Stress Due to Wave Forces in an Offshore Research Platform

Abstract

ABSTRACT This paper reports on a study of wave induced strain on a free standing, rigid, four legged research platform erected in 12 m of water approximately 1.1 km offshore at the west end of Lake Ontario. The platform design was based on wave loads estimated from a 6 m, 7 s design wave, applied to the tower structure using the commercial program STRESS 3.0 to calculate member loads. Estimates of platform resonant frequency were also made. Following installation; wave data and strain in five bracing members were recorded and analysed. Wave loads derived from these data were applied to the tower structure using the original design model. Calculated strains were compared to observed strains and the usefulness of the design model was evaluated on this basis. Resonant frequencies indicated by the observed strain spectra are discussed. The study indicates that the relatively simple modeling techniques used on this platform provide a. conservative engineering estimate of stress for design purposes. INTRODUCTION Extensive use has been made of light-weight guyed towers as supporting structures for instrumentation and sensors in lake studies at the Canada Centre for Inland Waters. 1 While offering certain advantages in cost and mobility, such towers have limitations, such as low payload, high risk of loss in winter storms, and poor stability as instrument platforms. The need to provide a fixed research facility has resulted in the design and erection of a free standing, four legged platform in 12 m of water, 1.1 km offshore at the west end of Lake Ontario (Figure 1). As part of the design program for this platform, a study was established to measure actual strains developed in five major braces. The objectives were to confirm the engineering estimates of stress used for design purposes, and to learn more about wave forces on such offshore structures. This report outlines the basic platform design, and describes the techniques used for estimating design loads. The program for measurement and processing of platform strain and wave data is described. The models for derivation of wave induced external loads and for analysis of structural loads and deflection are described. Comparisons between observed and calculated strain for given wave conditions are discussed. PLATFORM DESIGN The design concept for this platform was adapted from an earlier platform installed off the coast of San Diego, California, in 1960 by the U.S. Navy Electronics Laboratory (NEL).2 The structure was laid out to provide CCIW with a stable platform which could accommodate large instrumentation and sensor packages for year-round, multi-disciplinary research. The main design problem was to achieve a high degree of structural stiffness and platform stability without requiring cross bracing in the wave zone, which would interfere with the air water interface and sensors. The resulting design (Figure 2), provides a bottom support frame with relatively large diameter legs. All members are round hollow structural steel (HSS) sections, with the exception of the upper deck box frame members.