STEVTENSIONER®

ENABLING INSTALLATIONS

The proof loading of a permanent mooring system for CALM buoys and FPSOs is critical for a system’s certification. The proof loads are typically 80-100% of the design load so that substantial bollard pull capacity is required. The use of the STEVTENSIONER^® for subsea tensioning offers significant cost benefits as it reduces the required load by typically 60%. As a result mooring installations can be performed from modest anchor handlers or crane barges.

Double pull power

The STEVTENSIONER^® is basically a chain shortening clutch with the mooring chain connected on one side and a reaction chain running through it. A vertical pull can induce more than double that pull in the horizontal leg. Repeatedly heaving up and slacking the STEVTENSIONER^® in a yo-yo action builds up the load in the mooring chain until the required tension is achieved. Typically 5-7 cycles are sufficient.

Reduce installation time and cost

The STEVTENSIONER^® has been successfully used for over 25 years for the installation of mooring systems of Floating Production Units and Offshore Loading Buoys. It suits all types of anchors, be it drag, suction or driven pile and it also serves to install a complete pre-laid mooring before the unit arrives on location. STEVTENSIONER^®’s handle loads up to 1,250 tons and accommodate chain size of 76-152 mm for the work chain. Given sufficient heave height there is no limitation on mooring line length and can thus also be used for tensioning of systems that include wire rope.

Cross tensioning savings

By cross tensioning of two opposite anchor legs in the mooring spread, an even larger time efficiency is created. It reduces the number of tensioning operations required by 50% and may save as much as 35 to 40% on the total installation cost originally required for the operation.

An all-in package

The tensioning services are available on a rental basis. Dedicated engineering can be performed and depending on the complexity of the project may include a feasibility or suitability assessment and the preparation of suitable procedures. The rental package includes the STEVTENSIONER^® and the load measurement pin and data transmission via acoustic and umbilical cable to a read out unit. The package would typically also include the supervisor during the tensioning operation.

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Eirini Papapanagiotou

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Peter Foxton

• Reduces required load by up to 60%
• Makes the need for bollard pull or winch capacity in excess of the installation load obsolete
• Allows moorings to be installed with a smaller number of vessels, or lower capacity anchor handlers or crane barges
• For all types of anchor points and mooring systems
• Subsea tensioning equipment is compact in size, suitable for even mobilizing by air freight

Features & specifications of the STEVTENSIONER®

The STEVTENSIONER^® is basically a chain shortening clutch with the mooring chain connected on one side and a reaction chain running through it. A vertical pull can induce more than double that pull in the horizontal leg. Repeatedly heaving up and slacking the STEVTENSIONER^® in a yo-yo action builds up the load in the mooring chain until the required tension is achieved. Typically 5-7 cycles are sufficient.

Technical features
- Available for loads of up to 1,250 tons
- Accommodates work chain range 76-152 mm
- No limit to mooring chain size
- Operates on mooring chain without impairing chain performance, life expectancy or damaging links
- On average needs only 5-7 tension movements per mooring chain
- Integral load pin in tensioner records tension in the anchor line, recorded in real time on deck
- Designed to smoothly guide at least 5 links and therefore prevent chain getting stuck inside
- The integrated shape allows for smooth passage over stern roller
- Load measuring pin is equipped with two independent sets of strain gauges. These connections may be used for acoustic transfer of the signals
Stevtensioning modes

Cross tensioning
In the cross tensinonig mode, two opposing anchors in the mooring spread are simultaneously tensioned.

Tensioning against a reaction anchor
In this mode, a reaction anchor is placed to tension an anchor in the opposite mooring line or a mooring line cluster as shown in the illustration. Change over the STEVTENSIONER^® between the various mooring lines will signiﬁcantly reduce the required tensioning time.

3-way tensioning
When the mooring spread is composed of mooring lines that are equally spaced, identical in length and require identical tensioning loads, this mode serves to reduce spread installation time, such as for an SPM buoy, to a minimum.
Working principle

S-1
The active chain (2) passes through the Stevtensioner^® (1) on deck of the installation vessel. The passive chain (3) is connected to the measuring pin shackle (8). The dislock wire (5) is connected on shackle 4. The umbilical cable (7) is connected between the measuring pin (6) and the read-out system on deck.

The STEVTENSIONER^® is based on the principle that a vertical load applied to a horizontal string causes high horizontal loads. To achieve the required horizontal pretension load at the anchor points, the vertical pulling force only needs to be 40% of this pretension. The anchor line tension is measured by a measuring pin inside the STEVTENSIONER^®(fig. S-1) that is connected to a read out unit on the installation vessel by an umbilical or acoustic system for deep water. One anchor line (passive line) is attached to the tension measuring pin, while the opposite anchor line (active line) passes through the STEVTENSIONER^®.
When the STEVTENSIONER^® is lifted it locks the chain and lifts the full arrangement off the seabed (fig. S-2), building load at the anchor points or causing drag anchors to penetrate the seabed. The STEVTENSIONER^® is then lowered over the gained chain length, so that the same action can be repeated. Tensioning is applied by repeating the yo-yo movement (fig. S-3). The pretension is maintained for a certain time for certification (DNV 15 minutes, Lloyd’s 20, ABS 30).

Computer calculations
Computer software can calculate the tension in the chain. Parameters such as submerged chain weight, length of mooring line as well as height of the STEVTENSIONER^® above seabed and the vertical pull load are incorporated in the calculation. By using this method the tension in the chain can be calculated at any height of the STEVTENSIONER^® above seabed, independently of the waterdepth. Fig S-4 shows the 5 movements in which tension is build up during tensioning of chains on piles.
Reduced installation time and cost

The STEVTENSIONER^® has been successfully used for over 25 years for the installation of mooring systems of Floating Production Units and Offshore Loading Buoys. It suits all types of anchors, be it drag, suction or driven pile and it also serves to install a complete pre-laid mooring before the unit arrives on location. STEVTENSIONER^®’s handle loads up to 1,250 tons and accommodate chain size of 76-152 mm for the work chain. Given sufficient heave height there is no limitation on mooring line length and can thus also be used for tensioning of systems that include wire rope. Cross tensioning savings By cross tensioning of two opposite anchor legs in the mooring spread, an even larger time efficiency is created. It reduces the number of tensioning operations required by 50% and may save as much as 35 to 40% on the total installation cost originally required for the operation. An all-in package The tensioning services are available on a rental basis. Dedicated engineering can be performed and depending on the complexity of the project may include a feasibility or suitability assessment and the preparation of suitable procedures. The rental package includes the STEVTENSIONER^® and the load measurement pin and data transmission via acoustic and umbilical cable to a read out unit. The package would typically also include the supervisor during the tensioning operation.

DOWNLOAD PRODUCT BROCHURE

The proof loads of a permanent mooring system for CALM buoys and FPSOs are typically 80-100% of the design load so that substantial crane/winch capacity or bollard pull is required.

The use of the STEVTENSIONER^® for subsea tensioning offers significant cost benefits as it reduces the required load by typically 60%. As a result mooring installations can be performed from anchor handlers or modest crane barges.

Discover how these and more unique features will uplift your next project; download the STEVTENSIONER^®product brochure here.