Flowline/Pipeline Installation with Dual-Activity Rig
Three methods of pipelaying are considered viable for field development using the drillship: discrete lay, reverse lay and dedicated lay.
Discrete Lay. This method utilizes the second rotary table on the drillship to construct flowline segments while the primary rotary table is performing the drilling and completions operations. Figure 8 illustrates the concept. Pipeline segments less than the water depth are made up in the auxiliary rotary table. First-end and second-end skids are made up at the extremities of the segment. The completed flow line segments are transferred to an anchor handling vessel (AHV) which transports and installs the segments. Subsea jumpers connect the segments to form the completed flowline.
Disadvantages of this method are the additional cost of the jumpers and the limitation of the segment length to the water depth. Obviously this is not a useful method in shallow water. The hanging loads are quite large and, for lengths greater than 6,000 ft, the weights approach the limit for standard AHVs.
The advantages are great. Most of the spread costs is charged to the drilling operation and this results in an economic means to install a flowline, and the flowline is installed when needed rather than at the convenience of the lay barge. In the case where the wells are separated by distances less than the water depth then this method becomes very economical when the wells are daisy-chained together by flowlines. The lines can be laid while drilling and no extra jumper cost is suffered.
Reverse Lay. This is another method where pipe is laid from the auxiliary rotary table while drilling. The reverse lay method uses the AHV to pull pipe away from the rig while being assembled. Pipe lengths up to three times the water depth can be assembled, thereby eliminating one or two pipeline jumpers. To lay the tree end it is necessary to offset the rig in the direction of the lay and take a larger horizontal pull.
It is possible to run an even longer segment by combining the reverse lay with bottom tow. In this case the AHV pulls the pipeline along the seabed while being made up by the second rotary table. Figures 9 and 10 illustrate the reverse lay concept.
Dedicated Lay. The dedicated lay technique uses the dual- activity drillship to install a flowline or pipeline in a continuous manner, as does a pipeline lay barge. Both the primary and secondary rotary tables can be used simultaneously to construct and install dual lines. The dedicated lay method assumes the rig is used strictly for pipelaying operations, with no drilling or completions operations being performed. Dedicated lay can be applied simultaneously for two lines, or with one rotary table in dedicated lay mode and the other rotary table in a discrete lay mode. The most efficient use of the drillship is in a dedicated dual lay mode. The rotary table separation of 40 ft gives sufficient clearance to run two lines simultaneously. The hoisting capacity for both rigs are identical (2 million lb), and the structure is designed to take maximum loads simultaneously. The vessel would normally lay the pipeline transverse to the hull centerline (sideways). In the event of bad weather the rig can change heading±45° without danger of the two pipes interfering. Possibly up to±60°can be realized without problems. The Discoverer Enterprise is much larger than a typical deepwater J-lay barge and has superior stationkeeping capability. As such the downtime for weather should be significantly less.
During times when simultaneous lay is not possible, say with separated risers or single flowlines, it is possible to engage the second rotary table in the discrete mode or in helping the single lay rotary table by making up160 ft-180 ft stands. Risers can be run discretely or in the normal forward lay. The two 1 million lb motion compensators can be of benefit in emergency recovery operations. Use of the drilling riser as a running string allows a 2 million lb lift load.
Dual lay of risers to a floating production facility is unlikely owing to separation requirements of riser strings near the seabed. Nonetheless the second rotary table can be put to good use while laying a single riser with the other rotary table. For example, both rigs may be used for dual lay up to the time that the single riser lay operation begins. From that point onward the second rotary table pays out the second riser with a dummy line (drill pipe). Upon connecting the first riser the second is hauled in and its lay recommenced.
During the period when the first rotary table is free, it can be usefully employed making up pipe segments for the discrete lay. Pipe segments are passed to an adequately equipped AHV which in turn steams to the line under construction and lays that section. Alternatively the idle rotary table can be used to make up 180 ft stands (from two 90 ft sections) and pass the longer stand vertically to the second rotary table using one of the rig floor pipehandlers. Savings would result from faster running (180 ft joints vs. a standard “quad joint” length of 165 ft) and from lower onshore fabrication costs (90 ft joints vs. 165 ft joints).
Burial. The high pressure mud pumps used to power seabed equipment can also be utilized to bury flowlines, thus improving their ability to maintain transported fluid heat where flow assurance is a concern. A drill string can be run to the bottom by each rotary table and the pump hydraulics can be used to jet the pipe. The jets at the bottom of the string are oriented to keep the drill pipe pressed against the flowline. The drill ship can travel along the flowline at up to 4-5 kts during this operation. An ROV maintains position just ahead of the forward string and above the unburied pipeline for inspection purposes. The drillship keeps station off the ROV Position.
