Collaboration on special production riser coupling meets new challenges

The Campos Basin tension-leg wellhead platform (TLWP) project called for a high fatigue-resistant coupling to meet the environmental and system loading conditions of a top tension riser system. To meet flow assurance needs, the riser is coated with 2 in. of thermal insulation. The insulation and the requirement for cathodic protection created several challenges for running the riser and for attaching the earthing strap to each of the submerged riser joint couplings while ensuring electrical continuity.

Dril-Quip has previously developed internal running tools (IRTs) for use with its PR-80 riser couplings along with numerous other marine riser connections. The production riser systems for the TLWP are designed with weld-on and threaded-and-coupled riser joint connections. The use of two types of joint connectors results in a reduced string weight while meeting the high fatigue resistance and loading requirements.

Collaboration leads to new design

Seeing the opportunity to provide a single riser-joint handling tool to run both types of riser connections, Dril-Quip worked with Vallourec to develop the special VAM TTR coupling with an internal handling profile (Figure 1). The major benefits from the single IRT include minimized rig operations and reductions in required backup equipment. As the operator changes between the various riser joints (PR-80 and VAM TTR) and specialty joints, the same running tool is used to eliminate downtime in changing out equipment as well as reducing operator error by maintaining the same functionality in tool makeup.

FIGURE 1. The Dril-Quip riser handling tool is installed in the VAM TTR (top) and PR-80 (bottom) riser couplings. (Source: Vallourec)

Dril-Quip’s IRT is hydraulically operated and functions with an internal piston actuating radial segments into the internal handling grooves of the coupling. The tool is designed with mechanical secondary locks and visual indicators to provide positive tool makeup when fully installed in either the PR-80 or VAM TTR coupling. The PR-80/VAM TTR IRT was successfully load-tested to 2.25 times the tool rating in both couplings to industry requirements.

Based on the design proposed, Vallourec took the challenge of incorporating the new features into its 14-in. VAM TTR connection. This connection is a field-proven design that features an internal metal-to-metal seal, fatigue enhanced threads, fatigue bending profile (swoosh) and external metal-to-metal seal. It was first successfully run 10 years ago in the Gulf of Mexico and since on multiple products.

Analysis, testing

Through finite element analysis (FEA), the coupling design with internal handling grooves was modeled and tested to verify no adverse effects on the tension, compression or fatigue performance of the connection. The FEA simulated the load steps of an ISO13679 CALIII test on the pipe and connection using techniques developed by Vallourec within the elastic-plastic strain methodology. Comparison of the new coupling design to the company’s large library of FEA and physical testing of similar connections showed positive results. The stresses in critical areas of the special coupling are consistent with standard VAM TTR connections.

FIGURE 2. Vallourec VAM TTR riser coupling is shown during fatigue testing in a resonant fatigue load frame. (Source: Vallourec)

Following FEA evaluation, physical testing was completed to further validate the coupling design. Vallourec performed a full ISO13679:2002 CALIII test of the VAM TTR connection in addition to fatigue testing using six samples. The connection test results matched the FEA in tension—and most importantly in compression—as this was the area that could be most affected by the presence of the lifting grooves.

The internal handling profile and the anode-threaded attachment hole presented areas of high stress concentrations, which potentially could limit the fatigue life of the connection. Dril-Quip took this into consideration when creating the shape of the internal handling profile, allowing large radii and smooth transitions. However, the threaded area of the anode-attachment hole with its sharp thread start was still an area of special interest. To mitigate potential concerns, Vallourec subjected the connection to fatigue cycles using the eccentric mass cyclic bending method (Figure 2). In this method the pipe and connection experience a predetermined stress via bending induced by an eccentric mass. The resulting alternating bending motion resembles that of a school grounds jump rope at 20 Hz to 30 Hz.

By subjecting the VAM TTR connection and pipe to the fatigue stress cycles, it was demonstrated that the connection was able to achieve more than 6,000,000 cycles with a resulting stress amplification factor of less than 1.5 using the DNV B1 2005 curve (air). This high level of fatigue performance gave great confidence in the coupling design and allowed manufacturing to move forward.

Manufacture

Since the internal handling profile became an inherent part of the coupling design, Vallourec had to identify the best way to machine the profile using standard manufacturing processes. The most critical point was to machine the IRT handling profile in the correct location within the specified tolerances while maintaining the production efficiency. Typically the shoulder region does not require high tolerances, thus allowing the coupling-face-to-shoulder length to be controlled tightly. The key innovation to success was to design a single direction high-tolerance coupling. For standard couplings both sides A and B are identical; no distinction is made for which side can be made up to the pipe pin thread. However, to accommodate proper IRT makeup, the internal profile was indicated from one coupling side during machining. This “quality control by design” method drastically reduced the reject rate such that the coupling could be manufactured in an economically viable way. To ensure visual identification of the high-tolerance IRT engagement side, a special band was painted on the coupling along with an explicit instruction procedure for coupling to pipe makeup. The first manufacturing order of the pipe and couplings was completed in late 2012.

While the project presented challenges and required new developments, the design synergy provided the greatest benefit to the end user by reducing operational time and risk. The production riser coupling met all performance criteria and was successfully manufactured to meet the project demands.