Techniques and Developments in Underwater Structural Inspection

Abstract

Abstract The paper considers current methods of undertaking underwater inspections of North Sea platforms and discuss" some aspects associated with obtaining and recording data for certification purposes. purposes. In time it is expected that improved techniques and equipment will be developed to the point where monitoring will reduce some of the more onerous and costly diving elements in inspection work. Introduction There are two important reasons why the work must be carried out: - It is vital for a platform operator to know the validity of the structure as a whole and to know exactly where the problems might occur, so chat they can be tackled before they develop into a serious situation. - Because of government legislation which requires that all offshore structures must have a certificate of fitness and have been thoroughly inspected over a period of five years;the certificate to be renewed every five years thereafter. The task of inspection, mainly a diving operation, at first glance appears formidable. It is equivalent to a man crawling across a football pitch to ascertain the condition of every blade of grass. Inspections by British Petroleum have been programmed equally to cover a five year period; the cumulative result being a prepared report which indicates how the structure stands to date and the inspections carried out during that time. Information gathered revolves round three main areas: - The integrity of the structure as a whole - The corrosion - The marine growth. It is necessary to realize at this point that there is no recorded data relating to climatic or environmental effects on steel structures positioned for long periods in the northern North Sea and little research if any, prior to the locating of the Forties platforms, into corrosion problems, types species and forms of marine growth, and the effects that effluent etc., from normal human habitation aboard a structure have on marine growth and in turn the structure. In effect, BP, like other platform operators, have no guidelines to follow on how to rid structures of excessive marine growth, or how to overcome corrosion by other than traditional methods. The Forties platforms were designed to stand in 400 to 500 ft. water depths, to take the static deck load of a specific number of tons located high, and to withstand the 100 year storm. The platforms are monitored to record the state of the members, any storm damage, corrosion, the condition of surface coatings, marine growth and damage caused by outside sources, etc. Before drawing up an inspection programme there are many factors to be considered - type of diving spread; saturation, surface; age of structure; previous work undertaken on platform in earlier surveys; degree of coverage and results of previous work done on similar structures in earlier years, fatigue life of members - obviously these matters would need to be enlarged on. With visual inspection, only defects of gross notice would be detected. It should be directed towards the diver and the member recording marine growth, damage corrosion condition of anodes taking c.p. readings, etc. This type of survey is particularly important in the early years of the structure when a base line survey is needed to detect any major damage that could have occurred when placing the structure in the sea bed. This enables the company to establish early marine growth patterns which are important, thus ensuring that marine growth remains within design criteria. The efficiency of the cathodic protection can also be established and practical estimations of the sacrificial anode life can be obtained. Early inspection also enables a company to check on the early condition of the grout and that of the sea bed state. Faulty connections on supports can also be checked. If a painted structure, paint conditions can also be checked. If problems do occur at a later date, have the faults been there since construction, or since placement days? Early surveys usually answer such questions and serve as a base line survey. It must be decided before undertaking a close detailed examination of a defect what the survey is being used to detect. If a painted structure and the paint is intact on scaled members, there is little point in destroying it to take thickness measurements. The question also arises in the high stress areas where the fatigue life can be affected by the corrosive environment - is it sensible to destroy the protective coating to undertake magnetic particle inspection?