SYSTEM MEASURES ENVIRONMENTAL LOAD ON N. SEA PLATFORM

Feb. 5, 1990
Shell U.K. Exploration & Production is conducting an extensive project to measure wave and current loads on a platform in the North Sea. Site for the 750,000 ($1.23 million) program is Tern oil field's steel structure in Block 210/25a in the U.K. sector. Shell Expro, operator for the Shell U.K. Ltd.-Esso U.K. plc combine, said the monitoring system will provide data not previously available about the platform's environmental load. Data gathered from the project are to play a role in

Shell U.K. Exploration & Production is conducting an extensive project to measure wave and current loads on a platform in the North Sea.

Site for the 750,000 ($1.23 million) program is Tern oil field's steel structure in Block 210/25a in the U.K. sector.

Shell Expro, operator for the Shell U.K. Ltd.-Esso U.K. plc combine, said the monitoring system will provide data not previously available about the platform's environmental load. Data gathered from the project are to play a role in planning inspection schedules, extending the life of existing platforms, and improving the design of future platforms.

Tern platform was installed in 548 ft of water in 1988. It is designed to withstand waves of 96 ft and wind gusts to 120 mph.

MONITORING SYSTEM

Tern's monitoring system, produced in conjunction with W.A. Fairhurst & Partners, provides data on key things such as wave spectrum, wave direction, and current and wind speed and direction.

Included in the system are a wind sensor, water particle velocity meters, and a wave height sensor.

The bases of each of the four corner legs and the adjoining horizontal braces have strain gauges to provide a history of the cyclic loading exerted on the platform. These data will be processed to provide statistical parameters for the measured base shear and overturning moment.

Full strain cycle counts are also recorded, adopting the rain flow technique, for fatigue calculation purposes.

Accelerometers at deck level enable determination of the platform's natural frequencies and deck displacements in two directions.

SHELL'S OBJECTIVE

Shell Expro said offshore platform design relies on mathematical models developed and refined over the years. They aim to represent the natural laws and phenomena to produce a structure that can safely withstand natural forces. But because details of the underlying physical phenomena cannot be captured or are not fully known, the models are not an exact representation of reality.

Jan Vugts, Shell Expro's marine technology manager, said the industry had been gathering environmental information on a large scale since North Sea operations began. However, since the start of North Sea oil production in 1975, only limited information has been obtained on the effects of the environment on platforms.

Vugts said, "Companies have run extensive and ambitious projects, and there have been some joint industry and government sponsored programs, but so far most of them have achieved only limited results for a variety of reasons.

"We hope the Tern system will, for the first time, give direct and realistic measurements of the loads on a complete platform, which have not been attempted in the same way before."

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