Testing under tension

Alex MacKinnon

August 1, 2014

Using the tensioner test rig simulator can show how to improve tensioner pad design, says MacKinnon Marine’s Alex MacKinnon.

The technical demands on pipeline installers continue to grow, due to the need to service the deeper field depths being exploited. Equipment providers are responding, with the result that everything is getting bigger and heavier.

Unfortunately, in many cases this comes at the price of poorer operability, and tensioners are no exception. Tensioners are used to transfer rigid flowlines and risers to sea, while maintaining full control of the product catenary top tension. For tensioners, bigger can’t always be practically achieved, so bigger often means doubling up, and a number of pipelay systems are now operating with two large tensioners on the reel lay tower, giving up to 800-tonne dynamic capacity.

Double unfortunately means taller, and taller usually means wider, for stability. Wider leads to longer, for efficient hull shape for transiting, leading to increased costs. One potential driver to capex cost reduction is to look more closely at the tensioner pad itself.

The tensioner pad is at the heart of the tensioner. It is a small piece of rubber/steel, which is pivotal to assuring safe and efficient operations. At this critical and highly complex frictional interface between the installation machine and the product, it would be good to know what is actually going on. In deeper fields, knowledge in this area moves from being good to know to being essential to fully understand. This is a specialized area MacKinnon Marine has research and development experience in, as well as a test facility.

The best way is to trial a system is to replicate the offshore installation conditions in a simulation test facility, using the same full-size tensioner pads you are going to use and the actual product pipe that is going to be installed. The key measurements are the compression behavior of the rubber pad and, more importantly, the frictional performance under a range of loading.

Real-time monitoring of displacements, pressures, and loads provides useful information for extrapolation to the full tensioner size. An ability to see tensioner pad deformation behavior leads to a robust understanding of how the system will perform. Fast data acquisition is used to investigate the rate of change of slip and exhaustive test variables, such as pad size and material, pipe diameter and coating type, temperature, water presence, contamination, etc., provide a sound understanding of the frictional behavior and compression response.

The declining friction coefficient with increasing track loading indicates that efficient pad design is required to provide useful levels of friction at the high loads necessary to have relatively short tensioner lengths in deep water. This is particularly true when temperature variation and the presence of contamination, typical in spoolbase facilities, becomes present on the pipe coating, resulting in significant friction reduction. In many cases the design friction coefficient is not achieved at high load, so an 800-tonne system may no longer hold 800-tonne.

Field joint coatings, typically 12.2m apart, also need to pass through the tensioner. The radial upstand height overloads the limited suspension available within the tensioner, causing a severe limitation on allowable track loading and, consequently, the top tension that can be held.

It is sobering to learn that the friction level envisaged does not exist and to realize that even small changes in a couple of parameters can significantly reduce the operational margin that is available.

In a recent test, simulating a four track tensioner with 400-tonne capacity holding a 12in. diameter pipe, two designs of tensioner pad were tested back to back. The MacKinnon Marine design returned significantly higher friction coefficient (circa 80%) right across the range of track loading indicating that by simply changing the tensioner pads an immediate performance upgrade is generated. For an existing tensioner, that means more operational margin up your sleeve and for a new build vessel the chance to fit a shorter tensioner.

Using the tensioner test rig simulator can show how to improve tensioner pad design, offering more friction coefficient over a wide range of track loading, even when challenging field joints are present. It also allows a challenge to the tensioner design friction coefficients currently considered. If additional friction is present, through good tensioner pad design, the tensioner can be designed to be shorter for the same top tension capacity, an effect which cascades through the whole lay spread and vessel sizing.

Alex MacKinnon
is MacKinnon Marine’s managing director. He has more than 20 years engineering experience and has been involved in all aspects of subsea engineering, including detailed design, subsea construction, and pipelay, globally. He specialises in installation engineering and tensioner pad design and testing. MacKinnon has a PhD in Experimental Aircraft Aerodynamics from Cranfield University. He also has a BSc in mechanical engineering from Dundee College of Technology.