• N-Line Tensioner system

    A direct acting tensioning system that apply a constant tension to the marine riser and compensates for vessel movements.

    Description

    Tension members (cylinders)
    The NOV N-Line Tensioner system consist of six N-line tension cylinders with air/oil accumulators, anti recoil valve skid, air control skid, remote operated control panel, PLC and APV's. The tensioner elements are suspended from single supports with the cylinders connected to a specially configured tensioner ring, furnished by others. The tensioner system is also supplied with a riser anti recoil system.

    The tensioner system consists of several tension members. Each tension member is associated with a high-pressure accumulator bank, which provides gas expansion volume and act as a hydraulic fluid receiver for each cylinder. The gas volume is selected to deliver the required tension and to conform to the tension design criteria specified by customer.

    The riser tensioner cylinders are directly attached to the tension ring for the riser pipe.

    As upward heave of the rig tends to cause an increase in tension, the cylinder extends and downward heave of the rig tends to cause a decrease in tension.

    The tensioner elements are suspended in the trip saver trolley from single supports under drill floor with a connection link. There is also the same connection link between the piston rod and the tension ring. This will give the cylinder a high degree of free movement.

    During normal operation, no air is vented from the system. The air compressor is used to increase the air pressure (tension level) and to replace lost air. The application of an air dryer with each air compressor reduces the problems caused by water condensation.

    Complete control of the tensioner system is accomplished from the control panel, located in drillers control room and/or by local control panel located in the moonpool area. These controls enables the operator to start-up, set operation, monitor and shut down the complete systems

    The riser tension system is designed in such way that it is possible to operate the system if one pair of tensioner is out for maintenance. In order to keep the same tension with one pair of tensioner out of service, the pressure in the remaining units must increase.

    Air/oil accumulators
    In order to provide the tension characteristics shown, a given oil/gas volume for each tension member is required. Each tensioner cylinder is connected to two air/oil accumulators that are piped together in top and bottom. The top inlet is for air supply and the bottom outlet is for hydraulic fluid supply to tensioner cylinders. Each air/oil accumulator assembly includes an oil level measuring system.

    Shut off valve skid
    The two skids are an assembly of three shut-off valves and one pilot pressure circuit each.
    Each of the six valves is controlled by the PLC through signals from the valves and the piston rod measuring system. In the case of a riser disconnect the valve will limit the oilflow and the riser will not gain momentum as it does in a wireline system. During disconnect the control system controls the position of the valves and the riser is brought up in a controlled way.

    Air control skid
    In order to obtain a centralized control of the tensioners there is one air control skid, located in suitable position. By using all the remote controlled valves on this skid the operator can start, operate, monitor or shut down the total system. The air control skid includes all valves, tubing, filter regulator(s), pressure instruments, cabling and junction boxes as required to operate all riser tensioner units.

    The air control skid is designed for installation outside in harsh offshore environmental conditions.

    Air pressure vessels
    The purpose of the APV’s is to act as an energizer that can be compared to an enormous pneumatic spring. The large volume will ensure a "nearly constant" tension.

    Operators Tensioner control panel
    Centralized control of the riser tensioners are from local control panel, and drillers control room (computer control).
    The control panel includes required controls and indicators for the riser tensioners. The panel is an AISI 316 console certified for operation in hazardous area, zone 1, ready for bulkhead installation.

     

    Features/Benefits

    • Tension capacity: 2.500.000lbs – 4.800.000lbs
    • Stroke 40ft – 65ft
    • Advanced and proven Riser Anti Recoil system
    • Certification according to ABS, DNV

    Specifications

    NRT1800-50  

    NRT3600-50 

    NRT4200-50 

    Max tension load R/M/E 

    1500/1800/2000kips

    680/816/900t

    3000/3600/3968kips

    1361/1633/1800t

    3400/4200/4748kips

    1542/1905/2154t

    Cylinder stroke 

    50ft (15240mm)

    50ft (15240mm)

    50ft (15240mm)

    Normal operating pressure

    50-150bar

    50-150bar

    50-150bar

    Working air volume 

    12.000gal (45400l)

    14.900gal (56.400l)

    15.850gal (60.000l)