An ultrasonic inspection system is being developed to locate and size defects in the tension legs of the Hutton Field Tension Leg Platform (TLP), which will be installed in the United Kingdom sector of the North Sea. The ultrasonic inspection probe will collect and transmit analog data to topside computers by wire line for digitization, processing, analysis, and graphic reconstruction.
The mooring system for the Hutton TLP (Figure 1) has been discussed in detail in earlier papers. The basic components of the tension legs (Figure 2) are the tension legs elements, the cross load bearing, the tension adjusting elements, and the anchor connector. Sixteen tension legs, four in each corner of the platform, have been designed to withstand a very high cyclic tension load from the wave action on the platform hull. The design tension load per leg can vary from 0 to 3100 tonnes under extreme storm conditions, and over its 20-year design life the platform can be expected to experience approximately 108 load cycles of varying magnitude. Thus, fatigue is an important factor in the design of the TLP tension leg components. To ensure that these components are functioning as designed and, more importantly, that no critical damage has occurred, a nondestructive examination (NDE) system has been developed.
The ultrasonic NDE system, currently under final functional testing, will provide sizing information about defects or cracks that might develop along the load path so that the structural condition of the components can be assessed. Crack sizing information and growth measurements will be coupled with fracture mechanics analysis to determine fitness for purpose and schedule maintenance. This paper discusses the design and development program and presents some of the test results obtained with the prototype system.
The NOE system is comprised of three major subsystems: ultrasonic inspection probe, deployment, and data acquisition and analysis. The inspection probe is deployed inside the 75-mm tension leg bore, which is filled with a water-glycol mixture. This fluid serves as the transmission medium for the ultrasound that is emitted by the probe, reflected by defects, and echoed back to the probe. The ultrasonic immersion technique used for the inspection is complicated by the high-curvature tension leg bore surface, which has a strong acoustic focusing effect. This is counteracted by using special shaped transducers designed for the tension leg application. The received signals are complex and are analyzed and stored by a central computer (VAX 11/750). The computer also controls the probe so that operation is automatic and nominal operator interaction is required.
Equipment needed in the mooring compartments for deployment (probe, winch, portable control unit, and entry and calibration section) is designed and packaged in three modules that can easily be moved by crane from one compartment to another installation of the equipment on one leg is expected to take two people less than three hours, and transfer between legs in the same mooring compartment should take less than one hour.