Multi-purpose vessel with freestanding riser can cut deepwater well costs 15%

Upper riser package and near-surface disconnect package [13,303 bytes] A multi-purpose vessel with a free standing riser (FSR) system that employs proven technology along with innovative design can reduce deepwater well costs by 15% or more in an exploration, development, completion, or intervention mode.

Integrating well control with floating design

Joseph R. Roche
Friede Goldman International

Graeme E. Reynolds
Hydril Company

Kevin C. Robert
Maverick Offshore L.L.C.

A multi-purpose vessel with a free standing riser (FSR) system that employs proven technology along with innovative design can reduce deepwater well costs by 15% or more in an exploration, development, completion, or intervention mode.

Drilling and completing a well in deepwater presents many special technical and commercial challenges. Using a conventional marine drilling riser system to answer these challenges adds to the risk of equipment problems, increases operating expenditures, and prolongs the time needed to drill the well. Equipment problems can cause cost overruns and threaten the profitability of a deepwater play.

By integrating well control equipment with the floating vessel design, the time from spud to first oil can be reduced significantly. Proper design and integration of the marine riser, blowout preventor (BOP) and multiplex (MUX) controls into a newbuild or an upgraded second, third or fourth generation MODU adds market value by enhancing utility, safety, and environmental friendliness. This is all aimed at reducing the operator's well construction costs.

Use of a freestanding riser (FSR) can cut deepwater well construction costs. This configuration makes it easier to disconnect the riser at a shallow depth - 500-1,500 ft below the surface - and move the vessel off location, while leaving the positively buoyant riser in place. The riser is connected at the riser bottom to the BOP stack and wellhead, thereby preserving well integrity.

Riser evolution

In fact, the freestanding riser concept is not new. A major oil company specified a freestanding drilling riser system in the early 1970s. The manifest advantages of the FSR were seen as a natural complement to the use of dynamically positioned drilling vessels and electrohydraulic BOP controls in deepwater. However, construction of the riser ultimately had to be stopped when limitations in BOP structural strength and subsea electrical connector performance were found to be unacceptable.

Technological developments during the 1970s and 1980s brought about bigger rigs and better performance in harsh weather, but the status of riser technology languished. Flanged couplings were made bigger, joints were longer, and pipe wall thickness increased. But the major issues of time and weight savings were not adequately addressed and, more importantly, the integration of the riser system into the vessel design was basically ignored.

Certain technological advancements during the past 25 years have brought solutions to these problems of the past. Today's deepwater BOPs are sufficiently robust and the use of wet make/break electrical connectors is commonplace. The methodology set forth in this article embodies elements of already-proven technology.

FSR features

The features of the FSR include:

  • Upper marine riser package (UMRP) with a hydraulic riser connector mini-control pod
  • Near-surface disconnect package (NSDP)
  • Shear ram BOP to sever pipe in the riser and seal the riser bore.
A variable pipe ram is used for hangoff prior to shearing. An instrumented riser joint (IRJ) is used to measure riser tension and bending. Positively buoyant riser joints are installed as needed to free stand the riser.

There are major advantages of the FSR, compared with conventional drilling risers, related to planned or emergency disconnect of the riser in the event of a drive-off, a drift-off, inclement weather, or a well-control problem. These advantages include avoiding the need to hang a very long riser beneath the vessel during abnormal conditions such as high seastates, uncontrolled vessel excursions, or a well blowout. The FSR also offers substantial savings in time spent pulling and re-running the riser during a disconnect. Using this system also avoids the hazard and cost of dumping drilling mud from the entire length of the riser string into the subsea environment.

The FSR reduces the risk of violating the seal integrity of the riser coupling, choke and kill stabs, and auxiliary line stabs on most of the riser joints, since only the short upper riser is re-run to re-establish the riser. The FSR is subjected to minimal drag on the bare joints in the near-surface area of higher ocean currents, since only the riser joints below the NSDP are fitted with buoyancy.

The capacity requirement for the rig-mounted riser tensioner system is greatly reduced, as only the upper riser need be supported. Thus, the service life of the tensioner wire lines is increased accordingly. For most existing floaters, additional tensioner units need not be added to support a deepwater riser.

New enhancements

While the generic FSR has obvious advantages, new configurational features and applications options greatly reduce cost and time to first oil. A patented method of placing the MUX cable reels on the NSDP (illustration) eliminates the risk of slacking off and the costs of severing the control umbilicals during riser disconnect.

By employing this feature, MUX cables are spooled off the NSDP mounted reels while running the lower riser. The NSDP is stowed at the moonpool perimeter while it is aboard the rig. Fixing the MUX cable reels to the NSDP affords easy accommodation of drilling site-to-site variations in water depth. The MUX cable interface at the UMRP/NSDP is a field-proven wet make/break design.

The key to success of the rig as a multi-purpose vessel is the effective utilization of the deployed riser system as a multi-functional tool. To accomplishing this goal, a dummy wellhead is part of the temporary guide base (TGB). This allows the FSR to be "parked" - with or without the BOP stack - permitting unobstructed access to the wellhead and wellbore.

Consequently, the full riser system need only be run and pulled one time for the duration of drilling the well, bringing the well onstream and into early production. If it should become necessary to pull the BOP stack, the riser can be disconnected and parked, enabling quick tripping of the BOP stack using heavyweight drill pipe. This reduces physical risk, riser wear-and-tear, and time to first production.

Make/break connections

When the upper riser segment is dis connected, a clean disconnect of the MUX cables is made at the NSDP by way of wet make/break cable connections. This enables automatic re-connect when the upper riser is re-entered and re-connected. When the riser is disconnected, the drilling fluid saver (DFS) saves expensive drilling fluid, and protects the marine environment.

Special riser joints are positioned just below the NSDP, providing positive lift and overpull at the riser's lower extremity and keeping the riser erect when disconnected. The volume of air used to charge the buoyancy is variable so that the ideal amount of lift is provided for each site.

The standard riser joints below are fitted with conventional foam buoyancy modules. In the event of a catastrophic parting of the riser beneath the NSDP, lift reduction by ballasting is automatically executed to retard the vertical acceleration of the disconnected riser segment.

Field-proven threaded union type riser joint couplings (not flanges) significantly reduce the weight and save much make/break time. An optional annulus-closing device positioned just below the telescopic joint permits safe handling of a gas intrusion from the wellbore into the riser annulus.

Disconnect sequence

Riser disconnect at the NSDP can be either planned or unplanned and either manually or automatically commanded. Disconnect at the NSDP is integral to the emergency disconnect sequence. The general sequence of events after halting drilling is as follows:

  • Close the appropriate hang-off pipe ram in the BOP stack
  • Hang the nearest drill string tool joint above the closed pipe ram
  • Shear the pipe at the NSDP
  • Retract choke and kill and auxiliary stabs
  • Disconnect the MUX wet make/break connections at the NSDP
  • Disconnect the riser at the NSDP.
The driller must control the collapse of the telescopic joint and the reaction of the riser tensioner system. The crew will pull and store the upper riser joints, if appropriate. The wellbore below the mudline and the fluid in the riser annulus are dealt with separately. Re-entry and re-connect of the upper riser may be executed by reversing the above procedure. Naturally, the fish (if any) must be retrieved from the riser bore after re-connect.

Successful execution of the design and manufacture of the FSR system depends on an interdisciplinary team of project management, design, manufacturing, and shipyard conversion or new construction.

Project management is provided by Maverick Offshore. Hydril Company contributes the design and manufacture of the well control and NSDP equipment and the design of the riser. Friede Goldman International, through its subsidiaries BLM Offshore, Friede Goldman Offshore, and Friede & Goldman, Ltd., provides vessel adaptation for deploying and storing the FSR and fabricating major riser components. The synergy created by this teaming arrangement allows the group to achieve much more than the separate organizations could.

The bottom line

The FSR system enables a semisubmersible or drillship to perform more than just drill ing operations. Carrying out completion, development, and intervention operations adds substantial value to the vessel.

It is estimated that significant savings will result from running the riser only once per well. By outfitting the rig to efficiently handle specialized equipment and to serve multiple roles, the time to first oil can be reduced by as much as 15%, with concomitant cost savings. Thus, even during cycles of depressed oil prices, drilling, completing, and bringing large deepwater reservoirs onstream can be profitable.

Copyright 1999 Oil & Gas Journal. All Rights Reserved.

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