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378 Chapter 10 Marine Installations 19 Figure 9.7 An induced water pocket initiates the submersion of the pipe and as the Figure 7 An induced water pocket initiates the submersion of the pipe and, as the pocket pocket enlarges it allows the submerging to gradually progress forward Figure 9.7 AennlarIgnedsu, itcaelldowWs tahteesruPbmoecrkgiengt Itno igtriadtueasllythpreogSreusbs mforewrasridon of the Pipe and as the Pocket Enlarges it Allows the Submerged to potential risk during the submersion operation is that when the pipe sinking occurs too Gradually Progress Forward A potential risk during the submersion operation is that, when the pipe sinking uickly the bending of the pipe between the water filled and air filled portions may be occurs too quickly, the bending of the pipe between the water-filled and air-filled harp enough so as to risk the development of a kink, a form of localized pipe buckling. portions may be sharp enough to risk the development of a kink, a form of localized s a pipe is bent, its circumferential cross-section at the point of bending becomes Since the formation of a kink impedes the submersion process and it can also pipe buckling. As a pipe is bent, its circumferential cross-section at the point of creasingly ovalized. This ovalization reduces the pipe’s bending moment of inertia compromise the pipe’s flow capacity and its structural integrity – in particular, the bending becomes increasingly ovalized. This ovalization reduces the pipe’s bending us depcirpeea’sinregstihsetabnecnedtiongcoflolarpces.eUupnodnerseuxfftiecrineanltporveaslsiuzaretio–nitaishiensgse,notirakl tihnakt, dcaunrinfgorm moment of inertia, thus decreasing the bending force. Upon sufficient ovalization, a t the ptohientsuobfmearxsimonumprobceensdsinthge; abnenevdiengtothfatthealspoiplelaindesbtoealimsuitdedetnoraenduecxttieontothfathte hinge or kink can form at the point of maximum bending an event that also leads to endingwifllonrcoet.rSisiknctheethfoermfoarmtioantiofnaolfoacakliznekdimkipnekd.eTshethpeipsuebbmeenrdsiinognrpardoiucessastawnhdicithcan a sudden reduction of the bending force. Since the formation of a kink impedes the buckling is in risk is given by the following expression: lso compromise the pipe’s flow capacity and its structural integrity – in particular, the submersion process and can also compromise the pipe’s flow capacity and structural ipe’s resistance to collapse under external pressure – it is essential that during the integrity – in parti(cular, th)e pipe’s resistance to collapse under external pressure – DR−1 ubmersion process the bending of the pipeline be limited to an extent that will not risk R =D (Eq.7) it is essentibal thato during submersion the bending of the pipeline be limited to an 1.12 e formation of a localized kink. The pipe bending radius at which buckling is in risk is extent that will not risk the formation of a localized kink. The pipe bending radius at iven by the following expression: which buckling is in risk is given by the following expression: Where Rb = buckling radius due to bending of the pipe, in (7) Do = outside pipe diameter, in DR 1 DR = pipe diameter ratio = average outside diameter divided Rb Do 1.12 (Eq.7) by minimum wall thickness, dimensionless JWanhseorne’sRbre=labtieondsihnipg rfoardiduesteartmwinhaictihonbuocfkmliingimcuamn beucinkilitniagterda,diuns (Eq. 7) was WHERE R = bending radius at which buckling can be initiated, in derived oDn t=heoubtassidiseopfiapemdaiaxmimeutmer,piinpe deflection (ovalization) due to bending of o b D = outside pipe diameter, in the pipe of 7% and a maximum strain limit in the pipe wall of 5%. In actuality, the o DR = pipe diameter ratio = average outside diameter divided DR = pipe diameter ratio = average outside diameter divided by minimum wall thickness, dimensionless short term strain limit for modern polyethylene pipe materials is somewhat higher, by minimum wall thickness, dimensionless on the order of 7-10%. Further, we know that these pipe materials are capable of Janson’s relationship for determination of minimum buckling radius (Eq. 7) was long term service at higher degrees of ovalization in buried pipe installations. o ensure a sufficient safety factor the practice is to limit the bending radius to about two derived on the basis of a maximum pipe deflection (ovalization) due to bending of (Please see the chapter on Pipeline Design) As a result, the values presented in mes that predicted by the above equation. Table 9.3 lists the multipliers (the ratio the pipe of 7% and a maximum strain limit in the pipe wall of 5%. In actuality, the Table 9.3 are considered conservative guidelines for the short term bending R-1)/1.12 in Equation 7) for the estimating of a pipe’s safe minimum bending radius. short term strain limit for modern polyethylene pipe materials is somewhat higher, radius of polyethylene pipe during submersion of most marine pipelines. The hese multipliers incorporate a 2/1 “safety factor”. on the order of 7-10%. Further, we know that these pipe materials are capable of designer may want to utilize a higher minimum bending radius to compensate for long-term service at higher degrees of ovalization in buried pipe installations. additional factors such as extremely strong currents, tidal activity, prevailing marine(Plterasffeicr,eferer qtouCehnacpyteorf6boaflltahsist Hplancdebmooekn.t),Aosraortehseurlti,ntshteavllaltuioens pvraersieanbteledsin assocTiatbeled3wairtehcaonsspideecriefidccinonsstaerllvaattioivne.guidelines for the short-term bending radius of polyethylene pipe during submersion of most marine pipelines. The designer may 17 A q A n h b p h g T i D TPDF Image | Marine Installations PE
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