Marine riser measuring joint

A marine pipe riser, through which wells may be drilled during offshore drilling operations, is provided with an instrumented section equipped with stress-measuring elements so that loads and stress on the pipe riser may be measured.

BACKGROUND OF THE INVENTION 
The invention relates to a marine riser joint and in particular to a marine 
riser joint for measuring loads exerted on a marine riser. 
Drilling boreholes in underground earth formations situated below a body of 
water may be performed by mounting drilling equipment on a floating 
vessel, and arranging a marine riser between the vessel and a wellhead 
situated at a level near the sea bottom (or ocean bottom). Such a marine 
riser consists of a plurality of sections that are interconnected in 
end-to-end relationship. The sections consist of tubes having a relatively 
large diameter so that a string of interconnected sections forms a tubular 
communication between the wellhead and the floating vessel. Tubular 
equipment (such as a drill string) may be guided from the vessel into the 
well through this communication, which further forms a return passage for 
the drilling fluid that has been used in the well. 
The lower end of the marine riser is connected to the wellhead. The upper 
end of the riser is connected to the floating vessel and loaded with an 
upwardly-directed force to prevent buckling of the riser. A reduction in 
the magnitude of the required upward force can be obtained by arranging 
buoyancy members around the sections. 
It will be appreciated that the marine riser should preferably extend 
almost vertically through the water. To achieve this, the vessel is 
anchored (either by means of sea-anchors or dynamically by the action of 
propulsion means) to maintain a position that is (as nearly as possible) 
straight above the well. However, it will be appreciated that conditions 
may arise, wherein excessive wave or current action on the vessel 
displaces the vessel in a horizontal direction to an extent which may 
cause overloading of the marine riser. 
To prevent damage of the marine riser, the drilling operator should be 
continuously informed on the load condition of the marine riser to enable 
him to take the required steps for counteracting excessive displacement of 
the floating vessel, or to take any other measures to obviate excessive 
loads on the marine riser. 
Some indication of the load exerted on a marine riser may be obtained by 
calculating and/or measuring the curvature thereof. Information on the 
curvature allows the operator (in combination with additional data, such 
as the upward forces exerted by the buoyancy means, the weight of the 
marine riser, the lifting force exerted on the marine riser at the upper 
end thereof) to calculate the stresses in the marine riser at the 
location(s) where extreme load conditions exist. 
SUMMARY OF THE INVENTION 
An object of the invention is a more direct way of measuring loads exerted 
on a marine riser, thus enabling the operator to obtain the required 
information on the load conditions of the riser almost instantaneously, 
which information allows him to take the necessary counter measures for 
load relief of the riser without any delay before these conditions give 
rise to overloading of the marine riser which would create a situation 
endangering the drilling operation. 
The marine riser joint according to the invention comprises a first and a 
second tubular element, each element being provided with coupling means 
such as threads for coupling the joint to and between the ends of sections 
of the marine riser, a third tubular element extending between the first 
and second tubular element and in sealing contact therewith near its ends 
in a manner allowing relative axial displacement between the third element 
and at least one of the other elements, and a load-transmitting element 
forming a firm connection between the first and second tubular elements, 
said load transmitting element being adapted to carry stress measuring 
elements. 
The load-transmitting element may be formed by a fourth tubular element 
having the wall thereof positioned concentric to the third tubular 
element, and comprising areas of reduced wall thickness, which areas 
extend substantially parallel to the axis of the fourth tubular element. 
Said latter element has openings in the wall thereof, each opening 
adjoining two of the said areas in a manner such that the said areas are 
subjected to shearing stresses only when the fourth tubular element is 
subjected to a tensile load in the direction of the axis thereof. Means 
may be provided in a plurality of said areas for measuring shearing 
stress. 
The marine riser joint according to the invention allows the operator to 
measure the load conditions of the marine riser in a manner that is not 
influenced by the fluid pressure prevailing inside the marine riser. 
However, if the inner part of the wall of the third tubular element is 
arranged to be flush with the inner parts of the walls of the first and 
second tubular elements, means will be provided for measuring the 
compressive load exerted on the third tubular element. This compressive 
load results from the fluid pressure inside the joint and equals the axial 
load on the first and second element raised by the said fluid pressure. 
The value of the compressive load is to be deducted from the axial load 
measured in the load-transmitting element. 
By measuring the shearing stress in the load-transmitting element, 
temperature influences on the measuring means may be automatically 
compensated.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The floating vessel 1 (shown in FIG. 1) carries a drilling rig 2 and part 
of the other equipment necessary for marine drilling. Marine drilling 
equipment is known per se and therefore not shown in the drawing, with the 
exception, however, of the marine riser 3 and the submerged wellhead 4. 
This wellhead is situated at a level near the sea bottom 5, and the marine 
riser 3 extends from the wellhead 4 to a location near the sea level 6, 
and is supported by the vessel 1 in a manner known per se. 
The marine riser 3 is connected at the lower end thereof to the wellhead 4 
and consists of a plurality of large-diameter pipe or tubular sections 7 
that are interconnected in end-to-end relationship. To allow such coupling 
of adjacent sections, each section comprises coupling means at both ends 
thereof such, for example, as threaded joints. Such coupling means are 
known per se, and therefore not described or shown in detail. This also 
applies for all other auxiliary equipment that may be included in the 
marine riser such as ball joints, buoyancy means, expansion joints, with 
the exception, however, of the instrumented load-measuring marine riser 
joint 8 that is arranged between sections 7 near the lower end of the 
marine riser 3. This marine riser joint 8 is shown in greater detail in 
FIGS. 2-5 of the drawings, and is applied for measuring loads or stresses 
exerted on the marine riser 1. 
The marine riser joint shown in FIG. 2 comprises a first tubular element 9 
and a second tubular element 10, which elements are co-axially arranged in 
spaced relationship. 
The first tubular element 9 is provided at one end thereof with coupling 
means, e.g., threads 11, for coupling the joint 8 (either directly or 
indirectly) to a co-operating coupling means (not shown) of a marine riser 
section superimposed on the joint. Any type of coupling means may be used 
for the purpose. The lower end of the first tubular element 9 carries a 
flange 12. 
The second tubular element 10 is provided at the lower end thereof with 
coupling means 13 for coupling the joint 8 (either directly or indirectly) 
to a co-operating coupling means (not shown) of a marine riser section 
arranged directly below the joint. Any type of coupling means may be used 
for the purpose. The upper end of the second tubular element 10 carries a 
flange 14. 
A load-transmitting element 15 is arranged between the flanges 12 and 14 of 
the tubular elements 9 and 10, respectively. The load-transmitting element 
15 is connected to the flanges through the intermediary of bolts 16 (only 
one bolt being shown in each flange) which bolts transmit, in co-operation 
with the element 15, any load exerted on the first tubular member 9 to the 
second tubular member 10. To measure the loads passing through the 
load-transmitting element 15, this element carries suitable load-measuring 
or load-sensing means (not shown) as will be explained hereinafter. 
A third tubular element 17 is arranged between the first and second tubular 
elements 9 and 10 in a fluid-tight manner to allow any fluid flowing from 
the wellhead 4 to pass upwards into the marine riser 3 without any leakage 
and to protect the inner wall of the load-transmitting element 15 against 
damage by equipment passing through the marine riser. The upper end of the 
third tubular element 17 is thereto sealingly guided in a cylindrical 
depression 18 of the first tubular element 9, whereas the lower end of the 
element 17 is sealingly guided in the cylindrical depression 19 of the 
second tubular element 10. Sealing rings 20 and 21 are arranged for that 
purpose. Thus, the third tubular element 17 is allowed to carry out 
relative displacements with respect to the elements 9 and 10, in a manner 
such that elongation of the element 17 resulting from temperature increase 
thereof by the fluid flowing therethrough, will not exert any load on the 
elements 9 and 10. That is, the length of element 17 is less than the 
distance between the shoulders of depressions 18 and 19. 
The annular slit between the element 9 and the element 17, as well as the 
annular slit between the element 10 and the element 17 may contain a 
compressible rubber composition (not shown) to prevent entry of dirt 
therein, which dirt might settle within the slits and prevent relative 
displacements between the elements 9 and 17, and between the elements 10 
and 17. 
The load-transmitting element 15 consists of a tubular element, on which 
load measuring elements are attached. Such elements may consist of 
electric strain gauges, like elements 27 through 30 of FIG. 6, that are 
glued to the outer or inner wall of the element 15 for measuring the 
elongation (or compression) thereof at various locations distributed along 
the circumference of the element 15. The strain gauges are electrically 
connected in a well-known manner to suitable amplifying means (not shown) 
and the resulting signals are passed on to the vessel 1 through a suitable 
electric cable (not shown). All such equipment is known per se and 
therefore not shown in the drawings. 
The load-transmitting element 15 is designed for directly measuring the 
axial load exerted on this element. It will be appreciated that the 
measuring results obtained have to be compensated for temperature 
differences, since the electronic circuit is tested and adjusted at the 
temperature prevailing at the surface of the earth, but used at a 
relatively low temperature at underwater locations, which temperatures 
vary with the operational depth at each individual location. 
To obviate temperature compensations, the load-transmitting element of the 
joint according to the invention may be designed in a manner such that the 
axial load passing therethrough creates a pure shear load in particular 
locations of the load-transmitting elements. Measuring these shear loads 
by means of a pair of strain gauges arranged at 45.degree. with respect to 
the direction of the shear load automatically compensates the measuring 
results for temperature differences. 
A load-transmitting element designed for the above purpose is shown in 
FIGS. 4 and 5. This element is a tubular element with external dimensions 
equal to those of the load-transmitting element 15 shown in FIG. 2, and 
can be inserted between the elements 9 and 10 in FIG. 2 to replace the 
element 15 shown in this FIG. 2. 
For a schematic description of this alternative load-transmitting element 
22, a development of the inner wall thereof is shown in FIG. 4 of the 
drawings. FIG. 5 shows a cross section of this development taken along the 
section V--V in FIG. 4. 
It will be appreciated that the difference between the load-transmitting 
element 15 of FIG. 2 and the alternative element 22 of FIG. 4 exists in 
the presence of openings 23 and grooves 24. These grooves form areas of 
reduced wall thickness and the arrangement of the openings 23 and the 
grooves 24 is such that interdigitating areas 25 and 26, having the 
original wall thickness of the element 22, are being formed thereby. An 
axial load that is transmitted from the lower area 26 to the upper area 25 
then creates pure shear forces in the grooves 24. To measure the magnitude 
of these forces in each groove 24, each pair of adjacent grooves 24', 24" 
(see FIG. 6) has glued therein a pair of electric strain gauges 27, 28 and 
29, 30. These four strain gauges are electrically coupled in a bridge 
circuit and the resulting signal obtained therefrom is representative for 
the sum of the shear forces in the two adjacent grooves 24', 24" and free 
from temperature influences. Since the load-transmitting element 22 has 
four pairs of adjacent grooves, each group of four strain gauges in a pair 
of adjacent grooves measures that part of the load that passes through a 
quadrant of the cross-section of the load-transmitting element. The 
quadrants I-IV shown in FIG. 4 in relation to the element 22 correspond 
with the quadrants I-IV shown in FIG. 3 in relation to the 
load-transmitting element 15 of which element 22 is an alternative. 
The manner in which the measured data resulting from the four quadrants may 
be combined to obtain the value of the average tension in the marine riser 
joint according to the invention will now be explained with reference to 
the electronic block diagram shown in FIG. 7 of the drawings. It will also 
be shown in which manner the maximum bending moment to which the joint is 
subjected may be ascertained. 
The signals that are obtained from the measuring joint shown in FIGS. 2 and 
3 (which may alternatively be equipped with the load-transmitting element 
22 shown in FIGS. 4-6) are transmitted to the vessel 1 (see FIG. 1) in one 
of the manners known per se. 
The assembly of the electric strain gauges in each quadrant delivers an 
electric signal that is representative for the average value of the axial 
load to which the body portion in each quadrant of the cross-section of 
the load-transmitting element is subjected. The four quadrants I, II, III 
and IV are indicated in FIG. 3 (and also in FIG. 4) and the axial loads 
passing through these quadrants are correspondent with the signals 
T.sub.1, T.sub.2, T.sub.3 and T.sub.4, respectively (see FIG. 7). Each 
signal is to be compensated for the part thereof resulting from the mud 
pressure exerted on the annular portion 26 (see FIG. 2) of the first 
tubular part 9. Thereto, the mud pressure inside the third tubular element 
17 is measured (such as by a piezo-electric element), and the electric 
signal MP resulting from such measurement is multiplied by a factor to 
obtain a signal MP' that is representative for the upward force exerted by 
the mud in each of the quadrants I-IV on the ring surface 26. This signal 
is deducted from each of the signals T.sub.1, T.sub.2, T.sub.3 and 
T.sub.4 and the resulting signals TC.sub.1 TC.sub.2, TC.sub.3 and 
TC.sub.4, respectively, are representative for the average tension (or 
compression) in the quadrants I-IV, respectively. Addition of the signals 
TC.sub.1, TC.sub.2, TC.sub.3 and TC.sub.4 results in a signal AT that is 
representative for the average tension exerted on the lower end of the 
marine riser. The bending moments in X and Y direction can be obtained by 
combining the signals TC.sub.1 and TC.sub.3 to signal BM.sub.x, and the 
signals TC.sub.2 and TC.sub.4 to signals BM.sub.y, respectively. The 
signals BM.sub.x and BM.sub.y can be combined for calculating the maximum 
bending moment BM.sub.t and the angle .phi. thereof with respect to a 
heading .alpha. of the wellhead 4. 
Finally, the mud gradient MG can continuously be read off by the operator, 
by dividing the signal MP' (which is representative for the mud pressure 
at the level of the third tubular element 17) by a factor WD that is 
representative for the depth at which the element 17 is submerged below 
the sea level 6. 
It will be appreciated that the marine riser joint according to the 
invention allows the drilling operator to continuously watch the load 
condition of the marine riser, and to take any steps instantaneously that 
are required to prevent overloading of the said riser. 
If desired, more than one instrumented joint according to the invention may 
be used in a marine riser. These joints may be situated close to each 
other (in order to duplicate the measuring results for safety reasons) 
and/or be distributed over the height of the riser. Preferably, at least 
one of the joints is arranged near or at the lower end of the riser, or in 
any tubular extension thereof that forms a connection between the marine 
riser and the wellhead, or between the wellhead and the well. 
It will be appreciated that the invention is not limited to the use of 
bolts 16 for securing the load-transmitting element 15 to the first and 
second tubular elements 9 and 10. Any other means suitable for the purpose 
may be applied. Preferably, such means are adapted for transmitting the 
load to the cross section of the load-transmitting element 15 in a pattern 
equal to the load distribution over the cross section of the elements 9 
and 10. 
If desired, a protective sleeve may be arranged around the joint 8 to 
protect this joint and the signal amplifying means carried thereby against 
damage. 
Although the arrangement of the load-transmitting element 15 having the 
inner wall thereof flush with the inner walls of the elements 9 and 10 is 
preferred, the invention is not limited to such construction. If desired, 
the element 15 may be arranged partly inside the elements 9 and 10. The 
signals T.sub.1, T.sub.2, T.sub.3 and T.sub.4 are then not compensated by 
the mud signal MP (which varies during the drilling operation), but by a 
signal WD representative for the waterdepth which is often constant for 
each individual drilling operation.