The Stevtensioner
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The Stevtensioner Introduction The Stevtensioner is used for cross tensioning of diametrically opposed anchor legs moored by drag anchors or anchor piles. The Stevtensioner is generally used for the installation of (semi) permanent floating structures such as the SPM buoy, STL, TLP, FPS, FPSO, etc. After the tensioning operations the Stevtensioner is demobilised and ready for the next project. The Stevtensioner can however also be used for permanent tensioning purposes, becoming a part of the mooring system. The Stevtensioner can be deployed from a crane barge, AHV or any vessel having enough crane/winch capacity to pull the required vertical force. The existing models VA220 and VA500 were designed for handling a single size of chain. The new Stevtensioner models VA600, VA1000 and VA1250 can handle chain diameter ranging from 76 mm up to 152 mm. Because of this variety in chain sizes additional work chain may not be required (fig. 3-105). he working principle of the tensioner The Stevtensioner is based on the principle that a vertical load 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 located inside the Stevtensioner and as such well protected against damage caused by handling and lifting operations (fig. 3-106). The new Stevtensioner models offer the following features: • Smaller dimensions, reduced weight and improved handling, but heavy enough to easilty slide down the mooring line. • Designed to smoothly guide at least 5 links and therefore prevent chain getting stuck inside. • Due to economical volume/weight ratio, the new Stevtensioner models allow for containerised freight by either sea or, for rush deliveries, by air. • The integrated shape allows for smooth passage over stern roller. • Load measuring pin is equipped with two independent sets of strain gauges. The umbilical cable connections are protected against handling and lifting operations. These connections may be used for acoustic transfer of the s

The Stevtensioner One anchor line (passive line) is attached to the tension measuring pin at the Stevtensioner. The oppos-ite anchor line (active line) passes through the Stevtensioner. Tensioning starts by applying the yo-yo movement to the active line (fig. 3-107). When the Stevtensioner is lifted by the active chain, it blocks the chain. When the Stevtensioner is lifted from the seabed, the passive and active mooring lines are also lifted. Consequently the anchors or piles are loaded and cause an inverse catenary of the mooring line in the soil, as well as causing the anchor to drag and...

The Stevtensioner Measurement of the tensions applied Fig. 3-109 shows the curve recorded during tensioning of chains connected to piles for the Coveñas Pipeline Project in Colombia. The graph shows a total of 5 heaves (yo-yo’s), each resulting in a higher tension. When the Stevtensioner is lifted from the seabed, the passive and active mooring lines are also lifted from the seabed. Consequently the anchors or piles are loaded. The loading causes an inverse catenary of the mooring line in the soil, and also causes the anchor to drag and embed; in other words: chain length is gained. When...

The Stevtensioner Computer calculations The tension in the chain can be calculated by means of computer catenary calculations. Besides known parameters such as submerged chain weight, and the length of the mooring line, other parameters measured during tensioning need to be incorporated in the calculation: • Height Stevtensioner above seabed. • Vertical pulling load. By using this method the tension in the chain can be calculated at any height of the Stevtensioner above seabed. This method is independent of the waterdepth. Umbilical cable and measuring pin The chain tension can be measured...

The Stevtensioner Duration of pretensioning anchors and piles Once the required tension has been achieved, the tension has to be maintained for a certain duration. This period is described in the table below for various Certification Authorities. Certification Authority maintaining tension Lloyds Register of Shipping American Bureau of Shipping Det Norske Veritas (NMD) Required duration of 20 minutes 30 minutes 15 minutes

The Stevtensioner Handling the Stevtensioner Handling operations can generally be described as follows: • Positioning the anchors and paying out the chain • Hook-up all necessary hardware for tensioning operations on deck of barge or AHV • Deployment Stevtensioner to the seabed and positioning of the installation vessel • First lift (yo-yo) • Series of yo-yo’s • Maintain required tension for a specified period of time • Retrieve the Stevtensioner and disconnect • Prepare for next tensioning A Stevtensioner can be deployed from a crane barge, Anchor Handling Vessel or any vessel having...

Hook-up Pass the active chain (2) through the tensioner (1) on deck. Connect passive chain (3) to measuring pin shackle (8). Connect dislock wire (5) to shackle (4). Connect umbilical cable (7) to read-out system on deck and to the measuring pin (6). Lowering Fix active chain (2) to winch or crane hook. Slack dislock wire (5) and lower Stevtensioner to seabed. Stevtensioner will pass over active chain (2). Tensioning mode When Stevtensioner is on seabed, slack dislock wire (5) before the first yo-yo, and keep slack during all yo-yos! Tensioning is achieved by pulling on active chain (2)....

Retrieving When tensioning is completed be sure to lower the Stevtensioner to seabed and slack off active chain (2) before retrieving Stevtensioner with dislock wire (5). Pull on dislock wire (5). Stevtensioner will pass over chain (2). Disconnect Stevtensioner on deck of the barge or AHV. Stevtensioner Product Range The following Stevtensioners are available from vryhof anchors. Stevtensioner Maximum Suitable* for chain Suitable* for chain Size Weight model horizontal load size with Kenter size without Kenter Stevtensioner Stevtensioner The suitability only refers to the...