Electronic combined load weak link

Abstract

A safety device and method for protection of the integrity of well barrier(s) or other interfacing structure(s) at an end of a riser string or a hose includes a releasable connection in the riser string or hose, the releasable connection arranged to release or disconnect during given predefined conditions in order to protect the well barrier(s) or other interfacing structure(s). The safety device safety device includes at least one sensor to monitor at least one of tension loads, bending loads, internal pressure loads and temperature. The sensor provides measured data relating to at least one of tension loads, bending loads, internal pressure loads and temperature. An electronic processing unit receives and interprets the measured data from the sensor. An electronic, hydraulic or mechanical actuator or switch is arranged to receive a signal from the electronic processing unit and initiate a release or disconnect of the releasable connection.

Claims

1. A safety device for protection of the integrity of well barrier(s) or other interfacing structure(s) at an end of a riser string or a hose, the safety device comprising a releasable connection in the riser string or hose, the releasable connection arranged to release or disconnect during given predefined conditions in order to protect the well barrier(s) or other interfacing structure(s), wherein the safety device comprises: at least one sensor to monitor at least one of tension loads, bending loads, internal pressure loads and temperature, where said at least one sensor is arrangeable on a segment of the riser or hose, and where said at least one sensor is adapted to provide measured data relating to at least one of tension loads, bending loads, internal pressure loads and temperature, an electronic processing unit adapted to receive and interpret the measured data from said at least one sensor, an electronic, hydraulic or mechanical actuator or switch arranged to receive a signal from the electronic processing unit and initiate a release or disconnect of the releasable connection, wherein the electronic processing unit is configured to autonomously send the signal to the electronic, hydraulic or mechanical actuator or switch when the measured data is indicative of the given predefined conditions.

2. Safety device according to claim 1, wherein said at least one sensor to monitor at least one of tension loads, bending loads, internal pressure loads and temperature is arranged close to the well barrier(s) or the end(s) of the riser string or hose in order to allow reliable measurements of riser string or hose bending moments or deflection angles.

3. Safety device according to claim 1, wherein said at least one sensor to monitor at least one of tension loads, bending loads, internal pressure loads and temperature comprises any number and/or any combination of one or more of the following sensors or measuring devices: strain gauges potentiometers optic displacement sensors pressure gauges temperature gauges in order to ensure the reliability of the measured data.

4. Safety device according to claim 1, wherein the electronic processing unit, if it receives measured data from a number of sensors providing overlapping results, comprises a voting system arranged to select what results to apply in order to ensure that only reliable results are interpreted by the system.

5. Safety device according to claim 1, wherein the releasable connection comprises a split cam ring with a number of rotating connector dogs, where the releasable connection is arranged to hold together the flanges of two riser string or hose sections, and where the split cam ring of the releasable connection further comprises two or more hinges to close the split cam ring around the flanges, where one or more of the hinges comprises: 1) a removable locking pin so that the cam ring is split to release the grip on the connector dogs by removing the locking pin, or 2) a releasable latching mechanism so that the cam ring is split to release the grip on the connector dogs by opening the latch mechanism in one of the hinged elements of the cam ring.

6. Safety device according to claim 1, wherein it comprises a disengagement mechanism to ensure disengagement of any control umbilical running along the riser string or hose and which needs to be disconnected together with the riser string to protect the integrity of the well barrier(s) or other interfacing structure(s), the disengagement mechanism comprising one or more of the following: an electrically activated over-center mechanism to release a spring loaded cutting tool, an electrically driven release of an energized cutting tool, a hydraulically driven cutting tool, a clamping device for securely clamping the umbilical to the riser string or hose, and furthermore arranged to tear off the umbilical when the riser string or hose is separated.

7. Safety device according to claim 1, wherein the electronic processing unit is without any external power supply or control signals going into the electronic processing unit during operation.

8. Safety device according to claim 1, wherein the electronic processing unit is arranged in the vicinity of the releasable connection and/or said at least one sensor.

9. Safety device according to claim 1, wherein the electronic processing unit is arranged remotely from the releasable connection and/or said at least one sensor.

10. Safety device according to claim 1, wherein the electronic processing unit is connected to an actuator mechanism which upon signal will trigger a disengagement of the releasable connection in the riser string or hose, wherein the actuator mechanism is one or more of: an electric switch, electric or magnetic release of a spring loaded over-center mechanism, electric or mechanical opening or closing of hydraulic valves to trigger a hydraulic release mechanism.

11. Safety device according to claim 1, wherein the releasable connection comprises a number of connector dogs that hold the flange faces in the riser string together at a certain pretension level in order to provide the required seal pressure between the flange faces, and wherein the connector dogs are free to rotate in order to allow the flange faces to be pulled apart when the connector dogs are released, even under high loads.

12. Safety device according to claim 6, wherein the locking pin and/or latching mechanism securing the split cam ring during nounal operation is energized using either a mechanical spring or a pressurized hydraulic unit, where the energy in the spring or hydraulic unit is arranged to be released by the actuator, causing the locking pin to be removed from the split cam ring, thereby causing the split cam ring to separate and disengage from the connector dogs.

13. Safety device according to claim 2, wherein the electronic processing unit, if it receives measured data from a number of sensors providing overlapping results, comprises a voting system arranged to select what results to apply in order to ensure that only reliable results are interpreted by the system.

14. Safety device according to claim 3, wherein the electronic processing unit, if it receives measured data from a number of sensors providing overlapping results, comprises a voting system arranged to select what results to apply in order to ensure that only reliable results are interpreted by the system.

15. Method for providing protection of the integrity of well barrier(s) or other interfacing structure(s) at an end of a riser string or a hose, the method comprising the step of providing a releasable connection in the riser string or hose, where the releasable connection is arranged to release or disconnect during given predefined conditions in order to protect the well barrier(s) or other interfacing structure(s), and where the releasable connection is provided between two riser string or hose sections or between the riser and any other part interfacing the riser string or hose, the method being wherein it further comprises the steps of: a) monitoring and measuring loads in the riser string or hose related to at least one of tension loads, bending loads, internal pressure loads and temperature, and providing measurement data, b) determining a combined load on the riser string or loading hose, and the well barrier(s) or other interfacing structure(s) to the riser string or hose on the basis of the measurement data using a processing unit, c) comparing the determined combined load based on the measurement data with a pre-defined allowable combined load capacity using the processing unit, and, if the determined combined load based on the measurement data exceeds the pre-defined allowable combined load capacity: d) the processing unit autonomously sending a signal to the releasable connection, and e) disconnecting the riser string or hose from the well barrier(s) or other interfacing structure(s) in response to the signal.

16. Method according to claim 15, wherein the step of providing measurement data in the riser string or hose is continuously or discontinuously received and processed by an electronic processing unit, wherein the electronic processing unit continuously or discontinuously, respectively, determines the combined load in the riser string or hose, and compares the determined combined load with the pre-defined allowable combined load capacity of the well barrier(s) or other interfacing structure(s).

17. Method according to claim 15, wherein the capacity of the structure at either end of the riser string or hose is defined as a combined load capacity curve covering any relevant combination of tension load, bending load, internal pressure load and temperature in the riser string or hose, as well as the relative angle between the riser string or hose and the well barrier(s) or other interfacing structure(s).

18. Method according to claim 15, wherein the combined load in the riser string or hose is evaluated according to the following equation: $f=\frac{T_e}{F_s{T}_{\max }}+\frac{M_{\mathrm{tot}}}{F_s{M}_{\max }}+\frac{p_i}{F_s{p}_{\max }}$ where: F.sub.sis an overall safety factor as defined by operator or regulations, T.sub.maxis the maximum allowable tension in the releasable connection and typically set to the tension capacity of the limiting barrier component, M.sub.maxis the maximum allowable bending moment in the releasable connection and typically set to the bending capacity of the limiting barrier component.

19. Method according to claim 18, wherein the monitored and measured loads related at least one of tension loads, bending loads, internal pressure loads and temperature somewhere along the riser string or hose, are converted to local surface stress parameters according to the equations: $\begin{array}{c}{}_z=\frac{E}{1-v^2}\left({\mathrm{.Math.}}_z+v{\mathrm{.Math.}}_{}\right)-\frac{ET}{1-v}\\ {}_{}=\frac{E}{1-v^2}\left({\mathrm{.Math.}}_{}+v{\mathrm{.Math.}}_z\right)-\frac{ET}{1-v}\end{array}$ where: .sub.zaxial stress .sub.hoop stress .sub.zaxial strain .sub.hoop strain EYoung's modulus Possion's ratio thermal expansion coefficient Ttemperature difference relative to reference temperature these equations covering the situation with constant temperature over the cross section, and temperature induced strain compensated for in the equations by using the materials coefficient of temperature expansion and the measured temperature.

20. Method according to claim 19, wherein the local surface stress parameters are converted to internal pressure, effective tension and bending moment parameters according to the following equations, where an index 0, 90, 180 and 270 indicates the position around the circumference of the riser string or hose: $\begin{array}{cc}{M}_x=\frac{\left({}_{z,90}-{}_{z,270}\right)}{2}\frac{}{32D_o}\left(D_o^4-D_i^4\right)& \left(\mathrm{Bending}\mathrm{about}\mathrm{local}x\text{-}\mathrm{axis}\right)\\ M_y=\frac{\left({}_{z,0}-{}_{z,180}\right)}{2}\frac{}{32D_o}\left(D_o^4-D_i^4\right)& \left(\mathrm{Bending}\mathrm{about}\mathrm{local}y\text{-}\mathrm{axis}\right)\\ M_{\mathrm{Tot}}=\sqrt{M_x^2+M_y^2}& \left(\mathrm{Combined}\mathrm{bending}\mathrm{moment}\right)\\ T=\frac{\left(\begin{array}{c}{}_{z,0}+{}_{z,90}+\\ {}_{z,180}+{}_{z,270}\end{array}\right)}{4}\frac{}{4}\left(D_o^2-D_i^2\right)& \left(\mathrm{True}\mathrm{wall}\mathrm{tension}\right)\\ T_e=T-p_i\frac{}{4}{D}_i^2& \left(\mathrm{Effective}\mathrm{tension}\right)\\ p_i=\frac{\left(\begin{array}{c}{}_{,0}+{}_{,90}+\\ {}_{,180}+{}_{,270}\end{array}\right)}{4}\frac{1-{\left(\frac{D_i}{D_o}\right)}^2}{2{\left(\frac{D_i}{D_o}\right)}^2}& \left(\mathrm{Internal}\mathrm{pressure}\right).\end{array}$

Description

SHORT DESCRIPTION OF THE DRAWINGS

(1) The following is a detailed description of advantageous embodiments, with reference to the figures, where:

(2) FIG. 1 shows a vessel 3 during a workover operation, where a rigid riser 2 is suspended from a heave compensator 1 on the rig and is rigidly attached to a wellhead (well barrier(s) 5) on the seabed. The heave compensator 1 strokes up and down to compensate for the heave motion of the vessel 3 in the waves.

(3) FIG. 2 illustrates the accidental scenario referred to as heave compensator lock-up, causing a tension increase in the riser 2 when the waves lifts the vessel upward. The rapid increase in riser tension will typically result in excessive combined loading of the well barrier(s) 5.

(4) FIG. 3 illustrates the accidental scenario referred to as loss of position (due to loss of an anchor, drive-off or drift off) and how this will cause excessive bending in the well barrier(s) once the heave compensator 1 has stroked out.

(5) FIG. 4 shows a typical operational envelope of a vessel for a workover operation. The figure further illustrates how allowable vessel offset needs to be limited to protect the well barrier(s) from heave compensator lock-up when the weak link being used relies on failure of a riser component in tension. The figure shows how much the operational envelopes can be increased if there is a weak link that protects the well barrier(s) against any type of combined loading without regard for vessel position of system pressure.

(6) FIG. 5 illustrates the challenge of designing a weak link that fulfils all safety criteria in normal operation, but at the same time ensures a reliable release in an accidental scenario before the well barrier(s) is(are) damaged. The figure illustrates the problem related to the width of the band between the weak link fulfilling all design requirements and the structural failure capacity of the same weak link.

(7) FIG. 6 illustrates a typical defined combined loading capacity curve for well barrier(s) 5. The load capacity curve does not represent an actual break of the well barrier(s), but indicates the design curve that has been used for accidental scenarios where all safety factors have been removed. When the combined load in the well barrier(s) 5 exceeds this curve there is no guarantee for the integrity of the well barrier(s), and there is a significant risk of having damaged the seals or having caused some form of permanent damage to the well barrier(s) 5.

(8) FIG. 7 illustrates the problem of using a weak link based on structural failure in a riser component to protect the well barrier(s) in case of a heave compensator lock-up. The figure shows how the combined load in the well barrier(s) 5 will exceed its capacity curve before the structural capacity of the weak link is reaches typically due to the vessel 3 offset causing the angle which increases the bending loads on the well barrier(s) 5.

(9) FIG. 8 illustrates the problem of using a weak link based on structural failure in a riser component to protect the well barrier(s) in case of a loss of position accidental scenario. The figure shows how the riser 2 tension remains constant until the heave compensator 1 stroke out. At this point the tension will increase rapidly and the angle will cause high bending loads in the well barrier(s) 5, causing the load capacity of the well barrier(s) 5 to be exceeded long before reaching the structural failure of the riser weak link designed to fail in tension.

(10) FIG. 9 shows how the present invention would work to protect the well barrier(s) 5 in case of a heave compensator 1 lock-up. The figure shows how the combined load capacity of the weak link is defined to be just within the capacity of the well barrier(s) 5. Hence for any load combination induced on the well barrier(s) 5 the invention will ensure a controlled disconnect of the riser before exceeding the capacity curve of the well barrier(s) 5.

(11) FIG. 10 shows how the present invention would work to protect the well barrier(s) 5 in case of the vessel loosing its position due to a drive-off or drift-off scenario. The figure shows how the combined load capacity of the weak link is defined to be just within the capacity of the well barrier(s) 5. Hence for any load combination induced on the well barrier(s) 5 the invention will ensure a controlled disconnect of the riser before exceeding the capacity curve of the well barrier(s) 5.

(12) FIG. 11 shows a cross section of an embodiment of the present invention with a disconnectable connector 6, a sensor package 19 to measure combined loading in the riser 2, an electronic unit which interprets the information from the sensors and checks if the combined load in the riser is within the allowable limits and if not trigger a disconnect sequence.

(13) FIG. 12 illustrates the actuation sequence when releasing the locking pin 8 that holds the cam ring 7 of the connector 6 in place.

(14) FIG. 13 shows one possible embodiment of the actuator mechanism 20 for disconnecting the releasable connector 6 and some alternative release mechanisms that may be applied. In this possible embodiment of the actuator 15a, a spring 10 loaded locking pin 8, which locks the connector, is supported by an over-center mechanism which is balanced by a magnet or an electrical switch. When the electronic unit 20 recognizes that the measured combined load reaches the defined combined load limit curve the switch or magnet will release the over-center mechanism. The rotation of the over-center mechanism will release the spring 10, thereby releasing the locking pin 8 to trigger a disconnect of the releasable connector 6. Alternative configurations of the actuator is shown in 15b with an electric motor for releasing the locking pin 8 and in 15c where the locking pin 8 is removed hydraulically by opening an electric valve connected to a charged accumulator.

(15) FIG. 14 shows a disconnect sequence of the preferred embodiment of the present invention from the point where the spring loaded locking pin 8 is released. The spring loaded locking pin is pulled out from the connectors cam ring 7 by the force of the preloaded spring. When the locking pin 8 is removed, the cam ring 7 will open due to the tension forces in the system or by using a leaf spring in the cam ring 7. When the cam ring opens the upper and lower part of the pipe hubs in the connector will pull apart as the connector dogs 9 are free to rotate.

(16) FIG. 15 shows a 3D illustration of a disconnect sequence of the preferred embodiment of the present invention.

(17) FIG. 16 illustrates alternatives for disconnecting the control umbilical when the connector disengages in an accidental scenario. In the preferred embodiment of the invention the umbilical is clamped tightly to the workover riser on either side of the electronic combined loading weak link. This method relies on the tension forces in the system to ensure that the umbilical is torn off when the connector 6 is released. An alternative solution to cut the control umbilical is illustrated in 14a using an over center mechanism which is triggered electronically to release a cutting ram which is charged by a mechanical spring held in place by the over center mechanism. 14b is a similar solution where the cutting ram is released by an electric motor rotating a disk that holds the ram in place during normal operation. 14c uses a hydraulic principle to move the shear ram to cut the umbilical. In this case a valve to a charged accumulator is opened electrically to push to cutting ram towards the umbilical.

DETAILED DESCRIPTION OF THE INVENTION

(18) The safety device according to the present invention responds to bending forces in the riser system in addition to tension forces. Furthermore, the device according to the present invention preferably monitors the total combined load including tension, bending, internal pressure and/or temperature effects. All these parameters may continuously be monitored by an autonomous electronic unit 20 which evaluates the combined load on the system and ensures that the combined load is kept within pre-defined allowable limits. The electronic unit 20 compares the evaluated combined load with a pre-defined, limiting combined loading curve developed to protect the well barrier(s) 5 and which will be defined by the calculated relationship between the combined load at the position of the weak link and the combined load capacity curve for the well barrier(s). If the combined load measured exceeds the defined limit curve for the well barrier(s) 5 on the well in question the electronic unit 20 will trigger a disconnect of a releasable connector in the riser.

(19) One embodiment of the electronic combined loading weak link according to the present invention comprises a sensor 18 pipe with an electronic processing unit 20 which interprets the combined loading condition in the sensor pipe 18. The limiting combined load in the sensor pipe is developed to ensure the integrity of the well barrier(s) (ref. FIG. 9 and FIG. 10) and is given as input to the electronic processing unit. If the combined load in the sensor pipe 18 exceeds the defined allowable limit, the unit will activate a mechanical, electric or hydraulic trigger which will disengage a releasable connector 6 in the riser 2.

(20) A standard connector principle may be modified with a release mechanism 11 using a hinged and split cam ring 7 and a spring loaded locking pin 8 as illustrated in FIG. 11-FIG. 16. The locking pin 8 may also be energized using any sort of hydraulic arrangement. The split cam ring 7 is pre-tensioned to engage connector dogs 9 with sufficient force as for a normal connector design. In order to accommodate a disconnect function the split cam ring 7 is hinged in two or more locations. It is understood that the number of hinges may be higher or lower, for example 3, 4, 5, 6, or any other suitable number. At least one of the hinges is connected by an energized locking pin 8. The locking pin 8 is energized with sufficient force to ensure that the locking pin can be retracted from the split cam ring 7 when the split cam ring 7 is pre-tensioned up to it's maximum design load. According to one embodiment the locking pin 8 is energized by a loaded mechanical spring 10. Alternatively a pressurized hydraulic system with electronically actuated valves may equally well be used. Pure electric retraction of the locking pin 10 may be another option. Several alternative principles for retracting the locking pin are illustrated in FIG. 12. The locking pin 8 holds the split cam ring 7 together as long as the locking pin 8 is in place. In order to disconnect the riser 2, the locking pin 8 in the split cam ring 7 is released by releasing the mechanical spring 10, alternatively by opening a hydraulic valve, or any other suitable method for retracting the locking pin 8. The locking pin 8 is then pulled out and cleared from the split cam ring 7, which will then open up due to the tension forces in the system. The connector dogs 9, which hold the flanges of two riser sections together, are then free to rotate, and the tension in the riser 2 will ensure that the flange faces 11 of the riser sections are pulled apart, and the riser 2 is disconnected from the well. Radial springs (not shown) may be incorporated into the split cam ring 7 in order to ensure that the split cam ring 7 opens up when the locking pin 8 is retracted. It is understood that a releasable latching mechanism (not shown) may be used instead of locking pin 8.

(21) The disconnect sequence is illustrated in FIG. 14 and FIG. 15.

(22) In the case that an umbilical line 12 is deployed along the riser, for example during work over applications using a work over riser (WOR), umbilical release is ensured by applying tight umbilical clamps 13 in the region immediately above and below the electronic combined loading weak link connector, as shown in FIG. 16. This will ensure a concentrated load/strain in the umbilical 12 at the location of the connector. The strain concentration will cause the umbilical 12 to tear off when the electronic combined loading weak link connector is released. Tearing off the umbilical 12 will initiate a shut down sequence, securing the well barrier(s) 5. For umbilical designs not suitable for being torn off by axial loads, a spring loaded shear ram mechanism may be used to cut the umbilical. The shear ram may be triggered by an actuator similar to the one used to release the locking pin 8. Alternative configurations of such a shear ram for umbilical cutting are illustrated in FIG. 16.

(23) According to one embodiment of the present invention, again with reference to FIG. 11 a sensor pipe 18 may comprise a machined pipe section which is provided with for example three separate and complete instrument packages 19. The instrument packages 19 may for example comprise a number of strain gauges, a number of temperature gauges and/or a number of pressure gauges or strain gauges set to measure hoop stress used to deduct internal over pressure. Each instrumentation package 19 will primarily be fitted around the circumference of the sensor pipe 18, but may also be fitted in alternative configurations. An electronic processing unit 20 will continuously monitor signals from the sensors in each of the (e.g. three or more) instrumentation packages 19 on the sensor pipe 18.

(24) According to one embodiment, the signals may be processed by a voting system in order to ensure that only functioning sensors are interpreted by the system. The signals will further be used in an algorithm developed to monitor the combined loading in the pipe. Pressure measurements will be used in an algorithm to ensure that the device works equally well if the riser is unpressurized or if the riser is fully pressurized to its design pressure. The electronic processing unit 20 may be designed according to the appropriate Safety Integrity Level (SIL) as required by the relevant authorities to ensure sufficient system reliability. According to one embodiment of the present invention, the electronic unit may be designed according to SIL2 requirements to ensure sufficient reliability of the system, but higher or lower levels of safety performance may be chosen according to need, requirement and/or preference.

(25) According to the present invention, the measurement of the measurement data relating to at least one of tension loads, bending loads, internal pressure loads and temperature, may be continuously or discontinuously received and processed by the electronic processing unit (20). Furthermore, the electronic processing unit (20) may continuously or discontinuously determine the combined load in the riser string or hose (2), and compares the determined combined load with the pre-defined allowable combined load capacity of the well barrier(s) (5) or other interfacing structure(s).

(26) A release curve, of which two examples are given in FIG. 9 and FIG. 10, can be given as an input to the electronic unit 20 for each specific field or project. Thus the Safety Device according to the present invention is suitable for operation on any field, as the release curve may be tailored for each individual location and application.

(27) The purpose of the instrumentation packages 19 on the sensor pipe 18 is to capture the internal pressure, the bending moment and the axial tension of the weak link detector pipe. To do this, the following sensors would, according to one possible embodiment, be needed: For redundancy, 3 independent measuring sections are recommended. Each measuring section may contain: 4 strain measuring points including strain gauge rosettes located at for example 0, 90, 180 and 270 around the circumference of the sensor pipe 18. Each point must contain strain gauges in both the axial and the hoop direction. Temperature sensor(s). An electronic processing unit containing: Logics to process the strain and temperature measurements from each measuring section mentioned above; A voting system for selecting between the measuring sections.

(28) An example of each step necessary to carry out one embodiment of the present invention is outlined in the following. It is understood that the specific steps and methods to deduce the various results may vary and that the person skilled in the art with the benefit of the present teachings may chose to simplify, rewrite, add, or exclude certain terms and/or parameters in the following exemplary equations and steps.

(29) 1. Conversion of Measured Strain to Stress:

(30) The surface of the pipe where the strain gages are located is in a plane stress condition. The following equations apply for converting the local strain and temperature at the pipe outer surface to local stress:

(31) $\begin{array}{cc}{}_z=\frac{E}{1-v^2}\left({\mathrm{.Math.}}_z+v{\mathrm{.Math.}}_{}\right)-\frac{ET}{1-v}& \left(\mathrm{Axial}\mathrm{stress}\right)\\ {}_{}=\frac{E}{1-v^2}\left({\mathrm{.Math.}}_{}+v{\mathrm{.Math.}}_z\right)-\frac{ET}{1-v}& \left(\mathrm{Hoop}\mathrm{stress}\right)\end{array}$

(32) Where: .sub.zAxial stress .sub.Hoop stress .sub.zAxial strain .sub.Hoop strain EYoung's modulus Possion's ratio Thermal expansion coefficient TTemperature difference relative to reference temperature

(33) These equations will cover the situation with constant temperature over the cross section. The strain contribution from temperature changes will be compensated for in the algorithm based on the temperature measured by the temperature sensor(s).

(34) 2. Convert Surface Stress to Pressure, Tension and Bending Moment

(35) The following equations may be used to convert from stress at pipe surface to effective tension, internal pressure and bending moment (index 0, 90, 180 and 270 indicates position around circumference):

(36) $\begin{array}{cc}{M}_x=\frac{\left({}_{z,90}-{}_{z,270}\right)}{2}\frac{}{32D_o}\left(D_o^4-D_i^4\right)& \left(\mathrm{Bending}\mathrm{about}\mathrm{local}x\text{-}\mathrm{axis}\right)\\ M_y=\frac{\left({}_{z,0}-{}_{z,180}\right)}{2}\frac{}{32D_o}\left(D_o^4-D_i^4\right)& \left(\mathrm{Bending}\mathrm{about}\mathrm{local}y\text{-}\mathrm{axis}\right)\\ M_{\mathrm{Tot}}=\sqrt{M_x^2+M_y^2}& \left(\mathrm{Combined}\mathrm{bending}\mathrm{moment}\right)\\ T=\frac{\left(\begin{array}{c}{}_{z,0}+{}_{z,90}+\\ {}_{z,180}+{}_{z,270}\end{array}\right)}{4}\frac{}{4}\left(D_o^2-D_i^2\right)& \left(\mathrm{True}\mathrm{wall}\mathrm{tension}\right)\\ T_e=T-p_i\frac{}{4}{D}_i^2& \left(\mathrm{Effective}\mathrm{tension}\right)\\ p_i=\frac{\left(\begin{array}{c}{}_{,0}+{}_{,90}+\\ {}_{,180}+{}_{,270}\end{array}\right)}{4}\frac{1-{\left(\frac{D_i}{D_o}\right)}^2}{2{\left(\frac{D_i}{D_o}\right)}^2}& \left(\mathrm{Internal}\mathrm{pressure}\right)\end{array}$
3. Failure Functions and Weak Link Release Criteria

(37) To establish a logical signal giving failure/no failure, a range of failure functions may be used. These failure functions may trigger on single loads or a combination of different loads depending on existing limitations in the equipment. The following combined failure function may be used:

(38) $f=\frac{T_e}{F_s{T}_{\max }}+\frac{M_{\mathrm{tot}}}{F_s{M}_{\max }}+\frac{p_i}{F_s{p}_{\max }}$

(39) Where: F.sub.sAn overall safety factor (defined by operator or regulations T.sub.maxIs the maximum allowable tension in the weak link (typically set to the tension capacity of the limiting barrier component) M.sub.maxIs the maximum allowable bending moment in the weak link (typically set to the bending capacity of the limiting barrier component)

(40) Release should be triggered when the failure function exceeds 1. Typically T.sub.max and M.sub.max will be project specific and will be given as input to the weak link algorithm for a specific wellhead system to define the appropriate release limit for that well.

(41) The instrumentation of the riser can be performed with any type of commercially available measuring device. The measurement can be based either on systems measuring local strain on the riser surface or it can be a system measuring displacement/deformation of the riser structure over a defined length.

(42) Tension in the system is typically measured with strain gauges which are fixed to the riser surface and measures strain on the riser surface. Strain gauges are typically based on measuring changes in the electrical resistance in the material as the length and/or shape of the spools shown on the figure changes with material deformation.

(43) Tension can also be measured by measuring the global elongation of the riser of a pre-defined length segment. This can be done by measuring change in conductivity in a pre-tensioned electrical wire, optically with laser systems, or with other commercial systems that also are available.

(44) Bending moment in the riser can be done by combining strain measurements around the cross section of the riser to separate the bending strains from the axial strains in the pipe. Alternatively, the curvature in the riser of a pre-defined length segment can be measured directly by measuring changes in the electrical conductivity of specially developed curvature measurement bars.

(45) The pressure in the pipe can be measured through a conventional pressure gauge measuring the internal pressure in the riser. Alternatively, the pressure can be extracted by measuring the hoop strain in the pipe using strain gauges.

(46) According to one embodiment of the present invention, traditional strain gauges are used for all measurements as these currently are the most reliable over time. If or when other strain gauging devices prove to be as reliable or more reliable over time, these may equally be used to make the necessary measurements.

(47) When it comes to details around the arrangement of the split cam ring 7, the connector dogs 9 and the release mechanism 10, there are several alternative solutions according to the present invention. As an example, the actuator may be designed to give an instant release of a force up to 80 T. It is envisioned that the force of 80 T will primarily come from a pre-tensioned spring mechanism. Alternatively this force could also be provided by a hydraulic actuator or even from an electrical motor. To release the locking pin 8, one of the following principles may be utilized (as also illustrated in FIG. 12): An electric switch or a magnet that releases an over-center mechanism which triggers the release of the 80 T force. An electric motor which frees the locking pin 8. A hydraulic system that opens a hydraulic valve thereby applying hydraulic pressure from a pre-charged accumulator to release the locking pin 8.

(48) The electronic combined loading weak link according to the present invention may also find other applications. For a typical test production (extended well testing) through a drill pipe or a WOR riser the weak link may be directly applicable also for production risers. For offloading hoses the electronic combined loading weak link according to the present invention would need to be configured for relevant accidental scenarios for the particular application. However, the same principles for combining electronic measurements into a combined loading formula which is compared continuously against a defined limit, and for triggering a connector release when necessary, are generally applicable. It should be noted that in particular for offloading systems there is normally a focus on having valves on the connector to prevent pollution from the hose in a disconnect scenario. This is not required for a WOR riser as a weak link release would be the very last resort to prevent accidents at a much larger scale.

(49) The present invention offers a number of possible advantages as compared to the conventional solutions that are in use today. Operational envelopes can be increased significantly during C/WO operations as static offset in operation does no longer affect the weak links ability to protect the well barrier(s), ref. FIG. 4. Each supplier can in principle qualify one weak link which can be used on any C/WO system and the release settings can be set for each specific project. The increase in the operating envelope is particularly important for work over operations performed from a dynamically positioned vessel, but will also apply to anchored vessels.

(50) In the case of a heave compensator 1 lock up, which creates excessive bending in the well barrier(s) 5 with rig offset, the allowable offset is usually limited. With a combined loading weak link according to the present invention, this limitation can be removed, and the weak link will protect the well barrier(s) against any combined load scenario. Hence, the combined loading weak link according to the present invention will also cover excessive vessel offset and thus will protect well barrier(s) for all accidental scenarios requiring a sudden disconnect of the workover riser.

(51) The safety level during C/WO operations, in particular from DP operated vessels, will be improved considerably as the combined loading weak link according to the present invention monitors and considers the accurate combined load that arises in the riser 2 and well barrier(s) 5. The combined loading weak link according to the present invention is able to protect the well barrier(s) 5 in case of compensator lock-up, vessel drift-off or vessel drive-off or any combination of these scenarios.

(52) The combined loading weak link according to the present invention does not rely on structural failure in any component and is therefore not relying on specific material batches that need project specific qualification. Such project specific qualification schemes have proven to be expensive, time consuming and in some respects unreliable. With the combined loading weak link according to the present invention, stringent project qualification schemes can be carried out with only non-destructive testing.

(53) The combined loading weak link according to the present invention considers tension loading and bending loads as well as any combination of these loads with better accuracy than existing weak link designs which are primarily suitable for pure tension or pure bending loads only.

(54) The combined loading weak link according to the present invention uses the pressure in the system in the combined loading analysis. Thus, it is no longer a challenge to fulfill all design requirements when the system is pressurized and at the same time ensure safe release when the system is unpressurized.

(55) The release settings of combined loading weak link according to the present invention can be adjusted with push button functionality and is not reliant on any structural design work or manufacturing of new components when being used on a new project with new design criteria.

(56) The combined loading weak link according to the present invention can be electronically tested on deck to ensure full functionality on deck immediately before use.