Category: Featured

What is Vortex Induced Vibration and how can it be identified?

VIV analysis - turbulent flow

Vortex Induced Vibration (VIV) is motion created by the interaction of a fluid flow and a cylindrical profile perpendicular to it.

VIV is an important source of fatigue damage of riser systems and subsea equipment, which are constantly in contact with changing flows of seawater.

When the boundary layer separates from the cylinder and creates an asymmetric vortex, the pressure differences create motion transverse to the flow. As the body moves, the vortex and its pressure change again, feeding back to create further motion. When this happens quickly enough, it becomes vibration of the cylinder.

What is Riser VIV?

The flow of seawater around a subsea riser can create vortices resulting in VIV.  This is important because the vibration can be shed downstream and cause in line, or more damaging cross-flow vibrations, to be created, affecting more equipment.

Where is VIV a problem?

Because of the way VIV is caused, the faster the flow of water, the more likely it is to occur. As a result, deep water locations with a typically high current, such as the Gulf of Mexico or West of Shetland, are more prone to problems with VIV.

What is Riser VIV?

What can be done?

The risk of VIV is generally assessed by as part of a Global Riser Analysis, however, it can be investigated as a separate study.

AS Mosley recommends an initial screening. This will highlight if there is a problem and if so, a full assessment can be carried out along with recommendations to overcome VIV and/or maximise fatigue life.
Shear7 is the industry standard tool for investigating the presence of VIV and the consequential fatigue damage.

A linearised modification of the OrcaFlex or Flexcom global riser model can be used in combination with Shear7 to determine the fatigue life of individual components in the riser system.

In addition to this, OrcaFlex also has a wake oscillator module and this can use the full non-linear OrcaFlex global riser model to investigate VIV if a more detailed analysis is required.

 

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.

How does analysis of weak wellheads improve fatigue life?

Weak Wellhead Analysis

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.

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.