Full length articleComputational models for the structural crashworthiness analysis of a fixed-type offshore platform in collisions with an offshore supply vessel
Introduction
Offshore supply vessels (OSVs) regularly visit offshore jacket platforms to transport supplies such as food, equipment, and chemicals. In the past, several minor or major collisions between supply vessels and offshore platforms have been reportedly recorded, as shown in Fig. 1. Obviously, the platforms should be designed so that the safety should be sufficient enough with tolerance against such potential accidents. Structural consequences due to collisions involve highly complex and nonlinear failure mechanisms involving local denting, crushing, fracture, and permanent deformation [1].
Within the framework of quantitative risk assessment and management, one is asked to carry out the structural crashworthiness analysis at numerous collision scenarios, where the accuracy of resulting computations must be secured. As the structural responses are highly nonlinear associated with multiple physical processes and multiple criteria, a careful application of modelling techniques is required to capture realistic collision characteristics using numerical analyses such as nonlinear finite element analysis (NLFEA) software.
Numerous researches on ship collisions to offshore structures are available in the literature [5]. The structural responses of both striking and struck bodies closely correlate, as shown in Fig. 2 [6]. Based on the collision energy interaction between the ship and offshore platform, DNV GL [7] classified numerical analysis of a collision scenario into three design domains – ductile, shared-energy, and strength design, see Fig. 2(a), and suggested that the ship-platform collision falls in the ductile design region, where the striking vessel can be conservatively assumed rigid so that the analysis can be greatly simplified.
However, Storheim and Amdahl [8] pointed out that simplifying the analysis by treating the ship as infinitely rigid may prove to be overly conservative, especially for collisions involving high levels of impact energy. Moreover, with the introduction of the modern supply vessels of increased size, and new bow shapes for operations in deep and ultra-deepwater offshore operations, the relative strength of the ship to offshore platform becomes comparable. Furthermore, the ship absorbs considerable impact energy through failure mechanisms such as plasticity, large deformations, and rupture of plate and stiffener components. A typical example could be the collision accident between Big Orange XVIII and the Ekofisk platform, where significant damage was caused to the platform as well as the vessel's bow structure, see Fig. 1(c). In reality, there exist no definite limits of relative strength, which could differentiate the three design domains, and one should pay attention to the deformations in both ship and offshore platforms to understand real collision behaviour, see Fig. 2(b).
DNV GL [9,10] provided some specific guidelines and general requirements for the determination of structural capacity using NLFEA. Many researchers have performed the structural response analysis of individual tubular members against ship impacts, for instance Refs. [[11], [12], [13], [14], [15]]. Notaro et al. [16] performed the numerical analysis of OSV collision to a fixed offshore structure, considering both bodies as deformable. In general, the applicability of simplified analytical expressions depends on a particular class of load, boundary conditions, shape and size of the striking body, material properties, contact areas, etc. Most of the physical model tests were conducted on single tubular members with mid-span impact, fixed boundary conditions, rigid intender, and concentrated load. In reality, however, ship and offshore structures are large, and the collision is often eccentric, with multiple interactions between ship and tubular members combined with finite boundary stiffness at interconnected tubular members. Moreover, the collision interaction occurs under the influence of surrounding waters.
With the advancement of high-performance computers and the availability of sophisticated software, it has been realised that NLFEA is one of the powerful tools, especially for the analysis of large displacements involving geometric and material nonlinearities. However, research on numerical analysis considering such nonlinearities associated with dynamic fracture strain, structural deformation in both ship and platform, and hydrodynamic modelling with application to a full-scale model of a jacket platform is seldom available in the literature.
This paper aims to develop advanced computational modelling techniques in association with structural crashworthiness analysis of fixed-type offshore platforms in collisions with offshore supply vessels (OSV). A demonstration of the developed modelling techniques is shown with an illustrative example in collisions between a four-legged jacket-type offshore platform and a modern OSV. LS-DYNA software with an explicit finite element solver is used to conduct NLFEA [17]. The computational models take into account the effects of the interaction between striking ship and struck platform, material strain rate and dynamic fracture strain, surrounding water, and contact problem between two colliding bodies. A comparison of the results with analytical and recommended design load-deformation curves is made. A sensitivity study using the developed modelling techniques is also conducted to investigate the effects of collision load parameters on the structural damage.
Section snippets
Procedure for structural crashworthiness analysis
Fig. 3 shows a flow chart of the structural crashworthiness analysis for collisions between an offshore platform and an offshore supply vessel. The numerical analysis begins with the definition of a collision event, which includes the characterisation of structural topologies, such as the target structures, environmental, and operational conditions.
Within the framework of quantitative collision risk assessment and management, a number of collision scenarios are selected in a probabilistic
Details of target structures
An illustrative example is considered where a modern OSV with a mass of 8546 ton collides with a hypothetical four-legged jacket-type offshore platform installed at a water depth of 120 m. Fig. 4 shows the front and top views of the jacket platform, where the brace configurations are made of both horizontal and X-configured diagonal tubular members. The thickness of the leg and brace parts are selected in compliance with API [19] compactness criteria, i.e. , where is the
Analytical models of collision load analysis and their validation
In the absence of experimental test studies on ship bow collision to jacket platform, the validation of the present study is performed with various analytical and empirical expressions available for the structural behaviour of tubes subjected to the lateral collision load of a striking body. For instance, Furnes and Amdahl [63] provided an analytical expression for the force () required to locally dent () the tubular members based on experimental test results:where
Sensitivity study
Using the modelling techniques described in section 3, a sensitivity study was conducted to investigate the effect of various collision affecting parameters on structural damage characteristics. This includes different collision velocities - 0.5, 1, 2, 3, and 4 m/s, column thickness - 40, 50, and 60 mm, and impact location on the platform - column, brace, brace-brace joint, and brace-column joint.
Fig. 23 shows an example of a force and an energy deformation curve obtained for the jacket and OSV
Concluding remarks
The study developed computational models for performing structural crashworthiness of collisions between a fixed-type offshore platform and an offshore supply vessel (OSV) using the nonlinear explicit finite element code, LS-DYNA. Particular emphasis was given to the material properties for the different materials involved in the various structural components. The study considered collisions between a jacket-type offshore platform and an OSV and performed a sensitivity analysis of the impact
Author statement
The first author (M.P. Mujeeb-Ahmed) now works for University of Strathclyde, Glasgow, UK.
Declaration of competing interest
This article is not in conflict of interests at all.
Acknowledgments
This work was supported by a 2-Year Research Grant of Pusan National University, Busan, South Korea.
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