Development of Low Force Shear Blades for High Strength Coiled Tubing

Abstract

Abstract As coiled tubing grades have evolved over the past 20 years from 70 grade with a minimum yield strength of 483 MPa (70,000 psi) to 140 grade with a minimum yield strength of 965 MPa (140,000 psi) and wall thicknesses have increased, the resulting force required to shear coil has more than doubled. Most coiled tubing units have a maximum pressure of 20.7 MPa (3000 psi) available for the blow out preventers (BOP) hydraulic circuits. There was an industry need to develop a shear blade for BOPs that could cut high strength coiled tubing using legacy pressure control equipment already in use. Additionally, the new shear blades must create a fish that can be easily retrieved from a wellbore. Shear strength is estimated using the maximum distortion criterion (von Mises yield criterion) as follows:Shear strength/Yield strength=1√3=.577 Since the maximum distortion criterion is merely an approximation and unique blade geometries are difficult to take into consideration using the above calculation, a considerable amount of hands on lab testing was required to design and optimize an elegant shear blade for cutting high strength coiled tubing with minimal hydraulic forces. The paper will share the iterative process as novel shear blades were developed that significantly reduced shear forces. Multiple piercing tip geometries were tested, including embodiments with several piercing tips. Success criteria was reduced shear force, acceptable fish profile on the lower piece of coiled tubing, and no damage to the blades after use. The embodiment that showed the most promise, based on lab testing was further optimized to improve its performance over multiple cuts. The result was a novel shear blade that is able to cut high strength coiled tubing with 50% of the normal shear force. As the industry continues to push the limits of coiled tubing with extended reach applications thought impossible only a few years ago, higher strength coiled tubing with increased wall thicknesses will continue to evolve. The new shear blade geometry developed in this project ensures that not only can the latest grades of coiled tubing be sheared in legacy pressure control equipment, but also future grades of coiled tubing that are in development.