Numerical Investigation of Fatal Accident Involving Toppling of Hydraulic Workover Unit

Abstract

Abstract In 2022, a Hydraulic Workover Unit (HWU), operating in Gulf of Mexico, toppled while being erected on a caisson. Photographs taken on site suggest that conductor casing lost stability, while submitted to weight of the HWU, combined with environmental loads. Since HWU activity is expected to intensify in Brazilian shore, Petrobras team decided to study this accident and prevent similar events from happening on its fleet. A finite element model was constructed, based on photographic records of the accident site. The model was loaded with the expected weight on deck, as well as environmental loads (wind gusts and waves), calculated as per (DNVGL-RP-C205, 2014). Numerical simulations were performed to assess safety factors of operation, considering the 120 klbf weight-on-deck reported on the safety alert (BSEE, 2022). Operation was found to be safe, if weigh-on-deck is not exceeded, and conductor casing is integer. A secondary batch of simulations considered several types of degradation that could incur to the workover unit, such as maritime corrosion and factory imperfections. Depending on the corrosion state of the conductor casing, safety factors were as low as 0.58, indicating high risk of catastrophic failure. Simulations were replicated to Workover Units expected to be employed in Brazilian Coast. For these units, risk of catastrophic failure was found neglectable, while risk of excessive bending to wind gusts was found more likely. Preventive actions were suggested, concerning periodic inspection of installations and Personal Protective Equipment (PPE).