Design and Assessment of Offshore Structures for Extreme and Abnormal Accidental Loading
Authors: Sari Ali , GHONEIM Abdel , Ayhan Yavuz 
Published: May 2013

The design and assessment of offshore structures, fixed or floating, against extreme and abnormal accidental events require special advanced analysis capabilities and can be dimensioning in certain situations. Such accidental events entail vessel collision/allision, dropped objects, fire, and explosion conditions. Only vessel impact and dropped objects are addressed herein. Allisions involving high impact speeds are also discussed. The structures considered include fixed platforms, floaters (Spar, FPSO, and Semi-submersible), and mobile offshore units. The paper shows how nonlinear FEA and simplified methods are applied both at the design stage and as assessment tools following an incident of accidental loading. It demonstrates that by making reasonable assumptions concerning the energy dissipation parameters during a collision/allision scenario, significant savings in computational and analysis efforts are realized while the reliability of the results is maintained. Both traditional low kinetic energy and abnormal high energy scenarios are evaluated. The paper also presents proposed repair solutions to some typical damage conditions and compares the different codes and standards (ISO, NORSOK, and API) with regards to the acceptability criteria.

Introduction

The design of offshore structures must account for accidental loadings with low probability of occurrence. Such accidental loadings include vessel collision/allision, dropped objects, fire, and explosion conditions. This paper is dedicated to vessel impact and dropped objects. Fire and blast resistance will be covered in future publications focusing on offshore applications. The FABIG (2009) and ASCE (2010) are currently the two main bodies dealing with fire and blast resistant structures with emphasis on buildings and petrochemical facilities.

In addition to accidental supply ship impacts, some recent offshore facilities had to consider high speed ship bow impacts or allusions in order to ensure adequate safety especially when storage of petroleum or liquefied gas is involved. The objective of this paper is to present recent analysis capabilities that make it possible to realistically predict the energy absorption capabilities of offshore structures due to ship collision. This is important in order to ensure that the extent and consequence of the damage is acceptable. The acceptability criteria are set in the regulations and codes of practice as dependent on the associated collision risk level and consequence assessment. In the ISO standards; for example, hazards are categorized into three main groups, according to annual exceedance probability or return period:–Group 1 - hazards with 10–2 annual probability or return period of the order of 100 years–Group 2 - hazards with 10–3 annual probability or return periods of the order of 1000 to 10000 years–Group 3 - hazards with 10–4 annual probability or return periods well in excess of 10000 years

Extreme conditions correspond to the 100 year conditions and are treated as ultimate limit state (ULS) events while accidental or abnormal events are associated with the 1000 or 10,000 year abnormal limit state (ALS) scenarios. A review of requirements in offshore regulations, standards and codes is given in Ghoneim (2008).