Basic Components Of An Exploratory Riser System

Figure 2-10 illustrates the arrangement of the riser system. It is important to note that the riser is not a simple piece of equipment but rather a complex system of component parts. A list of the sources foT components of basic riser equipment and the manufacturers of each component has been prepared (Table 2-1). Many manufacturers produce individual parts of the sub-sea riser system but only a very few supply the entire equipment package.*

»Table 2-1 was compiled from the 1974 Composite Cataiog and represents only those manufacturers included therein. The authors would of course be grateful to hear from any supplier of riser equipment not listed.

TABLE 2-1 Standard Riser Equipment

Riser Disconnect Vessel

The riser siring for a floating exploratory vessel is usually made up of 50 ft. long joints which can be stacked and stored on deck during transit from one drilling location to another (Figure 2-11). The ends of each joint have quick-disconnect couplings permanently attached to the joint. The telescoping joint which is at the upper end of the riser string is usually designed for a maximum heave of between 15 and 30 feet,

A constant force tensioner system is attached to the top of the fixed outer barrel of the telescoping joint to provide enough axial force in the riser string to prevent buckling. The outer barrel and the riser string have lateral movement with vessel surge and sway but essentially no vertical movement with vessel heave. The vessel and the inner barrel of the telescoping joint move together vertically with vessel heave. The optimum ten-

Stacked Riser Joints Riser Tensioner y

Stacked Riser Joints Riser Tensioner y

Riser Tensioning System

Fig, 2-11 Pacesetter I {tin transit showing stacked and'S&red riser joints on deck and riser tensioners mounted on derrick structure- (Courtesy Rucker Control Systems and Western Oceanic, Inc.)

sion is a function of water depth and operating conditions (mud weight, etc.), as shown in Figure 2-12.

Riser Tensioner
Fig. 2-12 Optimization of riser tensioners.

Ball joints (Figure 2-13) on each end of the riser allow foT rotation in any direction up to about 7-10 degrees. Actually, only a few operators insist on two ball joints, which offer more reliability than a single ball joint, because the use of two ball joints incurs greater costs and greater running time. The usual arrangement for floating drilling operations (Figure 2-10) is a gimbal under the drilling deck and one ball joint attached to the top of the subsea BOP stack, which sits on the wellhead. The wellhead attaches to the base template that is set with the conductor type at the beginning of the operation.

As drilling progresses, casing supported by casing hangers in the wellhead is placed in the well bore. At intermediate depths, another casing string of smaller diameter is set inside the first casing string from other casing hangers in the wellhead. The

Drilling Riser Ball Joint
Fig. 2-13 Ball joint, (Courtesy Regan Forge & Engineering Co.)

depth and number of the various sizes of casing strings depend upon geological conditions.

Riser Coupling Joints

The first riser systems had the choke and kill lines strapped to the riser pipe. Pipe handling problems with this system proved to be extremely time-consuming. Most riser systems now use integral choke and kill lines (Figure 2-14) which are permanently attached to opposite sides of the riser and have their own connectors. When the riser joints are stabbed and quick-connected the receptacles allow the choke and kill lines to be stabbed and automatically connected at the same time.

The BOP requirements have been the decisive factor in determining the diameter of the riser pipe and hence the riser wall thickness, required tensioner force, etc. In early drilling operations, a two stack 20 inch BOP was used until the first few strings of casing were set. The 20 inch BOP was then changed out for a 13% inch BOP for making the rest of the hole. Although this procedure is still used, and preferred by some, the single stack is growing in usage. The first single stacks were 16% inch and required a 17% inch underreamed hole for setting 13% inch casing. Because of dissatisfaction with underreaming, a single 21V4 inch BOP was then used.

Drilling Risers Choke Line
Fig. 2-14 Riser joint/integral choke and kill lines. (Courtesy Regan Forge & Engineering Co.)

The heavy weight of these systems, which sometimes exceeded 200 tons, overstressed the system in rough weather. The proposed "North Sea riser system" is a single 183A inch stack which weighs half as much as the 21'A inch system, yet does not require underreaming for the 17V2 inch casing hole.

Choke and Kill Lines

The choke and kill lines run from the deck along the riser string down to the wellhead. At the lower riser ball joint there are various schemes, such as looped pipes, to get the required flexibility in a jump line arrangement running from the bottom of the riser string (top of ball joint) around the ball joint to the BOP stack. The choke and kill lines control kicks in order to prevent them from developing into blowouts.

When a potential blowout is detected, mud is pumped down the kill line at the BOP stack to restore pressure balance in the hole. When excess gases occur the bag and ram type BOPs are closed around the drill string. The gas is relieved at the choke manifold on the BOP stack by running up the choke line 011 the riser string. As the gas expands it proceeds up the choke, displacing more mud and traveling faster as the gas bubble or gas-entrapped mud approaches the mean water line. Without the choke, the gas would push out the annulus mud between the drill string and the riser control from the weighted mud would be lost,

Telescoping Joint

There are two basic types of telescoping joints used with marine risers. The constant tension system (remote axial tensioning system) is most often used because maintenance is easier {Figure 2-15). This method uses a linkage system at the base of the drilling Hoot to maintain equal force on the several wire ropes attached to the outer barrel of the telescoping joint.

An alternate design of telescoping joint uses the direct axial tensioning method. This is a procedure where the seals and guide rings on the telescoping joint are designed to compensate for internal pressure so that the telescoping joint has the dual

Offshore Riser Parts
Fig. 2-15

function of allowing vessel heave and acting as a direct tensioning piston.

A diverter is located at the top of the telescoping joint. Depending upon the magnitude of the kick, the gasified mud is valved either onto the shale shakers or to port (Figure 2-16).

Tensioners

The inner barrel of the telescoping joint is connected to the gimbal under the drilling floor of a floating vessel (Figure 2-10). Wire rope runs through pulley systems on the deck down to the

Telescoping Hydraulic Ram
Fig. 2-16 Telescoping joint schematic. (Courtesy Regan Forge & Engineering Co.)

top end of the lower (outer) barrel of the telescoping joint. In the past, when floating drilling operations were restricted to shallow water, dead weights were connected to the deck end of the wire rope to maintain riser tension and prevent buckling collapse of the riser string.

Constant tensioners are now used having a total capacity ranging from 240-640 kips. The capacity of each individual tensioner is between 60 and 80 kips. The wire rope from the lower barrel of the telescoping joint is fed around pulleys on each end of a main hydraulic cylinder. Fluid in this cylinder is ducted to an air-over hydraulic bladder type of accumulator. The air end of the bladder accumulator is then ducted to a bank of air pressure vessels and the relatively constant tension is maintained by constant pressure with large changes of air volume due to the compressibility of the gas. A typical tension force-stroke curve is given in Figure 2-17. The riser tensioner main cylinders mounted on the mast substructure can be seen in Figure 2-11.

Total mechanical and hydraulic friction in any tensioning system is about± 15%. The variation from constant tension due to nonuniform pressure with varying volume depends upon the volume of the air pressure vessels used and is usually designed for 5-10%. The total variation from constant tension is then usually less than about 20-25%.

Riser tensioners are required for water depths greater than 250 ft. The critical buckling length, Lcr in feet, can be calculated by:

where: E is the modulus of elasticity in psi I is the moment of inertia in in.4 w is the weight of the riser in water in lb./ft.

Riser Tension Guideline

W. L, Clark of Hydril has developed a useful procedure for sizing the guidelines for riser tensioners. His procedure is summarized as follows: The riser tension guideline is a rough order of magnitude

80,000 Lbs Tension 415 U.S. Gal Air Pressure Vessel - K = 1,1

2400 2338

2078

1818

1640 1560

1430

1300

1039

* Air Pressure Vessel

2400 2338

Tension Riser

125 150

Tensioner Stroke - Inches

125 150

Tensioner Stroke - Inches

Courtesy Hücker Control Systems

Fig. 2-17 Tension versus stroke. (Courtesy Rucker Control Systems)

calculation only. More complete and involved methods are required for complete accuracy.

This method of calculation is based on the weight of the riser pipe, the suspended pipe, and the mud column.

Nominal tension equals weight of the riser system in water plus 20% so that positive tension will be maintained at all times during heave motions.

Example:

Weight of 20 inch OD riser x lh inch wall on water =-33 Ibs./ft. 18 lbs,/gal, mud = 264 lbs./ft. between ID of riser and OD of drill pipe Nominal tension for 800 ft. of water, Tnom = (800) (264-331 (1-20)

= 222 kips

For purposes of safety, the tension system must be operated at a level high enough to provide the minimum tension level (that required to maintain the bottom joint of the riser pipe in positive tension) after losing one tensioner unit,

Safety Divisor

Minimum Tension = Riser System wt. plus 5%

~ i , , No. of Tension Units less one ~ „ Safety Divisor --——— —- = S.D.

No. of Tension Units

Therefore:

  • 800 ft.) [231 lbs./ft.) (1.05) OT, = -- Q ?5--- 259 kips
  • 800 ft.) (231 lbs ./ft.) (1.05)
Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

Get My Free Ebook


Responses

  • kaylin
    What do riser and guideline tensioners do?
    5 months ago

Post a comment