Marine Growth on the Hondo Platform in the Santa Barbara Channel

ABSTRACT Marine growth on the Hondo platform has been well documented in photography over the years. These photographs have been reviewed in order to understand the interrelationship between different foulers and the sequence of their growth. This understanding has been useful in developing a reasonable predictive marine growth model for future platforms in the vicinity of Hondo. In addition, marine growth thickness and roughness were measured on the present Hondo platform in May, 1981. These measurements are compared with the measurements on the near shore Chevron platforms. The reasons for differences in the growth patterns are discussed. The final marine growth and roughness profiles for environmental loading purposes were estimated from these measurements. I INTRODUCTION The effects of marine growth in increasing environmental loading had been generally known for some time but were not quantified by field measurements until the Ocean Test Structure program. 1,2 During that program wave forces and water particle kinematics were measured simultaneously in various stages of marine fouling. The marine growth effect was represented by the following two parametersMarine growth thickness to represent the increase in the effective cross-section of a member, causing increased projected area for drag force and increased volume for inertia force.Roughness height to represent increased surface roughness, resulting in a higher value of drag coefficient. The present paper summarizes Exxon's effort to estimate appropriate marine growth thickness and roughness heights for designing future SYU structures. In Section II we describe briefly the cyclic history of marine growth on Hondo 'A' from June 1976, when the platform was installed, to May, 1981, when the marine growth was measured. The description is mainly based on a photographic study. In Section III we present the cyclic variation in the marine growth thickness along a short length of a member over the period 1977-1981. These thicknesses were estimated approximately from the photographs taken from a fixed camera mount. In Section IV we describe the methods used and the data obtained in the marine growth thickness and roughness measurements on the Hondo 'A' platform during May 20-23, 1981. These measured data are compared with the data4 obtained by Professor Nath of Oregon State University on two other platforms in shallower water near Santa Barbara. Based on our photo interpretation and measurement on Hondo 'A' and OSU measurements on Chevron's Hazel and Heide platforms, we present our best estimates of climax marine growth thickness and roughness height in Section V. II. HISTORY OF MARINE GROWTH ON HONDO 'A' In this section we describe briefly the evolution of marine growth on the Hondo 'AI platform. The Hondo 'AI platform was installed in 850' water depth in the Santa Barbara Channel in June, 1976. Exxon, USA, Western Division commissioned a study to document the marine growth on the platform. Divers visited the platform periodically from September, 1977 to March, 1980; took photographs of macro growth and individual specimens; and collected samples for identification by marine biologists. On the very first visit in September, 1977 the divers found a well established marine growth community. The examination of specimens revealed over forty major species present on the platform. The bay mussels (mytilus edu1is) dominated the upper levels. This mussel normally grows in sheltered areas away from strong wave action. It has vigorous growth habits and