Jeom Kee Paik

Toward a Probabilistic Approach to Determine Nominal Values of Tank Sloshing Loads in Structural Design of Liquefied Natural Gas FPSOs

The aim of this study is to develop a new probabilistic approach to determine nominal values for tank sloshing loads in structural design of LNG FPSO (liquefied natural gas, floating production, storage, and offloading units). Details of the proposed procedure are presented in a flow chart showing the key subtasks. The applicability of the method is demonstrated using an example of a hypothetical LNG FPSO operating in a natural gas site off a hypothetical oceanic region. It is noted that the proposed method is still under development for determining reliable estimates of extreme sloshing induced impact loads. It is concluded that the developed method is useful for determining the sloshing design loads in ship-shaped offshore LNG installations in combination with virtual metocean data and operational conditions.

On the Crashworthiness of Steel-Plated Structures in an Arctic Environment: An Experimental and Numerical Study

Structural crashworthiness with regard to crushing and fracture is a key element in the strength performance assessment of ship collisions in the Arctic. The aim of this study is to investigate the crashworthiness characteristics of steel-plated structures subject to low temperatures that are equivalent to the Arctic environment. The effect of low temperatures on the material properties is examined on the basis of tensile tests. Crushing tests are undertaken on steel-square tubes subject to a quasi-static crushing load at both room and low temperatures. The crushing behavior of the square tubes in this test is compared with ls-dyna computations. It is found that low temperatures have a significant effect on not only the material properties but also the crashworthiness of steel-plated structures in terms of mean crushing loads and brittle fracture. It is suggested that the collision-accidental limit state design of ships intended to operate in the Arctic region should be carried out by taking the effect of low temperatures into account.

A practical method to determine the dynamic fracture strain for the nonlinear finite element analysis of structural crashworthiness in ship–ship collisions

ABSTRACT Ship–ship collisions continue to occur regardless of efforts to prevent them. The collisions involve highly nonlinear characteristics associated with structural crashworthiness, including crushing and fracture as well as buckling and plastic collapse. When applying nonlinear finite element analysis (NLFEA) to solve these problems, a reliable critical fracture strain accounting for strain-rate effects due to collision speed must be implemented. This study proposes a practical method to estimate the dynamic fracture strain to be used for the structural crashworthiness analysis associated with the collisions. For this purpose, the strain-rate characteristics in struck ship were investigated by NLFEA, in which the striking vessel was assigned various velocities in the range of practical ship speeds. Based on computations, an empirical formula was developed to calculate the strain rate at a given collision speed, allowing for a practical estimation of the dynamic fracture strain. The formula is validated by a comparison with experiment.

A study on factors affecting the safety zone in ship-to-ship LNG bunkering

ABSTRACT The objective of this paper is to examine the characteristics of leaked-gas dispersion in ship-to-ship liquefied natural gas (LNG) bunkering, thereby providing an insight towards determining the appropriate level of safety zones. For this purpose, parametric studies are undertaken in various operational and environmental conditions, with varying geometry of the ships, gas leak rate, wind speed and wind direction. The study applies computational fluid dynamics (CFD) simulations for case-specific scenarios where a hypothetical LNG bunkering ship with a capacity of 5100 m3 in tank space is considered to refuel two typical types of large ocean-going vessels: an 18,000 TEU container ship and a 319,000 DWT very large crude oil carrier. It is found that wind speed, wind direction, ship geometry and loading condition are important parameters affecting the extent of safety zones in addition to gas leak rate and leak duration. Details of the computations and discussions are presented.

On design criteria for a disconnectable FPSO mooring system associated with expected life-cycle cost

ABSTRACT Some floating production, storage and offloading units (FPSOs) possess disconnectable systems to avoid harsh environments. According to a literature survey, the practice is based on perceptions and experiences of operators to judge disconnection; however, this paper offers a rational approach. A life-cycle cost model is proposed to optimise (1) the disconnection criteria and (2) the design of mooring lines under reliability format. Relevant ultimate limit states are considered in association with hull, moorings and green water failure. Effects of future failure costs are considered (downtime, environmental damage, reputation, etc.). Disconnection criteria are then formulated in terms of significant wave height and wind speed limits. Because a permanent mooring system may exhibit excessive resistance, it is possible to reduce the lines’ thickness until the cost is optimised for non-permanent service. Results for an example in the Gulf of Mexico show that important savings can be achieved by implementing the proposed optimisations.

Effects of a deformable striking ship's bow on the structural crashworthiness in ship–ship collisions

ABSTRACT Ship–ship collision accidents continue to occur regardless of the continuous efforts to prevent them, and they essentially involve highly nonlinear problems associated with structural crashworthiness due to crushing and fracture. The nonlinear finite-element method is one of the most powerful techniques to solve the problems. In industry practice, the bow structure of a striking ship is often modelled as a rigid body as it is usually much stiffer than the side structure of a struck ship. However, reality is that the initial kinetic energy in a ship–ship collision accident can be absorbed by the damages of not only the struck ships side structures but also the striking ships bow structures because the striking ship bow structure is actually deformable. The aim of the present study is to examine the effects of a deformable striking ships bow on the structural crashworthiness in ship–ship collisions. In the present study, two scenarios are considered where the side structure of a VLCC class double hull oil tanker is collided by the bow structure of a VLCC class double hull oil tanker or a SUEZMAX class double hull oil tanker. All of the ships considered are real ships in operation. The structural crashworthiness in terms of the collision force–penetration relation and the collision energy–penetration relation is then studied with two cases in which the striking ships bow structures are either rigid or deformable. It is concluded that the striking ships bow may be modelled as a rigid body in minor collisions at a collision angle of about 90° where the initial kinetic energy is entirely consumed before the inner hull structure is ruptured, but it should be modelled as a deformable body in a major collision accident or at an inclined collision angle where the maximum penetration can be greater than the double side breadth.

Effects of structural congestion and surrounding obstacles on the overpressure loads in explosions: experiment and CFD simulations

ABSTRACT An experimental and numerical study was undertaken to identify the characteristics of overpressure loads in offshore platform models subject to hydrocarbon explosions, with a focus on the structural congestion and surrounding obstacles. A large-scale (one-half) test model of a FLNG (liquefied natural gas floating production storage and offloading unit) topside structure was used for the experiment. Computational fluid dynamics (CFD) were used to calculate the overpressure loads in explosions with varying degrees of structural congestion. The overpressure loads tended to be more significant with the increase in structural congestion because the ventilation of exploded gas was retarded due to the obstacles presented by congested structural elements. Also, the overpressure loads with the surrounding structures are much larger than those without them. Details about the test database are documented to provide a useful reference for other researchers to validate numerical and theoretical methods.