SPM CALM Systems – Ultimate Guide

In this guideline, we take you through everything you need to know about SPM calm Buoy System. You are reading the most visited article on our blog. Connect with the author on Linkedin. 

1 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 for use in various locations and purposes, such as Turret Buoys, Single Anchor Leg Mooring (SALM), Single Point Mooring Towers, Spars, and Articulated platforms.

This article will only focus on the SPM 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 if it is 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 negate 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 loads from wind and waves.

SPM calm buoy system 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 on position with composite legs, often containing high-tech synthetics 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 SPM CALM Buoy System: History

Since being introduced, SPMs facilitated economies of speed and scale in the international transport of petroleum. And as the market for these SPM systems developed rapidly the decision was taken in 1969 by N.V. Industrieele Handels Combinatie Holland (IHC) to create a separate business to market and further develop the products, called Single Buoy Moorings Inc. (or SBM). Their success has become eponymous, as SPM CALM type are now common, but technically incorrectly referred to as SBM or 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:

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 complete closed product transfer system from the tanker hold, through the various interlinking parts, to the buffer storage tanks onshore.

Once the tanker is moored and the floating hose strings connected, the tanker is ready to load or discharge its cargo, using either the pumps onshore or on the tanker 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 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 and not the side of the tanker, reducing operational downtime caused by excessive tanker movements. The product swivel inside the buoy allows the product to keep 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 SPM calm buoy system Step 1: Tanker Approach and Mooring

2.2.1 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 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.

2.2.2 Mooring

During mooring manoeuvres the tanker will approach the buoy, heading into the dominant environmental conditions ensuring maximum control, while minimising the need for constant mooring assistance from a tugboat. 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 coming adrift from an SPM can quickly end up on the beach as it takes quite some time to get engines ready.

As the pilot slowly brings the tanker to the buoy, its crew will pass down a smaller messenger line down 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 facilitate to keep the floating hoses clear from the tanker bow.

After passing the hose pickup ropes to the tanker team, the tug moves around to the stern of the tanker where its towing wire is hooked up. 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 amount of tension on the mooring hawsers to prevent contact of the tanker with the buoy structure, as oftentimes the tanker’s response time is substantially slower than the rapid change of weather direction.

The constant tension on the mooring hawser also reduces the wear and tear on the mooring hawser components and SPM main bearing, as well as 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 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 SPM calm buoy system Step 2: Loading/Discharging Product

2.3.1 Connecting Floating Hose Strings

Once securely moored to the SPM, the free ends of the floating hoses 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 a flanged connection which could be expedited by the use of a cam-lock system. In some cases, where a specific tanker is the only tanker to visit a particular SPM, they may even install engineered cargo coupling solutions which makes the connection task almost autonomous.

With the hoses connected and the necessary valves on the 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 biocide, oxy-scavenger and 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 the pig is pushed through one pipeline via the PLEM and back to the tank farm with the pipeline.

2.4 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 is free to go – on condition of formalities such as cargo acceptance and a few other quality and procedural checks.

Below – This SPM was converted to be a mooring point only for a unique offshore bulk cargo transfer operation where smaller Panama sized trans-shippers fed the bulk carrier by way of STS mooring arrangement.

This video serves only to show 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 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 further below in the article 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 one you will find in a large construction crane.

The bearing is robust, but a failure will result in dry-docking the SPM, with the possible replacement of a new bearing and all associated works.

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 and high strength synthetic fibre nylon rope capable of withstanding the enormous forces of moored VLCCs, sometimes over 1000 metric tons of force. This mooring arrangement is called a Mooring Hawser.

These synthetic fibre ropes are only manufactured by a handful of companies in the world in near-sterile conditions. The manufacturing is guided by international best practice with constant input from end-users, SPM operators, oil majors and contractors – to ensure that 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 either be single-leg or grommet leg type ropes. These are usually connected to an OCIMF chafe chain on the tanker side (either type A or B depending on the maximum tonnage of the tanker and the 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 consequence of failure if a fully loaded VLCC breaks from its mooring.

Mooring hawsers typically last up to 2000 hrs of use (tanker moored to the SPM) before they need replacement. This is because every time the mooring hawser undergoes tension there is internal friction wearing the rope down and placing 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, movement in the waves and getting tangled around the buoy are all factors which 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 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 depending on the local soil conditions and the holding force required.

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 is the anchor legs. In shallow waters, up to 150m, they typically consist of marine or offshore grade stud-link chain, similar to that of large tanker anchors or floating oil rig chains.

The forces required to keep the buoy in position increases with water depth, and therefore larger and heavier chains are needed. In deep water systems, the size increase would be impractical and 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 Polyethylene) or HMPE (High Modulus Polyethylene) 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 take into account that the buoy stays in position purely on the weight of the mooring chains and the anchor points is to keep the chain endpoint in position.

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

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 SPM calm buoy system Floating Hoses Strings

Floating hoses transfer the fluid product between 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 make up a string of adequate length for operations, taking into consideration the buoy design, metocean conditions and 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 will have connections available for 2 floating hose strings but can be less or more.

During operation, the hose strings need to float and stream in a specific bight, as seen 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 sound against leakages, flexible, positively buoyant and robust for harsh open sea conditions. Similar to the mooring hawser, hoses have strict international standards and requirements for manufacturing requirements and operational use. Typically, they yield 3 to 5 years worth of fluid transfer, but it is not uncommon to see end-users push this out 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 store them in the sun due to UV causing deterioration.

3.5.2 SPM calm buoy system Product Swivel, Piping and Valves

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

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

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

3.5.3 SPM calm buoy system  Subsea Hose/ Flexible Risers

The flexible marine subsea hose risers form the link between the underside of the buoy and the PLEM. They are installed in a few basic configurations, all designed to accommodate tidal depth variation and lateral displacement due to 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 its 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 through horizontal to a vertical attachment to the buoy.

SPM Calm Buoy System

The above image shows a Steep-S riser arrangement together 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 will have a different lifespan than that of 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 oftentimes have their subcontractors perform a complete service of the buoy which will include all the flexible hoses and other wet components in one project. This may sometimes fall within the scheduled dry-docking of the buoy itself.

3.5.4 SPM calm buoy system Marine Breakaway Coupling (MBC)

Marine Breakaway Couplings (MBCs) provide an identifiable safety fuse/ separation point inflexible parts of the 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 the tension and pressure in the transfer hose system before it can rupture. In the event of such a breakout, the device will operate without any outside intervention by personnel or 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 minimises risks in offshore liquid transfers. Such as product loss, damage to assets, contamination of the environment, injury to the workforce 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 out of two pieces, which are 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.

3.5.5 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 be used to improve the lateral stability and for 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 locations to allow maximum clearance of overpassing 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, pipelines class and the hose configuration. The PLEM should be designed in accordance with relevant structural, mechanical and geotechnical design codes. Anchorage of the end of the pipeline could be accomplished with gravity anchors. Where the seabed has a deep layer of mud, a piled foundation may be required to hold the pipeline above the mud line.

In its simplest form, the PLEM may consist of a valve, preferably a ball valve, a spool-piece incorporating a water injection point and 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 way of flushing the hoses prior to their removal for routine inspection and testing should be provided. It is preferable to clear the hoses by injecting water at the low point of the system and the PLEM design should accommodate this requirement. Sufficient rigging points and pad eyes should also be provided on the PLEM to facilitate the installation of the hoses.

The PLEM design should consider pipeline and hose flange loads that may arise from operational conditions as well as from any credible accidental condition. The PLEM should also be able to accommodate the expansion for from long subsea pipes thermal expansion.

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

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

The PLEM piping may include branches for the connection of multiple hoses of smaller diameter 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 the replacement of the subsea hose risers and may be configured for the connection of 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

3.5.6 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.


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 operational lifetime of the main bearing 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 is fitted over the full width of the buoy, ensuring safe access, even in non-perfect weather conditions.
  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 and minimalist deck structure combined with the enclosure of all equipment in the deckhouse reduces significantly the maintenance and repair costs throughout the lifetime of the buoy system.
  8. Operating, inspection and maintenance crews can board the buoy and can work under safe circumstances as most of the work is to take place inside the deckhouse. Downtime due to restriction in maintenance and repair is, therefore, lower than with other types of buoys.
  9. By locking the doors of the deckhouse, unauthorized access to the equipment can be prevented.
  10. The overboard piping is at the lower part, close to the waterline, directly supported by the buoy structure. An adequate fender system protects the piping from tanker collisions.
  11. With any CALM buoy, at 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 larger, thus reducing the risk of tankers running into and damaging the anchor chain layout.
  14. The centre of gravity of the turret buoy is relatively low, providing higher stability during installation and hook-up of the buoy.

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 for the user is the more reliable and higher availability of the turret buoy system, thus reducing cost on demurrage, and on delays in the transference of fluids.

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 the new market with more suppliers stocking and fabrication 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 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 might seem less, but it does not mean a Turret is a better solution. There are many factors that go into selecting a suitable type and supplier of a buoy.

5 Design Factors

5.1 SPM Mooring Downtime

The total downtime (where the tanker cannot moor or load/discharge the product) of an SPM needs to be assessed to determine the feasibility of this type of infrastructure. If it is more than the facility can tolerate, then 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 have the need for much 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) – typically software like SWAN is used.
  • 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 location of the facility, 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 of crucial importance to ensure continued smooth, efficient and controlled-risk operations.

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

The maintenance crew (often a band of divers) will set upon tasks such as closing subsea valves, subsea hose inspections and removal of mooring hawsers. They will grease bearings, clean solar panels, checking batteries, cycling deck-valves and remove flaking rust and touching 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 in way of inspection. With the various level of checking details for daily, weekly, monthly, etc. inspections. This will be laid out in an approved maintenance and inspection plans developed by the Operators and Client.

Typical activities during inspections would include looking and reporting on wear on anchor-chains and changing out anodes. Insofar as major maintenance, you would look to change subsea- and floating hoses anywhere from 3 to 5 years.

When SPM buoys receive adequate maintenance as prescribed by the OEM, bolstered with the owner/operator’s additional points of care, they have the ability to 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 bearing and product swivel, changing out of winch and ancillary gear and blasting followed with a fresh coat of paint.

As the buoy shares many characteristics with tankers at sea, they are managed as strictly as any other tanker in the Oil and Gas fleet. Classification societies such as Bureau Veritas, American Bureau of Shipping and Lloyds register all have sets of guidelines to ensure these buoys are being kept in a 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 + adequate cover, to ensure the waves do not damage the pipe.

Sometimes the PLEM is installed separately to the pipeline by either floating it out and sinking into position or lifting it from a barge/ support vessel and placing on the seabed. A diving team would be involved with the Pipeline End Manifold (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 the slack is out, the chains are not twisted and they are laying 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 it is towed into position by a tugboat. In a process taking a few days up to a week, the anchor-legs are pulled into the chain-stoppers and the buoy is itself 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, navigation aids, etc.

8 Costs


SPM CALM systems are provided by a handful of OEMs around the world. For large buoys, they typically cost in the range of $15m to $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

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

The cost will consist of market-related rates for the tug boat with sufficient bollard pull, plus consumables (fuel, lubes, 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 a tug.

9 SPM CALM Buoy Key Criteria – Summary

Tanker size Tanker sizes unlimited, including the Ultra Large Crude Carriers
Operating water depth Typically in shallow, nearshore waters < 100m. But, can be installed at any depth in which case the anchor legs design becomes composite assemblies
Mooring chains 4 to 8 mooring chains (most common are 6-8). On deepwater buoys, you will find typically find 9, 12 or 16
Tanker Approach Can approach from any position – and therefore can choose to approach into prevailing weather conditions
Mooring time (after arriving at mooring position) Typically 15 min.
Hose connection time Typically 1.0 h
Offloading time The function of tanker storage, pumps capacity, line size and distance
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 Product offloading possible with wind up to 40 knots and head waves of 3.0 m to 4.5 m
Mooring disconnection The tanker has to leave berth with winds of 60 knots and waves higher than 3.5 m to 5.0 m
Cost Approximately US $15m to $20m for SPM CALM buoy alone (excluding installation)
Tuggage Required for mooring. Tug required full time during mooring to assist with weathervane movements and to keep tanker from colliding with 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.

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