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.

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:


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.

AS Mosley welcomes two graduates to support growth

AS Mosley has invested in two graduate trainee programmes after welcoming engineering graduates, Kenneth Morton and Prasanth Thangaraj, to its team. The move comes as part of the firm’s growth strategy, following an uplift in contracts over the last six months.

Trainee Engineering Analysis Graduates, Kenneth Morton and Prasanth Thangaraj

Andrew Mosley, Managing Director, comments: “We’re pleased to see an upsurge in business, as our clients invest in their assets and new technology.  It’s an exciting time, but as a company, we need to think of the long term and make sure that we have the right skills in place to support future projects.  Finding staff with such niche skills can be difficult, so we feel training is our only option.  It can take up to two years to train staff to a competent level and we need to be sure to have enough work lined up to support recruitment, so it’s a tricky balance.  It’s nice to give graduates the opportunity to put their degrees into use and to give them practical experience in their subject.”

Engineering Graduate Trainee from the University of Aberdeen, Kenneth Morton, started with the firm earlier this year.  He said: “As an industry, Oil and Gas has been going through a period of restructuring and, as a result, recruitment of graduates has been very limited for the past couple of years. Things seem to be changing and I feel fortunate to be involved with a company that has strength and ambition, as well as great ethics. I feel very grateful, as many of my friends from University haven’t been as lucky in finding jobs in engineering.”

Starting at the same time as Kenneth, Engineering Graduate, Prasanth, who graduated from Robert Gordon University also comments:  “It’s great to have the opportunity to put the skills I learned at University into practice and to be working with a firm that is so well regarded in the industry and has so much expertise.  I’m looking forward to working with colleagues and learning from them.”

Currently employing 17 staff, AS Mosley plans to take on another two graduate over the next three months.  The company has good relationships with the University of Aberdeen’s School of Engineering and Robert Gordon University; it works closely with both universities to identify suitable candidates.

AS Mosley was established in 1997.  The company is an Oil & Gas subsea design and analysis consultancy, specialising in global riser design and analysis of shallow and deepwater systems.  Its expertise covers wellhead and conductor structural integrity, including wellhead monitoring. Furthermore, the firm has a long history with lightweight subsea intervention systems and provides support for the SURF markets (Subsea, Umbilicals, Risers and Flowlines).  It works globally for its clients, carrying out complex analysis and design of equipment using innovative technology and highly advanced tools.

For further information about AS Mosley please contact us directly.

Pinakin tackles Annapurna Circuit in Nepal

Congratulations to one of AS Mosley’s Analysis Engineers, Pinakin Patel, who recently completed the Annapurna Circuit in Nepal.

The trek, which encircles Annapurna Massif in the Himalayas, totalling around 250 km in length with the highest pass being a mere 5416m above sea level. It took Pinakin 13 days to complete the feat, which he’s been training for over the last 6 months. Tilicho Lake (pictured below), was one of the lakes he passed along the way; it is situated 4920m above sea level.

Commenting on the trek, Pinakin said: “I have always been fascinated with Himalayas; Annapurna is one of the most popular treks in the world, known for its ever-changing scenery, tall glacier mountains, quaint mountain villages and Nepali hospitality. Trekking in Annapurna was such an exuberant experience and one that I’ll never forget.”

Spectacular – well done Pinakin!