A new subsea package allows drillers to lower hydrostatic head in deepwater applications.

In certain sedimentary basins around the world such as the Gulf of Mexico, West Africa and Brazil, as water depth increases, the probability increases that the difference between pore and fracture pressures is reduced. When this occurs, the maintenance of wellbore pressure balance and borehole stability can be a problem and, in some cases, limit the ability to reach target depth. To drill these wells in a conventional manner, additional casing strings often are required to minimize risk and allow drilling ahead. As a result, it is possible for the borehole size to be reduced below that which can be economically produced, and in some cases, it can become impossible to reach total depth before hole-size reduction prevents drilling ahead.
In the 1960s, the idea of dual-gradient drilling was introduced. Essentially, this is the combination of a seawater head in the riser between the rig and the sea floor, and drilling mud in the wellbore below the mud line. By creating a wellhead pressure that closely matches the surrounding ambient pressure, drilling of the deepwater well becomes a lower-risk undertaking.
First-generation dual-gradient drilling systems used existing subsea stacks and risers modified to enable dual-gradient drilling with seawater-filled risers. The modifications included the addition of auxiliary lines for mud return, while the main riser tube contained seawater and was isolated from the wellbore. Additional subsea hardware for pumping the mud returns also was required.
DeepVision LLC is designing, testing and manufacturing a complete dual-gradient drilling system to be used with limited riser modification. This system is one of several key components of the dual-gradient solution identified early in the project.
Application
The system uses a subsea pump package of centrifugal pumps, a mud return line and a mechanical seawater-mud isolation system (SMIS). This combination allows the opportunity to not only manage bottomhole pressure but also to selectively control it, together with the pressure at any other point in the wellbore. The combination of a seawater gradient from surface to the mud line and a mud gradient to the bottom of the hole can reduce the hydrostatic head in the upper hole sections, as compared to conventional operations. Yet it still can maintain the desired bottomhole pressure, as defined by the pore and fracture gradient window. In addition, the bottomhole pressure is maintained in the correct window during a riser disconnect, without the need for overpressure at the blowout preventers (BOPs).
Once returning mud and cuttings reach the mud line in the annulus, the flow path enters the subsea pump station (SPS) and continues upward to the rig via the mud return line. The SPS is a pressure-boosting device, which adds energy to the return flow and lifts it to the surface, without imparting the hydrostatic head of the return mud column to the wellbore. A byproduct of the subsea pumps is the introduction of a hydrostatic head imbalance on the drillstring side of the wellbore, which must be managed as part of the overall dual-gradient solution. The methods available to manage this hydrostatic imbalance vary and may be altered during the drilling operation.
During certain wellbore operations, the seawater-to-mud interface, which defines the potential hydrostatic imbalance, may be allowed to move up or down in the riser from its nominal location just above the mud line. This can occur, for example, when making a trip, where steel displacement volume is added or subtracted from the wellbore below the mud line. In this case it is advantageous to have the interface level several barrels above the pump suction inlet in order to feed the well when pulling out of hole or allow mud to move up into the riser as pipe is run in. The position of the fluid interface is measurable and can be monitored in real time at the surface. However, during other operations, it would be an advantage to have available the added control of a mechanical SMIS separating and defining the exact location of the wellbore and riser fluids. This would limit mud contamination and ensure that return mud and cuttings enter the inlet suction of the subsea pumps. This defined interface also allows greater flexibility in user-defined bottomhole pressure management.
Rotating control head (RCH)
RCHs, when used in drilling operations, divert the mud and cuttings at surface to the mud-processing equipment on the rig, preventing the returns from entering the traditional flow tee path. Onshore applications number in the tens of thousands. Until the early 1990s, the tool was primarily used as a rotating flow diverter for air, natural gas and geothermal drilling applications. Subsequently, to meet the more demanding needs associated with the advent of underbalanced operations, models with higher-pressure capabilities were introduced.
Typical components of a standard RCH are:
• a bearing assembly;
• stripper rubbers within the bearing assembly;
• a bowl that serves as a diverter housing; and
• mechanical or hydraulic clamps for securing the bearing assembly to
the bowl.
Basic specifications include:
• pressure ratings in the range 500 psi to 2,500 psi;
• bowl bottom flange sizes to match annular BOPs; and
• maximum recommended speeds of 200 rpm.
RCH design technology fulfills many of the requirements of an SMIS. Although the SMIS is not operated in underbalanced drilling conditions, the concept works well in providing a mechanical annulus barrier within the marine riser near the top of the subsea BOP.
Functional needs of SMIS
The SMIS serves as a mechanical barrier between the seawater-to-mud interface. Differential pressure management, which is normal in underbalanced drilling applications, is not the main focus of the SMIS. The primary objective of dual-gradient drilling is to have the pressure at the wellhead equal to the ambient seawater pressure, and therefore the SMIS typically is operated in a pressure-balanced environment. The DeepVision project uses this premise and has defined the term pressure in the well annulus (PWAS) at the sea floor. If the PWAS is equal to the seawater head, the classic definition of dual-gradient, then the SMIS differential pressure is negligible. The inclusion of the SMIS as part of the group's subsea centrifugal pump station creates a closed-loop fluid circuit that optimizes the value of the centrifugal pumps. The closed-loop configuration allows these pumps to become simple energy input devices, where system flow rates are defined by the surface reciprocating pump on the drillstring side. The subsea pumps can be used to control bottomhole pressure by maintaining a preset PWAS. Typically, this set point will be equal to seawater head for most dual-gradient drilling operations.
The SMIS is in the SPS, directly above the pump suction inlet. The SPS is mounted above the lower marine riser package and the lower flex joint. For effective operation, the SMIS must maintain a fluid seal around the drill pipe, serving as a wiper in order to prevent unwanted flow of mud into the riser or seawater from the riser being drawn into the wellhead by the pump station. The seal system also must allow for drill pipe rotation up to 200 rpm and must not cause internal bore restrictions in the existing BOP assembly.
Modification to the RCH was required to customize the technology, in order to suit the functional needs of the DeepVision SMIS. The existing RCH components were redesigned, and the concept of the riser rotating control head (RRCH) was developed.
The standard design of the RCH was modified to specifically fit the function and performance requirements of mechanically separating seawater in the marine riser from annulus returns within the subsea BOP. The bearing assembly was modified to reduce the outside diameter to permit deployment within a 19½-in. ID marine riser with self-lubricating capability. And adding a threaded-on lower housing with an upset changed the bowl.
The RRCH assembly thus consists of a RCH bearing assembly with high-pressure operating capability, redundant passive stripper rubbers and a threaded-on lower housing (Figure 1). Within the bearing assembly, the stripper rubbers rotate with the drillstring and are stretch-fit such that their cone shape augments sealability as pressure differential across them increases.
To complete the seal in the riser, the RRCH must be landed in an annular BOP, which then can seal around its lower housing. The lower housing has sufficient length to pass through the annular BOP, while its upset is securely retained therein.
The introduction of the SMIS adds several benefits, and since sealing does not necessarily have to be around the RRCH, the annular can be used to seal around drill pipe or casing strings as required.
The RRCH is deployed by stabbing it onto a joint of drill pipe. The stripper rubbers' stretch-fit nature holds the RRCH in place and permits lowering the tool, with the drillstring, onto an open-position subsea annular BOP. The lower housing passes through the SMIS annular seal, and the upset at the bottom of the lower housing extends past the seals of the annular BOP, which securely retains the RRCH when it is closed. When the annular BOP closes around the lower housing's midsection, it is thereby converted to an RCH.
Tool retrieval is achieved by opening the annular BOP and tripping out. Worn stripper rubbers may have lost their stretch fit, but the next tool joint will serve as a lifting ledge, permitting retrieval. Regardless, the bottomhole assembly will serve to retrieve the tool from the marine riser.
Status
The DeepVision project is in the testing phase prior to manufacture of a complete commercial system. Flow-loop testing of the subsea pump package fluid ends and manifold components will be complete in the third quarter of 2001. Delivery of a commercial DeepVision dual-gradient system is expected by the close of the fourth quarter of 2002.