Weak or old-style wellheads have evolved over the years, as technology has advanced. But when it comes to intervention and abandonment operations, weak or old-style wellheads do not always provide sufficient strength and stability for modern BOP stacks. BOP tethering offers a solution. Here’s how:
Background
The mass and stack height of subsea BOPs have significantly increased since old-style wellhead systems were first introduced (Figure 1). Modern subsea wellhead systems typically have a good foundation, usually based on a 36” conductor. Many old wells have slim 30” conductors, and so do not always provide sufficient strength and stability for intervention and abandonment operations with modern heavy BOP stacks. This can be a challenge, due to the high loading induced on the well from the modern BOP stacks. However, BOP tethering is one solution that can significantly reduce the loads.
Figure 1: BOP size comparison of 4th Generation vs 6th Generation
When do we need BOP tethering?
We need to use a BOP tethering solution when either the fatigue life or operating limits of a planned well operation are insufficient. This happens when the upfront global riser analysis highlights an issue with the untethered BOP option. A feasibility study of a tethered BOP solution can then quickly be performed to identify the potential improvement that could be made by tethering the BOP.
How does BOP Tethering work?
Typically, four tether lines are attached to the BOP protection frame and connected to anchors or piles laid on the seabed away from the well centre. These are then tensioned, usually with an ROV, which provide a significant reduction in lateral motion of the BOP, consequently reducing the structural loading on the well. Figure 2 presents an overview of a typical tethered BOP setup.
Figure 2: Tethered BOP configuration example
Does BOP tethering help improve operating limits as well?
Yes! During accidental loading from a loss of station keeping, the tethers provide a restoring force to the BOP stack, reducing the deflection and consequently the loads on the well. The benefit to operability is a function of the tether stiffness and rating of the tether anchor system. Our analysis always checks that the tether system can sustain the loading from the accidental offset condition.
What results can be expected?
In a project run by AS Mosley, the peak wellhead loading was seen to reduce by a factor of 5 and the fatigue life improved by a factor of 200 when compared to the untethered BOP configuration. The improvements achieved were sufficient to enable the abandonment operations to take place with a large BOP stack on an old well. The wells in this project were successfully abandoned in the summer of 2016, with no incidents.
Having completed over ten BOP tethering analysis projects, AS Mosley has helped operators overcome wellhead loading challenges.
AS Mosley provides support to understand the loading on subsea wellhead equipment and design services for bespoke tethering systems. For more information visit our BOP Tethering page or contact us directly.
