R.J. Baker
Shell International Petroleum
Mij. B.V.
The Hague
The development and installation of constant-volume risers on floating drilling vessels would significantly improve operational safety and reduce the environmental impact of offshore drilling.
Because these are declared goals of Shell companies, it has been decided to place this concept into the public domain for further development.
MARINE RISERS
Marine risers currently deployed by floating drilling units incorporate a telescopic joint (Fig. 1) to accommodate vessel movement, primarily heave. This vertical telescopic movement changes the internal volume of the riser and causes fluctuations in the return-mud flow rate.
Flow fluctuations make accurate measurement of the return mud very difficult. The significance is that these measurements are vital for the early detection of well bore influx or downhole mud losses.
Erratic mud flow also adversely affects the efficiency of the solids-removal equipment and potentially increases the risk of discharging whole mud to the environment.
To overcome these adverse effects, a novel design for a telescopic joint is proposed (constant volume riser or CVR) in which the internal riser volume remains constant, irrespective of movement, thus permitting a uniform flow rate of mud returns. The CVR will permit flow meters to be installed that detect early changes in the mud in/mud out differential flow rate (_Q).
Early detection (minimum influx volume) is of prime importance for well control because it ensures that minimum pressures are exerted on the well bore and surface equipment.
A major contribution to safer operations, early detection of well influxes can lead to optimizing casing design, which sometimes may allow a casing string to be omitted.
The elimination of mud surges from the flow line will enable shale shakers with finer mesh screens. These screens will permit removal of a greater percentage of drilled solids, reduce mud costs, and improve mud quality.
With the present systems, whole mud is frequently lost across the top of the screens due to mud surges. This mud is normally lost to the sea, or in "zero discharge" areas has to be shipped to shore for disposal with the cuttings.
The uniform return-mud flow rate of the CVR will prevent these losses from occurring, thus reducing costs and the environmental impact of floating drilling.
SYSTEM LIMITATIONS
Floating drilling units employ a marine riser that has several functions. One function is to act as a conduit to permit the drilling fluid to be returned from the well bore to the drilling vessel. This riser is supported by riser tensioners, assisted by buoyancy modules in deeper waters.
To allow for vessel movement relative to the riser, flexible connections (ball joints or flex joints) are incorporated at the top and bottom of the riser. In addition, a telescopic joint is positioned at the top of the riser to allow for vertical movements of the vessel, caused by heave, changes in draught and tidal changes, etc.
As the vessel heaves down, the telescopic joint will act as a giant piston pump forcing drilling fluid to surge down the flow line. When the drilling unit heaves up, the flow out is reduced or even stopped altogether.
Surge magnitudes of 1,224 gpm (4,650 I./min) can occur in a typical riser with an inside diameter of 20 in. (508 mm). This assumes that if the drilling unit heaves through 10 ft (3 m), the internal volume of the riser changes by 163 U.S. gal (620 1). if the heave period is 8 sec, then this equates to 1,224 gpm (4,650 l./min).
Because a typical drilling fluid circulation rate when drilling an 8.5-in. hole is 250 gpm, the effect of the heave will be to give surges well in excess of 1,224 gpm, interspersed with periods without flow. This phenomenon is very visible on a heaving drilling rig when observing the flow line.
OPERATIONAL IMPACT
In recent years, modern flowmeter technology has greatly improved the detection of influx from (or losses of drilling fluids to) the well bore on wells drilled from stationary rigs. Accurate flow meters can be placed in the flow line and the mud pump discharge line (or pump suction).
The meters allow the mud flow in and out of the well to be measured precisely, thus permitting early detection of changes in rates of flow (_Q).
If an influx is suspected, it is normal practice to carry out a flow check. This involves stopping circulation and seeing if there is flow from the well annulus, which indicates influx from a downhole formation.
On a floating drilling rig the mud surges caused by even a small amount of heave renders the flow check almost useless. To prevent the possibility of continued undetected influx, some operators have adopted a policy of closing in the well and observing if there is a pressure buildup to indicate if an influx has taken place. However, this practice has operational drawbacks. All operators would prefer to have a safe alternative.
Detection of influxes or losses by means of observing changes in mud tank levels is another method that is widely used. But here again, vessel movement causes level fluctuations that put the floating unit at a disadvantage when compared to stationary rigs.
During round trips it is necessary to have very accurate control over the amount of fluid added to the borehole to compensate for removal of the drillstring, or the amount displaced by the drillstring when it is run back in the well. This accuracy is required to identify if formation fluids are being swabbed into the well, or if mud losses to the formation are being induced due to excessive surge pressures caused by tripping pipe.
Historical data show that more well control situations ("kicks") occur while tripping the drillstring than while drilling ahead. Controlling these kicks is also more difficult, and potentially more dangerous because the drillstring does not extend to the bottom of the well.
To be able to monitor continuously for swabbing or mud losses, rigs are equipped with "trip tanks." These tanks are constructed and connected to the well in such a way that very small changes in mud volume can be measured, and if amounts added to, or received from, the well differ from precalculated amounts then the driller is instantly alerted.
Trip tanks work well on stationary drilling units but the mud surges induced by rig heave on floating units considerably reduce their accuracy.
Early detection is therefore of paramount importance if safe well control procedures are to be effective and pressures exerted on the well bore and wellhead are to be minimized. As a result, the development of these flow metering systems has been considered to be a major safety advance.
This advance has not yet been successfully applied to floating drilling units due to the movement of the vessel (mainly heave) which causes significant fluctuations (surges) in the return mud flow.
Attempts have been made, and are still continuing, to use computers to correct for surges. This would enable a steady reading to be displayed to the driller. But so far these attempts have not been very successful.
The cumulative impact of all of these shortcomings is that on floating drilling units, relatively large losses or gains can and do occur before the driller is aware of the problem.
This is not only detrimental to operational safety, it can add significantly to casing setting depths, even to the point of requiring an extra casing string. Casing design is forced to take into account this potential for large undetected influxes for wells drilled from floating drilling units.
Casing-string design has to meet many design criteria with respect to strength and setting depth. One major factor is the maximum anticipated borehole pressure. This in turn is affected by a number of factors. Major factors are the formation pore pressure (over which we have no control) and the maximum anticipated influx volume (which we can endeavor to minimize).
In almost all cases, the maximum borehole pressures during well-control operations are directly proportional to the influx volume allowed to enter the well prior to closure.
Reducing the detectable influx volume to a minimum, therefore, can have a direct influence on the casing design adopted, and this in turn can have a major impact on well costs. The potential for savings of up to $500,000 on casing costs alone has been identified for some planned deep, high-pressure gas wells.
The mud surges in the flow line also cause problems with shale shaker operations. Much coarser-mesh screens have to be fitted than would normally be required to handle the circulation rate. The coarser mesh prevents whole mud from being lost to the environment over the top of the screens.
At times of severe heave, some loss due to this effect is almost unavoidable. If the mud is of a type that cannot be allowed to enter the sea, drilling operations sometimes have to be suspended.
Mud surges can lead to a significant increase in the amount of mud required to drill the well. In the case of environmentally harmful muds, this creates a big increase in volumes that need to be disposed of.
Efficient mud management is difficult to achieve due to the fluctuating flow, and this can add considerably to the cost of the well.
CONSTANT VOLUME RISER
The constant volume riser concept (Fig. 2) depicts a modified telescopic joint in mid-stroke. In this design, the telescoping member (outer barrel) is on the outside of the riser. In the normal case it is inside (inner barrel). The inner barrel moves in an annular chamber which is attached to the outside of the riser body.
At the lower end of the outer barrel is a piston which seals the annular chamber. Just above the piston, the outer barrel is perforated (transfer ports) to allow fluid to pass through in a virtually unrestricted manner.
At the top of the annular chamber is a packing which seals against the outer barrel, while at the bottom of the chamber are drain/vent holes open to atmosphere.
The annular chamber is sized in such a way that the volume per unit length shown as V2 in the sketch is equal to the volume per unit length shown as V1. The volume shown as V3 in the sketch remains constant and can be disregarded when calculating the dimensions of V1 and V2.
In use, the outer barrel moves up and down in the same way as the traditional inner barrel; however, the effective internal volume of the riser remains constant and the drilling fluid flowing through the flow line will no longer surge. This is because the fluid is transferred to and from the annular chamber via the transfer ports in response to vessel movement.
This then allows using flow meters as described above, with all of their advantages in respect to operational safety. Trip tanks can now be used effectively during round trips and flow checks can be made in the same manner as from nonfloating drilling rigs.
Casing design can be revised to take advantage of the improved influx volume detection, Much finer shale shaker screens can be utilized, improving cost effective solids control and avoiding the loss of whole mud to the environment.
The idealized CVR (Fig. 2) requires modification to cope with actual drilling fluid. The mud is laden with abrasive rock particles and will cause the moving parts to wear and possibly jam the joint, with disastrous results.
Therefore, the design shown schematically in Fig. 3 is suggested as a means of overcoming these shortcomings, while maintaining the advantages of the CVR.
In this design, the moving parts are kept separated from the drilling fluid and are able to operate in a clean fluid with lubricating properties, (e.g., water plus emulsifying oil). This fluid is isolated from the riser by an additional inner seal and transferred through pipes (or an outer annular chamber) to a lower chamber which is fitted with a floating piston, the purpose of which is to separate the operating fluid from the drilling mud.
At the base of the lower chamber, ports allow free movement of the drilling fluid to and from the riser bore. The base of the lower chamber is also slanted, and the ports are designed so that drill cuttings in the mud will not settle out and cause a blockage.
Because the drilling fluid will normally be of greater density than the operating fluid, there will be a natural tendency for the two fluids to remain separate.
Some drilling fluid, however, will tend to cling to the walls of the chamber so that without the inclusion of the floating piston some commingling of the fluids will almost certainly take place.
Any slight losses of the operating fluid that might occur can be countered by injecting more as required.
The CVR concept is seen as being eminently suitable for retrofitting to existing drilling units because in most cases only the telescopic joint will require replacement. No major modifications will be required.
Riser running procedures will also be unaffected. Because most floating drilling units already have large-bore rotary tables installed, it is expected that the CVR will be able to pass through these without difficulty.
Alternative configurations have been considered that enable using the advantages of the CVR concept. Undoubtedly, to arrive at the optimum solution, these will have to be considered during the development of the system.
The proposal, as outlined, is therefore not intended to be a definitive design, but is simply intended to demonstrate the concept.
ACKNOWLEDGMENT
The author thanks Shell Internationale Petroleum Mij. B.V. for permission to place this concept into the public domain and also thanks his colleagues for their contributions and support.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
