Wind Tunnel Investigation of the Harmony and Heritage Jacket Tows

Abstract

ABSTRACT A wind tunnel investigation was performed in support of the HARMONY and HERITAGE transpacific jacket tows. This study provided wind load measurements for use in establishing towing vessel requirements, and assessing hydrostatic stability of the barge tows. In conducting the wind tunne1 experiments, the jacket models were specially designed and constructed to address Reynolds number effects on tubular members. Sensitivity studies were also conducted to quantify wind loads affected by wind direction, barge heel, and mud mat area. Measured loads were compared to empirical calculations based on procedures defined by DnV and ABS. INTRODUCTION The HARMONY and HERITAGE jackets, at over 1200 feet and 1075 feet in length, respectively, are the two largest barge-towed structures ever transported across an open ocean. The jackets were fabricated in Korea and towed to California as single-piece structures (Fig. 1). Both structures were successfully installed in 1989 in the Santa Barbara Channel. During preliminary design, it was recognized that both jackets may be fabricated in the Far East, and transported as either single-piece or two-piece structures. Tow horsepower and barge stability were two of several transportation design considerations that had to be addressed. Both tow horsepower and stability are controlled by wind load considerations. BACKGROUND Standard design practice for estimating jacket tow wind loads is based on empirical calculation methods. Such methods are defined by the American Bureau of Shippingl, Det norske Veritas2, and other design codes. However, these methods are generally not applicable to dense tubular lattice structures representative of typical offshore platform jackets. This is particularly true for deepwater jacket tows where the jacket provides almost the total contribution to overall tow wind load; the barge contribution is minor. Flow field effects, such as member shielding and other interaction effects are not explicitly addressed. Nevertheless, the ABS and DnV methods have been applied to most jacket tows performed to date, and with reasonable success. This is aided by the fact that for smaller jackets, inaccuracies in wind load estimates may be accounted for in safety factors applied to the design of the tow. Empirical calculations were not considered adequate for HARMONY and HERITAGE due to the large size of the jackets and their wind-dominated towing characteristics. Inherent uncertainties in calculated wind loads could influence the size and number of towing vessels as well as the minimum acceptable launch barge size based on stability criteria. Therefore, an experimental Study was necessary to obtain reliable wind load estimates for design use. WIND TUNNEL TEST PROGRAM The test program was conducted at Texas A&M University's low speed wind tunnel facility. The wind tunnel has a test section measuring 7 feet in height by 10 feet in width. Of the two platforms, only the HARMONY jacket was tested. Results were later extrapolated to the similar but smaller HERITAGE jacket. Particulars of the HARMONY and HERITAGE jackets are presented in Table 1 and Fig. 2.