Ultrasonic Thickness Measurement in the Cromarty Firth at Invergordon

Inspection services for ships at Invergordon

Each service has its own page with the method, the class requirements and what the report contains.

Where gauging happens in the Cromarty Firth

Rigs are worked in two ways. Jack-ups can be brought alongside or jacked up close to the quays, while semi-submersibles are usually moored in the deep water of the firth, with crews and contractors reaching them by boat. A rig in for repair and maintenance has its own power, lighting and accommodation, and its preload, ballast and drill-water tanks can be emptied in sequence for gauging while other contractors carry out their work. Pontoon and column structure, bracings and the connections between them are where fatigue and corrosion meet, so thickness readings are paired with magnetic particle testing on cleaned welds at the nodes the owner's inspection plan identifies.

Ships use the quays at Invergordon and the Port of Nigg. Offshore vessels supporting the rigs and the wind farms come alongside between duties, and cargo and installation ships stay for days while components are loaded or discharged. Vessels needing a docking for a renewal survey usually go to yards elsewhere in the UK or on the Continent, and a period in the Cromarty Firth before that docking is a convenient time for pre-docking readings. Cruise ships at Invergordon are short calls, best used for external piping and deck work arranged in advance with the ship's technical staff.

• Semi-submersible moorings in the firth — pontoon, column and ballast tank gauging plus MPI at bracing nodes during repair and maintenance periods.
• Invergordon quays — jack-up preload tank and leg structure checks, and deck and tank gauging on offshore support vessels between duties.
• Port of Nigg quays — deck, crane-foundation and sea-fastening inspection on installation and cargo ships handling heavy components.
• Invergordon cruise berths — external piping and deck readings on cruise ships during summer calls, with tanks opened in advance by the crew.
• Rigs preparing for tow-out — condition gauging on towing points and hull structure before a unit leaves for a new contract or recycling.

Rig class surveys, offshore wind and the UK regulatory frame

Mobile offshore drilling units are classed and surveyed much like ships, but under rules written for their structure: special periodic surveys every five years include thickness measurement of pontoons, columns, legs and the members the class society treats as critical, along with close examination of welded joints. Many owners use a stay in the Cromarty Firth to bring this work forward, or to check the structure before a unit is reactivated after a period stacked. The offshore safety regime enforced by the Health and Safety Executive in UK waters, and the owner's own structural integrity plans, add further reasons to know the condition of steel before the unit returns to work.

Ships calling at Invergordon and Nigg come under Maritime and Coastguard Agency port state control, which the UK carries out within the Paris MoU. Offshore support and wind vessels here sail under a mix of national and open registers, and the wind market in particular expects documented hull and deck condition from the vessels it charters. Units leaving the firth for recycling, as the Stena Spey did, bring the UK's ship recycling rules into play, including the expectation of an Inventory of Hazardous Materials, so owners selling older tonnage often want a condition record made before the tow.

Ships trading to Invergordon are inside a sulphur emission control area, where fuel must not exceed 0.10% sulphur unless an exhaust gas cleaning system (scrubber) is fitted. Scrubber wash-water piping, the overboard discharge and the hull penetration around it are among the fastest-corroding parts of a modern ship, so they belong in the gauging scope at every docking and, on open-loop systems, between dockings.

How a thickness measurement job runs at Invergordon

A gauging job at Invergordon runs the same way whatever the ship type. The sequence below is what the superintendent, the attending surveyor and the yard can expect from us.

Inverness Airport, with connections to London, other UK cities and some European hubs, is the nearest arrival point, and Invergordon is reached from there by road. Aberdeen and Edinburgh airports are alternatives with more international routes, followed by rail or road north. The team travels from India on UK visas, so we ask for a rig's or vessel's expected time in the firth as early as possible. Quay passes, boat transfers to moored rigs and site inductions are arranged through the agent, the port and the rig owner before the team arrives.

• Send the vessel name or IMO number, ship type, class society, the survey due (annual, intermediate, renewal survey number, CAP or owner's condition survey), the port and dates, and the scope you expect.
• We confirm the scope against the class rules for that survey, prepare the thickness measurement plan with frame and section references, and say whether the measurement will be credited to a class survey.
• Mobilisation is planned around visa lead times and the vessel's schedule; the ship's agent arranges port access, and the yard or ship arranges staging, lighting, ventilation and gas-free certificates.
• Gauging with calibrated ultrasonic thickness gauges: through sound coatings only where the gauge's through-coating mode has been verified on a coated reference, otherwise on cleaned bare steel, with every reading tied to its frame, strake and location on the sketch.
• Readings are compared with original thicknesses as work proceeds; substantial and excessive corrosion and any cracks are reported to the attending surveyor and the superintendent the same day.
• The final report is issued on the class thickness-measurement formats with sketches, a summary of locations exceeding the allowable margins, piping tables where piping was in scope, and the gauge calibration and technician certificates.

Related