Dynamic Tow Motion Of a Platform In a Seaway

Abstract

ABSTRACT An analytical method for the prediction and analysis of the motion and stability of a platform under tow in a seaway has been developed. Equations of motion and hydrodynamic coefficients are derived, which account for the effect, on plat form motion and stability, of tow speed, waves and heading relative to the prevailing wave direction. This method is an extension of a previous simple method of prediction for zero tow speed. Predicted motion is presented for a multihull platform subjected to the wave frequency of interest. The results show that heave is increasingly coupled with pitch as tow speed into a head sea increases, whereas there is little heave pitch coupling at zero tow speed. Surge, sway, roll and yaw show little coupling effect compared with heave-pitch coupling. As speed increases in a head sea, amplitude increases near the resonant heave period, possibly reducing longitudinal stability. Heave pitch coupling effects are strongly dependent upon tow speed or damping. At tow speeds above certain critical values, reduction in pitching moment or decrease of longitudinal stability may require damping devices such as fins. There has been renewed interest or concern following the recent capsize of several jacket platforms when under tow in a seaway. Conventional analytical prediction or analysis of stability has not taken into account the effect of speed in the equation of motion. Present analytical prediction of motion is based on solution of coupled six degrees-of-freedom motion for a multihull platform at tow draft. The analytically predicted speed effects on the motion of the present semi submersible platform under tow in a seaway are qualitatively compared with model test results of a jacket platform under tow. The comparison shows good agreement in the trend of the speed effect. This method can be used to aid in the analysis of dynamic stability or to establish requirements for towing a jacket plaltform in the actual ocean environment, given prevailing weather or wave conditions along the tow route. Also, it can be used to aid in deciding the tow-out date by analyzing weather and sea state forecasts, and in predicting platform motion and stability at various times of the month or year. Towing of semi submersible platforms, semi submersible-type offshore supply vessels or similar floating structures can be analyzed directly by the present method. INTRODUCTION Renewed interest or concern has been developed following several recent capsizing incidents of platforms or tall jacket platforms under tow at sea as a whole or in pieces. Floating structures such as semi submersible or jacket platforms designed for stationary operation in the ocean have usually been towed to operational or installation sites. Safe towing depends largely on local weather conditions prevailing along the tow route. That is as much a question of halting the tow during bad weather, as it is of getting to the site a few hours earlier.