A Wind-Tunnel Investigation of Loads and Pressures on a Typical Guyed Tower Offshore Platform
Authors: Pike Paul J. , Vickery Barry J. 
Published: October 2008

The information being used in design codes to determine wind drag loads on offshore structures has been 1imited to experimenta1 data obtained from simplified wind tunnel models. There is, therefore, a need to improve the experimental data base from which design code recommendations can be evaluated. This is particularly true in the case of compliant offshore structures such as guyed towers where wind loads can contribute significantly to overall design requirements.

To gain a better understanding of the wind loads on guyed tower structures, a wind tunnel test was performed on a 1:120 scale model of the above water superstructure of atypical guyed tower offshore platform. Force and moment coefficients were measured under a simulated steady boundary layer flow. A parameter study was conducted to examine how these coefficients are influenced by various elements on the deck and exposed substructure. Finally, pressure measurements were taken to obtain a better understanding of the flow field around the deck.

This paper discusses the wind tunnel tests, emphasizing (i) wind tunnel modeling techniques for offshore structures; (ii) test results presented in the form of force, moment, and pressure coefficients; and (iii) a comparison of the test results with recommended design code practices. The results of the model tests indicate that current design code practices overset imitating loads on offshore structures under steady wind conditions. The results also show that interference effects among various elements of the structure are significant.

Introduction

In designing offshore structures, it is common to base estimates of wind loads on projected area and nominal shape factors without regard to interference effects among the elements of the structure. This approach is traditional in the design of conventional fixed-bottom offshore structures because, first, it is generally believed that neglecting interference and shie1ding effects will lead to a conservative estimate of wind loads; and second, wind loads are often much less important to the overall structure than design-level wave and current loads. Thus, if wind loads contribute to, say 10% of the total design-level overturning loads on a structure, a great deal of conservator ism can be tolerated without paying a substantial financial penalty.

The introduction of compliant offshore structure concepts in recent years has lead to a reevaluation of the projected area/nominal shape factors method for estimating wind loads. One such compliant structure addressed specifically herein is the guyed tower, a deep-water drilling and production platform concept first introduced to the energy industry in the 1970's [l]. The guyed tower is classified as a compliant structure because it is designed to move in response to dynamic wave loads, as compared to conventional fixed-bottom structures which "rigidly" resist applied loads. This motion creates structural inertial loads that counteract the applied wave loads and help reduce the forces transmitted to the platform restraints. Constant loads due to steady winds and currents are not counteracted by the inertial loads and must be fully resisted by the platform restraints.