Inspection of Concrete Offshore Structures
Authors: Browne R.D. , Doyle V.J. , Papworth F. 
Published: December 2007

The planning and inspection procedures used on Total Oil Marine Ltd.'s MCP-01 (manifold compression platform) minimize time spent offshore and ensure that the retrieved information accurately describes the structure's condition. Procedures include preparing a detailed field specification; visual, photographic, and nondestructive inspection photographic, and nondestructive inspection techniques; and a report designed for easy access to detailed information. Current research on underwater repair techniques is outlined.

Introduction

The Offshore Installation (Construction and Survey) Regulations 19741 require that a thorough examination (major survey) of an installation be undertaken to ensure compliance with Schedule 2 of that document before the 5-year certificate of fitness is renewed. In addition, annual surveys are required of a sufficient number of selected parts of the structure to give reasonable evidence of continued compliance with Schedule 2.Schedule 2 of the regulations outlines the factors to be considered for structural integrity of a platform.The first certificate of fitness ensures that the design and construction requirements are satisfied. Therefore, subsequent inspections need only to consider changes in the structure's state. For dead and live loading, these changes must be taken into account in the design limits. Consequently, both annual and major inspections are concerned primarily with material deterioration, with accident primarily with material deterioration, with accident damage included where a separate inspection is not required.The inspection program for Total's MCP-01 satisfies these regulations while minimizing the interference with platform operations and providing detailed, quickly accessible information in annual reports on the structure's condition. This program is considered in two parts:the planning approach to cover the major and annual inspection requirements andvisual inspection and nondestructive test methods.In the event of a deterioration rate that could lead to an unacceptable structural state in the design life or to damage caused by ship impact, dropped objects, fire, or explosion, repairs will be necessary.

Although methods of repairing small scale concrete damage underwater exist, there are no techniques for undertaking major repairs.To fill this gap in the offshore industry's capability, a research program aimed at developing such techniques, particularly where reinforcing and prestressing are involved, is underway. prestressing are involved, is underway. Description of MCP-01

The intermediate manifold platform, MCP-01, was positioned as the intermediate station between the positioned as the intermediate station between the Frigg field and the St. Fergus shore-terminal treating facilities in July 1976.MCP-01 (Fig. 1) is a concrete gravity platform, designed by C.G. Doris, comprising a solid raft 101 m in diameter with six tunnels radiating from a hollow central core. The core, 9 m in diameter, extends from the raft through the full height of the structure, which is 147.5 m and houses the 32-in. riser piping with two risers per pipeline. Each radial tunnel contains a seal caisson which provides the watertight sealing joint through the concrete lobed wall for riser entry.At approximately 30-m radius from the core there is a concrete caisson wall (lobed wall) which extends above the waterline. The grillage of concrete deck beams is supported off the core from columns located on top of the lobed wall.

JPT

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