A specially designed rig tested the bearings used in the Trunnion-Latch.

As production systems push today’s limits, riser technology has to keep pace. Equipment that was suitable in more shallow applications and in benign environments is not necessarily deployable under the conditions that define tomorrow’s operations. In addition to environmental issues, some regions of the world impose exacting standards for operation. One of those areas is the northern North Sea.

Breaking new ground

In an ongoing project in the North Sea, Venture Production needed to employ a high-pressure riser with full-bore access (183?4-in. internal diameter) capable of withstanding a 12,000 psi pressure. The riser, currently being manufactured by Subsea Riser Products (SRP) and due for delivery later this year, will enable Venture to drill and complete high-pressure/ high-temperature subsea wells in its central North Sea assets from a jackup rig equipped with a surface blowout preventer.

The biggest challenge posed by the project has been to devise a way of joining the various sections of the riser, which are necessarily fabricated from thick-walled, high-grade (80,000-psi yield strength) steel. Welding normal flange connections to the pipes is not a practical option for this kind of material. Instead, the engineering team at SRP has chosen to shrink fit the flanges to the pipe sections. According to the company, it is the first time that this thermal process has been used in the riser industry.

Shrink fitting the flanges relies on creating a mechanical interference fit between the two components. Qualifying the technique for use on risers required an exhaustive development and testing program before the project could be sanctioned.

According to Simon Luffrum, SRP managing director, although the process is straightforward, there were several design challenges that had to be overcome before the company could guarantee repeatable performance to the level demanded by the offshore industry. Manufacturing also has to be tightly controlled. Great care has to be taken to achieve the correct component profiles and surface finishes, and the heating of the flange body, which is done using induction coils, requires close monitoring.

Another area where shrink-fitting offers advantages, Luffrum said, is in the manufacture of tapered stress joints, which are normally forged in one piece, including the flanges. Shrink-fitting enables the tapered pipes to be forged without flanges, since these can be added easily afterwards without having to resort to welding. This approach reduces the complexity of the forging and means that smaller starting ingots can be used. Also, through the opportunity to use higher-strength steels, the stress joints can be made shorter or can be designed to withstand higher riser loadings.

Component innovation

In another development, the company has invested in R&D that has led to a new system for connecting free-standing risers to their seabed foundations and also to the air cans that provide the system buoyancy. The Trunnion-Latch is a simple, compact, and easy-to-manufacture alternative to the more sophisticated and expensive rotational latches often used for this purpose.

The unit combines a latch comprising conventional spring-loaded dogs with a rugged universal joint made up of two axial trunnion bearings. Even though it uses existing technology and the bearings are widely used elsewhere offshore — notably in mooring systems — potential users have called for thorough testing before considering it for use in critical riser applications. To date, the latch has come through extensive full-scale mechanical trials without incident; the bearings are still being assessed using a large-scale rig specially built by SRP.

The rig is designed to evaluate the wear suffered by the bearing’s shaft, or pin, within the bearing housing, which has a self-lubricating (polytetrafluoroethylene, a.k.a. PTFE), aluminum-bronze liner. Rather than the pin rotating completely in the housing, which is normal in bearing wear tests, the test being carried out by SRP requires the pin to make slow oscillatory movements through various small angles under loads of up to 700 metric tons. By examining the pin after testing, the engineering team has been able to calculate wear coefficients and from these to predict the fatigue life of the bearings.

Results obtained so far have been revealing.

“We have used our knowledge of riser structural response behavior to define a test program that faithfully reproduces the sort of conditions the bearing will be subjected to in real life. And the wear that we are seeing is not exactly in line with what we would expect on the basis of the industry literature,” Luffrum said. “We are using the findings to optimize the design of the Trunnion-Latch. Furthermore, once the testing is complete, the data will have an impact on other existing trunnion-bearing applications in the industry where long-term reliability is a key issue.”

Another area of focus is the marine drilling riser market — specifically the fast make-up connector business. In recent times, this area has been dominated by the Clip Riser, designed by IFP (Institut Français du Pétrole). The Clip Riser uses a breeched union nut to connect the individual riser sections. Requiring far less time to make up than a traditional bolted flange connection, the Clip Riser significantly reduces the time taken to run a drilling riser in deep water. However, many flanged risers are still being used by the industry, and they do have the benefit of rugged simplicity.

Recognizing the need for a quick connector for flanged risers, SRP set out to design a rapid flange connector — essentially a better nut and bolt — to speed the running of these traditional joints.

The resulting Quick-Flange concept is based on the use of breached bolts that can be locked in a fraction of a turn, which considerably reduces the time required to connect individual riser sections, making the whole exercise much less arduous than at present. The company already has done a good deal of development work involving finite element analysis to optimize the design of the bolts and is undertaking a thorough test program before it launches the product.