RISER DESIGN

The risers are designed to meet the following design requirements:


• Vortex Induced Vibration

• Equivalent Stress

The methods of the analyses are described in the following subsections.


Vortex Induced Vibration

The allowable span lengths for the vortex induced vibration criteria are calculated based on riser general arrangement drawings and DNV 1981, whereby the reduced velocity is defined as:

Another parameter controlling the dynamic vibration is the stability parameter (KS) defined as:

Based on the calculated stability parameter, the limiting reduced velocity can be obtained from Figure A.3 of DNV 1981. As per DNV 1981, the in-line oscillation of a free span are initiated at lower velocities than those required for the onset of cross flow motion. Therefore, the maximum allowable span length for the in-line motion criterion will automatically satisfy the cross-flow criterion. The equation for reduced velocity (Vr) can be re-arranged as follows:

Combining the two equations and solving for L:

The vortex shedding analysis is performed using in-house spreadsheet files. The spreadsheet calculates the allowable riser span length to avoid the onset of pipeline in-line and cross flow oscillations induced by vortex induced vibration, which complies with the DNV 1981 method. Based on the calculated span length, the riser clamp elevation is then identified such that the clamp elevation spacing is always lower than the riser maximum span length.

Steady current and wave velocity are considered in the riser vortex vibration analysis

Riser analysis have been analyzed using AUTOPIPE software

ORIGINATOR IS BACK

OGA PIPELINE is back!!!!

It's been a long time I didn't write someting in my blog. I'm so sorry to all followers who waiting info about oil and gas pipeline in this blog. To be honest, the year 2010 was a busy and challenge year for me due to handle a big project and also involved in project management.

Normally, few years after you involved in specializing work, you've been forced to jump into management. But, this is depends on work load, either you fully or partially involved. Samething happened to me, which is previously one of my project required additional pipeline design, and I has been selected to be a focal person (Contractor Representative) between design consultant and Contractor(TLO).

The job title still considered as a part of management, but the work scope was small which is Revisit Pipeline Detailed Design (Pipeline Route Analysis, Pipeline Bottom Roughness Analysis, Expansion Analysis, On Bottom Stability Analysis and drawings).

Doing a management work is not easy as what we expect.  I've produce ITB (Invitation to Bid Doc), check the progress report and design report (almost every week), find an info for consultant to to do their job, and manage to complete the job at the agreed date. Management work is something that you play with the time. Same like the time you come to the office, for sure you're not coming after 9am everyday right?.

But, to be honest, I'm still love my job as a Pipeline Engineer, because I love to do calculation and realized I'm specialist. Other than that, currently I'm in progress to pursue my dream, to get the Ir status (Professional Engineer). Part of the requirement are to attend a course, produce experiences report, and interview.

Back to our main topic, after this I'm promised to spent sometimes to update my blog. And I'll start finish the Pipeline Design first before I bring all of you to the other field which is Pipeline Installation.

PIPELINE EXPANSION ANALYSIS

Pipeline expansion analyses are calculated using the strain balance method.

The load such as internal pressure, temperature, friction and some effect on residual lay tension will cause to expand at its free end with includes the stresses and force in the pipe wall.



Analysis Methodology

The axial force (Pr) acting on the pipeline is taken as:-

At a distance x, from the pipeline free end (platform end), the soil frictional force (Pfsoil) preventing this movement is represented by the following equations:-

The pipeline end movement occurs up to a point when the expansion force is equivalent to the soil frictional force. This point is called the virtual anchor point. The x value can be solved to obtain virtual anchor point Lexpand1 (hot end) and Lexpand2 (cold end) from the following equations:-

Subsequently, pipeline end expansion is determined using:-
 
 

Strain occurs in pipeline as shows below:-
 
 

 
 

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