Modern subsea wellhead systems have a good foundation usually based on a 36” conductor. However, old wells often used 30” conductor which does not always provide sufficient strength and stability for intervention with modern heavy BOP stacks.
A recent development in stabilising weak or highly loaded subsea wellheads during BOP operations is to tether the BOP. Tether lines are run from anchors or piles to the BOP frame and tensioned, usually using an ROV. Normally four tether lines are used, and the pretension required is relatively low for short tether lengths, in the order of 30kN (3t) in each line. The tether lines greatly reduce BOP motion and hence structural loading on the wellhead and greatly improve fatigue life and operating limits. In some cases, with tethering, fatigue damage to the wellhead can be considered negligible.
AS Mosley provides support to understand the loading on subsea wellhead equipment and offers assistance with the design of tethering systems.