SPM Calm Buoy System – The Ultimate Guide

In this guideline, we take you through everything you need to know about the SPM calm Buoy System.

1. SPM Overview

A Single Point Mooring (SPM) Calm Buoy System is an offshore mooring point used to facilitate tankers loading or discharging various forms of liquid product cargo near onshore storage or production fields.

There are various types and configurations of SPMs used in different locations and for different purposes, such as Turret Buoys, Single Anchor Leg Mooring (SALM), Single Point Mooring Towers, Spars, and Articulated platforms.

This article will only focus on theSPM Catenary Anchor Leg Mooring (CALM). Also, commonly called Single Buoy Mooring (SBM) – although this is technically referred to as a proprietary name. We will use the term interchangeably throughout.

The SPM CALM philosophy consists of a tanker mooring securely to a single buoy in open waters offshore. After which the free ends of floating hoses connected to the buoy are connected to the tanker, and product is pumped through the hoses, buoy, subsea risers, manifolds and subsea pipeline. Either from the tanker to onshore storage or vice versa, depending on whether it is an import or export facility.

These buoys can be designed and fabricated to handle large-capacity tankers, including very large crude carriers (VLCCs).

The SPM calm buoy system is typically used nearshore to eliminate the need for larger marine mooring infrastructure, such as ports, jetties, and marine loading arms. It also offers an improvement over the fixed-position Multiple Buoy Mooring (MBM) methods, in which tankers are typically more adversely affected by wind and wave loads.

SPM calm buoy systems are also used further offshore, in deep waters, to facilitate the transfer of product between floating production facilities and tankers. In these instances, the buoys have a different design and are moored in place with composite legs, often containing high-tech synthetic materials and steel wire rope.

The owners and operators of SPM systems should adhere to a special offshore operation standard, established by the Oil Companies International Marine Forum (OCIMF).

Figure 1: Tanker moored to an SPM CALM buoy with mooring hawser arrangements, and floating hoses connected to the midship manifold

1.1 When is an SPM CALM Buoy the Right Solution?

SPM CALM buoy systems are most effective where conventional marine terminals are either impractical or economically inefficient. They are commonly selected when shoreline access is constrained, environmental permitting for jetties is complex, or rapid project execution is required.

Ideal use cases include locations with limited natural harbour protection, where dredging volumes would be excessive for a fixed jetty, or where seabed conditions allow reliable anchoring but do not favour piled marine structures. CALM buoys are also well suited to remote sites with limited construction infrastructure, where offshore installation using marine spreads is simpler than onshore heavy civil works.

They are particularly attractive for import terminals that require flexibility in tanker size, as well as for export facilities tied to floating production units or offshore pipelines. When metocean conditions are moderate and predictable, CALM systems offer high operational availability at a lower capital investment than traditional berths.

However, CALM buoys are not suitable for all environments.

Locations exposed to long-period ocean swell, strong cross-currents, or highly variable wave climates can experience excessive tanker motions and reduced operating windows. Very shallow water sites may not provide sufficient under-keel clearance for large tankers, while congested shipping lanes can complicate safe approach and exclusion zone management.

Poor seabed conditions can also be a limiting factor. If the anchor holding capacity is inadequate and alternative solutions, such as suction piles or driven foundations, are not feasible, the overall system reliability can be compromised.

For these reasons, early metocean studies, geotechnical investigations, and marine traffic risk assessments are critical in determining whether a CALM buoy solution is technically and operationally viable.

1.2 SPM CALM Buoy System: History

Since their introduction, SPMs have facilitated economies of speed and scale in the international transport of petroleum. As the market for these SPM systems developed rapidly, N.V. Industrieele Handels Combinatie Holland (IHC) decided in 1969 to create a separate business to market and further develop the products, called Single Buoy Moorings Inc. (SBM). Their success has become eponymous: SPM CALM types are now common but are technically incorrectly referred to as SBM (Single Buoy Mooring).

The history of single-point mooring in the oil and gas industry started in the late fifties with loading/ discharging terminals using single-point mooring systems. In particular with:

• 1959: Shell, Malaysia (48 ft water depth), CALM Type Gusto constructing the first single point mooring (SPM) facility under a license agreement with Shell, with the concept developed by Esso.
• 1969: Esso, Bregia, Libya (140 ft water depth) SALM Type (this was an old Exxon patent)

2. How does an SPM CALM Buoy system work?

2.1 Operations Overview

An empty or fully loaded tanker approaches the SPM and moors to it using a hawser arrangement with the help of a mooring crew. The floating hose strings, attached to the SPM buoy, are then hoisted and connected to the tanker manifold. This creates a fully closed product transfer system from the tanker hold, through the various interlinked components, to the onshore buffer storage tanks.

Once the tanker is moored and the floating hose strings are connected, it is ready to load or discharge its cargo, using either the onshore pumps or the tanker’s pumps, depending on the direction of flow. As long as the operational cast-off criteria (i.e. when the environmental conditions are too rough – see table at end of the article) are not exceeded, the tanker can stay connected to the SPM and floating hose strings, and the flow of product can continue uninterrupted.

During this process,s the tanker is free to weathervane around the SPM, meaning it can move freely throughout 360 degrees around the buoy, always orienting itself to take the most favourable position in relation to the combination of wind, current, and wave climate. This reduces the mooring forces compared to a fixed-position mooring. The worst weather hits the bow rather than the side of the tanker, reducing operational downtime from excessive tanker movements. The product swivels inside the buoy, keeping it flowing through the buoy as the tanker weathervanes.

This type of mooring requires less room than a tanker at anchor because the pivot point is much closer to the tanker – typically 30m to 90m. A tanker at a mooring buoy is much less prone to fishtailing than a ship at anchor, although fishtailing oscillations can still occur at a single-point mooring

The following three steps explain the process in more detail:

1. Tanker Approach and Mooring
2. Loading or Discharging Fuel Product
3. Tanker Unmooring and Departing

2.2 Operational Envelope and Cast-Off Criteria

Every SPM CALM buoy operates within a defined operational envelope. This envelope represents the environmental and mechanical limits within which a tanker can safely approach, moor, remain connected, and transfer product.

The primary factors that define this envelope are wind speed and direction, significant wave height and swell period, surface and subsurface current velocity, visibility conditions, tugboat bollard pull capacity, and the allowable working loads of the mooring hawsers and anchor system. In addition, tanker-specific limitations such as bow thruster capability, engine response time, and manoeuvrability also influence safe operating limits.

Operational limits are not defined by a single parameter alone. A moderate wave height combined with strong cross-current and limited tug power may create a higher risk than a higher wave height under favourable alignment. For this reason, operational criteria are normally established using combined metocean thresholds rather than individual values.

Cast-off criteria define the environmental or operational conditions at which product transfer must be stopped and the tanker disconnected from the SPM. In practice, this is the point at which continued connection would increase the risk of excessive mooring loads, hose overstressing, or loss of vessel control.

Typical cast-off triggers include rapidly rising wind speeds, increasing swell approaching hose or hawser design limits, abnormal tanker movement behaviour, tug capability being exceeded, or loss of critical systems such as power, steering, or communications. These limits are defined during the design phase and formalised in terminal operating procedures.

Clear definition and strict enforcement of operational and cast-off criteria are critical for safe SPM operations. Delayed disconnection decisions are a common contributing factor in offshore mooring incidents.

2.3 SPM Calm Buoy System Step 1: Tanker Approach and Mooring

2.3.1 Boarding the Tanker

At first light, the pilot and mooring team would board a launch (smaller support tanker) and depart out to the waiting tanker at a close anchorage location. The team and tool basket are transferred on board, and as the pilot gets the tanker underway, the mooring crew will prepare their tools and equipment for the connection. Sometimes the pilot and mooring crew will be transferred to the tanker by helicopter, depending on the area, risk and operational need.

2.3.2 Mooring

During mooring manoeuvres, the tanker will approach the buoy into the prevailing environmental conditions, ensuring maximum control while minimising the need for constant tugboat assistance. Although a tugboat must always be available in case the weather deteriorates, the tanker has a mechanical breakdown or any event requiring assistance. The tanker can cause significant damage to the SPM Buoy, even with minor contact. Likewise, a tanker that drifts from an SPM can quickly end up on the beach, as it takes quite some time to get the engines ready.

As the pilot slowly brings the tanker to the buoy, its crew will pass down a smaller messenger line to the launch, which is run towards the SPM mooring hawser. When close enough, the launch crew will connect the messenger to the mooring hawser pickup rope and the tanker will moor bow to the buoy by means of one or two mooring hawsers hauled in by the tanker crew. The tanker holds on to a chafe chain at the end of each mooring hawser.

During mooring, the tug will help keep the floating hoses clear of the tanker’s bow.

After passing the hose pickup ropes to the tanker team, the tug moves to the tanker’s stern, where its towing wire is connected. The tug will static-tow (the term for a tug towing a tanker moored onto an SPM) away from the buoy under the direction of the mooring master on the tanker. This maintains the nominal tension on the mooring hawsers to prevent contact between the tanker and the buoy structure, as the tanker’s response time is often substantially slower than the rapid change in weather direction.

The constant tension on the mooring hawser also reduces wear and tear on the mooring hawser components and the SPM main bearing, and assists with the weathervaning of the tanker.

The SPM operates in an exclusion zone around the buoy, with the radius of this zone consisting of the sum of the length of the mooring hawser + tanker + static-tow line + tugboat + safety distance behind the tug.

Mooring and unmooring operations are quick, and oftentimes all connection points are equipped with quick-release mechanisms (not to be confused with quick-release hooks).

2.3.3 Safety and Control Checkpoint Before Cargo Transfer

Once the tanker is securely moored, and before connecting floating hoses, a formal safety and operational readiness check is normally performed. This step is critical to ensure all parties are aligned and that the system is prepared for controlled product transfer.

Clear communication links must be established between the mooring master on board the tanker, the pilot, the tug master, and the terminal control room. Agreed handover points, emergency signals, and primary and backup communication channels should be confirmed before proceeding.

The exclusion zone around the SPM buoy must be actively enforced. No unauthorised vessels should be permitted inside this zone during hose handling or cargo operations. Support craft must maintain agreed stand-off distances and remain clear of mooring lines, hose bights, and anchor leg footprints.

All quick-release arrangements should be verified as operational. This includes mooring hawser release systems, hose emergency-release couplings, where installed, and emergency shutdown valves at the PLEM, buoy, and terminal. Functional testing of shutdown logic and confirmation of valve status are typically part of pre-transfer checklists.

Operational incidents during SPM operations most commonly arise from a small number of recurring causes. These include floating hose strings snagging or misaligning during pickup, tug towing-line failures or incorrect tow angles, sudden weather shifts exceeding forecast conditions, and delayed tanker engine readiness during emergency manoeuvres.

Strict adherence to pre-transfer checklists and conservative operational decision-making significantly reduces the likelihood of escalation once product flow has commenced.

2.4 SPM calm buoy system Step 2: Loading/Discharging Product

2.4.1 Connecting Floating Hose Strings

Once securely moored to the SPM, the free ends of the floating hose strings are pulled around to midship and hoisted up by the tanker’s crane or derrick and connected to the tanker’s midship manifold. This establishes a closed product transfer system from the tanker to onshore storage. Manifold connections are usually flanged, and their installation can be expedited with a cam-lock system. In some cases, where a specific tanker is the only tanker to visit a particular SPM, they may even install an engineered cargo coupling solution,s which makes the connection task almost autonomous.

With the hoses connected and the necessary valves on the Pipeline End Manifold (PLEM) and Buoy open (hydraulically or manual by maintenance crew/divers prior to tanker calling) the product is then transferred either to or from the tanker, depending on whether the tanker is loading or discharging.

Once the transfer is complete, the line should still be filled with product. This remaining product is then either pushed out with the next product transfer, with the receiving end handling the product interface slug, or separated with an interface pig.

Sometimes the product is also pushed out with treated seawater. The filtered, treated seawater is chemically treated with a biocide, an oxy-scavenger, and a corrosion inhibitor, and filtered to remove all particles down to 50 microns.

In general, SPM CALM buoys cannot be pigged through the whole system due to the swivel mechanism in the buoy. However, there are examples of complete pigging of the system through the SPM and pipelines. Mostly, the pig will have to be inserted at the tank farm and received at the PLEM. In some oil SPMs with two pipelines,s the pig is pushed through one pipeline via the PLEM and back to the tank farm via the other pipeline.

2.5 SPM calm buoy system Step 3: Tanker Unmoors and Departs

Once loading or discharging is complete, the above mooring tasks will be reversed, and the tanker will be free to proceed, subject to formalities such as cargo acceptance and a few other quality and procedural checks.

Below – This SPM was converted into a mooring point only for a unique offshore bulk cargo transfer operation, where smaller Panama-sized trans-shippers fed the bulk carrier via an STS mooring arrangement.

This video serves only to show the SPM approach and mooring, not any cargo handling – liquid or bulk

3. Description of SPM CALM Buoy System Parts

3.1 Turntable or Turret Arrangement

Although suppliers each design their own buoys with various pros and cons, SPM CALM Buoys come mainly in two variations:

1. Turntable
2. Turret

The difference between them is that, in the case of the turntable, the anchor legs are connected to the buoy, and the turntable atop it turns around. With the turret, the anchor-legs are connected to a spider turret arrangement below the water (which typically has no buoyancy), and the buoy that sits on the spider turret can rotate around.

Both have advantages and are commonly used. See below for some key advantages of both.

3.2 Buoy

The SPM buoy, regardless of turret or turntable arrangement, has two primary functions:

1. Provide buoyancy and structural strength to withstand the tanker mooring forces
2. To house the bearing, swivels and transfer pipework

3.3 Main Bearing

The main bearing is the pivot around which the static and rotating components turn. The load of the anchors holding on the seabed and the tanker pulling in the current are borne by the main bearing. Typically, the bearings are slow-rotating slew bearings with multi-race cylindrical rollers, similar to the ones you will find in a large construction crane.

The bearing is robust, but a failure will require dry-docking the SPM, with possible bearing replacement and all associated work.

This is not to be confused with the Product Bearing, also known as the Product Swivel (see further below)

3.4 Mooring & Anchoring System

3.4.1 Mooring Hawser

Holding the tanker to the buoy is an incredibly durable, high-strength synthetic fibre nylon rope capable of withstanding the enormous forces of moored VLCCs, sometimes over 1,000 metric tons. This mooring arrangement is called a Mooring Hawser.

These synthetic fibre ropes are manufactured by only a handful of companies worldwide in near-sterile conditions. Manufacturing is guided by international best practices, with ongoing input from end users, SPM operators, oil majors, and contractors to ensure new data is considered and improvements are implemented.

The hawser arrangement usually consists of nylon rope, which is shackled to an integrated mooring uni-joint on the buoy deck. At the tanker end of the hawser, a chafe chain is connected to prevent damage from the tanker fairlead. A load pin can be applied to the mooring uni-joint on the buoy deck to measure hawser loads.

Hawser systems use either one or two ropes, depending on the largest tonnage of the vessel, which would be moored to the buoy. The ropes would be either single-leg or grommet-leg type. These are usually connected to an OCIMF chafe chain on the tanker side (type A or B, depending on the tanker’s maximum tonnage and mooring loads). This chafe chain would then be held in the chain stopper on board the tanker.

A basic hawser system would consist of the following (working from the buoy outwards):

Buoy-side shackle and bridle assembly for connection to the pad eye on the buoy; Mooring hawser shackle; Mooring hawser; Chafe chain assembly; Support buoy; Pick-up/messenger lines; Marker buoy for retrieval from the water.

Under OCIMF recommendations, the hawser arrangement would normally be purchased as a full assembly from a manufacturer.

The mooring hawser is a single point of failure for the SPM CALM system, and the quality of the hawser is crucial, considering the consequences of failure if a fully loaded VLCC breaks from its mooring.

Mooring hawsers typically last up to 2000 hours of use (tanker moored to the SPM) before replacement is needed. This is because every time the mooring hawser undergoes tension, there is internal friction that wears the rope down and places a limit on its operational lifespan.

In order to prolong the hawser’s life, operators will often remove the ropes from the SPM during extended periods of disuse. Exposure to UV, saltwater, wave movement, and getting tangled around the buoy are all factors that would prematurely age the rope.

3.4.2 Anchor-Legs and -Points

The buoy is secured to the seafloor and held in position by an array of anchor chains and a seabed anchoring system. The anchor chains will take the shape of catenaries, from where the name of this buoy arrangement, Catenary Anchor Leg Mooring (CALM).

The principles of anchoring a ship remain the same when employed under an SPM. There is a subtle change, though, as your anchor needs to hold the buoy in a local, specific position so as not to over-tension the subsea hoses. It also has to be able to hold the tanker with a tug boat hanging off the stern. To do so, SPMs are held in position utilising multiple anchor points. They terminate in a suitable type based on local soil conditions and the required holding force.

Typical type conventional anchors used in SPM moorings are drag or plate anchors. Where this is not suitable or economical, then driven piles, concrete blocks or suction pile anchors can be used.

Between the Anchor Points described and the SPM Buoy itself are the anchor legs. In shallow waters up to 150m, they typically consist of marine or offshore-grade stud-link chain, similar to that used in large tanker anchors or floating oil rig chains.

The forces required to keep the buoy in position increase with water depth; therefore, larger and heavier chains are needed. In deep-water systems, the size increase would be impractical, so composite-material legs are used. This may contain any combination of Dyneema ropes, steel wire ropes, stud link chain, etc.

Dyneema is a UHMWPE (Ultra High Molecular weight Polyethene) or HMPE (High Modulus Polyethene) fibre developed by DSM in the Netherlands some 30 years ago. Known as the world’s strongest, lightest fibre – 15 times stronger than steel, yet floats on water.

For shallow water SPMs, you will typically find 200m to 400m long anchor chains. The design should account for the buoy remaining in place solely on the weight of the mooring chains, and the anchor points should keep the chain endpoint in place.

The figure above shows a typical Stevshark-type anchor used in CALM Buoy systems

3.4.3 Common Failure Modes and Degradation Mechanisms

The mooring and anchoring system of an SPM CALM buoy is continuously exposed to cyclic loading, saltwater immersion, UV radiation, and abrasive contact. Most failures do not occur suddenly, but develop gradually through progressive degradation mechanisms.

Mooring hawsers primarily degrade due to internal fibre abrasion and heat generation during cyclic tensioning. Each load cycle causes microscopic fibre movement inside the rope structure, which produces frictional heating and a gradual loss of tensile strength. External wear at fairleads and chafe points further accelerates deterioration. Chafe chains are subject to localised wear at contact points with tanker chain stoppers and buoy pad eyes, where repeated movement can lead to ovalisation and material loss.

Anchor chains experience their highest wear rates at fairlead interfaces and seabed touchdown zones. At these locations, chains are exposed to combined bending, tension, and abrasion against steel surfaces or seabed material. Over time, this leads to section loss, corrosion pitting, and elongation of individual links. In high-energy environments, chain twist and uneven loading between anchor legs can further accelerate fatigue damage.

Main bearing and swivel assemblies are sensitive to lubrication quality and water ingress. Loss of lubrication film thickness increases metal-to-metal contact and accelerates raceway wear. Water ingress can cause corrosion of bearing components and seal degradation, compromising both structural integrity and product containment.

Routine inspections focus on identifying early indicators of these degradation mechanisms. Typical inspection findings include external abrasion on synthetic hawsers, broken or fused fibres, corrosion pits on chain links, deformation or flattening of links at high-load zones, coating breakdown on steel components, and abnormal grease contamination in bearing systems. Early detection allows targeted maintenance and component replacement before operational limits are exceeded.

3.5 Product /Fluid Transfer System

The fluid transfer system includes, proceeding from the tanker manifold to the onshore storage:

• Floating hoses (making up a hose string)
• A product swivel housed inside the buoy that provides the fluid transfer path between the geostatic part and the rotating part of the buoy
• Subsea hose risers between the buoy and the PLEM
• PLEM housing the various valves
• Subsea pipeline/s between the PLEM and onshore storage

3.5.1 Flow Assurance and Product Management Considerations

Reliable product transfer through an SPM CALM system depends not only on mechanical integrity but also on effective flow-assurance practices. These measures ensure that fluids move through the system safely, predictably, and without creating conditions that could damage equipment or contaminate product.

Flushing is a critical part of offshore transfer operations. After cargo operations, residual product remains in hoses and pipelines. If left unmanaged, this can lead to product degradation, internal corrosion, wax deposition in crude systems, or cross-contamination between different cargo grades. For this reason, systems are designed to allow flushing using water, displacement fluids, or the following cargo batch. Proper flushing also reduces the risk of stagnant zones forming inside low points of the system.

Interface management becomes important when transferring multiple products through the same infrastructure. The interface slug between different products must be controlled and accounted for to prevent the delivery of off-spec cargo. Terminals typically manage this by directing interface volumes into dedicated slop tanks, blending systems, or controlled disposal routes. Accurate volume tracking and flow metering are essential to minimise losses and contamination.

Surge pressure events represent one of the highest mechanical risks to hose and piping systems. Rapid valve closures, pump trips, or emergency shutdown activations can generate transient pressure spikes that exceed steady-state design loads. To mitigate this risk, valve closure times are carefully engineered, and surge analysis is performed during the design phase. In some systems, pressure relief devices or controlled ramp-down procedures are incorporated into operating protocols.

Pigging through an SPM system is inherently limited by swivel assemblies and complex buoy-piping geometries. As a result, operators typically pig only the subsea pipeline section between the tank farm and the PLEM. Product remaining in hoses and buoy piping is managed through flushing, displacement, and controlled drainage procedures rather than full mechanical pigging.

Effective flow assurance planning reduces unplanned downtime, limits product losses, and extends the service life of hoses, seals, and internal piping components.

3.5.2 SPM calm buoy system Floating Hoses Strings

Floating hoses transfer the fluid product from the tanker manifold to the SPM. They are permanently connected to the buoy at one end. The loose end is picked up by the tanker crane/ derrick and connected to the tanker’s midship manifold during operation. The loose ends are not tied back to the buoy between operations but float freely.

Each hose string consists of many individual hoses (typically 9m to 12m in length) flanged together to form a string of adequate length for operations, taking into account the buoy design, metocean conditions, and the size of tankers visiting the SPM. String lengths are typically from 150m to 250m, but they can be made up to any required length. Usually, an SPM has connections available for 2 floating hose strings, but it can have fewer or more.

During operation, the hose strings need to float and stream in a specific bight, as shown in the opening SPM image next to the tanker, and an incorrect-length hose string will have undesirable consequences.

The hose strings need to be leak-free, flexible, positively buoyant, and robust for harsh open-sea conditions. Like the mooring hawser, hoses are subject to strict international standards and requirements for manufacturing and operational use. Typically, they yield 3 to 5 years of fluid transfer, but it is not uncommon to see end users extend this by a year or two. Although the hoses will come with usage and lifespan recommendations from the OEM.

When storing flexible hoses, it’s crucial not to leave them in the sun, as UV light can cause deterioration.

3.5.3 SPM calm buoy system Product Swivel, Piping and Valves

Product handling through the buoy involves piping similar to that found in any tank farm or liquid plant. There are elbows, spools, expansion joints, floating flanges, valves, etc.

However, one unique component of these buoys is the product swivel, which allows the system to weathervane while the product is flowing. This is also called a Product Bearing and has multiple seals between the inner and outer races to prevent product from seeping into the bearing or out.

Some buoys have a complicated bearing called Multiple Product Distribution Unit (MPDU), which, as the name suggests, allows simultaneous pumping of different products without intermixing.

3.5.4 SPM calm buoy system  Subsea Hose/ Flexible Risers

The flexible marine subsea hose risers connect the underside of the buoy to the PLEM. They are installed in a few basic configurations, all designed to accommodate tidal depth variations and lateral displacement caused by mooring loads. In all cases, the hose curvature changes to accommodate lateral and vertical movement of the buoy within an acceptable footprint, while the hoses are supported at near-neutral buoyancy by floats along their length.

Some common subsea hose string arrangements:

• Chinese Lantern, in which two to four mirror-symmetrical hoses connect the PLEM with the buoy, with the convexity of the curve facing radially outwards, giving the outlined shape of a traditional Chinese lantern
• Lazy-S, in which the riser hose leaves the PLEM at a steep angle, then flattens out before gradually curving upwards to meet the buoy approximately vertically, in a shape that looks like a flattened S-Curve.
• Midwater Arch, in which a steel arch with buoyancy is suspended from the seabed tied back to a gravity anchor, and the hose is run over the arch, down into a bight and up to the buoy again.
• Steep-S, in which the hose first rises roughly vertically to a submerged float, before making a sharp bend downwards, followed by a slow curve from horizontal to a vertical attachment to the buoy.

The image above shows a Steep-S riser arrangement, along with the PLEM and its frame.

On deepwater buoys (typically deeper than 60m), risers are often used instead of subsea hoses. They are lighter and have a different lifespan than the flexible marine hoses.

These subsea hose risers are installed and changed out by divers. To save on mobilisation and expenses, operators and owners will often have their subcontractors perform a complete service of the buoy, including all flexible hoses and other wet components in one project. This may sometimes fall within the scheduled dry-docking of the buoy itself.

3.5.5 SPM calm buoy system Marine Breakaway Coupling (MBC)

Marine Breakaway Couplings (MBCs) provide an identifiable safety fuse/separation point for inflexible parts of offshore fluid transfer systems. Commonly, in the floating hose string, but there are instances of them deployed in the subsea hose/ risers.

When the flexible hose string experience undue pre-determined tensile loads or a surge inflow the MBCs automatically activate; shuts down product flow and separates the hose string or riser. This activation relieves tension and pressure in the transfer hose system before it ruptures. In the event of such a breakout, the device will operate without any outside intervention by personnel or the system.

Events such as tanker movement, loss of Dynamic Positioning (DP), storms and pressure surge (such as an inadvertent downstream valve closure elsewhere in the system) could compromise hose transfer systems, and MBCs minimise risks in offshore liquid transfers. Such as product loss, asset damage, environmental contamination, workforce injuries, and extended downtime.

The MBC consists of flanged ends of the same size and classification as the hose in which it serves. It is assembled from two pieces, held together with calibrated bolts that will only separate within a specific pre-set range of tension. The mechanisms with which this MBC valve closes are either flaps (similar to a butterfly valve) or petals that will close with the aid of flowing product.

3.5.6 SPM calm buoy system Pipeline End Manifold

The PLEM facilitates the subsea termination of the pipeline and the connection to the marine hoses. A PLEM comprises a structural base supporting a piping manifold. The PLEM base structure is secured to the seabed as a gravity base (being heavy, or weighted down with concrete or chains), but may also be piled, depending on the soil, loads and installation preferences. Perimeter skirts can improve lateral stability and provide scour protection, especially in sandy seabed conditions. Typical PLEM footprint dimensions range from about 3×5 to 8×10 metres. The PLEM height should be kept to a minimum in shallow-water areas to ensure maximum clearance for passing tankers and other marine traffic. Although in the case of an SPM, this is less of an issue as the PLEM sits directly underneath the buoy and, theoretically, should not see overhead marine traffic. A protection frame is fitted over the manifold to protect the piping from falling objects or fouling ropes.

The PLEM design should be compatible with the pipeline size, pipeline class and the hose configuration. The PLEM should be designed in accordance with relevant structural, mechanical and geotechnical design codes. The pipeline end anchorage could be achieved using gravity anchors. Where the seabed has a deep layer of mud, a piled foundation may be required to support the pipeline above the mudline.

In its simplest form, the PLEM may consist of a valve, preferably a ball valve, a spool piece incorporating a water injection point, and a hose connection flange. More sophisticated systems may incorporate pigging facilities, remotely operated valves, multiple hose connections and emergency shutdown functions. Where surge pressure and the setting of the appropriate valve closure times.

Whatever the design, a method for flushing the hoses before removal for routine inspection and testing should be provided. It is preferable to clear the hoses by injecting water at the system’s low point, and the PLEM design should accommodate this requirement. Sufficient rigging points and pad eyes should also be provided on the PLEM to facilitate hose installation.

The PLEM design should account for pipeline and hose flange loads arising from operational conditions and any credible accidental conditions. The PLEM should also be able to accommodate the expansion from long subsea pipes’ thermal expansion.

The PLEM design shall facilitate the subsea termination of the pipeline and the connection to the subsea hose risers.

Corrosion protection to the PLEM is usually provided through a coating system supplemented with sacrificial anodes.

The PLEM piping may include branches to connect multiple hoses with diameters smaller than the pipeline. The hose connections should be angled to provide a favourable alignment for the hose excursion when connected to the buoy. The PLEM piping should include valves to facilitate replacement of the subsea hose risers and may be configured to connect a pig receiver/launcher to facilitate pipeline pigging.

The PLEM branch lines may also incorporate small-diameter connections to facilitate flushing of the submarine hoses.

Figure: Typical PLEM Layout

3.5.7 PLEM Valve Operations

Operators have a few ways to operate the PLEM Valves:

• Diver operated
• A double-acting subsea actuator powered by a buoy-mounted HPU
• Spring return subsea actuator powered by a buoy-mounted HPU
• Autonomous Shutdown Valve

3.6 Ancillary Items

Apart from the major items listed above, the SPM has many smaller, but essential, ancillary items needed for efficient operations:

• Boat landing – for the launch to dock against to gain staff access to the buoy
• Fendering – to avoid damage when the launch docks
• Lifting and handling equipment – for installation and when removing from the water for inspections
• Navigational aids – according to the standard given by the International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA)
• Subsea Control Systems
• Solar panels
• Etc.

https://youtu.be/MVUCv5m6-Hc

4. Turntable vs Turret CALM Buoys

4.1 Key Advantages of a Turret Buoy:

1. The hawser load is directly transferred into the buoy body. Due to the large mass of the buoy body (incl. deckhouse and all equipment) and the added mass of the water around the buoy body, loads and vibrations from the hawser are transferred to the main bearing in a significantly reduced way. As a result, the main bearing’s operational lifetime is better ensured.
2. The loads generated by tanker-buoy contact (tanker kissing) and by mooring a workboat against the boat landing are cushioned by the mass of the buoy body as described above, thus protecting the main bearing from excessive shock loads.
3. The turret buoy has a flat deck, providing a safe working area not obstructed by a turntable or any other moving parts and is easily accessible via a proper boat landing designed to moor a workboat alongside the buoy.
4. The shape of the turret buoy is not restricted to a cylindrical shape only. The square shape has become well accepted with many clients, as the boat landing spans the full width of the buoy, ensuring safe access even in less-than-perfect weather.
5. The turret buoy is provided with a deckhouse. All the equipment, including the main bearing, is positioned inside the deckhouse and is therefore optimally protected against the harsh environment.
6. Extra features, such as additional electronic (control) equipment, winch, instruments and valves are also fitted inside the protected area of the deckhouse.
7. The flat, minimalist deck structure, combined with the enclosure of all equipment in the deckhouse, significantly reduces maintenance and repair costs throughout the buoy system’s lifetime.
8. Operating, inspection, and maintenance crews can board the buoy and work safely, as most work will take place inside the deckhouse. Downtime due to restrictions in maintenance and repair is, therefore, lower than with other types of buoys.
9. By locking the deckhouse doors, unauthorised access to the equipment can be prevented.
10. The overboard piping is at the lower part, near the waterline, and is directly supported by the buoy structure. An adequate fender system protects the piping from tanker collisions.
11. With any CALM buoy, in slack weather, the floating hose string may wrap around the buoy body. The advantage of the turret buoy design – where the rotating buoy body can be seen as a reel – is that the hoses automatically unwrap when the weather picks up, or when the hose is pulled out by a small service craft.
12. As the chain stoppers are not fitted at the outer perimeter of the buoy body, but in the spider underneath and well within the contours of the buoy body, they are not prone to damage by tanker contact.
13. Also, due to the position of the chain stoppers, the clearance between the tanker bow and the anchor chain catenary is greater, reducing the risk of tankers colliding with and damaging the anchor chain layout.
14. The centre of gravity of the turret buoy is relatively low, providing greater stability during installation and hook-up.

As a result of the advantageous aspects described above, both the operational and maintenance costs of the turret buoy are substantially lower than for turntable buoys. More important to the user is the greater reliability and higher availability of the turret buoy system, thereby reducing costs for demurrage and delays in fluid transfer.

4.2 Key Advantages of a Turntable Buoy:

1. In general, the Turntable Type SPM is usually the cheaper option of the two types
2. They are easily and widely available in the new market with more suppliers stocking and fabricating them
3. They are easily available in the used buoy market, which in many cases is a feasible option for projects to refurbish an old buy
4. The Turntable Type has no enclosed spaces for access, which removes risk such as H2S.
5. It is more common and in longer use, and is known and understood by more operators worldwide.
6. The turntable is easier to install as the mooring chains are pulled directly into the skirt of the buoy, allowing for less and easier diver intervention

The advantages of the Turntable system may seem limited, but that does not mean a Turret is a better solution. There are many factors that go into selecting a suitable buoy type and supplier.

CALM Buoy Selection Logic and Decision Criteria

Selecting between a turret or turntable CALM buoy and defining the overall system configuration is not driven by a single technical factor. It is the result of balancing environmental conditions, operational philosophy, risk tolerance, and lifecycle cost.

Key criteria typically considered during concept selection include:

• Metocean environment
  Wave climate, swell period, wind regime, and current profiles strongly influence mooring loads, tanker motions, and operational availability. Harsher environments generally favour designs with improved load distribution and bearing protection.
• Water depth and bathymetry
  Shallow nearshore installations place different constraints on anchor leg geometry and under-keel clearance compared to deeper offshore systems. Water depth also influences the selection of anchor type and mooring line configuration.
• Seabed soil conditions
  Soil strength and stratigraphy determine whether drag anchors, suction piles, driven piles, or gravity foundations are feasible. Poor soil conditions can significantly increase installation complexity and cost.
• Design tanker size and fleet mix
  Maximum tanker displacement, bow geometry, manifold height, and mooring loads directly influence hawser selection, buoy sizing, and bearing capacity. Facilities serving multiple tanker classes require additional operational flexibility.
• Maintenance philosophy and access strategy
  Some operators prioritise minimal offshore intervention and protected equipment access, while others accept higher offshore maintenance in exchange for lower initial capital cost. This philosophy affects the selection of buoy types and equipment layout.
• Local marine capability and support infrastructure
  Availability of experienced tug operators, diving contractors, inspection services, and specialised lifting vessels influences long-term operability and maintenance planning.
• Hazardous area and H2S operating policies
  Facilities handling sour products or operating in H2S environments may require enclosed or protected equipment, restricted-access designs, and additional safety systems.
• Downtime tolerance and operational availability targets
  Terminals with tight shipping schedules or high throughput requirements often prioritise reliability and redundancy over minimum capital cost.
• Capital expenditure versus lifecycle cost balance
  Lower initial CAPEX does not always translate into lower total cost of ownership. Maintenance frequency, component replacement intervals, inspection logistics, and downtime risk must be considered throughout the facility’s service life.

A structured multi-criteria evaluation during early project phases is essential to avoid selecting a buoy configuration that is technically feasible but operationally suboptimal.

5. Design Factors

5.1 SPM Mooring Downtime

The total downtime (when the tanker cannot moor, load, or discharge product) of an SPM needs to be assessed to determine the feasibility of this type of infrastructure. If it exceeds the facility’s capacity, another solution or location might be needed.

The maximum downtime a facility can tolerate depends on the number of berths per year required to meet the minimum needed operational throughput. Ideally, you want a storage facility that can be turned over three times a month to operate efficiently. However, many facilities need far fewer transfers.

To determine this downtime, the following basic steps are needed

• Assessment of offshore wave and wind data
• Wave transformation modelling to determine nearshore conditions (non-hurricane) is typically done with software like SWAN.
• Determination of downtime based on tanker berthing and operational criteria (see table below article)

Hurricane/Cyclone modelling needs to be done separately, depending on the facility’s location, to determine the impact on operations.

6. Maintenance

An SPM CALM system is exposed to some of the harshest open sea conditions. Maintenance of the system is crucial to ensure continued, smooth, efficient, and controlled-risk operations.

The full spectrum of the maintenance of SPM CALM systems is guided by the Single Point Mooring Maintenance and Operations Guide, 3rd Edition (SMOG).

The maintenance crew (often a band of divers) will undertake tasks such as closing subsea valves, inspecting and removing subsea hoses, and removing mooring hawsers. They will grease bearings, clean solar panels, check batteries, cycle deck-valves, and remove flaking rust and touch up with new paint.

SPM CALM buoys operate without permanent human occupancy, and with no electricity or fresh water, they get damaged by the severe elements. It is needed to wash the entire buoy with fresh water as often as is possible.

Periodic diving maintenance is also carried out through inspections. With the various levels of checking details for daily, weekly, monthly, etc., inspections. This will be laid out in an approved maintenance and inspection plan developed by the Operators and Client.

Typical activities during inspections would include inspecting and reporting on wear on anchor chains and replacing anodes. For major maintenance, you would look to replace subsea and floating hoses every 3 to 5 years.

When SPM buoys receive the prescribed OEM maintenance and the owner/operator’s additional points of care, they can achieve a 30-year service life with ease.

6.1 SPM Classification

Depending on Classification Society and Insurance requirements, the SPM would need to be dry-docked periodically. Typically, this is once every 5 to 10 years. In special cases, permission may be granted for UWIILD (Underwater Inspection In Lieu of Drydock). This exercise can include a major overhaul of the bearing and product swivel, replacing the winch and ancillary gear, and blasting, followed by a fresh coat of paint.

Because the buoy shares many characteristics with tankers at sea, it is managed as strictly as any other tanker in the Oil and Gas fleet. Classification societies such as Bureau Veritas, the American Bureau of Shipping, and Lloyd’s Register all have guidelines to ensure these buoys are kept in safe condition.

7. Installation

There are three main groups of installation that typically need to take place:

• The subsea pipeline/ PLEM (sometimes the PLEM is pulled with the pipeline, and sometimes it is installed separately)
• The buoy, anchor legs, and anchor points
• The riser and floating hoses

Firstly, the pipeline is installed. This typically involved a shore-crossing section in which the pipeline must be buried below the lowest natural level of the beach and adequately covered to prevent waves from damaging the pipe.

Sometimes the PLEM is installed separately from the pipeline, either by floating it out and sinking it into position or by lifting it from a barge/ support vessel and placing it on the seabed. A diving team would be involved with the PLEM connection or securing.

After this, an anchor-handling tug and marine crew would lay anchors (or jet in piles, depending on the design) and lay down the stud link chains, referred to as anchor legs. These anchor legs would then need to be pre-tensioned by the installation vessel to ensure that all slack is out, the chains are not twisted, and they lie in straight lines from the anchor to the buoy centre position.

The buoy is then installed. Typically, SPM buoys are deployed to the water in the closest harbour, after which they are towed into position by a tugboat. In a process that takes a few days to a week, the anchor legs are pulled into the chain stoppers, and the buoy is moored to the seabed.

The final step would be to use a diving team to install the subsea/floating hoses and ancillary items, such as mooring hawsers and navigation aids.

8. Costs

8.1 CAPEX

SPM CALM systems are provided by a handful of OEMs worldwide. Large buoys typically cost between $15m and $20m USD when purchased new.

The following items (material and installation) will typically be priced for an SPM job to determine the CAPEX:

• Shore Crossing
• Offshore Pipeline
• Pipe Trenches (onshore)
• Pipeline (onshore)
• Pipeline/PLEM Launch
• SPM, PLEM & Hoses (+ numerous ancillary items)
• Servitude
• Design (installation engineering + permanent works), Project Management
• Environmental approvals

8.2 Opex

OPEX is linked to the size of tankers calling at the SPM CALM buoy and to the prevailing environmental conditions. A study is conducted to ensure that the support tug boat(s) have sufficient bollard pull and manoeuvring capability to tow the tanker out of trouble in the worst weather conditions expected.

The cost will consist of market rates for a tugboat with sufficient bollard pull, plus consumables (fuel, lubricants, etc.). Pilot and crewing also need to be priced.

If the client is prepared to charter for longer periods, i.e. 10 years, it may be more financially feasible to purchase a dedicated tug.

8.3 Budget Items Commonly Underestimated or Omitted

In early project cost estimates, several supporting cost components associated with SPM CALM buoy developments are frequently underestimated or excluded altogether. Metocean data acquisition, long-term hindcasting, and numerical modelling required to define operational envelopes and downtime expectations can represent a meaningful portion of front-end engineering effort. Regulatory permitting, marine traffic risk assessments, and installation of navigational aids compliant with local authority and IALA requirements are also often treated as secondary considerations despite their potential schedule and cost implications.

Operational readiness planning introduces additional lifecycle costs. Establishing a spare parts strategy for floating hoses, subsea hoses, and mooring hawsers requires upfront capital allocation, particularly given manufacturing lead times and limited global suppliers. Logistics associated with periodic dry docking, including towage, tug availability, port slot access, and lifting arrangements, can also be substantial depending on regional marine infrastructure constraints.

Finally, ongoing inspection and maintenance contracts should not be overlooked. Diver or ROV inspection programmes, classification compliance surveys, and specialised marine support services are recurring operational expenditures that materially influence total cost of ownership. Proper inclusion of these elements during early budgeting improves financial predictability and reduces the likelihood of mid-project scope adjustments.

9. Common Misconceptions About SPM CALM Systems

Despite their widespread use and relatively straightforward visual appearance, SPM CALM buoy systems are often misunderstood. Several recurring misconceptions can lead to unrealistic expectations during early project planning or operations.

One common assumption is that an SPM is simple infrastructure. While the buoy itself appears uncomplicated, safe operation requires coordinated marine logistics, environmental monitoring, specialised equipment maintenance, and strict procedural discipline. The operational complexity is comparable to that of a small offshore terminal rather than a passive mooring point.

Another misconception is that SPMs function effectively in any sea state. In reality, all systems are designed within specific metocean envelopes. Wave height, swell period, wind, and current combinations directly affect tanker motions, hose loads, and mooring forces. Exceeding these limits results in downtime and may require disconnection.

It is also sometimes assumed that tug assistance is optional once a tanker is moored. In practice, tugs play a critical role in maintaining controlled tension, assisting vessel positioning, and providing rapid response capability. Removing or undersizing tug support significantly increases operational risk.

Finally, floating and subsea hoses are occasionally viewed as long-life static components. These systems are consumable assets subject to fatigue, environmental exposure, and internal wear. Their service life is finite and managed through inspection, testing, and scheduled replacement programmes.

Recognising these realities early improves planning accuracy, operational safety, and lifecycle cost forecasting.

10. SPM CALM Buoy Key Criteria – Summary

CRITERIA	SPM
Tanker size	Tanker sizes unlimited, including the Ultra Large Crude Carriers
Operating water depth	4 to 8 mooring chains (most common are 6-8). On deepwater buoys, you will typically find 9, 12 or 16
Mooring chains	Mooring time (after arriving at the mooring position)
Tanker Approach	Can approach from any position – and therefore can choose to approach into prevailing weather conditions
The function of tanker storage, pump capacity, line size and distance	Typically 15 min.
Hose connection time	Typically 1.0 h
Offloading time	Product offloading is possible with wind up to 40 knots and head waves of 3.0 m to 4.5 m
Hose disconnection time	Typically 1.0 h
Mooring disconnection time	Typically 15 min.
Net total time difference with Multi Buoy Mooring	Typically 2.5 h quicker compared to CBM or MBM
Mooring conditions	Able to moor with winds up to 30 knots and head waves of 2.0 m to 2.5m
Offloading steps	Steam from anchorage, moor, connect hoses, pump product through, disconnect hoses, disconnects mooring, steams away
Operating limit	The tanker has to leave the berth with winds of 60 knots and waves higher than 3.5 m to 5.0 m
Mooring disconnection	Approximately US $15m to $20m for the SPM CALM buoy alone (excluding installation)
Cost	Required for mooring. Tug required full-time during mooring to assist with weathervane movements and to keep tanker from colliding with buoy
Tuggage	Required for mooring. Tug required full-time during mooring to assist with weathervane movements and to keep the tanker from colliding with the buoy
Under Keel Clearance	The function of the tanker, metocean conditions and local operational guidelines. Must be calculated.
Suppliers	Proprietary technology and limited suppliers
Track record	Since 1959 successfully installed around the world
Weather impact	Less prone to adverse conditions and swell delays than a CBM or MBM
Night operations	Possible limitation on night-time mooring depending on local operational, safety and environmental procedures. Can disconnect from moorings 24 h a day
Maintenance	Additional complex maintenance activities compared to a CBM or MBM – swivel, bearings, mooring line tensions, classifications, etc.