Leask Marine can offer a range of temporary HDD duct mooring systems as short-term engineered moorings used to secure HDD ducts after drilling and during float-out & pull-in.
They are designed for quick deployment & recovery, resist environmental loads, and control duct buoyancy. Ducts can be stored in water (moored with buoyancy or ballast), or onshore until final installation.
Design of Temporary HDD Duct Mooring Systems, and mooring analysis
When installing subsea HDD ducts (plastic or steel pipes pulled from shore to seabed), temporary moorings are required to secure the duct in place offshore until final cable or pipe pull-in. The Leask Marine in-house naval architecture, design engineering, and operations teams will take into account a number of key design considerations including:
Load Assessment
- Moorings must withstand environmental forces (waves, currents, tidal flows, wind on vessels).
- The duct’s buoyancy requires ballast & moorings to maintain position.
- Safety factors applied to line tension and anchor holding capacity.
Mooring Configuration
- Typically taut-leg moorings using chain, wire ropes, & synthetic lines.
- Anchors: Clump chain, gravity blocks, depending on seabed type.
- Spacing ensures ducts are kept clear of seabed obstructions, minimises bending stress, and avoids interference with vessels.
Temporary vs. Permanent
- Design life is short (typically from a few days to 9 months) until pull-in.
- Materials and layout are optimised for rapid deployment & recovery rather than long-term endurance.
Duct Storage and Handling
As temporary storage is often required offshore or nearshore until pull-in, and Leask Marine have used various sites around the UK for temporary storage of the ducts.
Offshore Storage (in water)
- The ducts are stored on moorings with buoyancy support (kept neutrally buoyant or positively buoyant).
- The ducts are stored with a controlled bend radius.
- The ends are capped to prevent water ingress or contamination.
Retrieval
- Once ready for pull-in, the moorings are released, and the ducts are recovered individually as required for towage and installation on site.
- The temporary mooring systems including clumps and buoys which are recovered for re-use.
- The ends are capped to prevent water ingress or contamination.
Duct Towage & Handling
Leask Marine can either collect the assembled duct from the land via the trailing end and tow to location ready for pull-in, or collect the duct from its temporary storage.
Generally, prior to the pull-in operation, the ducts will be filled with water to reduce the buoyancy. The Leask Marine crew will open the vents on the pull head of the duct before the filling starts to ensure the duct is filled completely during the filling operation and prevent air being trapped. The crew will then commence the filling operation by pumping in water from the rear until the foam pig has reached the pull head end of the ducts and water starts to come out of the air valves of the pull heads. The pumping is stopped, the hoses on the flanges will be removed and the plugs in the flanges will be closed installed. The foam pig is now at the pull head side of the duct which is now semi-submerged and pull-in can commence.
Duct Pull-In
Leask Marine provide a full marine spread to handle and align any size and length of ducts.
At the offshore HDD exit, the duct end is connected to the HDD drill string using a swivel and pulling head, to prevent torsional stress during pull-in. The HDD rig onshore pulls the duct from offshore to onshore through the reamed borehole, while the divers or crew gradually remove any buoys or water ballast added to control the pipe profile during the pull-in operations.
Once fully pulled in, the duct is secured at both landfall and offshore ends, and temporary moorings, buoys, and anchor systems are recovered.
Gauging pigs & Messenger Wire Installation
After pull-in, the ducts are checked with a gauging pig to prove integrity and internal clearances. The landward pulling head is removed, the Leask Marine divers remove the blanking plate, and a pig catcher is installed to safely catch the pig. During pigging, a messenger line is simultaneously installed to enable the future pull-in of the subsea cable or pipeline.
Offshore Bellmouth Installation
The bellmouth is fitted offshore after the duct pullback and pigging, but before the subsea cable or pipeline is pulled in. It ensures a smooth, safe transition for the product into the HDD duct.
Offshore Completion and Ballasting
The messenger line will generally remain in the duct, a blanking plate has been fitted to the flanged end of the ducts by the divers.
Generally, the offshore end section of the duct is ballasted with rock bags or concrete mattresses to secure the duct in position.
The remaining void within the Exit Pit is then infilled with arisings as appropriate.
Grouting Marine Support – HDD Landfall
The grouting operation seals the annulus between the HDD duct and cable, preventing water ingress, stabilising the cable, and ensuring long-term integrity. Leask Marine divers are required for inspection, installation of grouting hardware, monitoring during injection, and backfilling after completion.
Post-Cable Installation Preparation
Once the cables have been installed through the HDD ducts by the Leask team or the HDD contractor remobilises to perform annulus grouting (filling the space between the cable and the inside wall of the duct). The Leask Marine teams support this with onshore set-up with equipment & personnel as required and diver operations at the offshore duct exit.
Diver Inspection
The Leask Marine divers inspect the offshore duct exit, the installed cable, and the surrounding seabed area. Any damage, debris, or obstruction is reported, and if necessary, remove obstructions and prepare the duct exit for grouting.
Installation of Grouting Equipment
The Leask Marine divers install the flange plate on the duct end, valves to vent and for confirmation of grout arrival, flexible pipework if required for grout routing and secondary valves if required.
Grouting Operation
The grout hose is connected to the grout interfaces from the HDD contractor’s pumps onshore, Leask Marine team opens the offshore valves to prepare for grout flow. The HDD contractor begins grout pumping into the duct annulus, and the dive team duties during pumping are to:
- Monitor the flange plate, valves, and pipework for leaks.
- Communicate continuously with the land-based grouting team.
- Report and assist as required.
Completion
Once grout is observed at the vents,, the offshore valves are closed, the flexible hose and temporary fittings are removed, and the flange plate is left secured.
Optional Backfilling
If within scope, Leask Marine can perform backfilling of any seabed voids around the cable beyond the duct exit, ensuring full protection and stability of the cable.