Installation of large subsea structures: Lowering of suction anchors through the splash zone
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- Institutt for marin teknikk 
In this thesis the dynamic air cushion pressure inside suction anchors during low-ering through the splash zone has been studied. A non-linear continuity equationfor the air mass is used to establish a theoretical model for the dynamic air cush-ion pressure. Buoyancy forces due to dynamic air cushion pressure inside thesuction anchors are calculated until the wave elevation inside the suction anchorreach the top. At impact Wagner s wedge method together with Von Karman swetted length is used to calculate the impact force on the cone-shaped suctionanchor top.Important parameters aﬀecting the dynamic air cushion pressure and thus thebuoyancy forces have been investigated. Force measured in lifting wire duringsplash zone transition of a single suction anchor is used as basis for comparison.The parameters that are investigated are introduction of a time-varying loweringvelocity, changing characteristics of the ventilation hatch outlet and introducingregular and irregular waves. By varying these parameters a best-ﬁt comparisonwith 2.5 [%] deviation between calculations and measurements have been ob-tained.Further, a comparison between theoretical calculations and force measurementsfrom a lifting operation of a subsea structure with suction anchors as foundationhas been performed. The buoyancy forces calculated prior to impact are due tothe dynamic air cushion pressure alone. The total force in the lifting wire forthis phase are corresponding well to the magnitude of the measured force.During this comparison the eﬀect of changing perforation ratio clearly show anincrease of buoyancy forces for decreasing size of ventilation hatch. Includingboth inﬂow and outﬂow of air from the suction anchors when oscillating in largewaves have also been studied. When comparing with force measurements the im-portance of including both inﬂow and outﬂow of air to account for both suctionand compressive pressure has been shown. This lead to negative and positivebuoyancy forces respectively.Long crested waves accounting for wind and swell sea are calculated with theJONSWAP- and Torsethaugen-spectra. By using empirical formulas for the lim-iting case between wind and swell sea it is shown that wind sea is the dominatingpart for the sea state measured during splash zone transition of the suction an-chors.Using Wagner s wedge model together with Von Karman s wetted length for cal-culating impact force on a ﬂat suction anchor top is also performed. Impactforces with impact angles in the range of 5-15 degrees between the wedge-shapedwater and top overestimate the force compared with measurements. A best-ﬁtfor the impact forces with the measurements has been obtained with deadriseangle of 17.5 [deg] in addition to impact forces calculated with a time-varying slamming coeﬃcient.Possible resonant phenomena connected with oscillations of suction anchor andpressure measurements at impact have been studied. Simpliﬁed models for cal-culating A) natural period of oscillating air cushion, B) piston mode of watercolumn and C) coupled piston mode of air cushion and water column has beendeveloped. These have been compared with oscillation-period of pressure mea-surements observed after impact of a single suction anchor. These calculationshave also been compared with oscillations of the suction anchor observed fromvideos of experiment. Case B) proved to give the most similar results with thesuction anchor oscillations while case A) and C) were not in the proximity ofthe observed oscillations. This showed that the air cushion were not causing anyresonant motion.Using a common safety criterion for avoiding slack in lifting wire, the calculationmodel have been used to give a recommendation for perforation ratios for thesuction anchors used on the installation of the subsea structure studied here.Perforation ratios in the range 1-3[%] used on suction anchor with diameters inthe range 3-6[m] proved to be well within the slack-wire limit.When using the theoretical model developed in this thesis a design recommen-dation for using ventilation hatches with a certain height have been made. Alsouniform hatches are advised to ensure as similar conditions as possible for outﬂowand inﬂow of air.