Abstract:
This disclosure provides methods and apparatuses to monitor strain in a steel pipe with reduced or eliminated disruption of the insulative and anti-corrosive layers or coatings that cover the pipe. The methods and apparatuses can include an attachment scheme that is less intrusive and less sensitive to dilation of the insulation layer on the pipe than previous strain monitoring solutions. Additionally, methods and apparatuses can reduce corrosion by virtue of the reduction in the number and volume of metallic components.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application relates to and claims the benefit of U.S. Provisional Application No. 60/943,275, titled “Device and Method for Providing Strain Testing” and filed Jun. 11, 2007, the entirety of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The present development relates to monitoring equipment, for example, monitoring the strain on a section of underwater pipe. 
         [0004]    2. Description of the Related Art 
         [0005]    Gas and oil drilling is performed in many different ways, on land and at sea. In marine drilling operations, large sections of steel pipe, often referred to as “catenary risers,” are connected to stretch deep into the ocean and along the seabed. The length of this piping required to reach the sea floor, the pressure extremes, and the temperature extremes to which such pipe is subjected often result in undesirable strain and/or bends in the pipe. 
         [0006]    The resulting bending strains the tolerances of the pipe. Unanticipated failure of such pipes can result in severe pollution and heavy economic loss. Thus, the monitoring of this “bending strain” in submerged pipes enables a continuous assessment of the integrity of the pipe. 
         [0007]    These pipes are typically steel with several layers of insulation and anticorrosion protection to help reduce the effects of the pipe&#39;s submersion in water, particularly salt water, as well as the low temperatures and high pressures exerted at depth. The insulation and anticorrosion protection additionally help extend the longevity of the pipes, which may be in service in such harsh environments up to twenty-five or thirty years or more. 
         [0008]    Heretofore, the strains in submerged pipes have been measured by a variety of strain sensors that are either bonded to the steel pipe surface or clamped in intimate contact with the steel surface, often using metal bands around the steel pipe. Such approaches are disruptive or damaging to the insulation and anti-corrosion coatings on such pipes. It is also time consuming and labor intensive to have to remove the layers of insulation and anti-corrosion coatings in order to attach the gauges. The clamping mechanisms themselves were typically steel or some other metal which is itself subject to the corrosive effects of the salt water—or even fresh water—in which the pipes were submerged. These approaches also can create more risk to the integrity of the pipe and/or its insulation. 
       SUMMARY 
       [0009]    An aspect of at least one of the embodiments disclosed herein includes the realization that the damage caused to the insulated pipes noted above can be avoided by mounting the desired sensors to the outer surface of the insulation. The behavior of the interior pipe can be correlated to the behavior of the outer surface of the insulation. Thus, the behaviors detected at the outer surface can be used to determine the behavior of the pipe within the insulation. Thus, sufficient monitoring of the insulated pipe can be performed without removing or damaging the insulation in the same way the known techniques require removal and/or damaging the insulation. 
         [0010]    Some embodiments disclosed herein provide methods and/or apparatuses that can be used to monitor strain in a steel pipe with reduced or eliminated disruption of the insulative, anti-corrosive layers, and/or coatings that cover the pipe. The methods and/or apparatuses can include an attachment scheme that is less intrusive and less sensitive to dilation of the insulation layer on the pipe than previous strain monitoring solutions. Additionally, methods and apparatuses can reduce corrosion by virtue of the reduction in the number and volume of metallic components. 
         [0011]    Described below are embodiments of methods and apparatuses that can be used for attaching sensors such as, for example, but without limitation, Linear Variable Differential Transformer (LVDT)-type displacement sensors. Other strain sensors are also commercially available and can be used. In some embodiments, the sensor is attached to the surface of commonly used insulation and anti-corrosion coatings for submerged pipes, such as those for transporting gas and oil. However, other sensors and other materials can also be used. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    A general architecture that implements the various features of the present developments are described below with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the developments and not to limit its scope. Throughout the drawings, reference numbers are re-used to indicate correspondence between referenced elements. 
           [0013]      FIG. 1  illustrates an overall view of a pipe extending from a Floating Offshore Platform toward a wellhead for an oil production operation. 
           [0014]      FIG. 2  is an enlarged perspective view of an embodiment of a strain station and an associated strain sensor assembly in accordance with an embodiment. 
           [0015]      FIG. 3  illustrates an exploded view of the strain station shown in  FIG. 2 . 
           [0016]      FIGS. 4A and 4B  illustrate cross sectional views of two example pipes on which the embodiments can be used. 
           [0017]      FIG. 5  is a block diagram of a method that can be used to attach a strain gauge to a pipe. 
           [0018]      FIG. 6  illustrates a simple embodiment of an alignment jig and temporary strap that can be used to temporarily hold attachment clamps in place on a pipe. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    An overview of a typical layout of a pipe extending between a floating offshore platform and the well site of an operation for the production of oil or gas is illustrated in  FIG. 1 . As shown in the figure, in a typical configuration, insulated steal pipe  102  extends from an oil tanker  104  or other platform toward a well of a drilling operation. The pipe  102  extends downward from the boat  104  into contact with the seafloor with significant sag. The sag in the pipe  102  allows the boat  104  to move relative to the well without instantly breaking the pipe  102 . Such a pipe can be used for production, i.e., collection of gas, oil, or other fluids from the well, or for other purposes. 
         [0020]    The area of the pipe that nears and comes in contact with the sea floor  110  is often referred to as the touch down zone (or TDZ)  108 . It is at this general location that the largest strains of the pipe usually occur, as the pipe  102  is bent as the platform or ship  104  moves in response to the swell, or other force, such as ocean currents, that act on the pipe  102 . 
         [0021]    In order to measure the strains experienced by the pipe  102  and help ensure that they are within tolerances, strain stations  106  can be provided at intervals along the pipe in and around the vicinity of the TDZ  108 . These strain stations can include multiple strain gauges attached to the pipe with a protective plastic coating or protection blister surrounding them. 
         [0022]    Previous solutions to testing such strains required removing the pipe&#39;s protective insulation to attach strain gauges as close to the steel pipe as possible. This was a costly and time-consuming process and exposed the steel pipe to a greater possibility of corrosion and damage than a pipe with intact insulation. 
         [0023]    An aspect of at least one of the embodiments disclosed herein includes the realization that testing the strain on the outer insulation of the pipe  102  provides data that can be correlated to the strain of the inner steel pipe, sufficiently accurately for monitoring purposes. In some embodiments, data from strain gauges properly placed on the exterior of the pipe&#39;s insulation layers provides similar strain readings from strain gauges installed by removing or otherwise damaging the insulation, without much of the expense and damage of removing the insulation layers, attaching the sensors, and replacing some or all of the insulation. The attachment of strain gauges in accordance with the present embodiments requires less time and cost of both the materials used and the labor in attaching the gauges. The steel pipe also remains protected by having relatively undisturbed insulation. 
         [0024]    In some embodiments, the attachment mechanisms can be made of a plastic such as polypropylene or some other corrosion resistant material. The use of fewer corrosive parts also lessens the likelihood of the strain gauge assemblies themselves wearing out or being damaged from prolonged emersion in water. 
         [0025]      FIG. 2  illustrates a strain gauge assembly  212  connected to the pipe  102 . In some embodiments, an LVDT-type displacement sensor  214  can be used as a strain sensor. The displacement sensor  214  can include an armature  216  and armature attachment bar  218 . 
         [0026]    The LVDT  214  can be configured to measure the displacement of the armature  216  relative to the armature attachment bar  218 . Each end of the LVDT  214  can be held immobile with respect to the pipe being measured so that any displacement of the armature  216  relative to the armature attachment bar  218  would indicate that the pipe is bending under some strain. 
         [0027]    In some embodiments, this strain gauge assembly  212  can be connected to the pipe&#39;s  102  insulation using clamps  220  near opposing ends of the strain gauge. The clamps  220  can be a split-block type clamp, having a clamp base  222  and a clamp top  224 . However, other types of clamps can also be used. 
         [0028]    In some embodiments, the clamp base  222  can be bonded directly to the insulation, such as by using a thermoplastic weld  228 . The strain gauge assembly  212  can be held in place by securing the ends of the assembly  212  with two clamps, e.g., with the clamp tops  224  being screwed  226  or otherwise attached to the clamp base  222  with the corresponding portions of the assembly  212  clamped between the corresponding tops  224  and bases  222 . 
         [0029]    As shown in  FIG. 3 , bolts  226  and washers  330  can be attached to threaded dowels  328  to hold the clamp sections  222 ,  224  in place. The clamp sections may be composed of any suitable polypropylene (“PP”), polyethylene (“PE”), or polyvinyl chloride (“PVC”) material. The clamp base  222 , in particular, is preferably composed of a homopolymer polypropylene that is relatively noncompressible under pressure. In particular, embodiments of clamp sections  222 ,  224  preferably have any gas bubbles or pockets removed during manufacture to avoid compression and failure at the high pressures of deep water. Such material is available, for example, under the commercial name VERSADUR® Homopolymer Polypropylene 500 Natural. However, other materials can also be used. For example, in another embodiment, Moplen COAT EP/60 BIANCO, available from Basell USA, Inc., may be used for the clamp sections  222 ,  224 . Preferable materials have a relatively low water absorption rate and a relatively low Izod impact strength. In particular, for example, one preferred material will have a water absorption of about 0.0100% or less based on the ASTM D 570 standard known to those in the art. The fasteners  226 , dowels  328 , and/or other parts of the fastening components can be made from any suitable material, including but without limitation, stainless steel, Monel 400, etc. 
         [0030]    As such, in some embodiments, the clamp  220  can be configured such that a catastrophic impact to the clamp will break the clamp itself with minimal or no damage to the protective coating of the pipe  102 . Clamps  220  are most preferably made of a material identical to the insulation surface material to which they will be attached. However, it is understood that any co- or homo-polymer polypropylene material with strength, water absorption, Young&#39;s modulus, Poisson&#39;s ratio, and thermal expansion properties similar to or identical to the particular pipe insulation surface material are also suitable, and other plastics can also be used for the systems and methods disclosed herein. The use of matching or substantially matching materials for the clamps  220  and the insulation surface material of the pipe helps ensure the longevity of the strain measurement system. The insulative materials used with the described piping have already been tested to withstand the extreme conditions of great ocean depths. Embodiments of the welding technique described herein essentially makes the clamps  220  an extension of the insulation surface material, thereby reducing the complexity and failure potential rather than adding to it as in the prior method of adding new attachment points of different materials. 
         [0031]      FIGS. 4A and 4B  illustrate two insulated pipe configurations for which the strain gauge assembly and attachment methods disclosed herein can be used. Catenary risers (which can comprise sections of pipe  102 ) used in oil production often include five layers of insulation and anti-corrosion materials (often known as 5LPP). First the steel pipe  434  and internal cladding  432  define a cavity  430 . The insulation builds out from the steel with layers of fusion bond epoxy  436 , polypropylene adhesive  438 , solid polypropylene  440 , syntactic polypropylene  442 , and solid polypropylene  444  ( FIG. 4A ). Similar piping is generally used for gas export risers, except that only three layers of insulation are used (often referred to as 3LPP): fusion bond epoxy  436 , polypropylene adhesive  438 , and solid polypropylene  444  ( FIG. 4B ). Regardless of the exact insulation, it is generally preferable to match the material for the clamps  220  with the outer solid polypropylene layer  444 . 
         [0032]      FIG. 5  is a block diagram of an embodiment of a method of attaching a strain gauge to a catenary riser for strain testing. Typically, this process occurs on land before transporting a section of pipe to the installation location. It is also possible to attach strain gauges on the ship but the process generally occurs prior to lowering the pipe into the water. In some embodiments, Hot Gas Hand Welding can be used for attaching the LVDT fixtures to the surface of the 3LPP, 5LPP, or similar coated pipes. Hot Gas Welding is generally a manual process using a filler material to perform the weld. The gas (air) simultaneously transmits heat into the parent materials and the welding rod to allow molecular interlocking to take place. 
         [0033]    A Hot Gas Hand Welding system that can be used generally comprises a welding gun, a clean air supply unit, and a welding tip. The welding gun typically is provided with a means to adjust the welding temperature. One example device that is suitable for this type of welding is the Hot Jet S with speed welding nozzle available from LEISTER. Other suitable welding techniques to accomplish welds, particularly, for example, plastic welds, as described herein would be known to those of skill in the art. 
         [0034]    In some embodiments, the surface of the insulated piping can be prepared prior to welding the clamps  220  for mounting the strain testing gauge (block  550 ). During such a process, for example, the Polypropylene surface of the risers can be sanded or otherwise scraped down to virgin material and chemically cleaned. In some embodiments, the surfaces can be prepared such that they are coplanar with the lower surface of the bases  222  so that the clamp bases are more likely to maintain contact with the pipe&#39;s attachment surfaces and limit or avoid air bubbles between the two surfaces. It is also preferred to prepare the pipe surfaces so that the clamps will be placed along the pipe in a manner to hold the displacement sensor  214  in an orientation that is substantially parallel to the longitudinal axis of the pipe. Such a preparation process can be performed with an eye towards minimizing the disturbance of the insulation and so as not to damage the overall effectiveness of the insulation. For example, the insulation layers can remain substantially unaltered. 
         [0035]    In some embodiments, for example, a scraper can be used to remove the rough outer surface of the polypropylene insulation at each attachment location. Each location is then wiped with a clean cloth to remove dirt, oil, or loose scrapings to provide a better surface for welding. Each base can also be scraped in the area where it will contact the pipe insulation to remove oxidized molecules that may interfere with the welding process. 
         [0036]    In some embodiments, an installation or alignment jig  664  (see  FIG. 6 ) can be used to temporarily attach a first clamp base  222  and a second clamp base  222  to the attachment site at the appropriate distance and orientation (block  552 ). This can help align the bases  222  with their respective installation location marks. In some embodiments, the alignment jig  664  can be as a pipe having a length and diameter approximate to that of the LVDT  214  and the armature attachment bar  218 . Once the bases  222  are in position, the alignment jig  664  and clamps  220  can be temporarily secured to the pipe with a strap  666  or other securing method, such as adhesives, tacks, or the like, to hold the clamp bases in place at the prepared attachment locations. 
         [0037]    Once this is done, the welding gun, with a “tacking” tip, can be used to tack the perimeters of the bases  222  onto the outer surface of the insulation of the pipe  102  (block  554 ). In some embodiments, a common form of hot gas welding, known as “High Speed Welding,” can be used to complete the attachment of the clamp bases  222  to the insulation on the pipe  102  (blocks  554 ,  556 ). 
         [0038]    High speed welding tips are designed to guide the welding rod into the weld zone while simultaneously heating up the rod and the base material. A shoe at the end of the rod orifice allows the operator to apply the welding pressure. The welding pressure is dependent on material type and rod size. It is understood that other suitable forms of hot gas welding or other welding techniques can also be used. 
         [0039]    In embodiments where Moplen COAT EP/60 BIANCO is the outer layer of pipe insulation and/or the material used for the strain gauge clamps, it is preferable that the welder be set to between about 300° C. and about 340° C. with an air flow volume of about 45 l/min and 55 l/min. Other suitable settings will be apparent to those of skill in the art based on the welding equipment and properties of the materials used. Utilizing a tack weld helps assure that the bases are in proper place and alignment and that they stay in position when additional welding is performed. In some embodiments, the tack welding can be skipped. In other embodiments, the strap  666  and alignment jig  664  can be removed once tack welding is completed. This can help allow easier access to all sides of the clamp bases for the next steps. 
         [0040]    After tacking, it is preferable to allow three to four minutes for the tack weld to cool. In some embodiments, forced cooling with an air hose or other means is avoided as it can cause thermal stresses or weaken the weld. After an appropriate cooling period, the welding can be completed using regular plastic welding (block  556 ). The welding rod used is preferably scraped, before use to clean it and remove any oxidized material, as with the preparation of the clamp bases  222 . The regular weld preferably surrounds the clamp base to provide the most bonded surface area. 
         [0041]    In some embodiments, it may be desired to make multiple welding passes around a clamp base  222  to further strengthen the weld between the pipe  102  and the bases  222 . In some embodiments, a first welding pass can be allowed to cool before a subsequent pass. For example, a delay of approximately 3-4 minutes between passes will provide stronger welds—similar to the preferred time to cool the tack welds. 
         [0042]    Compatible materials between the clamp bases  222  and the welding rod filling material can be used to help ensure lasting welds. For example, the filler and the bases  222  material can have the same or a close melt flow index. One of skill in the art will has familiarity with proper conditions for thermoplastic welding as described herein. 
         [0043]    Once the bases  222  are welded in place, the LVDT  214  is placed in the slot of one clamp base  222 , while the armature attachment bar  218  is placed in the other (block  558 ). The clamp tops are then set in place over the strain gauge pieces and fastened to the clamp bases to hold the strain gauge in place (block  560 ). 
         [0044]    Fasteners  226 , which can be made from Monel 400, Monel 500, Inconel 625, Super Duplex Steels, or other long life corrosion resistant materials, can be bolts (as shown in  FIG. 3 ), screws, or other fastening mechanisms and can be used to close the strain sensor clamps  220 . Monel is also a preferred material for fasteners  226  because it is additionally resistant to hard marine growth. The clamp bases  222 , as noted above, can have two threaded dowels  328  inserted into holes  329 . The threads can be aligned with clearance holes  331  through the clamp base  222 . The clamp top  224  can have two clearance holes  331  which can be for ¼-20 socket head cap screws, which can serve as the fasteners  226 , although other sizes can also be used. Flat washers  330  can be used here to distribute the load and cross-drilled heads for Monel 400 seizing wire to reduce the likelihood of the fasteners  226  backing out. The fasteners  226  can thread into the threaded dowels  328  in the base and can be tightened enough to close the gap between the clamp top  224  and base  222 . 
         [0045]    In some embodiments, this procedure can be repeated one or more times with additional strain gauges set out at various locations around the circumference of the pipe  102 —generally approximately the same distance along the length of the pipe section. This would allow the collection of data indicative of a pipe bending in a multitude of directions. 
         [0046]    Optionally, the one or more strain gauge assemblies can be enclosed in a protective casing. This casing is meant to help reduce impact damage to the strain gauges. In some cases, these casings may further be waterproof or water resistant to reduce corrosion of strain gauge components. In some embodiments, the interior of the casing can be pressure neutralized. In some embodiments, the casing is a steel shell connected to the pipe with rubber bushings. 
         [0047]    There are numerous alternatives that may be employed without deviating from the spirit of this disclosure. For example, although  FIGS. 2 and 3  illustrate a particular embodiment of clamp  220 , the clamp  220  itself or one or more of its parts  222 ,  224  may take numerous other forms. For example, a top clamp  224  can include a flexible or rigid rubber piece that is bolted in place in a manner similar to that shown. Another alternative can be a generally U-shaped bolt that would fit around a portion of the strain gauge. It could, for example, be threaded at both ends that would fit through holes in the bottom clamp  222  and be secured by nuts. Epoxies or other glues that are degassed, that have a low porosity, and that will not break down in water can also be suitable forms of bonding the strain gauge to the clamp bottoms  222  and/or the clamp bottoms  222  to the insulated pipes. Importantly, preferred alternative epoxies or other glues should be able to maintain their bonds for the expected pipe submersion time of up to approximately 25-30 years or more. 
         [0048]    Although the foregoing has been described in terms of certain specific embodiments, other embodiments will be apparent to those of ordinary skill in the art from the disclosure herein. Moreover, the described embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosure. Indeed, the novel methods and systems described herein can be embodied in a variety of other forms without departing from the spirit thereof For example, mounting blocks can be constructed from materials other than those disclosed herein. Similarly, assembly of the strain gauge in the mounting blocks can utilize any of a variety of suitable fastening devices, including screws, nuts and bolts, adhesives, and the like. Accordingly, other combinations, omissions, substitutions and modifications will be apparent to the skilled artisan in view of the disclosure herein. Thus, the present disclosure is not limited by the embodiments described above.