Installing an offshore platform is a hugely complex, multi-disciplinary operation. The importance of centralization for the integrity of conductors and to extend the life of the wells is sometimes underestimated. Ignoring the issue at the start of any project or addressing it incorrectly can be costly, particularly with the trend nowadays showing new projects with more ambitious design life for the platform. Whilst appearing to be simple devices, centralizers provide a service at a critical interface between the platform structure and the well – where the conditions imposed upon them can be severe.
Operators often have to divide their attention and focus on so many other aspects of a new development project that conductor guide centralization is sometimes left till the last minute. This lack of planning will likely generate issues and additional costs for a project including:
- The jacket design does not account for centralization; therefore, limiting the design options available, potentially increasing the difficulty and the cost for hardware as well as installation
- Due to the late introduction of centralization there will be little time to design and build, forcing to pay for expedited fabrication increasing costs
- As it is too late to allow for pre-fitting of the centralizers onto the conductor, operators are forced to go down the retrofit route which is operationally more expensive.
Ann Vicens, Product Leader – Structures, said: “Often centralization is left until quite far down the chain. This can prevent us from being able to give the operator a fully holistic view on how the centralizers will interface with their guides and other equipment.”
For existing platforms, it is not uncommon to concentrate only on the most “visible” (e.g. cellar deck) guides of the platform. Issues at this top level are often caused, however, because of poor centralization at the splash zone or at the lower guides. Therefore, centralization must be analysed as a full system to take effective and efficient measures. Making changes without a proper understanding of the system can mean simply moving the issue to another section of the conductor. It is therefore important to consider the entire conductor centralization system when in the planning phases of your project.
Ann continues: “Ideally centralization of a conductor needs to be considered at the point of ordering the riser analysis and certainly before completing the design of the platform jacket. By ensuring centralization is considered at this point, Claxton can advise of achievable clearances for the analysis and dimensions required for the jacket guides”.
Loadings and clearance requirements sometimes can be non-achievable when installing a centralizer. The riser analysis is an extremely important first step to determine these requirements. The results should provide the maximum expected loads at each level based on a series of assumptions including the maximum expected clearance in each guide. This clearance should consider what is practicably achievable considering the range of tolerances that builds up by adding the tolerances of the conductor, guide and centralizers. If the requirement calls for a very small clearance, then the only option might be an adjustable design.
There is the view that sizing the guides to minimize its dimensions using the maximum OD of the running tool as a reference is 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 (e.g. rope access operations, divers, non-routine ROV ops).
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 provide a guidance of what the maximum allowable clearances should be at each level but at the same time consider best 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 axial loads to guides and decks.
misalignment of the conductor
Eccentricity and misalignment of the conductor within the guides can also be challenging, in particular when the level of misalignment is not always known and it can be specific of a slot within each well of a platform. A good understanding of the reason for the misalignment, what is required in terms of lateral support as well as operational challenges during installation is key for the successful installation of a centralizer.
It is also worth considering that there are steps that could be followed prior drilling and installing the conductors, that could prevent misalignment and eccentricity problems. An example of this would be to centralize the drill string within the platform guides or subsea pacing structure to prevent misalignment between the wells and the platform. The sooner a specialist is engaged to discuss the project the better chances there are to making sure the integrity of the conductor and hence life of the well is being looked after with a simplified and cost effective solution.
Conductor guide centralizers provide essential service at the critical interface between the platform and the well’s 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 today to discuss your platform and jacket project to see how our engineers can help to ensure maximum conductor integrity.