Analytical Risk Assessment And Risk Control Of Floating Platforms Subjected To Ship Collisions And Dropped Objects
Authors: Moan Torgeir , Karsan Demir , Wilson Terry 
Published: October 2008

This paper presents a method for assessing the impairment risk of a concrete TLP platform system subjected to impact hazards. Estimation of the probability of impacts due to ship collisions and dropped objects is briefly reviewed, and a probabilistic assessment of the possible associated impact damage, flooding and tether failure is made. These estimates are integrated to obtain the probability of impairment. The risk may be controlled by adequate structural design of the columns and pontoons, as well as choosing a proper subdivision of the buoyant elements. Design of the flat and curved walls of a concrete North Sea TLP hull with a given compartmentation to resist impact loads to achieve an acceptable risk level with respect to ship collisions and dropped objects, is demonstrated.

INTRODUCTION

Analytical risk assessment and risk control of platforms located on the Norwegian Continental Shelf have been an accepted practice since, about, fifteen years. The Piper Alpha accident resulted in the introduction of these requirements to other parts of the North Sea. The focus of this paper is on the marine risks associated with the buoyancy loss of floating platforms induced by ship impacts and dropped objects. Method for estimating the probability of occurrence of the initiating impact and the conditional probability of wall penetration flooding given the initiating impact, the conditional probability of progressive flooding given the wall penetration, and the tether failure of concrete TLPs are discussed. These methods are integrated to obtain a measure of the probability of total loss of a platform subjected to all impact events. Application of the risk methodology to evaluate and compare risks for different conceptual designs and especially, how an acceptable risk level can be achieved through the introduction of a strong outer shell (e.g. of concrete) and/or using appropriate subdivision of the buoyant volume, is demonstrated. In this way, an acceptable safety level can be achieved while maintaining acceptable cost levels.

In an example study, it is shown that such a risk analysis methodology provides a basis for defining design accidental loads and demonstrating compliance with acceptable safety levels without sacrificing the economics.

SYSTEM IMPAIRMENT OF TLP

Basic assumptions

The present analysis is concerned with the risk of system impairment due to mechanical impacts. Such events may be caused by ships or dropped objects. Event sequences that may follow after an impact and result in impairment are indicated by the event tree in Fig. 1. System impairment is defined by global structural failure of the hull or tethers. This critical event may take place immediately after a severe ship impact or later as shown in Fig. 1. After

SYSTEM FAILURE PROBABILITY

General approach

The following system failure modes (FSYS m) are considered: m = 1: global failure of the hull; m = 2: tether - overload; m = 3: tether group (tension leg) slack and subsequent failure. Here, slack is conservatively assumed to imply failure.