Deep water rigid production risers which terminate to flexible flowlines at shallow depths have been made for use with floating production systems. The dynamic behavior of such rigid production risers with flexible flowlines is determined in 947 meters (3106 feet) of water, and compared to surface-piercing rigid production risers restrained to the vessel by either a tensioning system or a yoke. The economics and relative cost comparison of the different systems is presented for the cases of 24 and 36 wells, both with and without manifolding.
Floating production systems (FPS) have been manufactured for use in the Gulf of Mexico with deep water production, non-surface-piercing, rigid risers using flexible flowlines. This type of riser, depicted in Figure 1, is an oil and gas conduit between the sea bottom template and the floating production facility.
The arrangement of the FPS/riser system shown in Figure 1 provides several advantages. The production riser is offset 24 meters (80 feet) from the moonpool toward the stern of the vessel to afford adequate clearance for drilling or workover from the vessel without interrupting production, if the riser is part of a well template system. With the rigid riser directly beneath the rig, the flexible flowlines that conduct fluids between the riser top and the vessel hang in catenaries from each side of the rig. Tensioners on the rig apply a total force to the riser top in order to reduce the horizontal distance between it and the rig. This reduces the risk of damage to the flexible flowlines and the capital cost of the flexible flowlines.
The buoyancy of the riser system, shown in Figure 2, satisfies a variety of constraints.
The syntactic foam modules provide slightly less lift than the net weight of the riser joints to facilitate installation of the riser. The foam also increases the mass of the system, which is usually beneficial in compliant structures. Integral to the 15-m riser joints are air cans, as shown in Figure 3, which provide sufficient lift to support the wet weight of the non-integral export lines, control lines and the non-integral steel production and annulus lines which penetrate the foam modules. Depending on the water depth, some of the top riser joints are encased in full-length flotation collars, as depicted in Figure 1. These air tanks provide the buoyancy needed to maintain acceptable stress levels and achieve the required design life.
The Upper Riser Connector Package (URCP) shown in Figure 4 can effect a hydraulic disconnection should it be needed for maintenance of the riser or the rig. The flowlines can be disconnected simultaneously or individually, even during an emergency.
A riser joint, assembled with the foam modules, is shown in Figure 5. The joint is designed to fit through the opened rotary table on the rig.
