Subsea service: Subsea integrity assessment for offshore foundations

The largest part of a wind turbine's foundation is hidden beneath the water surface or even buried in the seabed. Up to 95 percent of a wind farm's cables are not accessible for visual inspections. Yet both are crucial to a reliable operation of the wind farm, which is often for twenty-five years or longer. Deutsche Windtechnik has the operational equipment and experience needed to collect inspection data, and has furthermore the technical expertise to turn the collected data into conclusions and recommendations. The balance of system expertise is the key for our quality, and operational efficiency is the key for our commercial approach.

Deutsche Windtechnik provides:

Subsea visual inspections and measurements, e.g. using an ROV (Remotely Operated Vehicle) from a client's vessel or our own vessel
Diving services for delicate or complex inspections and repairs
Bathymetric scour and sea bed surveys, backscatter sonar images of seabed objects, from a client's vessel or our own vessel
Complex geotechnical surveys such as cable burial depth assessments using technologies appropriate for the local soil conditions

More information on this topic can be found in our infoletter or in this article from Offshore Wind Journal.

Geert Timmers

Managing Director Deutsche Windtechnik B.V. (The Netherlands)

Phone:+ 31 613 78 16 69
g.timmers[at]deutsche-windtechnik.com

Integrity assessments

Integrity assessments comprise several steps:

On-site visual inspections and measurements
Data processing, root cause analysis and assessment of the structural integrity
Preparation of conclusions, recommendations and, if applicable, mitigation plans

Operational efficiency and cost optimisation

Operational efficiency and cost optimisation are achieved using two methods:

Deployment of client vessel, combining several offshore operations in one efficient campaign
Clustering approach, combining similar scope of work for many wind farms in one campaign

Subsea services

Subsea services include inspection, integrity assessment and repair tasks.

Subsea visual inspections by ROV or similar cost-efficient technologies, both outside and inside the foundation, cover the inspection of:

Corrosion protection (CP), including Impressed Current Cathodic Protection (ICCP), sacrificial anodes, CP measurement
Coating
Marine growth
Welds
Damage, dents and debris
Additional components, such as sonar transponders and condition monitoring equipment
Scour protection
Cable exits, cable protection systems and cable burial

Subsea maintenance and repair work includes:

Simple operations by ROV, for instance debris removal and lifting of objects
Complex operations by divers, such as repairs and replacements

An overview of our experience can be found in the reference overview.

Scour and seabed surveys

Scour and seabed surveys include inspection, integrity assessment and repair tasks.

Scour and seabed surveys are carried out with proprietary multibeam echosounder equipment (MBES), which is installed on CTV (Crew Transfer Vessel)mobilised for survey applications. This CTV can be made available for simultaneous execution of other tasks, e.g. for logistical optimisation.

The applications include:

Scheduled high-resolution scour survey
Unscheduled scour survey after significant storm events
Regular seabed surveys to examine water depth and changes in cable burial depth along cable routes
Seabed surveys to identify cable free spans or dropped objects
Large-area, low-resolution surveys to study global seabed changes, especially in estuaries
Specialised surveys to measure cable burial depth

The surveys can be combined with subsea visual inspections to access further information, e.g. of cable free spans.

Repair activities can include:

Positioning of mattresses, rock bags and sand bags
Dumping additional rock for scour protection
Repair of cable protection systems (by divers)

An overview of our experience can be found in the reference overview.