Instrumentation of the SYU Jackets for Trans-Pacific Tow and Launch

Abstract

A comprehensive structural measurement program was carried out on Exxon's Harmony and Heritage platforms during their trans-Pacific tow and subsequent launch in 1989. The objectives of this program were to 1) provide the towmaster with wind and barge motion data during the tows, 2) collect jacket/barge response data to help define post-tow jacket inspection plans to account for actual tow conditions, and 3) collect a comprehensive set of oceanographic and jacket/barge response data for use in calibrating Exxon's tow and launch analysis procedures. Metocean conditions, jacket/barge motions, and loads in a broad range of structural members were measured throughout both tows. This paper summarizes how the measurements were performed, as well as, how the data was analyzed and used to accomplish the program objectives. INTRODUCTION In 1989 the Harmony and Heritage jackets were towed from Ulsan, Korea to the Santa Barbara Channel, offshore California. These jackets are the largest of all Exxon's platforms, standing in water depths of 1200 ft and 1075 ft, respectively. Harmony departed Korea aboard Heerema's 851 foot long launch barge on May 18, 1989 and completed the 5840 nautical mile voyage in 30 days at an average speed of 8.2 knots. After a successful launch, the barge returned to Korea for the Heritage towout on August 29, 1989. Following the same rhumbline course as Harmony, Heritage arrived offshore California 36 days later. Because of the great tow distance and the potential for severe weather (29 ft significant design seastate), much of the strength and fatigue design of the jackets was dedicated by tow conditions. In all, about 13% more steel was added to the jackets to accommodate the trans-Pacific tow, compared to a local west coast tow(1). With the potential for severe weather and the need for confidence that the jackets were fit for in-place service after completing the tow, Exxon chose to use a monitoring system for both tows. This paper describes the tow instrumentation program which addressed the tow monitoring needs as well as additional research objectives. A brief discussion of measurements conducted during jacket launch is also provided. TOW INSTRUMENTATION PROGRAM Objectives The tow instrumentation program was defined by two operational objectives and one research objective. The first operational objective was to provide the towmaster with quantitative data that could be used to decide if a change in barge heading or speed was required during severe weather. In this way, excessive jacket/barge motions could be prevented, reducing the risk of jacket overstress or fatigue damage. Referred to herein as "motion monitoring", this application of tow instrumentation has been used extensively in the past and is generally accepted as a valuable asset for large open ocean tows(2,3). The second operational objective was to collect jacket/barge response data that could be used to modify post-tow inspection plans to account for actual tow conditions. The importance of this objective was accentuated by the desire to expedite the Harmony launch so that the Heritage jacket tow could occur before encroaching too far into typhoon season.