Structural vibration surveys offer a low cost method of assessing structural integrity. The cost is low because surveys are done exclusively above water at deck level by a small crew using light weight portable equipment. It can augment or replace diver and ROV inspection surveys, thereby offering a significant reduction in inspection costs. Black Consulting (USA) is partnering with Somehsa Geosciences (Singapore) to provide this inspection service to offshore operators in the SE Asia region.

Vibration surveys monitor low frequency platform vibrations and then, through data analysis, deduce the platform’s global resonant frequencies in the surge sway and yaw modes of motion. Basic relationships between platform stiffness, mass (weight) and resonant frequency enable one to assess a platform’s condition and deduce its fitness-for-purpose. This technique can be useful for life extension assessments or assessing platform integrity following extreme loading conditions (storm, seismic event or foundation disturbed by drilling upset). The UK Health and Safety Executive has evaluated vibration monitoring and deemed it effective for assessing structural integrity (HSE Offshore Technology Report, OTO 97 040).

These techniques have been developed and applied for a number of years. Assessments have been made on platforms in Alaska’s Cook Inlet, California’s Santa Barbara Channel, the Gulf of Mexico, the UK sector of the North Sea, Indonesia’s Makassar Strait and SE Asia. Vibration surveys are particularly useful for monitoring aging facilities that are being modified for enhanced recovery (increased deck loads).

Black Consulting recently partnered with Somehsa Geosciences (Singapore) to conduct surveys on 95 platforms. Many of the platforms had been surveyed in prior years and this project was a significant expansion of the client’s existing resonant frequency database. These results provide guidance for the client to determine whether any frequency change was due to added deck weight or loss of structural stiffness. The field survey, data analysis and reporting costs averaged less than US$2500 per platform.