Reliability Analysis of a Wellhead Platform in Al Shaheen Block 5 Project

Abstract This study proposed the reliability analysis of a wellhead platform in Al Shaheen Block 5 Project. The reliability, optimal maintenance period, and appropriate redundancy policy were acquired by taking the failure rate of each component and methods from OREDA and PDS. The PDS is a method used to quantify the reliability of computer-based control and safety systems. The probability of failure on demand (PFD) was estimated for each safety function. The safety integrity level (SIL) of the overall system was evaluated to quantify failure risks. The SIL evaluation was applied to judging whether the reliability of process shutdown (PSD) functions of the present system was properly configured. In particular, the shutdown functions of the flare system and well isolation satisfied the minimum SIL requirement by IEC general recommendation. Introduction The Al Shaheen Oilfield is located in the Arabian Gulf, approximately 70km offshore northeast of the Qatar Peninsula. Hyundai Heavy Industries (HHI) has been awarded a contract for the Engineering, Procurement, Construction and Commissioning (EPCC) of a number of new platforms and additional facilities, by Maersk Oil Qatar (MOQ). This study gives a detailed risk assessment of the high liquid level shutdown to prevent liquid carryover to the flare stack in Block 5 Development Project. The overall system consists of two sub-systems equipped with safety instrumentation: one concerning the Production Wellheads with isolation valves, and the other concerning the Wellhead Platform with the Inlet Separator, Test Separator, and HP Flare K.O. Drum. Depending on the field development phase and operating philosophy, the number of working Production Wellhead varies up to 30. The HP Flare K.O. Drum is not designed to receive the total liquid flow from Inlet or Test Separators. This situation has raised a concern whether the safety instrumentation of the Wellhead Platform is of sufficient integrity. To analyze the reliability of the issued system, the functional boundaries are first defined in consideration of the design and operational philosophy. The reliability block diagram (RBD) is used to represent the system configuration. The failure rates of the overall instrumentation and equipment are obtained from the surrogated data such as OREDA and PDS. The SIL requirement by IEC is employed as the safety level determining the system integrity. Safety Integrity Level Assessment Safety functions and safety integrity level In the IEC standards, a safety function is considered as a function to be implemented in order to achieve a specified risk reduction related to a hazardous event. A safety function is thus specified in terms of the action to be taken and the required probability to successfully carry out this action. This probability is also referred to as the safety integrity, and in the context of IEC 61508 the safety integrity is classified to discrete levels as indicated in Table 1.