Steel catenary risers (SCRs) have been the preferred risers for deep and ultra-deep waters on account of their simplicity and lower cost. Over 275 have been installed globally in the past twenty years. Recently, with more extreme production conditions and a trend towards tiebacks to older floaters, lazy wave SCRs are gaining popularity due to their reduced payload on the host vessel and their ability to control strength and dynamic response along the riser. They do, however, come with their own set of design challenges:
1. Shape optimization
2. Potential for interference
3. Fatigue performance
4. Installation in shallow waters
In this article, we will explain how to address them.
Optimising the shape of a lazy wave riser
Compared to the “simple” shape of a regular SCR, the shape of a lazy wave riser is a combination of top angle, sag bend elevation, arch bend elevation and buoyancy length from the hang off point (shown below).
There are countless combinations of the four parameters that form the shape of the riser. Although numerous shapes will work in terms of response during extreme storms and fatigue sea state conditions, the shape is typically optimised to reduce vessel payload (especially when the riser is water filled), the quantity of buoyancy modules, lateral motions and vertical distance between the arch and the sag bend, as much as possible. Strength and fatigue analyses can be used to give you an idea of how these will be affected by a lazy wave shape. There is no mathematical solution or direct equation that will help determine the optimal riser shape. The shape is generally determined by a combination of engineering judgement, past experience and trial and error. This process usually requires multiple iterations before zeroing in on the best configuration.
Reducing the potential for riser clashing
One of the key benefits of a lazy wave riser, its reduced weight on the vessel, is also the cause for a major design challenge; its susceptibility to clash with adjacent structures, such as other risers, moorings, umbilicals or tendons. For a new development, this is relatively straightforward to address during design and can be mitigated by staggering the top angles of adjacent lines and spreading them far enough apart. An early interference analysis will also confirm whether the selected layout may result in a clash. For a tieback to an existing facility, however, a number of issues affect the riser configuration selection. Adding more lines to those already present will result in a lot of congestion and vastly increase the potential of clashing. Lazy wave risers are an attractive solution for tie-backs which need lighter additional payload, but careful attention to the routing helps to keep them away from adjacent lines by using steeper top angles and lower sag and arch bends. These characteristics are not always desirable from strength and fatigue perspectives. Additional hardware, like tethers, can also be used close to the touchdown area to reduce lateral motion and risk of clashing.
Improving fatigue performance
Lazy wave SCRs in deep water environments, for the most part, have excellent strength and fatigue response. For lazy wave risers installed in shallower water depths (~1000m), fatigue becomes an issue. The source of fatigue is primarily from wave induced vessel motion due to low hydrodynamic damping, which can be amplified by the presence of a sour production fluid. High-quality welding is typically required for all catenary risers, whether a ‘simple’ catenary or lazy wave, and a rigorous welding qualification program during the fabrication phase of a project will help with fatigue issues. Other fatigue improvements include the use of upset end joints, and internally cladding fatigue critical sections of the riser with a corrosion resistant alloy material. Tethering the riser close to the touchdown zone is also a good way of lowering fatigue damage in that region due to reduced motions. This has not been used for steel risers, but it has been used successfully for flexible risers.
Installing a lazy wave SCR in shallow water
Installation of lazy wave SCRs in shallower waters can be tricky. SCRs with buoyancy modules are often installed by J-lay method, but in shallower waters, it can overstress the pipe. If the pipe diameter allows, the S-lay installation method may be more suitable. If installation is going to be conducted using the S-lay method, the buoyancy modules will need to be designed to accommodate the high local bearing loads when they go over the stinger of the installation vessel. They can also be attached to the riser after it is exited the stinger, which, whilst feasible, does have an impact on installation times and increases risk. Installation method also has an impact on the lazy wave shape. The heavier the riser, i.e. fewer buoyancy modules, the easier the installation. This does, however, adversely affect the riser’s performance when it’s in service. The riser could be installed water-filled in order to accommodate the buoyancy effect during installation, but this is not favoured by installation contractors as it puts an excessive burden on the installation vessel. There needs to be a balance between installation requirements and in-service requirements with regard to buoyancy length selection.
Despite the challenges mentioned above, lazy wave risers are becoming more accepted and are quite attractive given the benefits they offer. As long as you are mindful of the issues they pose, and address them early in the design phase, you can reduce the possibility of surprises in later stages of development.
Madhu Hariharan, Senior Principal Engineer, Houston
Madhu has over 16 years’ experience in deep water riser systems. He specialises in design and component FEA, fabrication and installation verification, project management, field monitoring, offshore inspections and integrity management of all types of risers, including flexible and steel catenary production and export risers, drilling, completion and workover risers and top tensioned risers.