To support the safe and effective installation of subsea structures such as subsea templates, structural monitoring systems are frequently used to ensure there is no damage or stress during installation and structures are positioned in the correct location and orientation. Most subsea structures are installed using bespoke autonomous monitoring systems which are typically fitted to the structure onshore and removed once the structure is successfully positioned on the seabed.

Structure installations are usually assisted by a ROV where critical data from the structure monitoring system is communicated directly to the ROV subsea and then communicated via the ROV umbilical to the construction vessel control room. The data is then fed into a software application for interpretation and graphical display.

The real-time data received from the monitoring system is critical to the success of the overall operation. Historically, this would be sent acoustically from the structure mounted monitoring system to the ROV using acoustic telemetry modems. As the installation environment is often very noisy, due to the proximity of the ROV and construction vessel, acoustic telemetry can be unreliable with frequent data drop outs. The use of optical data telemetry modems is proving to be a very viable alternative, helping to enhance the accuracy of the data collected, ensuring maximum performance of the system.

Deployment of LUMA modems to help reduce project complexity, risk and cost

Ashtead Technology has been at the forefront of developing autonomous subsea structure monitoring systems for over 15 years and since 2018 has been working in collaboration with Hydromea to introduce subsea optical data telemetry to the subsea sector. Hydromea has developed a range of high bandwidth subsea optical data telemetry modems (LUMA) which are unique to the offshore industry. As LUMA modems transmit data optically they are not affected by acoustic noise which can cause data drop out. Based on projects successfully carried out in the field, Ashtead Technology has demonstrated that the LUMA modems can provide a much more reliable data link than the acoustic alternative when sending data via an ROV.

The LUMA modems transmit data at a much higher bandwidth than acoustic telemetry systems allowing multiple data set updates per second compared to one update every 3-5 seconds. With fast, real-time updates from the monitoring system at critical times during the installation, this increases customer confidence in the data.

The operating range of the LUMA modems varies from 7 m to 50 m dependent on modem version, which has been found to be ideal for communication with an installation ROV. Ashtead Technology’s experience in the operation of these new modems is that they work well in low visibility conditions. The LUMA modems also have very low power consumption enabling the autonomy of the monitoring system to be increased significantly.

Ashtead Technology grout monitoring system
Ashtead Technology’s grout monitoring systems track the below surface elevation of grout within the seabed sockets when cementing the piles in place during the deployment of LUMA optical modems offshore. (Source: Ashtead Technology)

Successful deployment of LUMA modems in the field

On two recent installation campaigns, Ashtead Technology integrated the LUMAs within its subsea structural Deflection Monitoring System (DMS) and found the LUMA modems provided significant advantages in data throughput.

The Ashtead Technology DMS monitors structural stress during installation along with structure position, heading, attitude and pressure within the structure suction anchors.

In soft seabed conditions, many subsea templates employ base suction anchors to provide the necessary stability over their operating life. Often two or four suction anchors are built into the template and where more than one suction anchor is employed, the structure has the potential to twist during the process of installation. This can be remedied using a structural DMS.

A DMS monitors the relative elevation of each of the structure corners and measures the out of alignment deflection during installation. If the measured deflection exceeds the design tolerance, the DMS indicates an alarm so that the installation contractor can individually adjust the level of suction in each suction anchor to reduce the deflection to an acceptable level.

All data output from the DMS is transferred via a ROV to topside in real-time and is critical to ensure the safe and efficient installation of the subsea templates. As such, it is essential that the data telemetry between the structural monitoring system and ROV is robust with a high data update rate providing reliable information about the structure condition to the installation engineers during the critical installation period.

Supporting offshore wind installation projects

Ashtead Technology recently deployed the LUMA optical modems on a major offshore wind installation project where the modems were built into its autonomous subsea grout monitoring and pile alignment systems for the installation of seabed foundations for wind turbine jackets.

Ashtead Technology’s grout monitoring systems tracked the below surface elevation of grout within the seabed sockets when cementing the piles in place. The monitoring system data was critical to the project to ensure that the piles had been grouted to the correct elevation ensuring foundation integrity. 

Ashtead Technology’s pile alignment system aligns the pile installation tool with the seabed socket to help prevent pile clash during installation and provide valuable information for the installation contractor helping to ensure efficient pile installation with no delays due to misalignment.

Both monitoring systems operate independently but use a common LUMA modem installed on the ROV for communication to the topside system monitoring software. This arrangement simplifies the installation on the ROV and the operation of both systems.

The windfarm installation was in shallow water, less than 60 m, and over the winter localized subsea visibility drops significantly. This is where the LUMA provided significant advantages, ensuring reliable communication even in murky water.