To ensure success of a subsea well abandonment, understanding the historic damage and loads at the wellhead during operations is obviously vital because it factors into its fatigue life.

The age and design of subsea wellheads is a major factor in their fatigue life. Early ones were based on land wells and very little modification for the subsea marine environment was made. They sometimes included multiple casing heads with no heavy wall extensions and had low capacity HPHs or weak LPH extensions.

Improved

Over the decades since, the design of subsea wellheads has improved beyond comparison. Modern ones have two housings locked together with preload and high-strength heavy wall extensions to enable load transfer.

The inner housing provides pressure containment with provision to support casing, while the outer housing provides the structural support to transfer the axial and bending loads into the soil foundation.

Riser factor

The marine riser equipment used during operations is also a key factor.
Older wells might have originally been drilled with small stacks (i.e. 80Te) and lighter risers. Equally, the modern 6th Generation drilling rigs could have 400Te stacks with heavy wall riser joints – which induce far greater loads.

Soil Strength

AS Mosley presented a paper  - Real time monitoring of subsea well foundation integrity - at the ASME 2020 OMAE Conference that showed soil integrity was critical to the integrity of a well. A number of subsea wells have had to be prematurely abandoned due to excessive soil degradation and therefore a strong understanding of site-specific soil characteristics is key during the well planning stage.

Wellhead analysis

So what can you do to understand the loads, soil support and ultimately fatigue life at the wellhead accurately given its age and design as well as the riser used? An important part of the answer is specialist weak wellhead analysis. A wellhead study using sophisticated global riser analysis techniques can be used to calculate the accumulated historical fatigue damage and then compared to a known safe fatigue limit based on industry standards to estimate the fatigue life for a proposed operation. The data captured is crucial to informing good risk-assessed decision-making.

Benefit

The benefit to the operator is having an accurate estimate for the fatigue life for the wellhead. This in turn allows you to maximise the operability of the well – by knowing how long the wellhead can safely be used with the proposed rig.

Get in touch

AS Mosley specialises in understanding wellhead loads during all operations and the effect this has on operability and fatigue. For more information get in touch.