4-track Offshore Pipe Tensioner

Offshore Pipe Tensioner

Present pipe lay track tensioner designs date back to the late 1960s when track machines first started replacing the original wheel designs. Basically all track tensioners have two vertically or horizontally opposed tracks that squeeze the pipe to generate the necessary traction for linear pull.

In order to achieve an equal circumferential squeeze on the pipe, and to maintain constant bottom of pipe elevation for all pipe diameter, the configuration shown in Fig. 1 was employed. Unfortunately there are lots of shortcomings to this design. Since pads would collide in the small pipe adjustment (pads in the lowest point of the “V”), while moving around the sprocket wheel axle, only alternate crossties can be used and thus halving machine capacity. Another handicap is the limitation of squeeze forces on those crossties. Manufacture costs of crossties make up a substantial part of machine costs. In big pipe settings traction pads are elevated far above the track chains and tension forces tilt back crossties especially when track forces are not in equilibrium above field joints. It took decades before industry tried to come up with a more satisfactory design. Crossties were made straight and cheaper, but traction pads became more sophisticated. Equal circumferential squeeze (Fig. 1) was sacrificed and the difference in bottom of pipe elevation had to be remedied by making the lower track unit adjustable in elevation. But one serious handicap remained in all those crosstie machines: During the transition from unloaded to loaded traction pads, said pads are skidding along the pipe circumference till the final compression is achieved, leaving visible skid marks on the pipe. The dimensions of those skid marks indicate that wear is caused during the pad engagement to the pipe and not by tension forces. The only remedy to this extreme wear and tear problem is a wheel type pad approach to the pipe as it is characteristic for 4-track machines.

Considering all above short comings, it is obvious that only a 4-track machines,  with squeeze forces pointing directly toward  the center of the pipe and with an appropriate and intelligent machine configuration, will have any future in this business. The pad contact capacity mounted on one crosstie (Fig.2) can be easily accommodated on one track of our design (Fig.3). This means that one machine of our 4-track designs  allows double the pipe contact as  a 2-track  machine, which implies that one of our machines replaces two of the conventional 2-track designs.

As can be seen in the crosstie arrangement in Fig.1, small pipe simply cannot be held because of the physical shape of the crossties and the limitation of the squeeze forces (Figure 2). Therefore a special small diameter 4-track tension machine has to be acquired to be able to handle  all product diameters, whereas our machine can hold any pipe size  from 3”  to 60” diameter. Consequently, one of our machines  replaces three conventional designs, two crosstie machines and one 4-track machine for small pipe. Our machine has an open top, which is important for lay down in S-laying. Big windows  in  the tension banks  allow  squeeze elements to be changed for a multitude of squeeze  configurations, i.e. rubber springs, hydraulic cylinders,  solid squeeze supports for low friction factors , or  combinations thereof. Any shape of traction pads can be adapted, from wide big pipe pads to narrow steel pads for 3” steel cable. Other unique features of our 4-track machines are noted in the following:

• We employ Diesel-hydraulic power with either a combined hydrostatic counter balancing propulsion or electric propulsion with payout pipe speeds 3 times the in haul speed.
• We prefer PD controller over PID for automatic controls since PD allows for a nearly perfect fail safe system without operator attendance.
• Worldwide patents are held on this machine.

Severe  pad abrasion on 2-track machines. In the following picture (Fig.4) a well known tension machine  manufacturer gives us involuntarily  a textbook demonstration on the excessive traction pad wear and tear on a 2-track machine.  During this particular test in Fig 4, two machines were arranged back to back to pull against each other in slow speed and unknown tension. The total pipe movement during this trial might have been  equivalent  to  100 yards of actually  laid pipe line. So it is easy to judge the pad consumption in real action by referring to the skid marks  on the concreted pipe. (Fig.4) Nowadays pads are mostly made from polyurethane etc. and are slyly colored to make abrasion particles invisible on concrete pipes, but as experience suggests,  wear is still the same or worse.

Here is why:On 2-track machines only some 70% or less of the overall track length are holding the pipe. The rest of the track length is wasted due to the unfortunate design. On either end of the machines, the tracks carrying crossties  with 2 pads in “V” arrangement ( Fig.1),  approach or leave the moving pipe on a sloped path. During the pad-to-pipe engagement  those “V” arranged pads are wedged perpendicular onto the pipe. Throughout the pad compression period till  final squeeze load   pads are rubbing  tangentially along the circumference of the concrete pipe causing a tremendous pad abrasion. This handicap is true for all 2-track machines. For better understanding an equivalent wheel type configuration to a 2-track machine (Fig.5) and to a 4-track machine (Fig.6) is shown below.