The bottom line advantage of Subsea Tensioners
Subsea tensioners are different in design than standard, topside tensioners. That may seem like an obvious statement. However, real differences have been engineered into the subsea tensioners with the operational environment in mind. Meaning, subsea tensioners are designed for working at depth on the seabed – this situation is very different than working topside. If you look at a subsea tensioner like the STU4-187 which is for 1-⅞” bolt you will find it has a little less capacity than a topside tensioner for the same size bolt. The topside tensioner you would use for the same size bolt would be something like the STS3-187B1. The STU4 has a capacity of 85.0 tons and the STS3 model has a capacity of 92.4 tons. However, the differences designed for the subsea are a bit easier to spot.
First the STU (Subsea Tensioning Unit) model subsea tensioners have built in lifting rings. This makes it much easier for them to be secured in place. They might be secured in a basket which is being lowered to the seabed or they might be secured on the flange itself. It will depend greatly on the task at hand. Since divers do not have the same leverage subsea as they would topside, it is important that their tools are easy to use. For example, they could be standing on a soft bottom, swimming or in an awkward position. As a result, these built-in rings can be very handy.
Subsea tensioners also have “Quick Fit Pullers''. These take the place of the standard threaded pullers. They are split into halves and lock when squeezed together. Then, springs inside push the two halves apart again when a release is pressed. This main function of this design is so that the divers can eliminate the time wasted by threading standard pullers onto bolts. Instead, the divers leave the two halves separated and slide it down over the bolt. When the puller gets to the right spot, the diver squeezes the two puller halves together on the bolt and gives it a single turn or two in order to make it snug against the tensioner.
Another difference between subsea and topside tensioners is that the subsea tensioners use holes drilled into the nuts whereas topside tensioners use nut rotating sockets. They both still use tommy bars but this is another design with the diver in mind. By not needing nut rotating sockets, the divers have fewer components to handle which will again make the job easier. Companies like Direct Bolt and Supply have the capability to drill all sizes of nuts.
Finally, the subsea tensioners also have more stroke length than the standard topside tensioners. This is designed with the divers and their valuable “bottom time” in mind. With longer strokes, the tensioners will require fewer actuations and will save time when making up the flanges.
Attached are pictures of a pipeline installation where the subsea tensioners were utilized in the “firing line” of a pipelay barge. In this situation, topside tensioners could have been used on board the boat. However, the decision was made for the subsea tensioners to be used during the pipeline installation and used for the “tie in” work as well. What this means is that once the pipeline was installed on the seabed from the barge, the subsea tensioners would be taken from the firing line and given to the divers. The divers would then use those same tensioners to install the spool pieces and make the final connections on platform risers. The real advantage to this type of tie-in is to get the Pipelay barge “off the clock” and perform the tasks with divers and a diver support vessel or platform support. The pipelay barge cost is the largest expense related to an installation and should be minimized. The only additional piece of equipment needed to move from working on the pipelay barge to subsea was a longer main line, high pressure hose from the pump to the first tensioner. The addition of a hose reel like the XHR1 made the organization of that hose much easier. The client was able to reduce overall project cost substantially while maintaining schedule.