Page 14 - Oil and Gas Simulations
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OCAS NV, BELGIUM

Making the Oil Supply Safer and
More Stable

When the supports or risers from oil platforms go down hundreds of meters, they
experience tremendous stress due to the motion of tides and waves. Rather than send divers
down to check welds, operators can use a new technique called ACFM (alternating current
field measurements) that has been refined with the help of multiphysics simulation.

by FILIP VAN DEN ABEELE AND PATRICK GOES, OCAS NV

Erecting offshore oil platforms is very           OCAS NV, which is a market-oriented          away from the deepest part of the weld
expensive, with costs often exceeding half        research center that provides innovative     and concentrates it near the ends of the
a billion dollars. Because of this, there         solutions and services to metal-processing   defect. As a result, one magnetic field
is a strong emphasis on the engineering           companies worldwide. Located in              contains information about the defect’s
effort needed to make sure they not               Belgium, OCAS is a joint-venture of the      depth and the other measures its length.
only are safe for operators, but have             Flemish Region and ArcelorMittal. Of our     	 This method is superior because, in
a reasonable service lifetime as well.            140 employees, roughly 15 percent are        contrast to other methods, it provides
Such semisubmersible platforms often              involved in simulation using a wide variety  information on crack shape and size as
use spars made of welded sections that            of codes, including some dedicated tools     well as position. Once the size of a crack
descend into the sea for stability and            for sheet metal forming and deepdrawing      is known, its growth can be monitored
welded risers to bring oil and gas to the         processes. For projects requiring a higher   over time, leading to an accurate
surface. Simulation is helping engineers          degree of accuracy, in particular projects   prediction of a structure’s useful lifetime.
find mitigation measures that extend              involving electromagnetics, fluid-structure  In addition, the ACFM method is able to
the lifetime of such structures as well as        interaction, welding processes that involve  detect cracks through several millimeters
refine nondestructive test methods that           multiphysics couplings, and heat transfer    of nonconductive coating, allowing
can detect and monitor the development            analysis, we turn to simulation with the     inspections to be carried out through
of cracks in welds, even at ocean depths          COMSOL Multiphysics® software.               paint, epoxy, oxide layers, fire-protection
far below what divers can reach.                                                               coatings, and even marine growth.
                                                  èCRACK DETECTION AT
èEVERY WELD HAS                                   GREAT DEPTHS                                 èSIMULATING THE ACFM
IMPERFECTIONS                                                                                  TECHNIQUE
                                                  When determining the lifetime of offshore
The challenge begins with the fact that           oil platforms, the challenge is performing   Understanding and interpreting the
the risers are made of tubular sections           crack detection and predicting failure       information in the measured magnetic
that are welded into a single piece. It is        modes in underwater welds—a task that        fields is where simulation comes into play.
important to realize that no weld is perfect;     is further complicated by vortex-induced     Using the AC/DC Module in combination
even in a virgin pipe or riser there will always  vibration (VIV) and the inevitable fatigue   with COMSOL Multiphysics, we simulated
be some defects. These include internal           damage of girth welds. Visual inspections    the ACFM technique and studied the effect
cracks (which are sharp and thus detrimental      looking for early cracks are not practical,  of crack size on the measured magnetic
to fatigue resistance), lack of fusion, voids,    especially since divers are limited to       components.
and porosity. In addition, a weld’s geometry      depths of only several hundred meters.       	 We created a model geometry where
gives rise to stress concentrations, again        Over the years, a number of alternate        a cylindrical air box confines the model
reducing the fatigue life.                        methods for nondestructive testing have      boundaries and the crack consists of an
	 Then add the fact that it’s not just            been developed, but one very interesting     ellipsoid. The Magnetic Fields interface
storms that put enormous loads on these           emerging technology is ACFM (alternating     was used to model a current-carrying wire
welds—even moderate sea currents can              current field measurements).                 and to inject an almost uniform current
create oscillations that lead to cyclic           	 In ACFM, a test engineer injects a         distribution into the thin steel plate.
loading, where tiny inherent defects              uniform current in a weld, thus inducing     	 With this model, we were able to gain
start to grow, making them vulnerable to          an eddy current in the conductive            a good understanding of the underlying
fatigue failure over time. Detecting welds        material. The presence of flaws in           principles and arrive at some formulas
that are developing dangerous fatigue             the weld cause a change in the eddy          and guidelines. Using ACFM, at the spot
cracking is crucial so that mitigation            current, which can then be measured          where the measurements are taken, the
measures can be taken to strengthen these         by two sensor coils. The coils record the    tangential magnetic field component Bx
weak spots or to determine when it’s time         magnetic fields that are parallel and        contains information about the depth of
to decommission a drilling platform.              normal to the flaws. The presence of         a possible crack, and the normal magnetic
	 This is a large part of our work at             a surface discontinuity diverts current      field component Bz reveals its length.

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