Big Size Bearings Boost Offshore Pipe Laying

SKF has supplied a huge order to Norwegian offshore equipment manufacturer National Oilwell Varco (NOV), amounting to more than 28 tonnes of bearings in three different types. The big size of these unique bearings boost off-shore pipe laying.

Equipment used in the offshore industry is so expensive, sometimes costing hundreds of millions of dollars or Euros, that it must last for many decades. If market conditions dictate that equipment must start to run more efficiently (in order to extract more oil, for instance), the challenge is on to enable this. A common approach is to replace major bearings within the equipment, allowing it to handle more rigorous conditions such as higher loads or faster speeds. Recently, SKF helped Norwegian offshore equipment manufacturer NOV achieve just that by supplying it with a unique set of bearings.
Among other things, NOV builds handling and pipe-laying equipment for offshore vessels. In a recent upgrade, it needed to ensure that the bearings on two cable drums could withstand the tough conditions of unspooling pipe into the sea from a pipe-laying vessel.

Challenging conditions: enormous loads and very low speeds

Conditions on this kind of vessel are hugely challenging, as the bearings are under constant loading, both radially (from the load itself) and axially (from the continuous motion of the ship on the sea). The application requires very complex engineering because of the uncertainty from the micro motions of the ship. The challenge for the bearings had nothing to do with speed, as the drums turn at around one-third of a revolution per minute. Instead, the difficulty was to design bearings that could take enormous loads while moving at such slow rotation speeds. As well as achieving this, SKF managed to design bearings in standard ISO dimensions, meaning they can be found within its catalogue. Strange as it may seem, these are standard bearings, the largest ISO-sized bearings that SKF has ever supplied.

The scope of supply comprised bearings in three different types

The order comprised six separate bearings in three different types: spherical roller bearings (SRBs); spherical roller thrust bearings (SRTBs); and self-aligning CARB bearings. The bearings were used on two different cable drums, one large and one small.
The large drum used four bearings, in an alternating SRTB/SRB/SRTB/SRB arrangement, which were mounted onto the main shaft. This in itself is a unique arrangement. The smaller drum used two bearings, an SRB, and a CARB. The CARB helped to correct axial and radial misalignment.
Overall, the total weight of the bearings was 28 tonnes, while the largest single bearing weighed eight tonnes.

Expertise in the field of huge dimension bearings

While this is a unique order, it is not the first time that SKF has supplied bearings of such huge dimensions: just two years ago it supplied a three-tonne sealed SRB to a copper mine in Peru. Here, the sealed bearing was fitted to a horizontal shaft on a roller press, which operates in very dusty conditions.