Underwater Inspection

Underwater inspection of concrete structures is normally undertaken using divers or remotely operated vehicles (ROVs).  Whichever method is to be used a simple rule of thumb is that:

• Offshore above water inspections take 3 - 5 times as long as onshore inspections
• Underwater inspections take 5 - 10 times as long as onshore inspections

Cost And Inspection Quality

In addition, the support cost and potential downtime associated with offshore and underwater inspections is significant.

Consequently the normally important item of planning becomes a critical task.

With the support cost, difficult working conditions and potential downtime, obtaining information underwater is most expensive.  Considering the expense of gaining information, it is essential to ensure that the information gained is accurate first time around.  Reinspection is generally not an option.

Planning Procedures

Planning is a normal part of inspection however with underwater inspection the following aspects are emphasised:

Proformas

Proformas are drawings of the structure that provide a visually simple means of locating elements within the structure and features on an element.  Proformas are developed that divers use underwater to assist in liaising with showing critical and non-critical features:

• Design review - Determine potential failure mechanisms and specify associated inspection requirements and survey methods.
• Logistics - Detailed logistics review to minimise downtime.
• Training - Ensure the dive team are fully trained in the inspection plan, system & techniques.

Design Review

On above water inspections it is normal to have proformas showing specific design and construction features to guide the inspector.  Having completed the inspection defects may be referred to a structural engineer to help establish the cause.  Because of the difficulties on inspection underwater (limited vision, surface covered, high cost) recording of defects across the whole surface is not generally undertaken.  It’s more appropriate to undertake the design review before the survey noting potential/common failure mechanisms and then specifically search for associated defects.  For example this avoids the problem of missing serious defects because the critical area was not inspected due to marine growth.

Logistics

The logistics of diver inspection are significant.  Weather windows, current restrictions, diver support requirements, accessing the site, equipment operations (dive and inspection) all present restraints to the straightforward task of noting a defect.  It is essential that the dive team and inspection management work closely together to determine how these aspects impact on the inspection programme and planning requirements.

Another issue is the location of defects. Underwater vision can be extremely limited.  Diver and ROV location is difficult and requires planning and location systems.  Whether by guide wire or acoustic positioning a methodology appropriate to the structure needs to be developed.

Training

Divers are multi-disciplined but have little knowledge of concrete and particularly defects in concrete.  Before arriving on site the divers should be trained in inspection procedures.
Defect classifications such as ACI 201 are comprehensive but too complex for inexperience concrete inspectors.  A simplified guide specifically for the structure to be surveyed should be developed (ref “Inspection of Concrete Offshore Structures” Papworth et al Journal of Petroleum Technology).

Inspection engineers may understand concrete and the significance of defects but they are generally not familiar with the difficulties of inspecting remotely or through third parties.  In the case of inspection reporting in real time using ROV’s or reporting on photographs, the inspector looses clarity, touch and is operating in two dimensions.  When guiding divers with audio only, resolution of defects identification and significance becomes impossible without training. Communications training between the diver and inspection support engineer is essential.

Operations

Rigid discipline is required to ensure information is gathered in a timely and consistent manner.  After each dive, the diver needs to be debriefed by an experienced engineer so that information is accurately recorded.  Videos and photographs should be immediately reviewed together to maximize accuracy of interpretation

Dive windows need to be carefully planned and logistical operations established to ensure these windows are maximised.  Daily planning based on weather, current, resources and key objectives are required.  Diving should generally take precedence over other activities due to the short windows available.