Pre installation of offshore pipelines

INTRODUCTION
Offshore pipelines are used for transportation of fluid from offshore fields to the shore. During the last decade, several major pipeline projects have been completed and are under construction several parts of the world.
Pipelines are built up by welding individual pipes into a continuous line. Quality of all welded joints is thoroughly inspected by X ray method. There after the pipe line is submerged into water and lowered onto the sea by selecting laying method one of which is the S-lay method.
S lay takes its name from the suspended shape of the pipe at the end of the barge, which lays in a gentle ‘S’ from the stringer to the sea bed. Usually a lay barge like the LB200 shown in the figure below is used to lay the pipeline.
This report talks about the step-by-step procedure of joining the pipeline aboard the lay barge LB200. A complete pipe handling system is installed inclusive two (2) double joint fabrication buildings located at the main deck port and starboard with transfer facilities to the main firing line. Additional outfitting facilities include welding, tensioning and stern ramp equipment in support of variable pipe lay activity up to 1727 in diameter in deep water.

ROUTE SELECTION
The starting point for the best route is a straight line. However very few pipelines go in straight line and there are numerous factors which lead us away from the straight line.
1. The oil industry shares the sea bed with the telecommunication industry. With the advent of sub sea fibers optics for international phone calls etc there are many cables currently being installed.
2. Existing pipelines are preferably crossed perpendicularly, while the minimum angle being 30 degree. The reason for this being that any shallower angle of approach would lead to a long and extensive crossing with a greater amount of protection needed over the area. With anchored lay barge like LB200, the anchoring procedure will be difficult at angles less than 30 degree.
3. There is a minimum radius for the curve that the lay barge can achieve. Typically this is 1 km radius for small pipelines and 2 km for large pipelines.
4. There are restricted areas where military operations are carried out and where dredging is carried out. Also sometimes national boundaries are avoided due to many reasons.
5. There may be environmental pressures to avoid fishing grounds, sites of special scientific interest and special areas of conservations.
6. Rock outcrops are avoided to permit trenching, which improves stability of the pipelines. Damage can occur to pipelines laid directly on rock so a layer of gravel is laid between bedding.







WALL THICKNESS
The principle for sizing force wall thickness is to make the wall thick enough to contain the maximum allowable operating pressure. This sets a minimum value for the wall thickness. There are many factors to take into account which may lead to the selection of a thicker wall. They are
1. In order to account for the internal pressure.
2. The wall thickness design must allow for corrosion for all manufacturing tolerance on the thickness of the steel.
3. The pipe must resist bending due to self weight or environmental loading.
4. It must resist running buckle. This is an effect where a single imperfection can allow hydrostatic collapse to start and to run both ways along the pipe until it is all flattened or runs into shallow water. The material needs to be strong enough to resist this.
5. In some cases the pipeline may need a thicker wall to increase its self weight to ensure that it remains stable on the sea bed.
The wall thickness needed for pressure containment is found using the following equation
tmin = [{(pe-pi).D/{2Aσyα}] + Ca
where,
tmin= minimum wall thickness
pi and pe = internal and external pressure
D= outside diameter
A= design factor (.72 for pipelines)
σy=specific minimum yield stress for the material
α = manufacturing tolerance on wall thickness (0.95 for welded)
Ca= corrosion allowance, typically 3 to 6 mm.
Having found the minimum thickness, the nominal thickness is found as the next standard pipe thickness above this (taken from API 5L or ISO 3183)







GENERAL ARRANGEMENT ON THE LAY BARGE
The lay barge operations can be divided into two main sections
The double joint line
There are two double joint located port and starboard side on the main deck with pipe transfer capabilities into forward conveyor houses for optional distribution onto main line pipe welding station. Each double joint facility is outfitted with handling equipment inclusive of- electro hydraulic track conveyors, pipe transfer trolley, pipe end preparation station, one line up station with internal clamp, three external, one internal welding station and one X-ray station.
Main line
the main line equipment and facilities include the main line conveyor system, line up station with internal clamp , 4 welding stations with automatic and semi automatic weld equipment, NDT testing station and 2 field joint coating station, 4 tensioners and a central stern ramp.


INITIAL PREPARATION
Leak test
Individual pipes have to under go a leak test where the ends of the pipes are closed and filled with water as in figure
Any leak if found, the pipe is rejected
Joint preparation
The operation on the lay barge begins with the joint preparations
The end preparation machine is used to prepare a special joint design on the pipe ends and is an essential part of the welding station. The joint design for the submerged arc welding is a V beveled on both the inside (root) and the outside (face) of the weld joint.
The preparation of the pipe provides uniform joint design; dimension control needed for automatic welding and provides for excellent cleaning in advance of cooling as in figure.
The end preparation machine is built in 2 main sections, the clamping section and the rotating cutting section.
The clamping section is built to be inserted into the pipe where two sets of clamping mechanism are hydraulically expanded radiall outwards against the inside surface of the pipe. This operation clamps the machine in the pipe, rounds the pipe in case of ovality and aligns the axis of the cutting machine with the pipe axis so that the machine cut out on the pipe end is normal to the pipe axis.
Induction heating
Also before welding the pipes are usually preheated using induction units. The induction type uses rapid alternating electric field to induce eddy currents in the pipe wall, which gives a uniform heat throughout the wall of the pipe. The preheating causes the evaporation of any moisture near the weld site and allows a slower cool down rate after the welding. This reduces the problems associated with rapid cooling, particularly hydrogen embitterment. Commonly the preheating temperature is between 60-140 degree centigrade.




THE DOUBLE JOINT
The double joint utilizes the submerged arc welding process in the double joint line. Here the two 12m pipes are welded to form 24m pipes. Each station consists of one powered turning roll and one idler roll. As mentioned earlier the double joint consists of 4 welding stations and the procedure is as follows
Initial root pass
At the first welding station the two 12m pipes are brought into alignment and clamped with a special double joint type line in clamp here the initial root pass is given. Now the 24m pipe moves to the second welding station. The figure below shows the internal clamp

Internal welding station
Here the weld pass is made on the inside of the pipe. The welding head mounted on the end of a boom and carriage assembly, is inserted into a pipe by an operator. When the weld head is in position at the weld joint the pipe is rotated by turning the rolls and the welding begins. The welding arc is maintained near the bottom dead centre of the pipe. A vacuum flux recovery system is included as a part of the internal welding equipment.

External welding station
The first weld pass together with the outside hot pass are welded at the stations 3 and 4. When the pipe is positioned to the proper location, and elevated onto the turning rolls, a floating weld head assembly is lowered into position on the pipe surface on the welded joint. The weld head rides the pipe surface on 4 small wheels and the floating action accommodates small movements on the pipe joint during rotation. This welding equipment also incorporates a vacuum flux recovery and filtering system.
Double joint testing
After the welding process the joint is transferred to the NDT i.e. the non destructive testing station. The two methods of welding employed on the lay barge are the X-ray method and the ultrasonic method, choice of method being the clients.
In the X- ray method X ray’s are produced by high voltage X ray machine. The x rays are placed close to the material to be inspected. The rays pass through the material and are captured on film, which is wound around the pipe. The film is then processed in darkroom situated there and the image is obtained as a series of grey shades between black and white.
While the film is being inspected the double joint is in stand by position. If the weld is rejected, the double joint is transferred to the reject line for cut out and recycles.
If accepted, the double joint is inserted in the line up, from starboard and port station. The double joint is inserted in the main line as require by production.

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THE MAIN LINE
The main line consists of 6 welding stations, 2 coating stations, line up station with internal clamp and a main line conveyor system the procedures at each stations is as follows
Welding station 0
The pipeline internal equipment is maneuvered from station zero. The buckle detector and the stop crawler are inserted at this station into the double joint. The movement of these equipment along with the X ray motion is controlled by the winch permanently installed at station zero.
Welding station 1
The pipe (24m) from the line up is put on the main line. The internal clamp is inserted as shown in figure
The pipe ends are brought together, aligned and butted tightly together. The clamp facing feature hydraulically separates the two joints to the pre set root opening after which the back up ring is expanded radially outwards against the inside pipe surface at the weld joint. After the root pass is completed, the copper back up ring is retraced from the weld joint, the space mechanism is retraced to its original position, and the clamping piston retraced.
As in figure, the weld repair team, interprets the film and estimates the position of the default. The weld if ground tills the defect (manually) and the grove filled again by series of weld. The operation is manual and not automated. After this the weld is shot again and the inspection is done gain. If the weld is accepted the procedure activities resume else the weld is rejected.
If the weld is rejected the pipe is cut out i.e. the pipes joined before it is removed. The barge moves back till the pipe is at WS1 and the whole process starts again.

Welding station 6 and 7
Pipe which is shop coated with concrete has a cut back at the field joint for welding. The cut back in the concrete leaves a space to be filled with anti corrosion coating till the concrete level which is done at station 7 and 8. The various steps are as shown in figure.





The pipe mastic station located midship. Aft on the work deck, supplies hot melted mastic to the field joint as they pass from the inspection station to the stern ramp. This mastic provides protection against corrosion and mechanical damage at the field joints and helps control the specific gravity of the pipe by adding weight.
The mastic station is a heating plant, which consists of two mastic chopper, two lower conveyor, two transfer bins and six mastic kettles.
The mastic chopper contains a rotating drum with hammers to crush the mastic if it is in the solid form. The lower conveyor has a chute with a moving chain which transports the chopped mastic to the transfer bin. The bin stores the chopped mastic until the kettle is ready for refilling. The bins are equipped with sliding doors used to direct the chopped mastic to the mastic kettle storage hopper. The kettle melts the chopped mastic and stores it until it is used to fill the transfer pot. Each transfer pot (two in number) is oil filled and electrically heated to maintain the mastic at the correct temperature. The mastic so obtained is used to fill the field joints.
BARGE MOVEMENT
The lay barge is equipped with 14 point mooring system which is used to control the barge movement during pipe laying operation in such a manner that the pipe is laid on the sea bed within the specified corridor
The mooring system must hold the barge in position and stationary during pipe welding operations and generate the required barge movement. In addition the mooring system must provide the reaction to balance the pull of the tension machines which may exert forces in excess of 340MT.
As in figure to move forward the barge track itself on the bow anchors lines while releasing the stern anchor cables. The barge moves under the pipe 24 m at a time, then another add on cycle starts at WS1.
At good conditions the barge lays 6.09 km of pipeline in 24 hours.

Anchor resetting
To reset the anchor, the tug lifts the anchor from the sea bed by means of a pennant line. The barge retracts the anchor line by pulling it with its anchor winches. The anchor handling tug re extends the lines at the desired direction and length and sets the anchor on the bottom.
TENSIONERS
The lay barge is loaded with a numerous length of straight pipe which are welded one by one board to the end of the pipe which are being fed into the sea. As more and more pipe is fed of the back of the ship, the weight of the unsupported pipe grows. The pull down on the end is called over bend.
The greater the depth of the water and the bigger the pipe. Furthermore the vessel will be pulling in the direction of the lay. The weight of the suspended pipe and tension imparted by the relative movement of the vessel away from the pipe laying on the sea bed will tend to pull the pipe towards the sea bed and off the ship. This is prevented by the use of tensioners.


the tensioner has or must have the capacity to match the tension trying to pull the pipe of the vessel or we can say that to prevent the pipe moving away from the ship or lay barge in an uncontrolled manner an equal an opposite pull has to be exerted by the tensioner
This gives it buoyancy. The pipe tensioning system aboard the LB200, is designed to automatically maintain a constant tension on the pipe string during operations. Tension on the pipe is required to prevent damage to the length of the pipe suspended between the barge and the sea floor. The amount of tension required depends on the water depth, pipe diameter, wall thickness and weight, and can be automatically maintained between pre set limit at all times once the required tension value is set.
Each pipe tensioner consists of an upper and lower track, supported by a frame assembly. Pneumatically actuated rollers within the track loops apply squeeze forces on the track, which in turn grip the pipe.
This automatic system compares the sum of the measured tension with the desired tension, the control system, the control system causes hydraulic pumps to haul in or pay out pipe as required until the actual tension equal the desired tension dead band controls allow selection of a range of tension within a given band instead of a single tension point. Each selection maintains within finely adjustable limit without constant correction.
The pipe tensioners are designed to allow passage of irregular coated surface and to absorb pipe flexing while maintaining constant grip and tension. The “soft” track for +- 30mm variations in the pipe coating thickness and +- 50mm pipe curvature while providing enough stiffness to keep traction pads from pressing into the connecting pipe joint and the hot weld surface. The track suspension system does not support the pipe, which actually floats freely to compensate for vertical misalignment.
The traction pad mounting and cross-tie configuration maintaining an equal four point distribution of clamping forces on the pipe thereby imposing lower stresses on both pipe and concrete.





STERN RAMP
The stern ramp functions to guide the pipe string over the stern of the vessel into the sea and onto the sea bottom. The ramp is attached to the vessel at the port and starboard carriage of the stern ramp retraction system and through an intermediate support by an ‘A’ as in the figure below.
The ramp is made-up of three hinged sections with a total length of 120m and overall width of 6m.The roller guide way on the stern ramp on both the side a rigid curved support in the over bend of the pipe as shown in the figure below
The ramp roller system comprises of the horizontal roller box assembly, 20 cable guide rollers. 2 cantilever track assemblies, 3 pairs of vertical rollers and a tip roller as shown in figure below



PIPE ABANDONMENT AND RECOVERY SYSTEM
It is necessary during pipeline installation works to be able to lower the pipe string to the seabed.. This capability is required at the termination of the pipeline, junction with a second pipeline or when deteriorating sea/weather conditions dictate, so as to prevent damage to the pipeline and to safely recover the abandoned line in order to continue the pipe lay operation.

Prior to abandonment, the end of the pipe is sealed by welding an Abandonment/Emergency head to the pipe. The pipe is then lowered to the sea floor. During the process of lowering the pipe and its subsequent recovery, it is necessary to maintain the tension on the pipe string that is normally exerted by the tensioners. To meet these requirements, the Abandonment and Recovery (A&R) winch is used. When the pipeline is to be abandoned, the pipe tension is transferred from the tensioners to the A&R winch, which then controls the pipe tension as the barge moves out from under the pipes string. Subsequently, when pipe laying operations are to be resumed, the A&R winch is used to recover the pipe. The figure shows the top and side view of the A&W winch on LB200



The initiation is laid into the water with the help of a tug boat. The initiation head is connected to the tug via a cable.The tug helps pull the pipeline to the starting point after which the laying procedure proceeds as explained earlier. Due to the large amount of force coming on the head the material of the head is made up of high carbon steel.
Similarly the last joint on the pipeline is the lay down head.The lay down head as shown in the figure below
Is connected to the A&R cable. the force imparted by the tensioner is transferred to the cable which slowly lays the pipe onto the sea bed. Once the pipe end is at the target the following steps are done
1. after the pipe is put in the right position the tug comes to the surface byou to lift the cable ( A&R cable)



2. the cable is lifted and the A&R cable disconnected
18
The heads contain valves which are connected by pipes to obtain or deliver oil.
Pigging
.Before the commencement of transportation of oil the cleaning of the pipe is done once by passing a pig through the pipe. Pig is a device containing metal wires as brushes on the circumfrerence to remove wax rust etc accumulated on the internal of the pipe after the welding process.a typical pig is given below

CONCLUSION
S lay method is the most widely accepted method for laying pipelines at high depth. The LB200 has a world record of laying 6.09 km in 24 hrs. The search for faster methods needed for laying pipelines is still on . The problem arising due to bad weather etc. are still being researched upon.




JOURNALS AND REFERENCES
1. Journal of petroleum technology .
2. Trevor Jee Associates- Overview of Pipeline Engineering
3. K Karal- Offshore pipelines
4. Stolt Offshore- Operational manual LB200