Polaganje podmorskih cjevovoda u obalnom području

Hidrodinamičko opterećenje valova i morske struje te savijanje uslijed zakrivljenosti morskog dna dva su bitna problema kojima u projektu podmorskih cjevovoda za transport nafte i plina treba posvetiti nužnu pažnju u obalnom području. U članku je opisan metodološki postupak dimenzioniranja i provjere stabilnosti čeličnog, betonom obloženog podmorskog cjevovoda, slobodno položenog na zakrivljeno morsko dno. Ravnotežni položaj savinutog cjevovoda koji je izložen radnom, gravitacijskom i hidrodinamičkom opterećenju, određen je na nelinearnom strukturnom modelu konačnih elemenata. Model uključuje geometrijsku nelinearnost uslijed velikih pomaka i nelinearnost uslijed kontaktnog problem između cjevovoda i morskog dna. Ilustrativnim primjerom prikazan je način definiranja projektnog stanja mora i dana je usporedba bitnih rezultata dobivenih vremenskom simulacijom s rezultatima važećih propisa

ADDED RESISTANCE IN WAVES OF INTACT AND DAMAGED SHIP IN THE ADRIATIC SEA

In this paper the ship added resistance in regular head waves and at different sea states in a certain frequency range at different forward speeds was calculated. Calculations were conducted using hydrodynamic software. Since after a maritime accident a damaged ship often has to be removed from the place where the accident occurred, the seakeeping characteristics of the damaged ship were also calculated. The response of the damaged ship was simulated using two models: damage simulated as an increase in the ship displacement mass and as a flooded tank within the midship area. Calculations are based on the linear potential flow theory and added resistance was determined by the wave drift force as the second order wave load. The quadratic transfer function QTF which describes low-frequency second order wave loads was approximated by its zeroth term only, i.e. by the drift load at incoming wave frequencies. The mean added resistance values of the intact and the damaged ship were compared. The calculation results show that added resistance in waves of the damaged ship with a flooded tank differs slightly from the added resistance in waves of the damaged ship with an increased displacement

Evaluation of the added resistance and ship motions coupled with sloshing using potential flow theory

Ship added resistance is a steady force of the second order which depends on the ship motions and wave diffraction in short waves. To predict the impact of liquid motions inside the ship hull, a flooded tank in the midship area is generated and the ship response to regular waves is calculated. Currently available mathematical models and numerical tools are used to enable new insights into the sloshing effects of a relatively large free surface of flooded water inside the ship hull. The liquid inside the tank is first considered as a solid body in order to make a distinction between the hydrostatic and the hydrodynamic effect on global ship motions. Source formulation panel method is used and wave loads acting on the ship hull are determined using the near-field formulation. Based on the potential flow theory, the influence of sloshing on the ship motions and added resistance is evaluated to get an insight into this complex hydrodynamic problem. The calculated results are compared with the available experimental data from the literature

Polu-spregnuta analiza dinamičkog odziva plutajućeg tijela i sidrenih linija pritegnutih pučinskih platformi

Matematički model nespregnutog dinamičkog odziva plutajućeg tijela i sidrenih linija pritegnutih pučinskih platformi (pritegnute plutajuće platforme, plutajuće pučinske vjetroturbine) unaprijeđen je uzimajući u obzir nelinearnu povratnu krutost kao posljedicu utjecaja sidrenih linija na dinamiku plutajućeg tijela. Osim nelinearne povratne krutosti model na jednostavan način uzima u obzir i inerciju i viskozno prigušenje prednapregnutih sidrenih linija što rezultira računalno efikasnim poluspregnutim matematičkim modelom. Točnost modela ocjenjena je na primjeru ISSC TLP-s usidrenog u moru srednje dubine usporedbom s potpuno spregnutim modelom konačnih elementa (teorija štapa) koji uzima u obzir deformacije prednapregnutih sidrenih linija. Analiza točnosti modela uključuje analizu statičkih pomaka i slobodnog njihanja plutajućeg tijela. Simulacija u vremenskoj domeni provedena je preslikavanjem linearnih hidrodinamičkih sila iz frekvencijske u vremensku domenu pomoću Cumminsovog konvolucijskog integrala, pri čemu je hidrodinamička reakcija određena programom HYDROSTAR. Neki važniji rezultati i zaključci navedeni su u radu

Integralni model dinamičkog odziva pučinskog plutajućeg proizvodnog objekta

U ovom radu razvijen je integralni model za proračun dinamičkog odziva pučinskog plutajućeg proizvodnog objekta (eng. Floating Production Storage and Off-loading). Navedeni objekt često je usidren na velikim dubinama te služi za crpljenje, rafiniranje, skladištenje i prekrcavanje nafte i plina. Integralni model opisuje gibanje pomorskog objekta u sprezi s gibanjem sidrenih linija i proizvodnih podizača (eng. marine riser). Model se rješava u vremenskoj domeni zbog izrazito nelinearnih karakteristika. Drugo poglavlje opisuje dinamiku sidrene linije i proizvodnog podizača. Gibanje ovih elemenata je u potpunosti trodimenzionalno te se razmatraju veliki pomaci. Navedeni problem se rješava metodom konačnih elemenata. U obzir se uzimaju inercijske, prigušne i povratne sile. Sile opterećenja sadrže hidrostatski tlak okolnog fluida te sile uslijed gibanja fluida. Sa stanovišta uzdužne deformacije materijal sidrene linije ili proizvodnog podizača može biti nelinearan. Pretpostavljaju se male savojne deformacije odnosno linearan odnos moment-deformacije. Visoki iznos istezanja uzima se u obzir kod određivanja svih navedenih sila. U trećem poglavlju opisana je oscilatorna dinamika pomorskog objekta. Polazni podaci za ovaj proračun dobiju se u frekvencijskoj domeni. Prikazan je postupak preslikavanja iz frekvencijske u vremensku domenu. Pretpostavljaju se mali pomaci pomorskog objekta. U obzir se uzimaju inercijske sile vlastite mase i dodatne mase, prigušenje uslijed radijacijskih valova i viskoznog otpora te hidrostatička krutost. Opterećenje se sastoji od valnih sila prvog i drugog reda u odnosu na valne frekvencije te sila vjetra i sila morske struje. Proračun integralnog modela opisan je u četvrtom poglavlju. Unutar ovog proračuna sadržane su sve karakteristike sidrene linije, proizvodnog podizača i pomorskog objekta. Sprega se ostvaruje ravnotežom sila u svakom hvatištu te jednakošću pomaka vrha sidrene linije ili proizvodnog podizača i pripadnog hvatišta. Prilikom definiranja ravnoteže sila kod pomorskog objekta i kod sidrene linije ili proizvodnog podizača u obzir se uzimaju sile međudjelovanja koje nastaju u hvatištima. U petom poglavlju rezultati opisanog integralnog modela uspoređeni su s približnim analitičkim rješenjima te s dostupnim rezultatima iz literature

An Improved Stiffness Model for Polyester Mooring Lines

The stiffness model of highly extensible polyester mooring lines is studied. Mooring lines are considered within coupled dynamic model of a moored fl oating object. In more detail, deepwater mooring with taut polyester mooring lines is observed. In this case mooring line is modelled as an extensible cable without bending and torsional stiffness. Movements are assumed to be three-dimensional, so it is necessary to examine large displacement model. In longitudinal strain calculation the material of the mooring line is considered as nonlinear. A large elongation value is examined within the stiffness model. Inertial forces of the mooring line are also considered. Hydrodynamic loads due to surrounding fl uid are taken into account with the Morison equation. Due to nonlinear properties of mooring lines calculations have to be done in time domain. On these assumptions, derivation of a mooring line fi nite element is presented for static and dynamic analysis. A fl oating object is modelled as a rigid body with six degrees of freedom and with small displacements assumption. Hydrodynamic coeffi cients are calculated in a specifi ed frequency domain; therefore, mapping from the frequency to the time domain is necessary. Comparison between the improved model developed in this paper and current equivalent model is done. A simple mooring line that can be analytically described was the base for comparison. The improved model achieved better agreement with the analytical result

FLOATING CRANE RESPONSE IN SEA WAVES

Many activities present in the offshore technology are affected by sea waves and their safety strongly depends on the sea state. The fundamental question in operation risk analysis is the limit sea state which enables the safe operation controlled in all phases: from the sea surface to the sea bottom. The safe limit sea state is especially important when some lifting operation is performed in sea waves. This paper presents a dynamical model of a floating crane in sea waves with a hanging load at a high altitude above sea level. The model is defined as a linearized coupled double pendulum where the usual 6 DOF of the floating crane oscillatory motions in waves are extended by additional 3 DOF of the swinging load. The numerical model of the floating crane is calibrated and tuned according to the measured model in a towing tank. The significant values of the crane boom top accelerations were estimated for the given design sea state with different vertical load positions. The problem is illustrated by the example of a floating crane consisting of a simple single pontoon with an ordinary land crane mounted on the deck. The reliability of the proposed method is evaluated on the basis of correlation between the calculated and measured data