API Standards are generally reviewed, updated, and redrawn at least every 5 years, and API 4F is no exception. In January 2013, the 4th Edition of Specification 4F was released and came into effect in August 2013. In this new edition, there are several key updates that will affect the design of service rigs. These updates include :
- Differentiating between service rigs (workover rigs) and drilling rigs. This edition highlights the difference between service rigs and drilling rigs, including mast nameplates and design loading. The same loading scenarios apply (Operating, Expected and Unexpected Winds, etc.), however, the loads on the mast and percentages of each have changed considerably. For example, the rotary loads have been removed, and setback has been split into both rod and tube loads for service rigs.
- The inclusion of non-shielded members in the wind analysis. In 3rd Edition, a global shielding factor is applied to the wind load on all components. In 4th edition, the designer must differentiate between members that are shielded and ones that are not and remove the shielding factor from those components. This is the most critical change between 3rd and 4th Editions. In our analysis, this change has led to an increase in mast wind loading of about 5-10%, depending on the mast structure. This has a large effect on the design of a freestand package, as well as on torque in the mast. Freestanding service rigs are more efficient than their guyed counterparts. The smaller footprint on the ground allows the rig to be setup in tighter areas, relying less on the area surrounding the well site, and decreasing the time required for rig setup.
- The increased wind loading in 4F 4th Edition has a major effect on the freestand. While a 5% increase in force does not seem like much, most of that increase is applied high on the mast, greatly increasing the tipping moment. This increase necessitates either a large increase in the load beam width, or a requirement for more stabilizing weight. For rigs designed to operate in Period 1 or 2 conditions, increased weight is not always an option.
- Other changes to note include provisions for hydraulic cylinder selection and manufacturing processes requiring validation.
While some of the changes in the 4th Edition are significant and make designing service rigs more difficult, the increased safety and reliability of rigs designed to 4F 4th Edition is worth the challenge.
Read more about API 4F here: API 4F and the Design of Service Rigs, Designing a New Steel Mast Using High Tensile Steel, High Tensile Steel in Design