform and well conductor where the forces imposed upon them can be severe. Correctly designed and installed centralizers are therefore operator’s ‘front line’ assurance of conductor integrity.
Claxton’s team have installed more new and retrofit centralizers than any other North Sea supplier – contact us to discuss your platform and jacket project to see how our engineers can help to ensure maximum conductor integrity.
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