The Front End Engineering Design (FEED) for the Pearl GTL Offshore Platforms was based on a successful central North Sea sweet gas analogue with a few additions intended to address the problems of working with a highly sour process gas. As the detailed design progressed it became clear that although many of the features introduced by the FEED team were effective, certain aspects of the platform’s design could not be considered to have As Low As Reasonably Practicable (ALARP) residual risks since the initial assumptions could not be sustained. The Risk Reduction Philosophy harmonized the various engineering and operations philosophies that had been issued on the project and provided an overview of how the major hazards would be managed to ALARP. Of particular note is the drive to design a facility with maximum inherent safety, minimum personnel exposure and ensure that Operators can access and escape the facility with minimal risk.
A facility that is inherently safe is one that is impossible to have an incident with – a facility with a design pressure of 301bar connected to a well with a maximum closed-in tubing head pressure of 275bar cannot be over-pressured. If it cannot be over-pressured there is no need for a relief valve, so the number of flanges (leak paths) is reduced therefore reducing the probability of a leak. Additionally the maintainable equipment count is reduced, thereby reducing the personnel exposure hours – a double knock in reducing individual risk per annum.
The demands on the Pearl platforms are very tough; very high (~98%) availability production with a minimum intervention concept targeting maintenance only every 90days with the intent of using only the safest form of transport; boat access. Metocean forecasts have shown that with a conventional boat landing the expected waiting-on-weather time for a twelve hour bunkering operation could exceed a week in winter and the probability of getting a week long period to carry out routine maintenance was around 15%. Shell’s experience in the southern North Sea in using a heave-compensated gangway, deployed from a dynamically-positioned support vessel, provided the solution and due to having an operational window of up to 2.5m significant wave height the percentage of time in which the platform is inaccessible by sea drops from 30% to 3% during the winter months. Ready access also means a ready escape in case there is a gas release on the platform thereby increasing safety.