How to overcome Landing String System Challenges

How to overcome landing string system challenges | AS Mosley

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.

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

COVID-19: Business Continues for AS Mosley

In light of the current COVID-19 outbreak, we would like to reassure our clients and stakeholders that our day-to-day business will continue as normal.

Our staff’s health & wellbeing and those around us are important to us. Following the guidelines from the World Health Organisation (WHO), we have put measures in place to allow for business continuity.

Our engineers and admin staff are now all working from home and will continue to do so until further notice. All meetings with clients and other stakeholders will be held virtually.

Over the years we have continually invested in technology. We therefore have the latest hardware and software, which allow us to run simulations on at least 24 cores. This allows us to work at ease remotely and we trust it gives our clients confidence in our work. We will continue to monitor the situation and follow government advice as the situation develops.

For new enquiries, please contact our office or get in touch with us directly.

In the meantime, please keep safe.

AS Mosley to benefit from funding award for digital twin of Floating Offshore Wind Turbines

Floating offshore wind turbines

AS Mosley, along with consortium partners, Fugro and Strathclyde University, has been awarded funding from The Carbon Trust, to develop new technology for monitoring Floating Offshore Wind Turbines (FOWT).

As one of eight projects to benefit from a share of £1m from The Carbon Trust, the project will see the development of a highly efficient method for measuring fatigue and detecting anomalies in real-time for the renewable energy sector. The system has the potential to vastly reduce operating costs by lowering – or potentially replacing – the need for subsea visual inspection of mooring lines for FOWT.

The funding, that was announced on Saturday (14 Mar 2020) was won as part of a competition run by The Carbon Trust, with the objective of accelerating the development and commercialisation of floating offshore wind technology, with particular emphasis on mooring systems and Operations & Maintenance (O&M).

AS Mosley will now work alongside its project partners to develop the system, which will take around 12 months to design.

David Bolger, Principal Engineer at AS Mosley, comments: “We are really excited about this project and are looking forward to the challenge of developing a digital twin for these impressive offshore floating wind systems. The new method could potentially be rolled out across the renewable sector, bringing with it improved efficiency, safety and substantial savings for energy companies. We’re also looking forward to working alongside project partners, Strathclyde University and Fugro.”

AS Mosley is an engineering analysis consultancy based in Insch, Aberdeenshire. The company works worldwide and specialises in Surface and Subsea design. For more information please get in touch.