Quantitative collision risk assessment of a fixed-type offshore platform with an offshore supply vessel
Introduction
Offshore installations depend on offshore supply vessels (OSVs) for various supplies such as food, chemical, and equipment. Consequently, they are prone to collision accidents during vessel approach, the transfer operations, or while leaving the platform. Various factors causing an accident include platform or vessel oriented human or equipment errors combined with harsh environmental conditions.
The design principle against the supply vessel includes calculation of both collision frequencies and its associated consequences, which are expected to occur during the operational period of the platform. In other words, the situation calls for a risk-based design approach.
The risk of the offshore installation against potential accidents shall be best estimated using quantitative risk assessment (QRA). Typically, the risk is defined as the product of the frequency of occurrence and its associated consequences [1]. Depending on the type of consequences incurred, three types of risk exist, namely asset, fatality, and environmental risk. Generally, the collision of supply vessel to fixed platforms results in minor consequences, which include significant damage to the substructure of the platform, such as local denting and fracture of legs or braces [see Fig. 1 (a) and (b)], except a case where the total loss of the MHN (Mumbai High Northern) platform [see Fig. 1 (c)]. However, the consequences are primarily governed by the financial cost to repair the damaged parts [2]; therefore, it is crucial to evaluate asset risk and implement suitable measures to mitigate the damage against potential ship impacts.
The success of a risk assessment methodology depends on how well it incorporates various uncertainties associated with the collision scenarios. For instance, the identification and quantification of all the collision-affecting parameters (such as statistical interpretation of collision accidents and operational conditions), selection of credible collision scenarios, and determination of the load parameters such as ship impact velocity, angle, and location.
A fundamental challenge in risk assessment is to select credible collision scenarios [1]. These scenarios represent to occur during the design life of the platform. Generally, collision scenarios are selected using either deterministic or probabilistic methods. Often, the former involves the selection of a worst-case collision scenario from past accidents combined with expert judgments. At the same time, the latter employs more sophisticated statistical and probabilistic techniques to arrive at the scenarios. In most cases, the deterministic approach becomes conservative when the platforms are designed based on the selection of the most prominent accident, [such as in the case of the total loss of the MHN (Mumbai High Northern) platform [see Fig. 1(c)], thereby incurring a huge capital loss for the platform owners for ensuring high safety and intact in the event of a collision.
Moreover, the increasing size of the supply vessel, [as observed in regions of Norwegian Continental Shelf (NCS)] and the changing design of supply ship bow and the platform, demands a frequent revision of the industry standards and guidelines [6], which again may prove conservative for other platforms and regions where smaller supply vessels are in operation. Hence a careful selection of the possible and potential collision scenarios should be exercised. Such analysis should be performed separately based on factors such as types of offshore platform and supply vessels, geographical location. In this context, the probabilistic analysis proves to be a more viable option, albeit it involves extensive computational methods.
The fundamental steps and general guidelines on performing QRA for ship-platform collisions have been developed by prominent organizations [2], [7], [8], [9], [10]. Using AIS (automatic identification system) database, Mujeeb-Ahmed et al. [11] performed passing vessel collision risk to an offshore platform. However, research on the practical application of risk assessment of offshore platform against infield vessel collisions is seldom available.
In this context, the main aim of this study is to develop an advanced and practical risk assessment method for ship collisions to an offshore structure. A demonstration of the developed procedure is carried out with an illustrative example consisting of the collisions between a jacket-type offshore platform and an OSV. A combination of probability and sampling techniques is employed to select fifty credible collision scenarios. Based on the collision loads obtained from the vessel motion analysis, extensive structural crashworthiness analyses using NLFEA (nonlinear finite element analysis) have been performed using advanced computational modelling techniques. The repair cost of damaged tubular members is calculated using a simplified cost model. Combining collision frequency calculated based on historical accident database, various exceedance diagrams are established for structural damage parameters and their repair costs. The design values of the parameters are determined and compared based on the NORSOK (the Norwegian shelf’s competitive position) and HSE (Health and Safety Executive) acceptance criteria. Further, A sensitivity analysis of collision load parameters to the structural consequences such as absorbed energy and column and brace deformation have been performed to identify critical collision scenarios. The risk assessment procedure developed in this paper will be helpful for the platform owners and analysts in their decision-making process towards the risk-based design of structural requirements and the calculation of asset risk.
Section snippets
Framework for quantitative collision risk assessment
A systematic and well-defined procedure forms the backbone of a QRA study. Fig. 2 shows an overall flow chart of the different steps involved in the collision risk assessment procedure.
Key challenging tasks identified in the procedure include:
– hazard identification
– selection of credible collision scenarios
– calculation of collision frequency
– collision load analysis
– structural crashworthiness analysis
– risk assessment
The current study is the continuation of a series of tasks involved in QRA,
Target structures
A demonstration of the applicability of the procedure has been carried out using an illustrative example comprising of collisions between a four-legged offshore jacket platform installed at a water depth of 120 m and a modern 8546-ton OSV, see Fig. 10. Table 2 gives the main characteristics of the target structures.
The jacket substructure consists of both horizontal and X-configured diagonal braces. For simplicity, the complexities at tube joints arising from increased thickness due to joint
Results and discussions
Following the procedure described in Section 2, NLFEA was carried out for 50 collision load scenarios (provided in Table A.2). Fig. 16 shows an example of a collision scenario-27, depicting the enlarged view of measuring global deformation and local denting characteristics of a damaged horizontal brace member. The local dent characteristics — dent depth, breadth, and length, are monitored based on the nodal displacements of the elements. The global deformation is measured with respect to the
Summary
The main focus of the study was to enhance the safety of offshore platforms against ship collision accidents using a risk-based design process. An advanced procedure for the QRA of offshore platforms in collisions with incoming supply vessels was developed. A practical demonstration of the developed method was carried out with an illustrative example, considering collisions between an OSV and an offshore jacket platform.
Based on a set of fifty collision scenarios selected using the
Conclusions and future works
The main focus of the study was on how to enhance the safety of offshore platforms against ship collision accidents using a risk-based design process. From the results obtained, the following conclusions can be drawn:
- 1.
A novel risk-based methodology was proposed to perform a quantitative collision risk assessment of OSV to an offshore jacket platform.
- 2.
The severity of collision consequences was measured in terms of structural damage and repair costs using extensive nonlinear structural
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was undertaken at the Korea Ship and Offshore Research Institute (KOSORI), ICASS (International Centre for Advanced Safety Studies) at Pusan National University, which has been a Lloyd's Register Foundation (LRF) Research Centre of Excellence since 2008. The support of Dr. Serdar Turgut Ince and Dr. José Manuel Cabrera-Miranda are greatly acknowledged.
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