Rough Weather Drilling Vessel Simulation

Abstract

ABSTRACT The Gulf of Alaska may have the roughest wave climate ever faced by the offshore oil industry. The high cost of time lost due to these rough seas was a major factor in selecting a floating drilling rig. This paper describes a simulation method that was used to estimate down times of floating rigs in the Gulf of Alaska and can be applied to other roughweather areas. The simulator has two parts: one that simulates operations and another that simulates weather interference with those operations. While the operations model "drills a well," a novel weather model generates both wave heights and periods. Wave heights and periods determine vessel motions, which are checked against operating limits. If motions exceed the limits, work is "slowed down" or "stopped." By "drilling" - simulating - a large number of wells, reliable statistics on the down time of a particular rig are developed. The weather model in the simulator can be applied to other offshore areas provided enough wave data exist. This approach also can be used for simulating offshore tanker loading, pipelaying or platform installations. Furthermore, the simulator can assess the value of hardware improvements that permit operations to continue in rougher seas. Statistical tests on the weather model showed that it adequately simulated Gulf of Alaska sea conditions. Furthermore, rig down time calculated with the simulator proved to be more realistic than down times calculated with models using just wave height. INTRODUCTION While waiting for rough weather to subside, a floating drilling rig will still cost up to $100,000 per day even though no work is being accomplished. Down time is expensive. And weather down time is even more important in selecting rigs for rough weather areas. Minimizing this expense can be done best by selecting an optimum rig design, one with the least down time under given sea conditions. This is not an easy task, given the many different rig designs proposed for rough-weather areas like the Gulf of Alaska. This task was simplified by a drilling vessel simulator that calculates down time and pinpoints operations particularly sensitive to weather-caused interruptions. The simulator used two interacting parts: one that simulates offshore drilling operations, and another that simulates weather interference with the operations. While the operational model "drills" a well, the weather model generates wave heights and periods. The wave heights and periods determine vessel motions that are checked against operating limits. If the motions exceed the limit, work is "slowed down" or stopped. By drilling many wells, the simulator develops reliable and relistic downtime statistics. The novel weather model simulates both wave height and period, whereas previous weather simulators generate only wave height. This model also can be expanded to include additional parameters such as current and wind speeds. This advanced weather model was needed because motions of drilling vessels depend on both wave height and period.