A Variable Reluctance Means for Determining Tendon Loads in c Tension Leg Platforms

ABSTRACT This paper describes an instrumentation system for measuring axial tension and bending loads in the tendons of a Tension Leg Platform. The heart of the system is a set of 12 variable reluctance sensors that interpret tendon stretch under load as inductance changes in a magnetic circuit. The sensor inductance is used in a L-C analog signal conditioning circuit. A digital computer converts resonant frequency of this circuit to displacement using a set of measured displacement-frequency calibration tables. The digital computer also performs a least mean square fit of the displacement data to an axially displaced and related plane. These displacements multiplied by the appropriate stiffnesses are obtained from physical load calibration to determine the loads. The system incorporates multiple levels of redundancy to achieve along service life and is designed for minimal loss in accuracy in the event that any of the system components are replaced. A single prototype tendon instrumentation system (TIS) was physically calibrated both in axial tension and in bending. This system was accurate to within 30 KIPS for axial loads up to 4500 KIPS and to within 15 ft-KIPS for bending loads to 600 ft- KIPS. INTRODUCTION The Tendon Instrumentation System (TIS) is designed to provide a highly reliable system for accurately measuring axial and bending loads in the tendons of a tension leg platform. As shown in Figure 1, the TIS consists of underwater components including a Load Measuring Unit (LMU) in series with each tendon, underwater-mateable connectors and cabling, and dry components consisting of signal conditioning, data processing and communication hardware and software. Each LMU contains 12 variable reluctance sensors that are enclosed in a sealed, oil-filled cavity. Outputs from these sensors are carried by cables to two underwater mateable connectors that are in turn connected to two umbilical cables that bring the outputs to the analog signal conditioning circuits in the sensor interface units (SIUS) on the platform. The four SIUS are coupled to a digital computer or Central Processor (CP) that calculates the axial tension and bending moment in each tendon and communicates this information to other monitoring systems on the platform. The TIS minimizes the probability that the underwater portion will fail byincorporating seawater barriers for each underwater component, fourfold sensor redundancy, and twofold cable and connector redundancy. In the event that an underwater failure does occur, the sensors, cables, and connectors can be replaced with minimal degradation inthe accuracy of the measurement. Signal conditioning electronics consist of customized analog circuitry and commercially available hardware. Signal conditioningsoftware is designed to self-monitor sensor faults and to automatically eliminate bad sensors from TIS output calculations. The components, and the means for theirphysical calibration, are described in the following paragraphs. LOAD MEASURING UNIT The Load Measuring Unit consists of a modified segment of tendon pipe, called the spool piece, and the variable reluctance sensors mounted in a sealed cavity within the spool piece.