Full-Scale Structural Testing of Deep Drilling Masts

ABSTRACT This paper discusses analysis and full scale testing of deep drilling masts. Offered are methods for analysis and design as well as a method for applying a simulated heavy casing load to verify the analysis and overall performance of the structure. A specific case history of a 1,200,000 pound test application is documented. INTRODUCTION The design and fabrication of masts and substructures must be adequate to insure that each member has sufficient strength to resist all loads that are imposed during operation of the rig. For proper design, loads must be determined for the particular operating conditions of the structure. Engineering analysis and design must incorporate a variety of load combinations that are a result of drilling, casing running, setback loads, erection (rig up), earthquake, and wind on the structure. API Spec 4E provides a guide for loads and load combinations for the design of drilling Structures. A major consideration in analysis and design is member arrangements in the structure to assure adequate load paths through the structure. The nature of a drilling operation requires frequent relocations of the rig during its useful life. During relocation and rig up of the mast, members may be damaged which causes a reduction in their resistance to loads. In addition, damage to the structure can occur during operation of a rig during years of service. The result is an increase in the risk of potential failure. Thus a reliable and thorough structural evaluation is important should be performed after fabrication periodically during the life of the structure. Although masts with open faced configurations toward the v-door (commonly referred to as jack knife masts) have the appearance of a space truss, they behave structurally as a space frame in resistance to load. The design of many drilling structures that are in operation today have been done using simple methods of analysis assuming a distribution of load through the structure by simple truss action. Most masts designed before the development of computer techniques used the truss analysis methods. This can result in the fabrication of a structure that is insufficient for its intended design use. In addition to the simplistic approach for the design of drilling structures, there has been minimal full scale testing of drilling rig structures. Most tests of drilling structures have been conducted by applied loads that simulate the actual loads during a drilling operation. This paper describes the space frame engineering analysis and full scale test of a 3000 HP drilling rig structure. A special test assembly was designed and constructed between the lower sections of the substructure at the centerline of the well for application of the 1,200,000 pound test pull. The test load was applied by the drawworks and traveling equipment. Strain gages and deflection targets were placed at strategic locations on the mast to measure the loads and deformations during the test. Eight strain rosette gauges were symmetrically placed on the 13 3/8 inch casing to monitor the hook load during the test.