Abstract:
A method of reducing stress and wear on one or more components in a keel joint assembly in which a cobalt-based, wear resistant alloy coating is applied to the surfaces of one or more components. The use of the coating reduces stress and wear and achieves improved corrosion, galling, erosion and abrasion resistance as compared to other currently known and applied methods. In the present invention, the coating would preferably would be applied to the surfaces of the mating components of the keel joint.

Description:
RELATED APPLICATIONS 
     This application claims benefit from U.S. Provisional Application No. 60/506,793, filed Sep. 29, 2003. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to offshore drilling and production platforms, and in particular to the application of a wear resistant coating to components of a keel joint used with such platforms. 
     2. Description of the Prior Art 
     In certain types of offshore oil or gas production wells, a riser assembly is used to connect a floating drilling and/or production platform with a stationary subsea wellhead. The riser assembly passes through an opening in the bottom of the platform. The riser is subject to bending movement where it enters the floating platform caused by wave action and the like. Such movement can result in stress on the components of the riser assembly. A keel joint is often used to absorb and reduce this stress. The keel joint typically includes a housing that surrounds a portion of the riser assembly. The housing includes mating keel joint components that flex or move relative to one another. The movement from the floating platform is translated to these mating surfaces. While the stress on the riser assembly may be reduced, typically there is a corresponding increase in stress on the mating components and other components of the keel joint. 
     The harsh environment can also cause wear to the keel joint components. Seawater, entrained sand, chemical contamination, mud and other damaging elements can corrode the component surfaces and result in unwanted galling, erosion and abrasion, as well as increase the likelihood of component degradation and eventual failure. These drawbacks are in addition to the stress and wear on the components caused by normal bearing loads and work requirements. Other offshore drilling and production components are also subject to similar conditions. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to the application of a cobalt-based, wear resistant alloy coating to the surfaces of the offshore drilling and production components, particularly those in a keel joint, to reduce stress and wear and achieve improved corrosion, galling, erosion and abrasion resistance as compared to other currently known and applied coatings. In the present invention, the coating would preferably would be applied to the surfaces of the mating components of the keel joint. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a keel joint housing surrounding a riser assembly with a bearing element. 
         FIG. 2  is an enlarged sectional view of the encircled portion of  FIG. 1  with an applied coating in accordance with this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows an example of a keel joint  20  located at the bottom of a tubular conduit  10  in an offshore platform. The keel joint  20  is generally comprised of a housing  60  which surrounds a riser assembly  40 . Housing  60  extends a short distance below conduit  10  and a selected distance within conduit  10 . Keel joint  20  serves to reduce bending stress where riser assembly  40  passes into platform conduit  10 . Conduit  10  has a downward facing guide funnel  30 . Keel joint  20  is submerged in the sea during normal use. 
     The riser assembly  40  includes a plurality of tubular individual riser segments, typically secured by threads.  FIG. 1  shows a flanged connection point  15  between two individual riser segments. Flanged connection  15  forms a part of keel joint  20 . An upper riser segment  41  has a mating flange  43 . A lower riser segment  42  has a mating flange  44 . The mating flanges  43 ,  44  of the upper  41  and lower  42  riser segments are held together by bolts  45 . 
     The mating flange  43  of the upper riser segment  41  has an upper shoulder portion  46  on its outer diameter. The mating flange  44  of the lower riser segment  42  has a lower shoulder portion  47  on its outer diameter. An annular recess  48  is located between the upper  46  and lower  47  shoulder portions. A metallic bearing element  49  fits closely within recess  48 , sandwiched between the shoulder portions  46 ,  47 . The bearing element  49  has a spherical surface  50  along its outer diameter. 
     The housing  60  is sized so that platform conduit  10  may move slidingly up or down relative to housing  60 . The housing  60  has an upper section  61  and a lower section  62 . The upper section  61  has a lower mating metallic element  63 . The lower section  62  has an upper mating metallic element  64 . The mating elements  63 ,  64  each have an inner surface that is generally spherical in shape. The housing  60  has a generally vertically aligned interior portion. 
     When the housing  60  is assembled and surrounds the segment of the riser assembly  40 , the generally curved-shaped inner surfaces of the upper and lower mating elements  63 ,  64  of the housing  60  closely fit with the outer spherical surface  50  of the bearing element  49  of the riser assembly  40  creating a flexible ball joint. It is within this ball joint region, i.e., upon the closely fitted surfaces of the bearing element  49  and the inner diameter of the mating surfaces  63 ,  64 , where the majority of wear and stress within the keel joint  20  occurs, and where a wear resistant coating can provide the greatest benefit. 
     In the preferred embodiment of the present invention illustrated in  FIG. 2 , a first coating layer  70  is applied to the outer spherical surface  50  of the bearing element  49 . A second coating layer  72  is applied to the inner surfaces of the mating elements  63 ,  64  of the housing  60 . In general, and in accordance with the present invention, one or more layers of coating can be applied to any one or more of the surfaces of the keel joint  20  which can benefit from the coating&#39;s stress and wear resistant properties. 
     The coating can be applied to the surfaces of the keel joint  20  by a cladding process, which is preferably performed under high temperature and/or pressure conditions. The cladding process can involve, for example, a laser or tungsten inert gas (“TIG”) welding process. Laser welding utilizes energy from a concentrated coherent light beam to melt and fuse metal. Tungsten inert gas welding utilizes energy produced by an electrical plasma arc to melt and fuse metal. The electrical arc is formed between a tungsten electrode and the work piece. Shielding gas is used to protect the weld pool and electrode from the atmosphere. A filler rod is dipped into the molten pool or a filler wire is continuously fed into the molten pool. 
     Laser welding is the preferred process because of lower manufacturing costs and because laser welding is a faster process than TIG. The width of the coating layer tends to be larger for laser welding (up to 1 inch for laser versus about 0.25 inch for TIG). Also, laser welding provides lower weld metal dilution than the TIG process and the travel speeds are greater for laser welding. Lower weld metal dilution means that a thinner weld layer is required to achieve a corrosion resistant chemistry. For example, it is possible to achieve a maximum iron dilution of 12% with the laser process at a clad thickness of 0.025 inch. On the other hand, the same iron dilution requirement takes a minimum clad thickness of 0.050 inches with a TIG welding process. This is important in keel joint applications, which require both wear and corrosion resistance, because a smaller clad thickness is required to achieve the required corrosion resistance properties. This potentially reduces the number of weld passes required. 
     The preferred coating of the present invention is a wear-resistant, cobalt-chromium-nickel alloy with high tensile strength, when compared to stainless steels, and good resistance to aggressive, oxidizing and reducing substances. A preferred coating is marketed under the trademark Ultimet® by Haynes International, Inc. of Kokomo, Ind. Preferably, the Ultimet® alloy contains, by weight percent, approximately 23.5–27.5% chromium, 7.0–11.0% nickel, 4.0–6.0% molybdenum, 1.0–5.0% iron, 1.0–3.0% tungsten, 0.1–1.5% manganese, 0.05–1.00% silicon, 0.03–0.12% nitrogen, 0.02–0.10% carbon and the remainder cobalt. Also, the coating may optionally contain no more than 0.030% phosphorus, no more than 0.020% sulfur and no more than 0.015% boron. In one embodiment, the Ultimet® alloy contains, by weight percent, approximately 54% cobalt, 26% chromium, 9% nickel, 5% molybdenum, 3% iron, 2% tungsten, 0.8% manganese, 0.3% silicon, 0.08% nitrogen and 0.06% carbon. 
     In an alternate embodiment, the coating is a wear-resistant, cobalt-chromium-nickel alloy preferably containing, by weight percent, approximately 26.0–29.0% chromium, 8.0–12.0% nickel, 3.0–5.0% molybdenum, 0.4–1.0% tantalum, no more than 2.0% iron, 3.0–5.0% tungsten, no more than 1.0% manganese, no more than 1.0% silicon, 0.12–0.20% carbon and the remainder cobalt. 
     Combining the relative percentages of the common components of two previous examples yields the following: 23.5–29.0% chromium, 7.0–12.0% nickel, 3.0–6.0% molybdenum, 1.0–5.0% iron, 1.0–5.0% tungsten,  0.1–l.5 % manganese, 0.05–1.0% silicon and 0.02–0.20% carbon, and an amount of cobalt. 
     In certain embodiments, the amount of nitrogen, sulfur, boron and/or phosphorus in the coating may be regulated in order to avoid weld quality problems associated with use of the alloy. For example, excess nitrogen in the weld filler increases the probability of solidification cracking. In certain embodiments, if nitrogen is added, it shall not exceed, by weight percent, 0.090%. High levels of phosphorus, boron and/or sulfur tend to segregate grain boundaries and cause embrittlement, which results in increased cracking sensitivity, reduced fracture toughness and lower Charpy V Notch impact values. In certain embodiments, if phosphorus is added, it shall not exceed, by weight percent, 0.030%. In certain embodiments, if sulfur is added, it shall not exceed, by weight percent, 0.020%. In certain embodiments, if boron is added, it shall not exceed, by weight percent, 0.015%. 
     Preferably, the alloy has a density of 0.306 pounds per cubic inch and a melting point of approximately 2505 degrees Fahrenheit. The thickness of the coating layers  70 ,  72  is preferably at least 0.025 inches. 
     The coating has excellent wear resistance properties as well as a high degree of resistance to corrosion and other forms of environmental degradation. The coating can be easily weld-repaired, and in addition to the proposed use in a keel joint assembly, can be used in a variety of subsea oil field applications involving metal components that slide against one another, for example metal seals, ball joints and guide rods. The coating may be applied to different types of keel joints. 
     While the invention has been described herein with respect to a preferred embodiment, it should be understood by those that are skilled in the art that it is not so limited. The invention is susceptible of various modifications and changes without departing from the scope of the claims.