Underwater Pipeline Inspection and Repair

By Dutch Hollander, Offshore diving superintendent. Reviewed by Dana Whitcombe, technical reviewer.

Underwater Inspection | Commercial Diving Contractors Alabama I was sat in the dive control van on a DSV in the Shetland Basin, watching the ROV camera wobble over a 12-inch export line. The survey said 'minor coating damage' but the video showed a concrete weight coat hanging off like a loose tooth. We called it a moderate risk. That's what 19 years offshore teaches you: the paper never matches the sea floor. Everyone wants to replace divers with ROVs. And we use a lot of them. But for a pipeline inspection job that demands touch, you still put a human on the bottom. This is how we do it – from the first desktop kickoff to the last grinder spark on a pipe sleeve. Underwater pipeline inspection is a how-to thing, but it's also a knowing-when-to-stop thing. If you push a dive window, the sea will humble you. I've seen too many young superintendents learn that lesson the expensive way.

Why Visual Inspection Isn't Enough

Plain visual inspection gets you maybe 60 percent of the story. You'll see the coating holidays, the free spans, the scour. But you won't see the wall thinning on the bottom of the pipe. That's why we always run a CP survey alongside – a close-interval potential survey with a silver-silver-chloride reference electrode. And we use ultrasonic thickness gauges – a Cygnus 1, thumping the wall from outside. You get real numbers, not 'looks okay'. It's a judgement call. A fixed line with decent concrete and CP current isn't going to suddenly fail. But if you find a fault in the coating and a dead anode, you've got a problem that's going to eat the pipe. Give the client a clear breakdown: visual findings, CP data, wall thickness measurements, and a risk rating. They don't like surprises. We give them a chart and a plain-English bottom line.

The Tools: What We Actually Use

On an IRM spread – inspection, repair, maintenance – you've got a dive bell, a saturation system, and a rated ROV. We run a Schilling Robotics HD work ROV with a tether management system, and we keep a Saab Seaeye Falcon for the light recon. Divers use John Kinergetics helmets, a KMS-48 or the newer Mk II, and the Divex helmet for mixed gas. Mix is usually heliox for the deep stuff, around 10% oxygen and helium balance, at 40 metres for a typical North Sea gas line. For NDT, the workhorse is a Cygnus ultrasonic thickness gauge – you can get a direct reading on a dirty pipe if you grind a spot clean. For CP, we tow a small cell and read potential through a calibrated wire. And we always carry a magnetometer for locating buried sections. These aren't wild toys; they're the bare minimum to do an honest job.

Planning the Job: The Paperwork That Keeps You Alive

Dive plan, risk assessment, method statement, permit to work, gas management, and a contingency for lost bell or injured diver. We work off a 5-year inspection calendar, but every survey is a separate animal. You need to know the pipeline's weight coat type and thickness, the burial depth, the pigging history, and the last CP readings. And you need to know the weather – a southwest storm in the Gulf can blow out a window faster than you can say 'abort'. One time we sat on the back deck of the boat for three days waiting out a northeast gale in the Gulf of Mexico. The client was on the sat phone every two hours. We can't control the weather, so we wait. That's not an excuse, just a fact. You learn to build weather days into the contract, or you'll eat the cost.

The Dive: What It Looks Like on the Bottom

The diver travels down in the bell, locks out, and swims the short hop to the pipeline. We're in constant comms on a hardwire – no break. The dive supervisor is in charge, not the client, not the bridge. The diver runs a small video camera, a T-probe for CP readings, and the ultrasonic probe. It's a slow, methodical swim along a marked section. You're not making time; you're making sure the numbers make sense. Visibility is a killer. In the north sea, you're often working in zero viz – black water. You'd think that's impossible, but you go by touch and by the little friction marks you leave on the pipe. You have to trust your tools and your training. It's not glamorous; it's a job. But when you find a free span that's five metres long, and you measure it, and the client's database says it shouldn't be there, that's why you're in the water.

Making Repairs: From Grinder to Sleeve

Say you've got a damaged concrete weight coat – you might knock off the loose bits and recoat with a marine-grade grout. For actual pipe wall loss, we sometimes strap on a split-sleeve repair clamp with a synthetic rubber gasket. It's a mechanical repair that holds the pressure rating, and it's properly engineered. For coating holidays, we clean to bare metal and apply a cold-applied corrosion coating – a tape or a two-part epoxy. These aren't permanent fixes in the sense of a new piece of pipe, but they'll keep the integrity for years if the CP is right. Every repair has to be signed off by an engineer. We give them the measurements, the material spec, and the installation photos. They do the fracture mechanics. I don't pretend to be an engineer; I know what's safe to touch and what's not. The sea floor is unforgiving, and paper is cheap. So we document everything, take the video, and file the reports. That's the real work – making sure the next team knows exactly what they're looking at.

Frequently Asked Questions

What's the difference between a visual inspection and a CP survey? Visual inspection finds coating damage, free spans, and trash on the pipe. CP (cathodic protection) survey measures how well the anode system is protecting the pipe from corrosion. You need both to understand the real risk. How often are underwater pipeline inspections required? It depends on the operator, the regulator, and the risk. Most fixed pipelines get a baseline inspection, then a full IRM survey every 3 to 5 years. High-risk areas – like river crossings or heavy current zones – might see a diver or ROV every year. Can ROVs completely replace divers for pipeline inspection? For simple visual checks, an ROV with a camera and sonar is a great tool. But for complex repairs – fitting a clamp, replacing an anode, or doing a precise ultrasonic test – you still need a human with a thumb and a brain on the other end of the tool. The sea floor is messy. How much does an underwater pipeline inspection cost? There's no sticker price. You're paying for vessel time, dive or ROV spread, personnel, and the engineering. Typically it's quoted per day or per job based on water depth and pipeline length. A small shallow-water job might run a few hundred thousand; a deepwater sat spread goes into millions. Get three quotes.