E01 OrcaLay Plus Lateral Restraint

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Pipelay and Recovery: E01 OrcaLay Plus Lateral Restraint Page 1 of 3

E01 OrcaLay style model with additional

lateral restraint

X

Z

10 m

OrcaFlex 9 .3a48: C02.01 OrcaLay Model plus lateral re straint.sim (modified 15:27 on 08/07/2009 by OrcaFlex 9.3a47) (azimuth=270; elevation=0)Statics Complete

This model is based on the simple method used in OrcaLay for multiple static analyses of lay

configurations. The reactions between the lay pipe and the stinger rollers are generated by Tethertype Link objects, which have strength in tension only. This has the advantage of speed and

simplicity over more detailed treatments, but does not allow for movement at the tensioner. Ittherefore represents the common situation where the pipe is held fixed during welding operations.

The OrcaLay model is two-dimensional and consequently has tethers in the vertical plane only.

This model extends the idea by adding tethers to provide lateral restraint as well as vertical andcan therefore deal with three-dimensional motions due to out-of-plane environmental conditions.

This type of model is not suitable for modelling the situation where the tensioner allows axialmovement of the pipe because the roller locations (Link connections) would move with the pipewhich is unrealistic.

The method allows for lift off the rollers, but in setting up the geometry implicitly assumes thepipe lies on the stinger over its entire length. Any significant departure from this condition will

lead to some inaccuracy in the geometry. Nonetheless, it does provide a simple and robust

technique for modelling many pipelay operations.

1. The ModelThe active parts of the model consist of a Lay Vessel, the pipe being laid and a set of roller boxes.These are best seen with the model browser set to View by Groups.

Each roller box is represented by three Tethers and a roller shape. The latter has no stiffness andserves purely as a visual indicator of the roller position. The physical restraint is provided by the


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three links. These are positioned so that they become taut when the pipe comes into contact with

a roller. Because tethers have no strength in compression, they limit movement in one sense ofdirection. The tethers are extremely long so that the tether tension has a negligibly small

component along the axis of the pipe. In principle the two lateral tethers could be replaced by a

single spring/damper with appropriate non-linear properties, but the two-tether approach isconceptually simpler and, as a result of the symmetry of the system, easy to build.

The lay pipe is built in at its connection to the vessel with infinite stiffness and declination 90.The target segment length is set to 2 m over upper section of the line and 5m in the sagbend and

seabed sections. Note that the lengths of the sections that lie over the stinger have been set tomatch the roller spacing allowing for the stinger radius. The length of the first line section is setto the distance between the connection point on the vessel and the first roller.

The vessel is an OrcaFlex default vessel which is treated as fixed during the static calculation.The lay pipe and its supports are placed on the vessels axis of symmetry but can easily be

repositioned by using the Move facility in the model browser.

The model also contains a 6D Buoy placed at the centre of the roller radius and used to positionthe solids and links representing the roller boxes. The procedure to add an extra roller to thestinger is as follows:

1. Duplicate one of the groups representing the roller boxes,2. Change the connection for the 6D Buoy from Vessel to Free,3. Change all the Vessel connections in the new group to 6D Buoy4. Change Rotation2 for the Buoy to position the new roller box along the stinger radius5. Reconnect the new Links, Shape and finally the 6D Buoy to the vessel.6. Adjust the line length to achieve the desired top tension, as described below.

2. Initial Set UpIn general a pipelay configuration will be set up to achieve a specific level of top tension. InOrcaFlex this is done via the Line Setup Wizard which is accessed through the Calculationmenu on the OrcaFlex main window. This allows either the lines length or its anchor position to

be altered to achieve a target condition. In this model the geometry of the upper sections of theline must not be changed, as this will affect the link connection locations, so the Wizard was used

to alter the anchor position to achieve a top tension of 150 kN in still water.

The environmental conditions were then set as follows:

Slab Current 0.5 m/s 20 off the bow of the vessel.Jonswap wave, Hs 1m, Tz 6s, 45 off the bow of the vessel. The wave preview facility was usedto identify the largest event in the first three hours of the simulation and the Simulation Time

Origin was set accordingly so that a 150 s simulation includes the largest wave. The wavehistory applied can be seen by clicking on the View Profile button on the Waves Preview pageof the environment data form.


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3. ResultsLoading the simulation file also loads the default workspace so results summaries areautomatically generated. The tension range graph (top left) shows that the maximum tension at

the tensioner is about 220kN, which is close to 1.5 times the nominal tensioner load. It alsoshows that the entire pipe remains in tension throughout the simulation. The time history ofEffective Tension at End A (top right) shows how the tensioner load varies. The range graph of

Maximum von Mises stress (bottom left) shows that peak stresses are below 400 MPa at all times.

Loading the workspace file OrcaLay Model plus lateral restraint Roller4 Loads.wrk shows what

is happening at the fourth roller box. The tension in the upper link (bottom right), representingcontact between the laypipe and the lower roller, remains positive throughout showing that the

pipe never lifts off the roller. Contact with the starboard link and the pipe is much more chaotic,

with the pipe moving from side to side throughout the simulation as a result of the out-of-planeenvironmental conditions. The time histories show that there is frequent intermittent contact with

the starboard link, but no contact with the port link.

Finally, note that the analysis uses a constant normal drag coefficient of 1.2 for the lay pipe. Thissimple approach is likely to be conservative and the use of a variable drag coefficient based on

the local instantaneous Reynolds number would probably reduce stress levels.

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E01 OrcaLay Plus Lateral Restraint