CALM

A CALM consists of a floating buoy anchored to the seabed by catenary chain legs, which are secured to anchors or piles. One or more elastic mooring hawsers hold the tanker captive to a turntable which is mounted on top of the buoy by means of a slewing bearing. This bearing allows the turntable to freely weathervane so that a tanker can take up the position of least resistance to the prevailing weather at all times. Fluid product is transferred via the CALM to or from the tanker by floating and subsea hose systems.

When the tanker moves off station, due to the effects of wind, wave and current, certain anchor legs are lifted. This generates a restoring force and returns the system to an equilibrium position, thus limiting the tanker’s excursion.

CALMs are usually located in water depths between 20 to 100 meters and are connected to a shore storage facility (tank farm) or to offshore production platforms by means of a subsea pipeline. Since early 2000, the CALM design has been used and adapted to deepwater conditions, greater than 1, 000 meters. For this application, the CALM is used as an offloading system for a deepwater Floating Production Storage and Offloading unit (FPSO). SBM is currently the market leader of deepwater CALMs having supplied eight out of ten deepwater CALM systems to date.

When SBM Offshore invented the turret concept, its application to CALM buoys was considered. This idea was rejected, however, in favor of continued use of the turntable configuration for the following reasons that still hold true today, even for deepwater.

The most prominent advantages of the turntable-type buoy are as follows:

• A naturally ventilated turntable and center well offer safe and easy access to maintenance personnel. The entire piping path is open to air and there is no enclosed area, which could potentially trap explosive gases.
• The working area for maintenance operations is unobstructed.
• The safe and easy re-tensioning of moorings is accomplished through the use of automatic ratchet type chain stoppers positioned at the periphery of the buoy.
• The main bearing is mounted above the upper deck and supports an above-water turntable. This dry turntable arrangement offers free and easy rotation with minimum break-out and running torque requirement.
• The main bearing is only loaded when a tanker is attached and is not directly affected by wave loading, impact from workboats or from tankers riding up.
• The main bearing is easily accessible in a dry and open environment and can be fully maintained or even replaced offshore in case of damage by collision.

Long Life

Recently, the life span of SBM CALMs was increased to 30 years. Based on offshore platform standards, the paint system and corrosion allowance of the CALM were updated permitting the buoy to remain in the water for such a significant period of time. Additionally, the SBM CALM Buoy does not need to be brought to shore for overhaul and repair during this period. Special tooling and procedures are now available for the removal of the main bearing on site in the event of a failure due to a vessel collision with the turntable.

All mechanical components, Central Pipe Unit bearing, seals and valves, can be overhauled in-situ by using either the lifting equipment available on the CALM buoy, or replaced using much smaller lifting means such as the “A” frame of a workboat. This feature represents a significant advantage over other designs, which require regular in-shore overhauls, resulting in associated costs for the removal of the buoy body, overhaul and re-installation. These associated costs include the following:

• Direct Costs - Mobilization and work costs of the barge needed to disconnect the buoy from the chains and the hoses, the shipyard bill and the barge cost of reconnecting the buoy to the chains and the hoses, plus the barge demobilization cost.
• Indirect or Additional Direct Costs - The indirect costs associated with the unavailability of the terminal, or the direct cost of hiring a replacement buoy, plus its related installation and uninstalling.

The Turntable design and outfitting are adapted to the specific requirements of the project and Client. Product flow dictates buoy piping requirements and on-deck equipment can either be simple and manually operated, or more complex with remote control from shore station or ship by the use of telemetry.

Multi-Product Systems

CALM terminals are typically designed for the transfer of one product, e.g. for delivery of crude oil to a refinery. However, they can be used for the transfer of multiple products. CALMs have been designed to handle as many as five products simultaneously and discretely without co-mingling.

These products include:

• Crude oil or condensates
• Refined Petroleum products (e.g. fuel oil, diesel, aviation spirits, etc.)
• Chemicals and fertilizers in liquid form  
• LPG
• Fresh water
• Minerals in slurry form (e.g. iron, sand, or coal)

Benefits for the Operator

CALM terminals do not usually require tugs for berthing or un-berthing, only a small rope handling boat. The tanker turn-around times are generally less than 24 hours (i.e. no demurrage charges).

Other advantages to Clients include:

• Capital cost and lead time to system commissioning are significantly less than for a fixed-jetty facility
• Operating costs are lower due to the “no tug” requirement
• CALM systems are designed to comply with the rules and regulations of all of the leading Classification Societies such as: the American Bureau of Shipping, Lloyd’s Register, Det Norske Veritas and Bureau Veritas, among others.

Additional Equipment

While the original concept remains the same, a number of safety features and special equipment have been developed, which can be made available for all multi-purpose buoys, including the following:

• Telemetry and Berthing Aids - to provide the Master and Operators with information on process and ancillary equipment, together with berthing data
• Load Monitoring Systems - to provide a constant display of the tension in the hawser
• Breakaway Couplings - to provide the means to prevent overstretching of the floating hoses in the event of overload (broken hawsers) or pressure surges, thus avoiding any risk of significant pollution
• Electric Swivels - to enable power and instrument signals to be transferred between the fixed and rotating parts of the buoy
• Hydraulic Power Units and Hydraulic Swivels - to provide hydraulic power supplies for sub-sea applications, i.e. PLEM control valve(s)
• Solar and Wind Power Systems - to power the navigation aids, telemetry and berthing aids
• Outboard Product Swivels - to limit the stress on the offloading hoses and thereby increase hose life
• Surge Relief Systems - to provide a means to contain any fluids that may be relieved due to pressure surges during loading/offloading operations
• Subsea Wave, Tide, and Current Sensors - to monitor the long-term tide height, significant wave height and wave period.