Tapered stress joint

3 Things to Consider When Using Tapered Stress Joints on a Tieback

Tiebacks are very common when it comes to new offshore developments, substantially lowering CapEx by using existing infrastructure. Many tiebacks use steel catenary risers to connect the tieback flowline to the host facility. There are a couple of well-established options when it comes to securing the new SCR to an existing vessel, including steel or titanium tapered stress joints and flexible joints. Both have advantages and disadvantages with respect to cost, manufacturing lead time, long-term performance, material compatibility, internal fluid characteristics and interface loads to the host vessel. In this article, we will discuss what to keep in mind if you decide to use a tapered stress joint to connect the SCR to the facility.

1. Stress Joint Sizing

Steel catenary and lazy wave risers produce high bending moments at the vessel attachment point which can be accommodated by a special joint with a wall thickness that gradually increases towards the attachment point. The “tapered stress joint”, or TSJ in short, geometry including overall length, wall thickness and taper profile (single, double, hyperbolic) is heavily dependent on the global vessel motions and riser structural response. In riser analysis work, the TSJ model needs to be included with a fine mesh in the riser global FEA to verify its strength and fatigue performance as well as optimize its geometry (the number of tapers, taper profiles and lengths). Titanium forgings can be used if the steel TSJ length/weight exceeds the manufacturing limits of a single steel forging, or when the interface forces with the vessel are too high for the existing structure. Both of which are common issues with steel TSJs for larger diameter risers.

3D Model of Vessel Hull, Porch, Basket, Adaptor and TSJ
3D Model of Vessel Hull, Porch, Basket, Adaptor & TSJ

2. Vessel Porch Structure Design

Another critical element that needs to be checked and proven to be adequate for the new tieback is the vessel porch structure. This was likely designed years, if not decades, earlier and possibly for flexible joints which tend to impose milder forces on the vessel. The vessel porch structure may also belong to another entity, such as the vessel operator. In such cases, as the owner of the tieback, you would have to prove that the new tieback does not jeopardize the overall structural integrity of the vessel. Reinforcement of the porch structure may need to be implemented to meet strength and fatigue requirements.

FEA of vessel porch
Stress Hot Spots on Vessel Porch

3. Adaptor Bushing Design

Once the tapered stress joint is sized appropriately and it has been verified that the vessel porch structure has sufficient structural capacity, what’s next? It is time to mate the TSJ and the vessel porch structure together. The riser requires a certain hang-off angle relative to vertical and azimuth angle based on the riser performance and subsea pipeline routing for the new field. An adaptor bushing needs to be designed and inserted to the vessel porch structure to orient the riser in the desired direction and to provide a resting surface for the TSJ bushing.  The design and fabrication of the adaptor bushing is intricate as it should fit tightly to the existing vessel structure, consider the fabrication tolerances and have a proper conic cut out with dual inclinations to simultaneously account for riser hang-off and azimuth angles.

Adaptor Bushing with Dual Inclination Conic Cut-out
Adaptor Bushing with Dual Inclination Conic Cut-out

Summary

Tiebacks to existing floating infrastructure is a common solution for new offshore developments if there is a host vessel nearby with adequate capacity. Tiebacks are a lower CapEx investment and the time to first oil is shorter, in comparison with a stand-alone development. Tapered stress joints offer an effective solution to attach steel catenary risers to existing facilities, however, the design of the TSJ is a complex process involving both global and local FEA in order to properly size the TSJ, the adaptor bushing and to verify the structural integrity of the vessel porch structure.

2H Offshore has designed over 25 TSJs that have been installed offshore and has completed detailed FEA of the vessel porch structure and adaptor bushings for many tiebacks. We have also managed the fabrication and delivery of TSJs and adaptor bushings. If you need help with your vessel tieback, please get in touch with us at houston@2hoffshore.com.

Yusuf Arikan

Yusuf Arikan, Principal Engineer, Houston

Yusuf has 15 years’ oil and gas experience with in-depth knowledge of the analysis, design and project management of subsea flowlines, jumpers, SCRs & TTRs. He has particular expertise in flowline and riser components such as subsea manifolds, PLETs, ILSs, VIV suppression devices, buoyancy modules, steel and titanium stress joints, and tensioners.