A monoboya or SPM is an open-sea loading terminal that lets a tanker load or discharge without entering port. Underwater it has a mooring system, a seabed manifold and subsea hoses that need periodic inspection and repair. It's a stable niche of work for commercial divers in crude-exporting regions.
A sizeable share of the crude oil moving around the world isn't loaded at a quay. It's loaded offshore, at a buoy moored to the seabed that the tanker ties up to the way you'd clip onto a ring. That system — the monoboya, or SPM, for single point mooring— is critical infrastructure, is permanently submerged over most of its structure, and needs someone to go down and take a look at it.
It's an unglamorous but very stable niche within commercial diving. It doesn't show up in documentaries, but it provides recurring work every year in the same places, with multi-year maintenance contracts and an inspection workload that doesn't depend on the oil price the way new field construction does.
What a monoboya is and what it's for
The problem a monoboya solves is one of draught and cost. A large loaded tanker needs a lot of depth and a terminal able to take it. Building and dredging a port like that is expensive; on many coastlines, impossible. The alternative is to leave the ship moored a few kilometres offshore and bring the product to it by pipeline.
The system has three parts:
- The buoy, at the surface, with its mooring system for the ship and a rotating body that lets the tanker weathervane freely with the wind and current without twisting the hoses.
- The mooring system, submerged, which keeps the buoy in position against current, waves and the pull of the moored ship itself.
- The product circuit: a subsea pipeline from shore, a manifold on the seabed, subsea hoses running up to the buoy, and floating hoses that reach the ship.
Practically everything that matters is underwater and subject to fatigue, corrosion and marine growth. Hence the maintenance.
CALM and SALM: two architectures, two inspections
The two classic configurations differ in how they're held to the seabed.
In a CALM —catenary anchor leg mooring— the buoy is anchored by several catenary chain legs arranged around it, each anchored to the seabed. The chains run from a structural fitting on the underside of the buoy and curve down to the seabed. Underwater work is concentrated on those legs, their connections and the area where they touch the seabed.
In a SALM —single anchor leg mooring— the buoy is held by a single vertical leg connecting it to a base anchored on the seabed, with an articulated joint that allows movement. There are fewer components, but each one is more critical: there's less redundancy.
For the diver the difference is practical. On a CALM you have to inspect multiple legs and their terminations, each with its own wear history. On a SALM you have to examine the articulation joint, the base structure and the small number of connections carrying the whole system in detail.
What gets inspected underwater
The mooring system
This is the heart of the inspection. The chains are checked link by link along the critical sections, along with the shackles and connecting elements, the terminations at the buoy and at the anchor, and the area where the chain touches or enters the seabed, which is usually the most punished part because it combines abrasion, corrosion and fatigue.
The defects being looked for are chafe wear, section loss, link elongation, cracks, loose or displaced shackle pins and coating loss. It's a slow, methodical task that shares technique and judgement with mooring chain and line inspection for any moored floating structure.
Cathodic protection
All the submerged steel carries sacrificial anodes or an impressed current system. The diver measures potentials with a reference electrode at points set by the inspection plan and visually checks how consumed the anodes are. A potential outside range or worn-down anodes are early warning of structural corrosion, and replacing them is a recurring task on these assets. It's the same procedure described in cathodic protection and sacrificial anodes.
Subsea hoses and connections
The hoses carrying the product from the seabed manifold up to the buoy are life-limited items under constant flexing. They're inspected for deformation, bulging, damage to the outer cover, crushing from contact with the seabed or the structure, and the condition of the flanges and their fixings. It's also checked that the hose's geometry is as designed and that it hasn't taken on a curve that fatigues it.
Manifold, structure and seabed
The subsea manifold — the point where the pipeline coming from shore splits out to the hoses — is checked for structural integrity, the condition of accessible valves, leaks and cathodic protection. Around it, the seabed is examined: scour under structures and pipelines is a classic problem, because a section left spanning free starts to vibrate and fatigue.
Biofouling
None of this can be inspected under a layer of marine growth. A large share of diving hours go into cleaning before you can even look, with brushes, scrapers or pressure-water equipment. It's physical work, unglamorous and absolutely necessary for the visual inspection to have any value.
Typical failures and why they matter
Understanding what breaks helps explain why the inspection is organised the way it is. There are four recurring failure modes on these terminals.
The first is a mooring line failure. When a chain gives way, the load redistributes onto the remaining lines, which had a design margin but weren't meant to carry that load indefinitely. An undetected failure turns into a cascading failure, and a buoy adrift with a tanker moored to it is a serious scenario.
The second is hose failure, with product spilling into the sea. It's the environmental risk these installations are best known for, and the reason hoses have a rated service life and are replaced on a schedule, not just by apparent condition.
The third is structural corrosion from insufficient cathodic protection, which advances silently for years until the loss of thickness is significant. It's countered with systematic potential measurements and anode replacement.
The fourth is scour and seabed movement, which leaves pipelines spanning free and anchors undermined. It's detected by comparing successive seabed surveys, and it's one of the reasons the inspection covers the surroundings and not just the steel.
How maintenance is organised
Terminal operators work with tiered programmes. The most frequent checks are surface checks done by the terminal's own crew: condition of the buoy, floating hoses, mooring fittings, marking. The full underwater inspection is done in periodic campaigns, on cycles that depend on the component, the age of the system, the operator's judgement and what the insurer and local regulator require.
The industry's good practice for operating and maintaining these terminals is published mainly by OCIMF, the oil companies' forum for marine matters, whose guidelines are the usual reference in contracts. On top of that comes the diving safety framework itself, with IMCA codes as the standard in most offshore markets.
A defining feature of these campaigns is the operating window. The terminal is in service: while a ship is moored and product is flowing, there are no divers in the water. Inspections are fitted in between loading operations, which demands fine-grained planning and making efficient use of the gaps.
What kind of diving this is
Most monoboyas are installed at moderate depths, within reach of surface-supplied diving. The work is usually done from a support vessel or a workboat, with the classic kit: helmet, umbilical, gas panel and recompression chamber on deck.
The conditions are the hard part. These terminals are in open water by definition, exposed to waves and current, often with poor visibility from sediment. Operations depend on sea state, and weather cancellations are common.
The skills asked for are those of an underwater inspector more than a construction diver: the ability to document rigorously, handling underwater video and photography, ultrasonic thickness measurement, potential measurement and report writing. A recognised inspection certification, of the kind detailed in the guide to certifications, makes the difference when it comes to getting into this niche.
Where the work is
The geography of monoboyas follows that of crude exported by sea from coastlines without major oil ports. Markets with recurring activity include West Africa, the Middle East, Southeast Asia, parts of Latin America and the Gulf of Mexico, plus import terminals tied to coastal refineries in many other countries.
Access to these contracts usually runs through two routes: local diving contractors with the maintenance contract, and international inspection companies that mobilise teams for campaigns. In the company directory and in the section on jobs and agencies you can see how that market is structured.
Why it's a good niche to start in
For a commercial diver in their early years, terminal maintenance has concrete advantages. It's repetitive work in the good sense: the same procedures, the same components, learned thoroughly. It builds up documented diving hours and inspection experience, which is exactly what higher-level offshore contracts ask for afterwards. And it happens at depths where a mistake still has room to be corrected, unlike at two hundred metres.
It doesn't pay like saturation — you can compare ranges in the guide to how much a diver earns— but it's one of the realistic entry points into the sector, and one of the few that offers continuity without depending on a new construction project getting under way. If you're still in training, the map of commercial diving schools is the place to start.
Frequently asked questions
What exactly is a monoboya?
It's a buoy moored to the seabed that works as an open-sea loading terminal. The tanker moors to it and connects a floating hose; the crude flows between the ship and a subsea pipeline running to shore. It allows large ships to operate without needing a berth with enough draught.
What's the difference between CALM and SALM?
On a CALM the buoy is held to the seabed by several catenary mooring legs spread around it. On a SALM the attachment is concentrated in a single vertical leg anchored to a base on the seabed. The mooring system changes, and with it the points the diver needs to inspect.
How often is a monoboya inspected?
The usual approach is a layered programme: frequent checks on the surface components and the hoses, and a full underwater inspection on annual or multi-year cycles depending on the component, the operator's judgement, the insurer and local regulations. Any damage detected triggers extra inspection outside the schedule.
What tasks does a diver do on a monoboya?
Visual and thickness inspection, cathodic protection potential measurement, checking shackles, links and mooring connection elements, inspection of subsea hoses and their flanges, biofouling cleaning, and replacing anodes or damaged components.
Is saturation diving needed for this work?
Usually not. Most monoboyas are at moderate depths, reachable with surface-supplied diving. Saturation comes into play when the system is in deeper water or when the volume of ongoing work justifies it.
Where is there most work of this kind?
Wherever crude is exported by sea without deep-draught ports: West Africa, the Middle East, Southeast Asia, parts of Latin America and the Gulf of Mexico. Also at import terminals in countries with coastal refineries.
Published 2 September 2026 · By Arthur More, professional saturation diver
Photo: ST33VO · CC BY via Openverse