dual wireline riser tensioners

RIG/PLANT

PDC No 5 REFERENCE

F3376 REFERENCE DESCRIPTION

Dual Wireline Riser Tensioners ADDITIONAL CODE

SDRL CODE

TOTAL PGS

10 This document contains proprietary and confidential information which belongs to National Oilwell Varco; it is loaned for limited purposes only and remains the property of National Oilwell Varco. Reproduction, in whole or in part; or use of this design or distribution of this information to others is not permitted without the express written consent of National Oilwell Varco. This document is to be returned to National Oilwell Varco upon request and in any event upon completion of the use for which it was loaned. © Copyright National Oilwell Varco - 2011

National Oilwell Varco Norway AS Postboks 401, Lundsiden N-4604 Kristiansand Norway Phone +47 38 19 20 00 Fax +47 38 19 26 04

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DISCIPLINE

CLIENT PO NUMBER

CLIENT DOCUMENT NUMBER

DOCUMENT NUMBER

F3376-Z-SA-001B

REV

0

Dual Wireline Riser Tensioners PDC 5

Technical Description

Document number F3376-Z-SA-001B Revision 0 of 10

TABLE OF CONTENTS

1 INTRODUCTION ............................................................................................................... 4

1.1 Purpose................................................................................................................... 4

2 BRIEF OVERALL DESCRIPTION .................................................................................... 4

3 DETAILED DESCRIPTION ............................................................................................... 5

3.1 Design Features ...................................................................................................... 5

3.2 Technical description .............................................................................................. 5

3.3 Riser Tensioner Cylinder ........................................................................................ 6

3.3.1 Principles of operation ................................................................................. 6

3.4 Air Control Skid ....................................................................................................... 7

3.4.1 Principles of Operation ................................................................................ 7

3.5 Control System / PLC Software description ............................................................ 9

3.5.1 Principle of operation ................................................................................. 10

3.6 System safety devices .......................................................................................... 10


1 INTRODUCTION

1.1 Purpose

The purpose of this procedure is to describe the design and function of the Riser Tensioner System, to give a better understanding of the different component interactions and to act as an introduction to the operating instructions.

2 BRIEF OVERALL DESCRIPTION

The Riser Tensioner System contains: Tensioners: (8 dual units) Control Equipment: (1 unit, Riser Tensioner Air Control Skid) (1 PLC control panel) (1 Remote control panel) Idler Sheaves: (8 dual units) The Riser Tensioner provides positive tension to the marine riser, and compensates for the relative motion between the riser and the drilling rig. The tensioners are combined in pairs, with the units installed diametrically opposite to each other. Each diametrically opposite installed cylinder assembly is piped together and acts as one tensioner unit. This is to avoid any lateral forces in the riser tensioner ring when reducing tension in one unit (bleeding off air for wire cut/slip etc.). The tensioners maintain tension in each support wire, which is connected to the support ring on the marine riser pipe. The wires from the support ring runs over the pivot hinged idler sheaves, via the fixed lead-in sheaves and then around the two double sheave assemblies on each end of the tensioner cylinders. The wires are then attached to the wire rope anchors. When the wire is reeved around the fixed and the movable sheave assemblies on the cylinder, a four - to - one mechanical advantage is obtained. This means that a cylinder stroke of 12.5 feet is transferred to 50 feet of wire rope travel. Tension in the support wires are maintained by hydraulic pressure in the tensioner cylinder. The accumulator pressurises this fluid. A fluid connection block is installed between the accumulator and the cylinder. This block incorporates a hydraulic flow shut-off valve, the National Oilwell Varco riser anti-recoil valve. In the case of wire rope breakage, this valve will close immediately and the cylinder will extend to its full stroke at a very low speed. This is to prevent damage on the cylinder and other equipment. In order to obtain a more or less constant tension in the marine riser pipe, the accumulator air side is connected to an air reservoir. This means that the riser tension unit acts like a hydro-pneumatic spring.


To obtain a centralised control of the tensioners there are an air control unit located in a suitable position. The Drillers Control Panel is interfacing with this unit for the operator to start, operate, monitor or shut down the total system. This unit is hooked up to a pipeline between the air reservoir and the tensioner.

3 DETAILED DESCRIPTION

3.1 Design Features

The Riser Tensioner design utilises high pressure acting on the blind side of the cylinder resulting in a compression loaded cylinder, rather than a tension loaded cylinder. The cylinder type is a plunger cylinder. This cylinder operates without low pressure side. It is thereby the same pressure on the rod side and the piston side of the cylinder piston. Cylinder push is maintained by different area on the two sides of the cylinder piston. This configuration removes the potential risk for leakage across the cylinder piston. The Riser Tensioners are mounted with the rod end up (REU). The benefits with this design is that when all air pressures are relieved on the REU tensioner, the weight of the rod and rod end sheave assembly ensures that the rod retracts back into the cylinder where it is submerged in fluid and protected from corrosion and mechanical damage.

3.2 Technical description

The sheave house for the fixed end includes four sheaves, grooved for 2,5" wire rope, snubber drum and wire rope anchor for each wire rope. The sheave house is equipped with a flange for attachment to rig substructure.

The sheave house for piston rod end consists of two sheaves grooved for 2,5" wire rope. Lubrication to sheave bearings through flexible l hose terminated at suitable position.

The idler sheaves consist of two hinged sheaves grooved for 2,5” wire rope.


3.3 Riser Tensioner Cylinder

The tensioner cylinder is a plunger cylinder, and is fitted one high-pressure accumulator. The piston rods are made from carbon steel material with Ni/Cr layer. The anti recoil valve is mounted between the high-pressure accumulator and the cylinder.

The high-pressure accumulator provides an air-fluid interface between the air pressure vessels and the cylinder.

3.3.1 Principles of operation

The riser tensioners maintain tension in the wire ropes by taking up or paying out rope in response to rig motion. The rope being controlled is reeved over multiple sheaves at the fixed and movable ends of the tensioner, over idler sheaves and connected to the tension ring on the riser. Extension force of the cylinder maintains tension in the wire rope. Pressure in the cylinder maintains the extension force. The extension force (rope tension) is directly proportional to the air pressure magnitude. As upward heave of the rig tends to cause an increase in rope tension, the cylinder rod retracts (thus increasing the length of rope) to maintain the selected rope tension. When the cylinder rod retracts, fluid in the cylinder is shifting through the riser anti-recoil valve and in to the fluid accumulator and air in the accumulator will be compressed into the air pressure vessels. As downward heave of the rig tends to cause a decrease in rope tension, the pressure force in the cylinder causes the cylinder to extend, thus maintaining the selected rope tension. When the cylinder extends, air expands from the air pressure vessels to maintain pressure in the cylinder. The Riser Tensioner is fitted with one high-pressure accumulator. The high-pressure accumulator provides an air-fluid interface between the air pressure vessels and the cylinder. Air pressurises the fluid in the accumulator, and the fluid (at the same pressure) pressurises the cylinder to cause the cylinder extension force. This technique ensures full lubrication and prevents cylinder barrel corrosion from the constant high-pressure air. The piston rods are made from carbon steel material. The cylinders are provided with a position measuring system. By use of this system the operator can monitor the piston rod movement of each individual tensioner cylinder. This system is an important part of the riser anti-recoil system. The riser anti-recoil system also consists of a PLC and a valve assembly mounted between the cylinder and the high-pressure accumulator. The rod end of the cylinder is filled with fluid. As the cylinder rod extends, the fluid flows through the rod piston into the piston side of the cylinder. Retraction of the cylinder rod permits causes the fluid to flow the opposite way, into the rod side of the cylinder. The rod side is thereby constantly filled with fluid. The hydraulic fluid used in the tensioners is a water/glycol-based, fire resistant fluid. The pressure (tension) variation due to compression and expansion of the air is related to the volume of the APV’s. The APV's volumes are selected to provide a variation of


approximately +-4% with heave of 4 meters and the cylinder stroking about its midpoint. It is important to note that the area of concern should only be within the expected heave conditions and not over the complete stroke capacity of the tensioner. During normal operation, no air is vented from the system. An 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 a tensioner system is accomplished at the centralised and common tensioner control panel, located in Drillers Control Room. This panel enables one single operator to start up, set operation, monitor and shut down the complete systems. Only infrequent attention by the operator is required.

3.4 Air Control Skid To obtain a centralised control of the tensioners there is an air control skid, located in a suitable position. By using all the remotely controlled valves on this skid the operator can start, operate, monitor or shut down the total system. This skid is installed in the pipeline between the air reservoir and the tensioner. The air control skid includes all valves, tubing, filter regulator, pressure instruments, cabling and junction boxes required to operate all the Riser Tensioner Units. The air control skid is designed for installation outdoors in harsh offshore environmental conditions. All instrumentation is certified for operation in hazardous area zone II.

3.4.1 Principles of Operation

The air control skid is mounted between the tensioner cylinder and the APV skid. As the platform moves up or down the tensioner cylinders rod retract or extend. The cylinder rod movement causes compressed air to flow through the air control skid. If the cylinder rod extends, compressed air will go from the APV's through the air control skid and into the cylinder, and the opposite way if the tensioner cylinder rod retracts. By opening or closing valves in this control skid, pressure and tension can be adjusted to suit the desired tension. The air control skid can also be used to separate one or all of the tensioner cylinders from the APV’s. All the valves which need to be operated during operation are remotely controlled from the panel located in the drillers control room.


By pressing various buttons on the drillers control panel, signals are sent to the solenoid valve in the air control skid which will open, and allow instrument air to be supplied to the valve actuator, which again will open or close the associated valve. The actuators for the 1" valves that are used are of the spring return type. All the 1" valves are “fail to close”. These valves shall normally be closed. All the 4" valves will remain unchanged, either in open or closed position, if control air pressure or electrical power fails. If this valve is being closed during operation the actual tensioner cylinder will be separated from the APV’s and no compensating can be performed. The air control skid is also provided with one pressure transmitter and one pressure gauge on each side of the 4" main air valve. These pressure transmitters send a 4- 20mA signal to the PLC cabinet and then to the drillers control room where the pressures can be monitored. The pressure gauge mounted next to the pressure transmitter is only for reference. The function of the equipment described above is for controlling the high-pressure air system. The air control skid also has an instrument air system. The instrument air system is furnished with an air pressure regulator and two filters. The instrument air system is for opening or closing the valve actuators, controlled by solenoid valves. Pressure in the APV’s or tension in the cylinders can be adjusted by pressing down one of the below listed push-buttons. The buttons are located on the screen in Drillers Control Room. APV Fill:

By operating this push-button pressure will increase in the APV's as long as pressure in stand-by bottles is higher than pressure in the APV’s. The push-button has a built-in lamp which will light up when the valve is in open position. To close the valve, the same push button must be pressed again and the light will go off. Valve is now closed. The valve is equipped with a spring return actuator and will close if a air or electrical failure should occur.

APV Vent:

By operating this push-button pressure in the APV's will be vented to atmosphere. The push-button has a built-in lamp which will light up when the valve is in open position. To close the valve the same push button must be pressed again and light will go off. The valve is now closed. The valve is equipped with a spring return actuator and will close if an air or electrical failure should occur.

Activate Tension:

Provided that the pressure difference on each side of the main shut-off valve is less than 10 bar the following will happen if this button is operated:

Belonging Bypass valve and main shut-off valve will open and the built in lamps in each push button will light up.

When main shut-off valve is fully open, the bypass valve will close and the light for the bypass will go off.


If the pressure difference on each side of the main shut-off valve is more than 10 bars, only the bypass valve will open. The lamp for the bypass will light up while the lamp for the activate tension will be flashing. When pressure difference on each side of the main shut-off valve is less than 10 bar the activate tension valve will open, and the flashing light will become a continuous light. The main shut-off valve will now stay open independent of any pressure differences in system To close the valve, the same push button must be pressed again and light will go off. The valve is now closed. Actual valve is equipped with a double acting actuator which will ensure that the position of the main shut-off valve remains unchanged even with an electrical or air failure.

Tension Vent: By operating this push button, pressure in the tensioner cylinders will be vented to atmosphere. Depending on the position of the main shut-off valve, pressure in the actual APV’s can also be vented at the same time. The push button has a built-in lamp which will light up when the valve is in the open position. To close the valve the same push button must be pressed again and the light will go off. The valve is now closed. The valve is equipped with a spring return actuator and will close if a air or electrical failure should occur.

Manual Tension increase: This is a manual function only to increase the pressure in the tensioner cylinders directly from the standby APV’s and is intended for service and start-up operations only.

3.5 Control System / PLC Software description

The Tensioning System PLC/Termination Cabinet includes the PLC control logic for the tensioning system. The cabinet is installed in the local instrument room (LIR). This Room is classified as safe by ventilation and will have operating temperature; min. 5 oC and max. 35 oC. The cabinet will be powered with a 230 VAC 60 Hz supply from the Platform UPS-system. Signals on profibus include signals for load calculation purpose to the Rig Management System and MCC Start/Stop/Running/Available signals, air supply pressure alarm(s) and system failure alarm as relevant for the equipment to be controlled.


3.5.1 Principle of operation

The PLC can be compared with a communication central where all the below listed signals will be processed through.

Pressure high alarm

Broken wire alarms from all pressure transmitters

Position of Cylinder rods

Monitoring of piston rod speed. Too high speed indicates that a wire has broken. PLC will send a closing signal to the valves between the cylinder and the accumulator to ensure that fluid flow to cylinder is stopped.

Riser anti-recoil system

3.6 System safety devices 1. Riser Anti-recoil system valve (Flow shut-off valve) This valve is installed between the accumulator and the cylinder port. In the event of

a wire rope breakage, a riser disconnect or riser failure, the valve will restrict the flow to the cylinder and slow it down. Detailed description of it is operation and function is found in Technical description, Anti recoil system, F3376-Z-SA-001A.

2. Bypass valve

This valve is located on top of the accumulator, and is a needle valve integrated with the 3” main air valve. The purpose is to equalise pressure on both sides of main shut-off valve before this valve is moved to open position.

3. Vent valve This valve is located on top of the accumulator. The purpose of this valve is to vent air pressure in accumulator to atmosphere when main shut-off valve is in closed position.

4. Relief Valve on Accumulator. This relief valve is located on top of the accumulator and prevents the air pressure, in

the accumulator from exceeding a preset safe value @ 230 bar. 5. Air Isolation valve. This valve is located on top of the accumulator. The valve makes it possible to isolate

the tensioner from the high-pressure air lines. Normal position during operation is fully open.

dual wireline riser tensioners

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