33 



A variety of commercial sub-bottom profilers are available where both the pulse 

 length and frequency are selectable for operations in different types of sea 

 bottoms. 



2.1.7.J Corrosion Potential (c-p) Monitoring 



Corrosion potential measurements are taken to monitor the effectiveness of 

 a cathodic protection system (i.e., determine the need to replace the 

 sacrificial anode) . The instrument employed is some form of half-cell 

 (silver/silver chloride, copper/copper sulphate) which measures the potential 

 between the structure and ambient sea water. Several commercial organizations 

 manufacture c-p monitors for application from ROVs (Corrosion and Welding 

 Engineering Ltd., Sidcup, Kent; Sabines Industries, Inc., Long Beach, Ca. ; 

 Morgan Berkeley and Co. Ltd, Winchester, Hants). The procedure involves 

 making contact between structure and c-p probe and reading or recording the 

 data (digital or analogue) . 



Sonarmarine Ltd. employs a technique which involves holding the reference 

 cell (silver/silver chloride) in the claw of one manipulator which is connected 

 through the umbilical to a high impedance digital voltmeter. The circuit 

 is completed by attaching the other side of the voltmeter to the structure. 

 The potential values are displayed on the video monitor. 



2.1. 7. k Ultrasonic Thickness Measurements 



Thickness of structures is being measured from ROVs using ultrasonic techniques. 

 In this technique an electric pulse is converted into a mechanical vibration 

 by a small transducer which is held against the cleaned structure. The vibrations 

 are transmitted into the object being measured where they are scattered, 

 attenuated, reflected or resonated. A portion of this energy returns to the 

 transducer where it is reconverted to electrical energy and subsequently 

 processed to be displayed either digitally or on a CRT. By knowing the 

 sound velocity characteristics of the material in question and the round 

 trip time of pulse from the transducer head through the test material, it is 

 possible to determine material thickness. 



Several commercial firms manufacture ultrasonic thickness probes which are 

 adaptable to ROV application Sonarmarine Ltd. , the only ROV operator known 

 to offer this service, has developed a probfe specifically for use from their 

 vehicles. 



2.1.7.1 Radiographic Testing 



Radiographic techniques are used for flaw detection in metals. The components 

 of this technique consist of a radiation (gamma) source and a photographic 

 film in which the voids or flaws in the test material appear as shadows. The 

 technique can also be used for thickness measurements. 



Sonarmarine Ltd is the only ROV operator known to offer this service. Their 

 technique is, at present, applicable only to risers and involves placing a 

 radiographic source and photographic plate around the riser for some specified 

 time before recovering them. Details of the technique are not available. 



