Development of a Specification for Christmas Tree and Wellhead Components

Abstract

Summary The development of a specification for wellhead and Christmas tree components exposed to aggressive production environments is described in terms of supplemental requirements to the API Specs. 6A and 14D for wellhead equipment. The supplemental requirements provide for material selection and confirmation of the integrity of provide for material selection and confirmation of the integrity of these materials. Implementation of this quality control program has been effective in preventing offshore wellhead failures. Introduction The continuing search for and development of oil and gas reserves has resulted in exposure of production equipment to increasingly aggressive production fluids and high wellhead pressures, The environments are characterized by high temperatures and often copious amounts of hydrogen sulfide (H2S) and CO2 gases. Metals exposed to these conditions are subject to cracking and accelerated corrosion. To ensure safe operation, great emphasis must be placed on the material integrity of the production equipment. placed on the material integrity of the production equipment. Valves, chokes, and tubular hangers are of particular concern in these aggressive environments because their construction involves several manufacturing techniques and a range, of materials, often of high strength and hardness. Unavoidably, valves and other wellhead equipment contain geometries conducive to crevice corrosion, fluid stagnation, and turbulent flow. These conditions promote leakage, erosion, cracking, and corrosion. To perform satisfactorily, valves must be pressure-tight, retain close perform satisfactorily, valves must be pressure-tight, retain close tolerances, and remain free of corrosion products between wear surfaces. Corrosion and/or erosion rates that may be acceptable in other production equipment will make a valve or choke inoperable. When production operations encounter these aggressive conditions offshore, the minimum requirements of the API Specs. 6A and 14D for wellhead equipment are inadequate. This paper relates some additional material properties, examinations, and testing applicable to wellhead components used in critical service conditions. This paper also describes laboratory and field data used in developing a procurement specification for wellhead and Christmas tree components and the cost/benefit of specifying supplemental quality control. Motivation for supplementary Specifications In 1978, a platform wellhead experienced leakage through the body wall of a cast 3-in., 10,000-psi [7.6-cm, 69-MPa] main wellhead block valve within hours after initial installation. The valve was replaced with a valve of the same model from the same vendor with identical results. The initial investigation revealed the cause of leakage to be internal, interconnecting, casting defects undetected by the vendor's inspections. Further investigations revealed the vendor had not tested the casting procedure for this valve design fully and had limited production examinations to a single full-body radiographic production examinations to a single full-body radiographic exposure. A complete radiographic examination identified the cause of leakage to be internal shrinkage resulting from inadequate risering of the flask. In addition to material failures, other difficulties with both cast and wrought wellhead components had been experienced. The extent of problems was neither catastrophic nor epidemic but included a continuing nuisance of leaks, misalignment, and mismatch of the tree's mating elements. Consequently, a procurement specification supplementing the requirements of API Specs. 6A and 14D, to be applied to all tree components of 5,000-psi [35-MPa] and higher working pressure from the tubing hanger to the outlet of the choke, was prepared, with the objective of minimizing these problems. problems. Scope of the Specification In addition to examinations, testing, and inspections, the specification addresses material selection and acceptable flaw criteria and the required identification and documentation of material to ensure traceability. Inspection of internal parts, such as valve stems and seats. is outside the scope of the specification. Hydrostatic and gas tests are applied to individual valves, and the final tree assembly is subjected to hydrostatic test-all under witness of a third-party representative. supplementary requirements are contained within the specification for welded components and repairs of manufacturing defects. Implementation of the Specification Initial implementation of the specification was a timeconsuming process of reviewing inspection. manufacturing, testing, and process of reviewing inspection. manufacturing, testing, and welding procedures. At the program's onset in 1978, conferences were convened with most of the major vendors to define the intent and requirements of the specification. JPT P. 655