Slimline rigid lockdown wellhead

The new product range is built round a single universal wellhead, other components such as a conductor housing are specified from operational requirements and the system is quickly defined from a stock list of building block parts.  There are eight configurations in total.

The system was designed with wellhead fatigue at the heart of the design philosophy and as such the wellhead fatigue life is considered to be at least a factor of ten superior to the industry’s previous generation of wellhead designs.  This has the potential to vastly increase the scope of intervention and workover packages an operator is able to consider for field development and potentially transforms the tail life of future assets.

SRLD wellhead system, wellhead fatigue was recently identified as a serious problem across the industry.  Aker Solutions has reacted exceptionally quickly to this issue and has undertaken many extensive studies utilising several independent analysis groups to rapidly gain a full understanding of what has since revealed itself to be incredibly complex and convoluted problem, residing at the backbone of all subsea architecture.  Fatigue failure occurs as a result of normal operating conditions, it happens without ever approaching the yield stress. This problem is compounded by the fact that it is all but impossible to inspect a working subsea wellhead for evidence of fatigue. 

For the reasons highlighted above, a specific approach was taken to redesigning the structural components of the wellhead system in such a way as to eradicate fatigue as an operator concern.

The initial means of reducing fatigue crack propagation was to improve the machined profile of the structural components with more rounded features, which incur much lower stress concentration factors.  This process involved several steps and the result was a much simpler load transfer profile.

The largest improvements in wellhead fatigue were made during the design of a mechanism for preloading the wellhead into the conductor housing.  By preventing any slippage or lift-off of the wellhead under large bending conditions the load path through the system is strictly maintained within a low stress concentration condition.

The mechanism itself contains several highly innovative features including a reduced backlash breaking mechanism.  Backlash in the activation system can lead to loss of preload under several load cycles, so reducing the backlash prevents the loss of preload.

One of the pillars behind the design philosophy was standardisation across the new wellhead family.  For example, all eight of the slimline rigid lockdown wellhead systems are built from three fully interchangeable sub assemblies and every system combination has the same wellhead and conductor housing datums when stacked up.