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The J-Lay test rig applies a 20 te vertical load via the rollers whilst pulling the product through. Image from Trelleborg Offshore. |
Specifying the right vortex induced vibration suppression solution is vital to ensuring pipeline longevity. Jonathan Fox explains.
The offshore oil and gas environment has experienced times of change and development. About 30% of the world’s oil production comes from offshore areas* and with more exploration and production activity taking place in remote locations with deeper waters and harsher climates, subsea challenges are increasing. Although production platforms historically resided at distances of 6mi from shore in 1970, projects are now more often based a significant 200mi from shore and in deeper waters.
These developments mean deepwater flowlines are faced with greater pressures and temperatures, all while demands for longer lasting products continue to grow. With this in mind, subsea solutions are required to perform more efficiently and robustly to ensure that performance is guaranteed for the lifetime of the project.
A remaining challenge is the impact and severity of vortex induced vibration (VIV) on deepwater pipeline applications. This includes steel catenary risers and rigid steel flowlines, which are unsupported over free spans.
The issues with VIV
In offshore conditions, VIV fatigue on a cylindrical structure is caused by the regular shedding of vortices from the pipe when subjected to a steady current.
As a fluid flows around a cylinder, separation of the wake cycles from one side to the other, coalescing along the length. This leads to an alternating transverse force whose frequency is dependent upon the fluid flow.
At certain water flow rates, the frequency of the vortex shedding can match the natural frequency of the pipe length, causing it to vibrate. This reaction can become more or less severe, depending on the current and also the pipe’s position in the water; as currents are more prominent closer to the surface of the sea.
It is this reaction of vibration or, more specifically, vortex induced vibration, which causes accelerated fatigue damage to the structure and gives rise to problems such as pipe girth weld failure, premature pipe failure or end connection malfunction. By suppressing VIV, fatigue failure of the pipe’s structure can be reduced or even eliminated.
VIV suppression
A high-performance VIV suppression system will primarily restrain the damaging vibrations to an acceptable level, while providing impact and abrasion protection benefits. Specifically, a solution which has been designed to stop the formation of vortices will efficiently prevent the detrimental VIV behavior.
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Stinger load test equipment. Images from Trelleborg Offshore. |
The right VIV suppression solution should incorporate overlapping and interlining moldings which wrap around the pipe and include three-start helical strakes. This will provide the triangular / trapezoidal strake profile required to prevent the formation of vortices and thus eliminate VIV. It is important that the product is designed to be independent of direction, as the flow can pass by the pipe on any route or path.
Traditionally manufactured using reaction injection molded polyurethane, each section of the VIV suppression product would be molded and then allowed to cure for hours. However, more recent practices have seen the product manufactured through thermoforming, using thermoplastic materials. This process is much simpler than the traditional techniques and is six times faster, resulting in reduced lead times, as well as project cost and time savings. Thermoplastic materials also exhibit the additional benefit of being recyclable.
Similarly, with the ability to produce a thinner product under the latest manufacturing practices, less material is required to produce the same high performance product. This not only makes the solution more sustainable but also lightweight, ensuring that it is easier to handle and pre-install onshore, or install offshore. The debilitating effect of drag on the structure is also reduced due to the smaller effective hydrodynamic diameter.
The product design should also enable each single component to be stacked efficiently. This, in turn, can drastically reduce the amount of required storage space so that transportation and containerization is much more efficient and cost effective.
Once an installation vessel has reached the field, the single component pieces are lifted from the deck, ready to install into place. Traditionally, the industry would install the pipelines via the S-lay method, where the pipe is installed over a steel structure hanging off the vessel. This meant that the total weight of the pipeline would be hanging off the structure, and during installation, the strake would have to withstand the full weight of the pipe.
However, as the industry has developed to move away from this type of installation method, pipes are now often installed using a J-lay method. This allows the pipes to be installed vertically in individual sections. Each piece is welded to the next and then lowered down into the sea, which reduces the loads experienced by the strakes. The J-lay load can still be fairly significant, so it is important that the strake solution is load bearing. Up to 20tonnes is ideal.
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| Left: Lightweight, stakable VIV suppression strake; Right: Overlapping and interlocking moldings, with three-start helical strakes. Images from Trelleborg Offshore. |
Testing
As a critical solution in an increasingly more challenging environment, it is vital to fully test the VIV suppression system.
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A hydrodynamic tow tank test of the strakes on a pipe. Image from Trelleborg Offshore |
Consultations with industry-renowned hydrodynamicists, alongside computational fluid dynamics analysis will enable the calculation of fluid forces, helping to determine the impact of a liquid or gas on product performance. This analysis should be available for any VIV suppression system as well as the results of physical hydrodynamic testing.
On a project-by-project basis, a VIV suppression manufacturer should provide sufficient impact, axial slip and load bearing capacity testing results. Specifically, an efficient J-lay test should be capable of applying a 20tonne vertical load along the length of the suppression system via a roller which simulates a small stinger or J-lay pipe installation.
All materials and geometries should be fully-qualified for long-term subsea use.
Conclusion
The unpredictable nature of an offshore environment means that the industry must remain flexible and responsive. Vortex induced vibration will always naturally occur where water flow meets a cylindrical pipe under the sea; however, the severity of this reaction will change as surroundings become harsher. The specification of the right VIV suppression solution is vital to ensuring that pipes can be protected for the project’s lifetime.
A product which not only does the job it is specified to do, but also offers benefits such as ease of use, more cost effective transportation and faster manufacture, will bring greater advantages to the project.
The exploration of oil and gas comes with its challenges and if contractors don’t sufficiently plan for these, they could fall victim to colossal financial implications and time delays. The assurance that comes with a high performance product is invaluable and is vital to providing that all important peace of mind, now and in the future.
*From Infield System’s Global Offshore Oil and Gas Outlook, Gas/Electric Partnership, 2013.
Jonathan Fox is a senior product development engineer at Trelleborg Offshore. Fox has seven years’ experience at Trelleborg, working in polymer engineering design and specializing in the design of offshore products, such as vortex induced vibration (VIV) systems, buoyancy, clamping solutions, and bend protection. Jonathan, a chartered engineer, holds a BEng (Hons) in Mechanical Engineering and is based in Skelmersdale, England, UK.
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