When the prototype testing was done for the new ram tensioner, the company carried out factory acceptance tests, which took place early this year.

The track record of ram-style tensioners is well on its way to being established in deepwater applications. Direct-acting hanging tensioners have been used in a range of applications for floating production systems worldwide.

A newly patented ram-style tensioner introduced by DTI is changing the way the industry thinks about motion compensation, particularly in the US Gulf of Mexico (GoM) and offshore Brazil, where operators have to contend with ultra-deep water and high-pressure/high-temperature reservoirs.

Pushing the limits

According to David Trent, president of DTI and the developer of a number of tensioner designs, the new patented design is a push-up tensioner that comprises off-the-shelf components arranged in a way that has been seen in the industry in the past. The impetus for designing the DTI ram tensioner was not only the need to make a better design, Trent said, so much as there was a need for another product in the market.

“There are a lot of similarities to crown mounted compensators,” Trent explained. “I wanted to keep it simple and inexpensive. The cylinders and accumulators are standard. I just arranged them differently. I’ve tried to stick with accumulator and barrel material sizes we can get off the shelf so they will be easy to procure, service, and maintain.”

Part of the objective in this design approach was to simplify repair. “If repairs need to be made in the field, they can be carried out without a lot of difficulty,” Trent said. “Part of the ease of repair is a result of selecting off-the-shelf components. For example, the bottles are light as possible, and the design is simple. It does not include a lot of components that can wear out or break in the field.”

Ram style systems have become the operator’s preference, particularly for spars that are working in extremely deepwater environments.

The tensioner takes the place of the air can, Trent explained. “If operators want to take spars into deeper waters, they won’t be able to do that with air cans. In deeper waters, the air cans become so enormous that it is almost like constructing another hull. Building and handling them is just not economical. That makes the ram tensioner a more viable solution for a deepwater spar.”

Ram tensioners are more cost-effective than air cans as well. According to Trent, the technology will provide the means not only for spars to reach greater depths but for semis to go deeper as well. “Production vessels that have longer stroke issues will be able to resolve them with this ram-style system.”

Another of the advantages of the ram tensioner is that it reduces weight.

Most tensioner systems are installed on the production or drilling deck, which means the deck has to be able to support the systems. The new DTI tensioner, on the other hand, is installed onto the hull with a conductor system, so the deck does not have to be designed to support the tensioners. “This construction is unique in that the load is actually transferred to the hull, which lowers the center of gravity,” Trent said.

Following Hurricane Katrina, design criteria changed for the GoM. “We had to deal with greater wave impact,” Trent explained, “which means the wave could come up as high as 19 ft (6 m) on some installments, requiring us to ensure that the connection is solid where the ram tensioner mounts to the hull.”

The DTI ram tensioner has guide posts that span between the decks so that all of the lateral support needed by the tension deck is provided by the guides. This allows the cylinders to act only as members in compression that do not have to take any side loads. “That will extend the life of the cylinders,” Trent said.

Maximizing design

From a design standpoint, the ram tensioner is simple and robust, with a service life of 20 years. “That is typical in this industry,” Trent said, “but until now, there has not been a lot of verification on components.”

DTI uses Inventor software from Auto- Desk to build 3-D components that can be put together in the shop so the design or concept can be tested manually before the unit is machined. “We put the parts together and test the concepts in-house,” Trent said. When the prototype testing was done for the new ram tensioner, the company carried out factory acceptance tests, which took place early this year.

Load testing will be carried out on the units this summer. Testing will involve performing stroke tension cycles and a full load test.

In the course of the test, the unit will be locked into position and loaded. The tensioner will then support the full load while the hot areas—identified by earlier finite element analysis—are evaluated to ensure there is no yielding or cracking in the welds. This will be followed by a stroke tension test under loading to extract stiffness values. Testing will be verified by a third party.

“When load testing is complete, we will have enough information to ensure the ram tensioner is built to support the loads the unit will see in the field, which will satisfy the operator that the unit is ready for installation in the field.”

ATP Oil & Gas Corp. will be the first customer to put DTI’s ram tensioner to work on a drilling and production floater in the GoM in 4Q 2009.

“This is our first ram-style tensioner coming out, our serial number one,” Trent said.

One DTI ram tensioner supports a full riser system, which means there is one required for each well bay. In the ATP application on the MinDoC floating production system, there will be four ram tensioners installed and four more on the Telemark platform.

What is unique about this project is that initially the production system was going to be loaded out with the ram tensioners installed, but the unit could not be righted offshore with the tensioners in place. “Since that wouldn’t work, the tensioners instead will be hanging inside the topsides. When the hull has been righted offshore, the tensioners will lower themselves into place,” Trent said.

Looking ahead

The ram tensioner is one of a number of products designed by DTI. Others include a wireline tensioner system and the industry’s first dual wireline system, which on the 250 k model decreases the overall length by 6.5 ft (2 m) and weighs only 42,000 lbs.

Innovations are always needed in the industry, Trent said, because as E&P activity extends into new frontiers, the market needs options. “The whole point of taking this to the next level is learning something new,” Trent said.