ABC Diving® Ltd.

International Cy
Registered in Port-Vila P.O. Box 3242
VANUATU South Pacific
Ph: +678 5684277 or 7100230

~ COMMERCIAL DIVING ~

Inspection Diving

Inspection is a diver task traditionally applied to exploration well-head diving, and has been based almost exclusively on visual methods such as photography, video and the diver's eyes.
However, the emergence of certification requirements for fixed production structures, in some areas, has meant that the requirements for inspection of a hight quality has been established. To cater for this, the traditionnal tasks and methods are now supplemented by additional specialised activities, which include:

Non-destructive testing (NDT) of nominated areas
Corrosion damage inspection
Cathodic protection potential surveys: anode inspection measurement
Marinegrowth inspection (and removal)
Debris inspection (and removal)
Scour and stability inspections.

The basic reason for inspection is to help assess the engineering integrity of a structure. It must be remembered that inspection is data-gathering task only, and does not normally include interpretation of data significance. Divers inspectors have no qualification to interpret structural integrity from the inspection data that they gather.

The various inspection activities have a basic logical order. Some tasks have to be done prior to cleaning, while others must be done afterwards. For example, most NDT cannot be performed before cleaning, while obviously a marinegrowth survey cannot be performed afterwards.

Once this is understood, the various inspection requirements of a client can be arranged so as to create the most efficient inspection programme.
In general terms, programmes are arranged so as to perform:

A general initial survey, before any cleaning, to see broadly if there is any obvious damage or problem.
General tasks, such as seabed surveys, marinegrowth surveys and corrosion inspection.
Appropriate underwater cleaning.
Critical inspection tasks in likely areas, in relation to less obvious (or even invisible) defects. The most important of these is cracking. This critical inspection is to detect defects and then, if found, to accurately size them.

The inspection is a complex subject, some aspects of which involve theories and formulae beyond the scope of this page. More detailed information may be obtained from other sources or Contact us.

Visual Inspection

The bulk of inspection data is derived from visual inspection techniques. Visual inspection is used to inspect structures for obvious damage such as missing parts, dents, buckles and tears; and to observe and record underwater conditions and features.
Underwater visual inspection is carried out by the human eye, still photography and closed circuit television (CCTV) or video.
These methods are used in conjunction with verbal and written reports from the underwater inspector, who may be a diver or submersible pilot. Both manned and remotely operated vehicules are normally fittted with still and video cameras.

We offer digital High Definition photography and video together, as each has its own advantages.
Still photography (Nikon camera D200 + Sea & Sea housing) provides the necessary high definition required for details analysis and easy access and share on the Web or printed report. All format available (jpeg, gif, raw...)
Video (Sony FX1 HD + Sea & Sea housing) on the other hand, provides a continuous real-time image of events and a wide angle of view with zoom possibility, efficient even with a poor water visibility. All format available (PAL, SECAM, NTSC) including Web.

It must be remembered that visual inspection can only detect visible defects. While this seems obvious, cracking defects can be very difficult to find by visual methods (although once found they may then be able to be 'seen'), and may indeed be invisible to the naked eye or camera.

General Survey Inspections

Seabed surveys

Tide and current movement around the base of a structure or pipeline may produce shifting of the seabed levels. Scour may leave portions of a structure (particularly leg bases) or a pipeline unsupported, resulting in movement, rocking, displacement, or even rupture of the structure or pipeline. Scour prevention devices, such as artificial seaweed mats, can effectively prevent or control scouring problems, but regular seabed inspections are still required to check their continued effectiveness.

Debris surveys

Debris means any material that should not be there, such as lost fishing nets, anchor cables, wire ropes, girders and scrap material dropped over the side from offshore platforms and vessels.

Debris is a serious problem, not only because it may create a hazard to divers, submersibles and ROVs, but also because it can interfere with corrosion protection systems and may cause fretting, wear and resultant accelerated corrosion.
For these reasons, debris surveys, to locate and record the positions of all debris so that it may later be removed efficiently and safely, are essential.
Every offshore structure has a limit to the amount of weight and wave loading that it can safely endure.

Marinegrowth surveys

Marinegrowth can jeopardise the safety of a structure by increasing not only the weight of the structure itself, but alos its surface area, which means that the impact of waves and currents is increased. If marinegrowth becomes excessive, important equipment may have to be removed from the deck, or the structure may become unsafe. Regular surveys are needed to help determine when the removal of marinegrowth will be necessary.
Such surveys need to measure the thickness of the growth, the amount of the structure that is affected, and whether the growth is 'soft' or 'hard', as each type has different significance due to different weights per volume.

Corrosion Inspection

Corrosion surveys are essential to any inspection programme. As steel and salt water are very corrosive combination, much attention is directed to the problem.
They are many different types of corrosion, each produced by different causes. They may occur alone or simultaneously. Some of these types are listed below:

Dissimular metals in close proximity (galvanic)
Fretting corrosion
Biological corrosion
Stress corrosion and fatigue
Erosion corrosion
Crevice corrosion and concentration cell corrosion
Hydrogen attack and embrittlement, etc, etc.

The two that are the most interest to the diver are 'general corrosion' (an even loss of metal) and 'pitting corrosion' (localised areas of concentrated metal loss).
The present engineering approach to these problems has two prime aspects:

To built-in at the design stage, an extra thickness of steel as a corrosion 'allowance', normally about 20-30 mm in the splash zone.
To attempt to prevent corrosion. The main protection methods are cathodic protection (sacrificial anode, impressed current) and protective coatings (electrically insulating cover).

Corrosion inspection has three important objectives:

To find out if the corrosion protection systems are actually working, by checking:
1. That the coating is intact
2. That the coating remains bonded to the steel
3. That the anodes are still in place
4. That the electrical circuit still exists between the anode and the structure (check continuity strap)
5. That the electrical potential between the structure and the seawater is adequate by comparison against a reference half-cell indication, using a CP meter.
To find and measure corrosion damage by:
1. Measuring the thickness of steel for general corrosion, using ultrasonic thickness meters, commonly called 'D' meters
2. Measuring the extent and depth of pitting corrosion, using pit gauges.
To help plan the maintenance of corrosion protection systems by:
1. Measuring the dimensions of anodes to find the rate of erosion and thereby estimate the remaining life of an anode
2. Assessing the condition of the protective coating to find out if and when maintenance is required.

We can provide you with thickness of steel measure, until 500 m depth, without the need to remove coating (use of multiple echo) and even on the most heavily corroded metals.