Category: BOP Tethering

Case Study: BOP Tethering for the abandonment of two weak wellheads

BOP Tethering helps overcome issues with wellhead and conductor loading, and is particularly beneficial when dealing with older wells. Old wells often have 30” conductors, which do not always provide sufficient strength and stability for intervention with modern heavy BOP stacks.

This process involves tethering the BOP to reduce structural loading and results in improved fatigue life and operating limits.

With a lot of interest in BOP Tethering, AS Mosley has put together a case study, based on analysis we did for the abandonment of two weak wellheads.  The analysis showed that tethering the BOP stack significantly improved the operability and fatigue life of the weak wellhead and conductor system. Peak wellhead loading was seen to reduce by a factor of 5 and the fatigue life improved by a factor of 200 for the fully tethered system.

The improvements achieved were sufficient to enable the abandonment operations to take place. Based on the support of AS Mosley, the two subsea wells were successfully abandoned in the summer of 2016 with no incidents.

To read or download this case study click here.  For more information please contact us directly.

 

Image credits: Trendsetter Vulcan Offshore

BOP Tethering: How to overcome weak or old-style wellhead loading problems

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.

BOP size comparison 4th vs 6th generations

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.