Extreme Responses of Turret Moored Tankers

Abstract The extreme responses of a turret moored tanker are sensitive to non-aligned wind, wave and current conditions. Such conditions commonly occur in the Gulf of Mexico during the passage of the eye of a hurricane. Conventional design practice often relies on a collinear or, at best, a "guessed" non-collinear combination of 100-year environmental return period wind, wave and current conditions. Hence there is a need to derive response-based design criteria, i.e. that particular combination of wind, waves and current which most likely yields the 100-year return period response. The long term response characteristics of a turret moored tanker in deep water Gulf of Mexico conditions are investigated through the use of a comprehensive hurricane hindcast database. The effects of turret location and wave spreading are considered. The 100-year long term responses are compared against the short-term 100-year design responses derived from a 100-year hurricane design analysis. Response-based design criteria are then derived. Introduction Turret moored tanker based FPSO systems are widely used in many deepwater areas. Conventional design of such systems often relies on the assumption of a design storm event comprised of a collinear (or at best a guessed non-collinear) combination of 100-year environmental return period wind, waves and current. However, it is well known that the extreme responses of a turret moored tanker are sensitive to non-collinear wind, waves and current (Refs. 6-8). With few exceptions, the effect of non-collinear wind, waves and current has received little attention. Such events commonly occur in the deepwater Gulf of Mexico during the passage of the eye of a hurricane. The resulting effects on the motions and mooring line tensions may be significant as such systems have a natural tendency to weathervane, i.e. align themselves against the prevailing direction of wind, waves and current. This then poses the question of how well the "conventional" design recipe really works for these systems in significantly non-collinear environments. In order to address this problem the actual long-term response characteristics need to be investigated and the 100-year return period responses need to be derived. Response-based design criteria may then be stipulated to capture specific response characteristics, e.g. the 100-year maximum offset storm is that particular combination of wind, waves and current that most likely yield the 100-year return period offset. Notice that the associated wave height, wind speed and current speed for such a non-collinear design event may well be lower than those normally referred to as 100-year return period environmental criteria. Also, the 100-year return period offset storm, say, is merely intended to estimate the 100-year offset while other responses (e.g. roll or mooring line tension) should be ignored. Theory The long term Gulf of Mexico environment is described by means of a hurricane hindcast database of 35 storms over an 85 year period since 1900 (Ref. 3). The original database contains some 240,000 records with the hourly values of wind, waves and current parameters.