Steel Catenary Riser (SCR) Studies

In ultra-deep water, beyond 2,000 m (6,600 ft), riser systems become increasingly technically challenging and comprise a major part of the overall field development costs. Large external pressures and high production temperatures in these great depths cause traditional flexible solutions to run into weight, temperature and cost problems. Steel pipes do not have these temperature limits.

The great depths enable steel pipe configurations to remain within acceptable bending tolerances when following floater movements and thus makes them suitable for deepwater riser use. The FPSO with its large displacement is ideally suited to carry a large number of such deepwater SCRs. SCR bending fatigue concerns in this use have been addressed and shown not to be a problem in moderate environments, and deepwater SCRs are to be deployed in 2009 on the 'Normand Installer' with the J-Lay system for SCR installation Company’s leased FPSO Espirito Santo.

Traditional installation of SCRs is done with welded pipe using expensive S or J-lay vessels. The normal weld procedures used with these vessels limit the pipe steel strength. In deep to ultra-deep water with high pressure / high temperature wells this pipe strength limits the use of these welded SCRs.  Internal R&D studies have been focusing on means to reduce the expensive offshore time required for SCR laying and also increase the pipe strength that can be used. This will be accomplished with the use of fatigue resistant threaded mechanical connectors to connect the SCR joints rather than with the traditional use of field welded joints. A qualification program for this type of connector was started in 2007. Twelve of these connectors were built in 2008, and have undergone a series of fatigue and strength tests per the ISO/DIS 21329 standard, test reports are available.  The connector is now available in sizes ranging between 8 and 20 inch, a larger size connector is possible.

A separate program is developing a high speed, efficient J-lay system for the installation of these mechanically connected SCRs. This lay system will be modular and able to be used on vessels such as the Company’s ‘Normand Installer’, or cantilevered over the side of a deepwater FPSO. When the SCRs are J-layed from the FPSO, anchor handling type tugs will be used to pull out the SCR to subsea wells or manifolds.

The above described threaded pipe installation method is also suitable for use in the installation of mid-water Oil Offloading Lines (OOLs). These large OOLs are suspended mid-water between an FPSO and export buoy and suffer bending fatigue due to the wave induced floater movements.  The use of the mechanical connectors in the OOLs improves their fatigue life and allows for the use of larger diameter lines thus reducing oil export pumping costs.

SCR analytical studies have been completed for the development of clamp on, buoyant and non-buoyant, added mass and damping devices. These devices are used to make slight SCR configuration changes, which greatly reduce dynamic bending response at the SCR touchdown point.

This development shows significant improvement of the SCR fatigue life and model tests will be carried out to confirm these findings in 2009. Additionally, analytic studies of low stress steel flex-joints for SCRs suitable for high pressure, high temperature and sour (H2S) service were completed in 2008. A prototype will be built of this steel flex-joint for confirmatory testing during 2009.

The objective of the various SCR developments described above is to enable the use of SCRs on semi-submersibles and FPSOs for service conditions found in virtually any deep to ultra-deep development area.  To further enhance understanding of riser response, the Company continues its research into Vortex Induced Vibrations (VIV) of risers and tendons.

This work continues to deliver better solutions for suppression of VIV loads and motions in deepwater, high current areas. Recent work includes development and testing of a new and improved fairing with a fairing/strake supplier, and continual update of the MIT/SBM-Atlantia Shear7 software for the more than 40 industry users that currently subscribe to this industry standard software.