7.1 INTRODUCTION
7.1.1 The following types of visual inspections are specified:
General Visual Inspection (above water);
Close Visual Inspection (above water);
Underwater General Visual Inspection;
Underwater Close Visual Inspection.
7.1.2 For all inspections, it is essential to set up a proper
referencing system to establish the locations of all defects, close visual inspections and tests.
For sheet pile walls, a chainage regime shall be set up for each structure.
For bearing piles each pile shall be given a unique reference.
A tape measure shall be used to measure the vertical location.
In the case of raking piles, it shall be made clear whether distances are measured vertically or parallel to the pile axis.
7.2 GENERAL VISUAL INSPECTION
7.2.1 General visual inspections are required for all elements of the structure at and above Mean Low Water Springs, under all inspection regimes.
7.2.2 General visual inspections shall be a brief qualitative visual assessment of the condition of the steel, without removal of marine growth. The method shall be suitable for the conditions.
As a minimum, it shall be undertaken at a distance of not more than 5 metres from the steel, or not more than 15 m with the aid of binoculars.
7.2.3 For individual piles, the inspection shall cover the element from Mean Low Water Springs to the visible top of the pile.
Ideally it should cover all sides of the pile, but where access is difficult and for the Regular Visual Survey, one side is sufficient.
7.2.4 For sheet pile walls, the inspection shall cover the exposed face only, from Mean Low Water Springs to the top of the wall.
7.3 CLOSE VISUAL INSPECTION
Piled Maritime Structures
7.3.1 A Close Visual Inspection involves careful inspection of the element at specific points.
7.3.2 All marine growth shall be removed at each point to expose the protective coating if present or, if there is no protective coating, the steel or rust products.
It should be noted that marine growth provides some protection to the steelwork, and therefore no more than is necessary for the purposes of the inspection should be removed.
Care shall be taken to ensure that the removal of the marine growth does not damage any protective coating.
Where there is a protective coating, the steelwork should preferably be cleaned using a low pressure water jet, at a pressure of around 1,000 to 1,200 psi. This will remove loose products but is less likely to damage the coating compared to mechanical methods.
If any protective coating is damaged, this shall be made good at the time of the inspection.
7.3.3 Any rust products shall be investigated during the inspection to establish whether they are as a result of MIC or normal corrosion.
7.4 UNDERWATER GENERAL VISUAL INSPECTION
7.4.1 Underwater General Visual Inspections (UGVI) are required for all elements of the structure below Mean Low Water Springs (or from where the above water inspection could not be done) for Standard Surveys. UGVI’s shall be a brief qualitative visual assessment of the condition of the steel.
7.4.2 The method of UGVI shall be suitable for the conditions, and agreed by the Professional Team Leader. Where visibility is very poor, a UGVI may not be possible or be too time consuming. In these cases, it may be necessary to restrict the extent of the UGVI, or rely on UCVI’s at discrete points, subject to the approval of the Professional Team Leader.
Where turbidity varies significantly over time, the UGVI should be scheduled to make use of the best conditions. For example the diving may have to be delayed during heavy rains, if the latter increases the amount of sediment in suspension in the water.
7.4.3 For individual piles, the UGVI shall cover the element from Mean Low Water Springs to the seabed. Ideally it should cover all sides of the pile, but where access is difficult, one side is sufficient.
7.4.4 For sheet pile walls, the UGVI shall cover the exposed face only, from Mean Low Water Springs down to the seabed or estuary bed.
Particular attention should be paid to the areas at LAT and also the bed level.
7.4.5 The UGVI shall identify the general condition and features on every structure sufficiently closely to detect major and minor defects or abnormalities. No removal of marine growth or deposits is required.
7.5 UNDERWATER CLOSE VISUAL INSPECTION
7.5.1 The Underwater Close Visual Inspection (UCVI) shall entail close inspection of specific areas.
7.5.2 All marine growth shall be removed at each point to expose the protective coating if present or, if there is no protective coating, the steel or rust products. The area shall be a minimum of 150mm by 150mm.
It should be noted that marine growth provides some protection to the steelwork, and therefore no more than is necessary for the purposes of the inspection should be removed.
Care shall be taken to ensure that the removal of the marine growth does not damage any protective coating, see clause 7.3.2.
7.5.3 Any rust products shall be inspected to establish whether it is MIC or normal corrosion.
7.6 PHOTOGRAPHY
7.6.1 A comprehensive photographic record should normally be taken for all Surveys, subject to MOD Regulations. Above water, this shall include colour still photographs of major defects and general photographs of the structure to give an overall
impression of the typical condition of the structure.
The photographs shall generally be taken in the same locations and of the same areas at each survey, to give a regular record of the condition of the structure.
7.6.2 All shots shall be recorded on suitable log sheets, with their locations clearly identified.
7.6.3 For underwater work, stills photographs or video records should be made, subject to available underwater visibility. For visibility less than 1.0 – 1.5m, stills photos are not normally useful. For underwater visibility less than 300mm video
recordings are also not practical. Low light subsea cameras can focus to 100mm but the diver in the water cannot.
There should not be a reliance on still and video cameras to carry out or assist inspection work of this nature. However if it is possible to provide stills or video, all shots and footage must be recorded on log sheets with their locations, to complement the Ultrasonic Test and Cathodic Protection logs at the same location.
Piled Maritime Structures
Section 8 – Requirements for Non-Destructive Testing
8.1 ULTRASONIC THICKNESS MEASUREMENTS
8.1.1 Residual steel thickness measurements shall be made as specified using an ultrasonic thickness (UT) meter.
8.1.2 The diver held underwater UT meter shall be the multiple echo single probe type with remote read out between 1-99mm range to 0.1mm accuracy.
8.1.3 The UT meter shall be used in accordance with the manufacturer’s recommendations and the following practical procedures.
Procedure
8.1.4 The following procedure should be used. Where appropriate, it may be adjusted slightly to suit site conditions.
(a) Calibration – Pre-calibrate the meter with the blocks supplied with the UT Meter. The UT meter shall be calibrated on the surface before and after the main measurements with 10, 15, 20 or 25 mm calibration blocks. Record the results in the correct place on the UT Log sheets (see sample in Appendix).
(b) Readings - At the specified position, clean off the marine growth using a wire brush and/or scraper and remove any loose flaking steel. The cleaned area shall be a minimum of 150mm by 150mm. Squeeze some ultrasonic couplant (not too much) on to the area (note: couplant is not required underwater). Attempt to obtain a reading without any grinding. This is preferable, but if it is not possible grinding will be required, see 8.1.5 below. Take 4 no. readings at each localised spot within a 50-60mm square. Write the four readings in the UT Log Sheet. Check if there are any
spurious readings (i.e. any wildly out) and re-do again to end up with 4 no. readings. Check the results against the original steel thickness. Take the average of the four readings to give the result.
(c) Recording - Use appropriate UT Log sheets (see Appendix for example) to record the results. Draw a sketch of the steel sheet piling/ section to illustrate where the measurements were taken. Measure up the overall dimensions of the section to check that it is as recorded on the drawings.
Record the original steel thickness.
8.1.5 Usually the use of a Multiple Echo Measurement Technique (such as a Cygnus 1 underwater UT meter) will negate the requirement of grinding. However when using other systems, if corrosion has caused pitting of the surface, it may not be
possible to obtain a UT reading. If this is the case grinding will be required to obtain reading from the UT meter. Grinding should be done very carefully and slowly in 0.5mm stages to minimise the removal of structurally intact steel prior to a successful UT reading being completed. Grinding is stopped either when the bottom of any corrosion pit is reached and the probe can take a reading or when the entire steel surface is visibly bright steel.
Above water an electric grinder can be used. Below water either an air grinder or a hydraulically powered grinder will be required.
8.1.6 Assuming that the age of the structure is known, the readings shall be converted to an estimated corrosion rate. Take the original wall thickness (WT) minus the remaining WT and divide it by the age of the structure to give the corrosion rate in mm/year. This can be compared with typical values.
Where there are previous readings, the corrosion rate between each reading shall also be calculated, to establish whether this is consistent.
8.2 MARINE GROWTH MEASUREMENTS
8.2.1 The thickness, percentage cover and density of the soft and hard marine growth (MG) shall be recorded on MG Log sheets (see Appendix for an example). The equivalent marine growth thickness shall be calculated from the equation
ET =[ (A * B) + 0.5 (C * D)]/100
where ET = equivalent marine growth thickness in mm A = %cover of hard MG
B = thickness of hard MG in mm C = % cover of soft MG
D = thickness of soft MG in mm
Marine growth measurements will act as a record for future inspections determining the level of cleaning that may be