Different types of risers are used to evaluate the production, water injection and gas export risers:
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Steel Catenary Riser:
Steel Catenary Riser (SCR) is a comprised of steel pipe that is suspended from a host facility in a catenary configuration to the seabed. The pipe itself can be insulated with wet insulation or be housed in a pipe-in-pipe configuration. At the vessel the hang-off can consist of a tapered stress joint or an elastomeric flexible joint to handle the weight and moment. At the touch down point of the SCR the pipe continues for a required on bottom length to a flowline or is terminated in a pipeline end termination skid (PLET). Depending on field layout and on-bottom length requirements the PLET may require anchoring.
Pipe for SCR is supplied as coated or insulated in a pipe yard. Pipe is then loaded on a barge or directly to an installation vessel for welding of the joints to form the riser in the field. For reel lay the pipe is stalked at a reel base and transferred in long lengths to a vessel reel carousel where it is then taken to the field and installed.
Hybrid Riser Tower:
A Hybrid Riser Tower (HRT) consists of number of riser pipes bundled together around a core pipe. On the top of the HRT flexible jumpers pass fluids to the host facility. A buoyancy tank supports the weight of the riser and may have product lines running through it or be externally tethered. At the bottom a flexible joint or flexible jumpers provide connections to flowlines. The HRT is anchored to the seabed with a pile.
The riser bundle is fabricated onshore and to towed to the field. At the field, the riser is up-ended and connected to the seabed. Flexible jumpers, at the top of the tower, are then connected to the floating production facilities. Since the HRT can be installed without a host, these connections can be made after host arrival allowing the decoupling the schedule of host arrival and riser installation. These connections may also be disconnected later if host facility changes or moves.
Single Line Offset Riser:
A Single Line Offset Riser (SLOR) is generally a free standing piece of pipe that is anchored at the bottom to the seabed and supported at the top by a buoyancy can. The top of the riser is 150-300 feet below water surface and connected to the host vessel by a flexible jumper. The jumper connects to a gooseneck assembly on the riser. A buoyancy can supports the weight of the riser. A tapered stress joint (keel) transfers the load from the can to the riser. The riser continues down to an offtake spool and lower stress joint. The lower stress joint allows the load to be transferred to the anchoring system and the offtake spool contains a connector for a jumper to a flowline PLET.
The SLOR can be installed from a pipeline installation vessel in J-lay, S-lay or also from a drilling derrick. The riser and lower assembly are lowered down and connected to the seabed while the buoyancy is attached to support the riser. Since the SLOR can be installed without a host, topside flexible jumper connections can be made after host arrival allowing the decoupling the schedule of host arrival and riser installation. These connections may also be disconnected later if host facility changes or moves.
Flexible riser (FLEX) consist of pipe cross section that is suspended from the host vessel in a catenary configuration, much the same as a SCR. The flexibility of the section allows the riser to hang in a nearly vertical position. The flexible riser can be configured in a way where buoyancy is added to the pipe allowing larger vessel offsets and smaller loads on the riser. At the vessel hang-off the flexible riser can use a bend stiffener that fits over the pipe cross section. At the touchdown the flexible riser may require anchoring and can be terminated in PLETS or fitted or attached via connector to a flowline PLET or manifold.
The flexible pipe itself is qualified for different water depths, pressures, temperatures, and services by the equipment maker with some limitations. It can be transported on large vertical reels in cargo vessels or in longer continuous lengths in dedicated carousels for transfer to installation vessel carousels. The flowlines for the field can also be comprised of flexible pipe if synergies/benefits exist.
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