Horizontal trees are run to the sea bed on drillpipe and landed without having to pull the BOP to the rig floor. Vertical trees can likewise be run by the auxiliary rig with a separate riser system. The second rig also provides the opportunity to carry out activities not normally considered possible due to the cost of the operation spread. The rig can be used to run pipeline bases and to pull in the pipeline. Heavy gear can be moved subsea, and the mud pumps can provide hydraulics for seabed work such as trenching or pipeline pull-in. The offline rig can run tree jumpers, pipeline jumpers and manifolds all while the main rig is drilling through the BOP.
Subsea Tree. Normally the subsea tree is run after securing the well and pulling the BOP stack. The stack or high pressure riser is then rerun and landed on the tree. Deepwater tripping of the BOP and riser is time-consuming and comprises a significant percent of the well completion costs. Often most of the tree preparation work commences only after rigging down from pulling the BOP and the moonpool area under the rotary table becomes free. To pull the stack, rig up and run the tree, and then rerun and test the BOP takes a minimum of 10 days in deepwater. This does not account for the risk of failure of the BOP or high pressure lines to test once the riser system is re-deployed.
The dual-activity rig runs the tree on drillpipe while the BOP stack is on the wellhead. Care and planning are necessary to execute this maneuver without problems caused by interference with the drilling riser. First, it is required that the tree, tree running tool or other bottom string equipment be provided with fenders or rails to keep that part of the tree susceptible to hang up off of the marine riser. The fenders themselves need to be smooth and snag free. Second, the drillship has to be oriented so that the second string is pushed away from the marine riser. This is accomplished by first acquiring a current profile and then calculating the trajectory of the second string as it runs to the bottom. If the displacement for all positions and all times is less than 40 ft, the rotary table separation, then the heading for best vessel motion can be maintained. If the displacement is more than the rotary table separation and would lead to interference with the marine riser, then it is necessary to orient the relative position between the running string and the riser by changing heading so that the displacement, whatever it is, runs away from the riser. In the rare event where the displacement is more than 40 ft and cuts through an arc greater than 180° (when viewed from above), then it is necessary to also offset the vessel, increase the riser angle at the BOP, and increase the separation between the two strings. It is also possible that a series of heading changes are necessary as the trajectory shifts due to increasing hanging weight and alterations in current. Finally the position of the bottom of the string is continuously monitored while lowering. A bottom string transponder gives the absolute position of the tree and is used to check against that calculated from the trajectory analysis. An ROV follows the tree to give both a visual and acoustic picture of the situation.
Other Subsea Equipment. Besides the tree there is other subsea equipment that can be deployed with the offline rotary table and help reduce the field development costs. Figure 7 illustrates a typical deepwater subsea setup.
Pipeline Manifold Base. Often a base is deployed as an anchor for pipeline termination or as a tie-in point for the tree jumper and the main production flowline or simply to function as a subsea manifold. As such this base requires a foundation that can support the weight of the base and piping and take side loading from pipeline pull-ins or from pipe lay initiation. The offline rotary table can be employed to install both the foundation pile and the manifold base while conducting normal operations with the main rig. This is best done during the early part of drilling a well, when there is some short slack in the auxiliary rotary table use and when still rigged up for drilling in top hole casing. Deploying the pile can be temporarily interrupted at any time except after its drilling has commenced. There is a means to hang the pipe string on the spider beams under the auxiliary rig and skid it out of the way. When the auxiliary rotary table again becomes available the string can be skidded back into position, and the operation can be restarted. Drilling piles is a function for which the rig is specifically specialized, and it is especially economical when this can be done offline to the main operation. The costs for this are reduced to that of the hardware with the rig spread costs going against the well.
The moon pool of the Discoverer Enterprise is 60 ft x 30 ft and there is a clear height of 50 ft under the substructure. This allows manifolds of up to 25 ft x 35 ft to be easily run from the aft station. Again planning is required to minimize the risk of interference with the deployed riser.
Tree Jumper. Once the manifold base is set and the tree is landed, it is possible to run the flowline jumper from the tree to the base with the auxiliary rig. Measurements are made by the shipboard ROV and the jumper fabricated onboard. Jumpers less than 60 ft in length can be assembled on the pipe rack and run directly through the moonpool. Jumpers over 60 ft but less than 125 ft in length can be assembled in one piece and passed over the ship side for keelhauling to the running string of the auxiliary rotary table. Jumpers longer than 125 ft must be assembled vertically in the moonpool under the auxiliary rotary table and then rotated 90° when clear of the hull. The jumper can be tested once it is installed.
Pipeline Jumpers. In addition to the tree jumpers the pipeline jumpers shown in Figure 7 can be run if the pipeline landing skids are in place or if there is another pipeline manifold base nearby. Even after the rig has moved off location the pipeline jumpers can continue to be fabricated on the drillship and passed to a separate installation vessel. This saves on handling costs and, once set up, provides a very economical means for jumper fabrication. Most importantly, this gives the flexibility to cope with the inevitable changes that come up in any short field construction period.
