SEMTEC performs offshore subsea works on pipelines, cables and foundations, combining commercial divers, ROVs and the explosive ordnance expertise of the DEMINETEC Group. Inspection, free span correction, stabilisation, scour protection, repair, tie-ins and offshore UXO survey and clearance: SEMTEC covers underwater interventions from the harbour to offshore.
Subsea pipelines
Pipeline inspection
Visual and video inspection by ROV or diver, position and burial survey, detection of free spans, external damage, lateral displacement and coating defects, cathodic protection potential along the line and at anodes. Data are georeferenced and delivered as event listings, profiles and indexed video.
Free span detection and correction
A free span occurs when a section of pipe loses seabed support through scour, sediment mobility or uneven relief. Exposed to vortex-induced vibration, the pipe accumulates fatigue. Spans exceeding the allowable length are corrected with grout bags, mechanical supports, concrete mattresses or rock placement.
Stabilisation, protection and repair
Concrete mattresses and ballast bags, crossing protection, coating repair, repair clamps, anode replacement, flange connections and valve work, also on nearshore intakes and outfalls.
Subsea cables
Route reconnaissance, position and burial surveys, post-storm and post-incident inspections and protection works on power and telecom cables: mattresses, cable protection systems, rock placement and landfall works, for inter-array and export cables as well as coastal and island links.
Offshore wind and marine renewables
Fixed and floating offshore wind farms require subsea works at every phase. Before construction: geophysical survey, target identification and UXO clearance. During construction: support to foundation, transition piece and cable installation, scour protection around monopiles and jackets. In operation: inspection of foundations, mooring lines and anchors of floating units, cables and cathodic protection, marine growth removal and repairs.
Offshore UXO: risk assessment, survey and clearance
Offshore developers manage unexploded ordnance (UXO) and explosive remnants of war (ERW) through a staged process widely based on CIRIA guidance C754 for the marine environment: desk-based UXO threat and risk assessment, definition of the target object and detection requirements, magnetometer and acoustic survey, target listing and prioritisation, then identification of targets by ROV or diver, with avoidance, relocation or disposal of confirmed items. SEMTEC delivers the identification and clearance stages and, with DEMINETEC, the risk assessment and survey interpretation, producing ALARP sign-off documentation for the client.
Offshore structures
Visual and instrumented inspections, thickness measurements, cathodic protection surveys, cleaning, cutting and underwater welding, and decommissioning of redundant structures and pipelines.
Frequently asked questions about offshore works
Why is a UXO survey needed before offshore construction?
The seabed of the Channel, the North Sea, the Atlantic and the Mediterranean contains ordnance from both World Wars and from post-war dumping grounds. Before cable lay, trenching, piling or anchoring, a UXO risk assessment, a geophysical survey and target identification are required to reduce the risk of contact with ordnance to a level that is as low as reasonably practicable (ALARP).
How is a pipeline free span corrected?
After measuring the span length and gap and comparing them with the allowable span defined by the operator's assessment, support is restored with grout bags, mechanical supports, concrete mattresses or rock placement, followed by a verification survey.
Divers or ROV offshore?
ROVs are preferred for deep water, long inspections and reconnaissance. Divers work within the depth range of surface-supplied diving for tasks requiring dexterity: grout bags, bolting, cutting, welding and tie-ins. The two are frequently combined.
Does SEMTEC handle the UXO clearance itself?
Yes. SEMTEC integrates the pyrotechnic expertise of the DEMINETEC Group: desk study, survey data review, target investigation by diver or ROV, and disposal or relocation of confirmed UXO in coordination with the competent maritime authority.
See also commercial diving services and underwater inspection, or contact SEMTEC.