Riser considerations for an ultra deepwater FDPSO

TheAzurite FDPSO arrived offshore Congo in March for charter to Murphy Oil Corp. First production is expected late in the second quarter of this year. Photo courtesy of Prosafe Production.

When an FDPSO produces from two to four pre-drilled wells and continues drilling, it reduces capital expenditure prior to first oil – especially if leased rather than owned. Early production and sequential tie-in of extra producing wells should allow rapid growth of income, to improve rate of return and project value. Satellite well drilling, tie-in and workover costs escalate rapidly as depth increases beyond 1,500 m (4,291 ft), so that the economic advantages of a stand-alone FDPSO are likely to become more important in ultra deepwater – especially on smaller, remote fields. Later in field life, it may be possible to remove some drilling equipment to add extra production facilities.

An FPU with direct access to local wells needs a riser system that is held clear of the drilling or workover riser. Although the flowline plus riser length of any riser configuration will be much shorter than those needed from satellite wells, it is beneficial for the flowpath to be as short as possible to reduce cost and to minimize wax and hydrate problems. Following are the options for risers from local cluster wells to an ultra deepwater FDPSO.

SCRs

A number of deepwater developments have steel catenary risers (SCRs) from remote wells, and the natural “minimum length” shape for a flowline approaching an FPU is the J-catenary. The SCR has been used from local wells, but an outgoing length of pipe is required across the seabed before turning through 180° to approach the J-catenary in a similar way to a line from a remote well. The seabed U-turn can be done using a bend radius that avoids overstressing the pipe, or at a junction with pre-bent spools. Both arrangements take a relatively large area of seabed, and a significant length of line is exposed to lowest ambient water temperature.

If there is a need to remove the FPU, multiple SCRs are difficult to disconnect and lay down on the seabed. Each SCR is likely to apply a large hanging load to its support at the FPU.

Risers with integral buoyancy

An article entitled “Lazy-S risers offer advantages in the ultra deep,” published inOffshore magazine September 2008, described three types of riser that have integral buoyancy:

• Lazy- or pliant-wave, with a series of small buoyancy collars around the overbend
• Free-standing multi- or single-line tower, with a large buoy at the top of the steel column
• Lazy-S, with a tethered buoy between the lower J-catenaries and the upper U-catenaries.

Wave-type risers

For a “wave” configuration, a flowline from local wells needs to reverse its direction so that the buoyant overbend is directed towards the FPU.

A clump weight often is used near the touchdown point with wave-type risers to stabilize touchdown position on seabed. Wave configurations then are called “pliant” rather than “lazy.” Wave configurations have the buoyancy set relatively low down, with a significantly larger portion of riser weight applied to the FPU.

Tower risers

Free-standing multi- or single-line tower risers need anchored bases, lateral spools probably more than 200 m (656 ft) long from the wells, and short connecting spools between the lateral spool ends and the bottom of the tower. A flexible U-catenary “jumper” is used to connect each flowpath from the buoyant top of the tower to the FPU. Lateral current may cause a large “leaning” deflection, and/or oscillations of the buoyancy tank and tower called vortex-induced vibration (VIV). Anti-VIV strakes increase current drag and overall deflection from vertical. For multiple single-line tower risers, 2H and Subsea 7 introduced the Grouped SLOR (single line offset risers) concept to overcome problems of widespread seabed connections, and buoyancy tank and mooring line spacing. It may be possible to control lateral deflections of Grouped SLORs in current by installing guy lines to the upper riser guide frame. Tower risers usually do not include umbilicals, which have been arranged separately in either lazy-S or lazy-wave configurations.

Lazy-S risers

The Deepwater Lazy-S Riser (DLSR) has a tethered buoyant Deepwater Riser Support (DRS) between lower J-catenaries and upper U-catenaries. The DRS with TLP-type tendons can be set well below the wave and high current zone. For an FDPSO or a semi FPU with local wells, it is convenient to reverse the J-catenaries beneath a long beam-type mid-water buoy.

The reverse lazy-S or J-S configuration begins with J-catenary pipelay between the tethers, a pull-in connection to a cluster well or manifold, and ends with hang-off at the tethered buoyant beam. If needed, 20 or more hangers can be spaced along the beam to support the J-catenaries; which can be flexpipes, Integrated production bundles (IPBs), umbilicals, or SCRs. IPBs from Technip Flexi France - as used for the Dalia and Pazflor oil developments off Angola - have integral temperature monitoring, heating, and gas lift tubes in the annulus around the main pipe. This complex bundle has relatively large bending radius and a heavy end fitting, but these properties are easily catered for in the almost static J-catenary and beam hang-off.

A second DRS-type mid-water support can be installed later to allow tie-in of more local or remote wells. Standard flexibles would be used for the U-catenary jumpers to the FPU. Connections at the FPU can be arranged along the sides in a line, or if necessary to allow +/-90° weathervaning, can be to outrigger turrets – as used with the DP FPSOMunin at Xijiang field in 2004.

DRS installation by drill rig, DSV

The tubular steel tethers needed to moor the DRS can be installed by the drilling rig of a drilling vessel, or by the rig onboard the FDPSO. Buoyancy tanks to keep the tethers close to vertical would be sized to pass through the rig moonpool, but tanks around 5 m (16 ft) diameter x 10 m (33 ft) long should be possible.

A DSV (or riser installation vessel) would tow the DRS with integral main buoyancy tanks to the site, then lower and align the ends to the tethers (possibly using guide wires and posts) before final lowering and connection. Vertical stub ends to the beam could compensate for any difference in length between the two tethers, or TLP-type adjustable gripping devices could be used for the connections.

After de-ballasting the main buoyancy tanks and adding guy lines for lateral stability, the smaller buoyancy tanks on the tethers can be removed for re-use. Riser installation and hang-off can begin, and the FDPSO can assist with pull-in connections of J-catenaries to seabed equipment.

FPU removal

Reasons to remove an FPU (FPSO, FDPSO, or semi) from its offshore field location include:

• Extreme storm or large iceberg avoidance
• Topside repairs after severe damage by fire or explosion
• Major topside modifications (de-bottlenecking or satellite field tie-in)
• Hull repairs
• Poor reservoir performance
• Political upheaval (nationalization threat)
• Terrorist threat.

After disconnect from any type of FPU, the DLSR configuration provides controlled support for the complete riser-plus-umbilical system. As no seabed laydown is required, disconnect and re-connect operations can be done with a relatively small support vessel. The DLSR allows pre-installation and hang-off of the major part of riser weight, or complete pre-installation to a disconnectable FPU buoy.

About the author

Keith Shotbolt gained 32 years experience in oil and gas projects and is now an independent consultant. Questions/comments on this article can be addressed to[email protected]