As a major offshore subsea umbilicals, risers and flowlines (SURF) contractor Stolt Offshore has interests in all types of offshore riser systems. It has acquired unique knowledge and experience in the delivery of hybrid riser towers (HRT), flexible risers and steel catenary risers (SCRs).

Each of these riser concepts has its positive and negative aspects for any given project. In ultradeepwater situations SCR and HRT solutions are main contenders. The company has built upon experience gained in the design, build and installation of three HRTs and has developed a new generation tower system; the Hyperflow Riser Tower system.

The hyperflow tower system consists of a vertical column made of one or several pipes used for transporting the fluids or providing some mechanical stiffness to the structure or both. The vertical column is tensioned by means of a top buoyancy structure or distributed buoyancy elements along the column or both. The interface with the seabed is static and consists of an anchor base onto which the riser column connects by means of a mechanical device designed to absorb the dynamic inclination motion of the tower - either a taper joint or flex joint. At seabed level vertical risers and horizontal flowlines are interconnected by means of spools and remotely installed API flanges or proprietary connectors. The interface with surface process facilities is made through short flexible dynamic jumpers.

Suspended

An SCR is a single pipe suspended from the surface support facilities in a catenary configuration and it can be laid down to the seabed where it can either continue directly into the horizontal flowline or it can be connected mechanically. Interface with the floating production unit (FPU) consists of a hang off structure and a flex or taper joint to absorb the dynamic moment variations generated by the motions of an FPU. SCRs are sensitive to dynamic motion of the surface facility and this interface is critical in the design of SCR solutions and the host facility itself. An HRT, however, is self supporting and this is a major benefit of the technology as this independence from the host facility removes large vertical loadings and horizontal motions, enabling a cost saving to be made in the host facility structure and mooring.

Fatigue

The hyperflow system is subject to little fatigue damage when in operation. The fatigue consideration is mainly associated with the surface towing operation and the Girassol experience - offshore Angola - has proved that in-house numerical models, developed to assess this fatigue, are reliable and accurate. Fatigue considerations consequently impose little constraint on the fabrication process.
On the other hand, SCR fatigue damage dominates all aspects of the riser integrity, dictating the choice of material for the riser structure and driving the high quality welding requirements for the fabrication process, which will ultimately take place offshore in a pressure environment. Fatigue damage is driven by the dynamic response of the SCR, where the hydrodynamic loads from waves and current, combined with the complex stresses induced from vortex induced vibration (VIV), produce significant fatigue loading. The fatigue sensitive areas of the SCR are the upper section where the riser is connected to an FPU, and the lower touch down point where the riser contacts the seabed. In addition, depending on the submerged weight of the coated pipe and support surface conditions, the dynamic response of SCRs may induce compression loads at the sag bend, which are not considered by design codes.

Seabed

With regard to the interfaces at the seabed, the HRT tie-in spools require remote installation between a hyperflow tower base and flowline. This is now a well-proven procedure. The final riser system is in essence static, reducing risk of issues from riser with seabed. Generally speaking, a riser tower solution leads to a simpler field layout compared to an SCR option.

Interface between an SCR and the seabed, however, may raise some concerns since the consequences of the technical issues associated with touch down point (TDP) excursion and embedment/trenching are not yet fully understood in the long term. In addition, phenomena such as longitudinal ratchet motion of the flowline due to cyclical thermal expansion can induce loads on the SCR, which are not acceptable. This has resulted in the use of anchors to secure the long-term stability of the riser and flowline on the seabed.

Flow

Flow management techniques such as gas lift, heating systems, methanol injection and other such systems can be readily incorporated into an HRT system , but these are more complex to address with SCRs and can complicate the field architecture. Special devices such as riser monitoring can also be easily accommodated in the new generation Hyperflow system.

Fabrication

With the exception of double or quad jointing, fabrication of SCRs is done offshore from a pipelay vessel. The interface between the SCR components and the installation equipment requires careful consideration and management to ensure that the integrity of the riser is not compromised. Any contact point between the two systems must be analyzed to check compatibility of dimensions, static and dynamic loads and material. The quality of the welding performed offshore must be to the highest standard in order to control the fatigue damage experienced by the SCR. For some SCRs on recent deepwater projects clad pipe and duplex/super duplex material have been specified, and this has required the development of new welding and non-destructive tests (NDT) procedures, which can sometimes prove difficult to control under offshore conditions.

Fabrication of the Hyperflow new riser system is performed onshore at a dedicated fabrication yard. The onshore environment offers good conditions, allowing easier quality control of the work performed especially where exotic materials are required such as clad pipe. Given the relatively straightforward process involved, it is possible to utilize suitable fabrication facilities in the country of the field development, maximizing the benefit to the local workforce and community. Riser towers for Girassol were all built at the Sonomet yard in Lobito, Angola.

Installation

Installation of both SCR and the new riser tower are considered as critical operations and expose the project to a certain amount of risk. In the case of the SCR, the overall installation, which can often be seen as an extension of the pipelay operations, takes longer and is more complex due to the combined fabrication/installation activities and the probability of an undesired event is higher. SCR installation requires the mobilization of very a sophisticated reel lay or J-Lay installation spread. In general SCRs need to be installed once the FPU is in place, and this involves a complex handover operation between the pipelay ship and the FPU, where the pipelay ship is working in close proximity to the FPU. It is possible to consider pre-laying the SCRs. However, this requires a significant amount of space on the seabed, which may not be available within the mooring pattern of the FPU, particularly when a large number of risers is involved.

With the new Hyperflow riser technology, the tower installation can be performed with high standard tugs that are available worldwide. The overall installation scope for the HRT must also consider the installation of a riser anchor base and the spools at the riser foot, which again can be addressed with readily available equipment. The hyperflow solutions can be installed prior to the arrival of any floating facilities at the offshore field development location, and therefore it allows an earlier schedule for first oil.
The three riser towers installed at the Girassol field were all fabricated onshore, towed to the field, and then installed prior to the arrival of the FPU. This is a significant advantage as it allows more flexibility and less risk to the overall project development schedule.

In water depths less than 3,280 ft (1,000 m) the general view is that SCRs will be more competitive than the hyperflow solution. However, as a project increases in complexity the hyperflow option is able to accommodate the complexity in a less costly manner. For example, strict flow assurance criteria; high corrosion or erosion scenarios; and the incorporation of either gas lift or active heating are situations more easily incorporated into the Hyperflow riser tower, with the overall result that it could move the tower solution into a more competitive standing.

As the industry moves to ever-increasing water depths, the increased pressures and top tensions call for significantly larger equipment to handle the required SCRs. The result is a corresponding increase in cost, mainly on installation. Furthermore, as top tension weight increases, it has an adverse economic effect on the host facility that must be designed to address loadings and motions.
Deeper water situations suit the riser tower solution well as its almost static operational situation and the installation method enable diameters to be maintained to much greater depths than an SCR solution within an acceptable commercial model.

The differences between riser towers and SCRs become more pronounced for ultradeepwater field developments.

Based upon development work performed, the hyperflow concept is a riser tower solution that uses proven technology and methods to efficiently address ultradeep riser system requirements.

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