Tensioners on right track
  from: Offshore Engineer
  by: Sergio Bogge
  Wednesday, March 19, 2008

Pipelay equipment specialist Remacut has built both two- and four-track tensioners for offshore pipelayers. Here, engineer Sergio Bogge explains why the new generation of tensioners with two vertical tracks is now the company’s favoured option for larger pipe diameters, with four-track systems ‘relegated’ to flexible or small reeled pipe work.

As a general rule, Remacut equipment is sized with suitable built-in safety factors for all mechanical, pneumatic and electric components. This allows the use of the machine continuously up to its rated performance, accepting overloads up to the peak holding force without using any component’s overload capacity.

During pipelay operations, due to weather conditions, emergency situations and other factors, the equipment works at high dynamic loads so that the tension can get higher than the nominal value of the machine. For this reason, Remacut tensioners are designed to work at the maximum rated tension continuously, using less than 80% of the motor torque, while the peak pull capability corresponds to 100% of the motor torque. The overload capacity of motors and the drives is never used.

Squeeze system

Remacut tensioners are equipped with ‘elastic suspension’, a simple, reliable and maintenance-free clamping system powered by air spring actuators. Hydraulic technology is not in use anymore unless required by particular machine configuration or special requirement.

he elastic suspension and spring actuators allow the machine to absorb distributed and local pipe OD variation, keeping the contact pressure applied to the pipe constant. This allows the absorption of roughness, stick out, non-weight coated field joints, concrete cutbacks up to 300mm on the OD, whereas most competitors’ machines can absorb only a few millimeters of distributed pipe OD variation.

Due to the very high stroke, the elastic suspension can be used to absorb pipe curvature along the firing line. This means the tensioners can be installed in the firing line even where the pipe is in curvature, which is not possible with horizontal, three-track or four-track tensioners.

Oscillating saddles

The latest generation of clamping system developed by Remacut is the oscillating saddle, designed to extend the working range of the pipe tensioner and improve both performance and the maintainability. Its advantages are:

• �high contact surface between pipe and saddle rubber pads;
• �self-alignment to the pipe, obviating the need to re-position within the working OD range of the saddle;
• �substitution of worn pads takes just a few seconds; and
• �long rubber pad life.

So-called V saddles offered by competitors touch the pipe only over a small surface, basically clamping it at four corners. This means that the concrete can easily break if squeeze pressure is high and pipe can be oval.

Combining elastic suspension with the oscillating saddles allows the tensioner to work with very low real friction coefficients. Each motor is controlled by a standalone AFE technology AC drive as standard, so a failure in a drive will affect only one motor. Moreover, the A/R winch and tensioner drives and control system are independent, so any failure on the tensioners system will not affect the A/R winch and vice-versa. Hydraulic drive systems are no longer installed, unless required by the client.

Tensioner comparisons

The advantages of the new two-track tensioners over four-track machines are illustrated in the following table, based on Remacut�fs experience in the construction of both types. It can be seen that the fourtrack tensioner technology is relegated to the laying of flexible pipes or small reeled pipes for which the main limits of the machine concept are mitigated by the particular pipe configuration. OE

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