Some recent experience is presented of the structural analysis of subsea completions subjected to bending loads. The paper looks at the stresses and internal loads induced near the mudline in the wellhead, conductor and casing and, in particular, the effect of cement shortfalls and different soil conditions are explored.
The insight obtained to structural performance by this type of analysis is performance by this type of analysis is shown to be invaluable and can lead to the design of more efficient structures. As a result, safety can be improved.
The move towards drilling wells in deeper water has resulted in a re-assessment of well completion structures and their design. Whereas reliance could be placed on a wealth of previous experience where installations lay within well-established water depth and well pressure limitations, this is no longer so as these bounds are being exceeded. Furthermore, it is now considered that reduced costs can be achieved and safety improved by a more thorough study of how the conductor, casing and template (if present) interact.
For most locations, with standard wellhead configurations, stress limits will only be exceeded if the drilling rig mooring system fails. Operations should have ceased with the riser disconnected prior to the onset of weather conditions which could lead to critical conditions. However, in areas of known high currents, deep water, poor seabed conditions, or with non-standard wellhead design, unacceptable loads may occur. In these cases serious consideration should be given, at an early stage of well planning, to a rigorous analysis of the completion structure in order to identify critical conditions which may arise within the operating envelope of the drilling vessel and during its production life. production life. When it is considered that abandonment of a poorly designed well could cost several poorly designed well could cost several million pounds sterling, and perhaps much more should environmental pollution result, a detailed structural analysis could be financially well worthwhile and should be regarded as essential to successful design. This is now common practice with at least one operator.
Environmental loads, transmitted through the drilling or production riser, have an impact on casing design only in the vicinity of the mudline to a depth of a few tens of metres, depending upon the attenuating effect of the soil. Above this level, the interaction of the casing and conductor is potentially very complex. At depth, the casing design is governed by formation geometry and pressure.
Note that the objective of the type of analysis being discussed herein is to validate the design of the conductor and casing rather than of the wellhead and conductor housing.