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.

Using a Landing String System to run a Subsea Test Tree (SSTT) is a complex operation. Different stages will throw up different challenges, all of which need to be properly planned for to deliver a safe and efficient well completion campaign.

What is a Landing String System?

A Landing String System is an assembly of large bore valves and a latch, which are hydraulically controlled. Steel pipe with threaded connections is used to connect the surface and subsea equipment. This assembly provides access to wells for testing and completion operations.

The Challenges

Maintaining restrictive flex joint angular limits
The large diameter SSTT requires relatively restrictive flex joint angles to be maintained in order to prevent lock-up. These angle limits should therefore be maintained whilst running the system and preparing to unlatch.

Heave Limits

Stroke out of the heave compensator has the potential to impart significant tensile loads into the system, causing potential equipment failure. It is also important to ensure that the surface test tree does not impact the drill floor during large vessel heave. By specifying clear and unambiguous heave limits, these scenarios can be de-risked.

Determination of Adequate Fatigue Life

It is important to ensure that the fatigue critical components within the system do not exceed the allowable limits. Landing strings often have multiple contact points (at the drill floor and within the BOP stack), which can impose high loads onto the system. Some threaded connections such as the latch and retainer valve are prone to high stress concentration factors, resulting in the requirement for detailed analysis to determine usable fatigue lives.

Accidental Events

Accidental events should be carefully analysed and planned for. Accidents such as a loss of vessel station keeping (either a single mooring line failure or DP Drift off), heave compensator lockup and a loss of top tension all need to be considered to ensure safe operations. As the landing string contains high pressure well bore fluids, it is critical that the system does not suffer any leakage or failure since the large diameter marine riser which surrounds the landing string is not rated for high pressure.

To overcome these challenges, detailed analysis must be completed. A full global riser analysis of the marine riser and landing string will help determine operating limits and the fatigue life of the system.

For further information, visit our Landing String Analysis page or contact AS Mosley directly.