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VESSEL IMPACT ANALYSIS FOR RISER PROTECTION FRAME AND 3
PROTECTION NET SUPPORTS ON SEMI-SUBMERSIBLE OFFSHORE STRUCTURES

VESSEL IMPACT ANALYSIS FOR RISER PROTECTION
FRAME AND PROTECTION NET SUPPORTS ON SEMI-

SUBMERSIBLE OFFSHORE STRUCTURES

Özgür ÖZGÜÇ*
*INPEX Exploration and Production, South Korea

ABSTRACT

The objective of this paper is to assess the collision strength of the riser protection frame (RPF) and
protection net (PN) supports of CPF (Central Processing Facility) of Semi-Submersible types. The PRF
and PN supports are assessed for their collision strength when they are being collided by a supply
vessel. The vessel colliding from north-side is assumed to be 18,000 tons and the one colliding from
east/west side is assumed to be7000 tons. The focus of this paper is placed on general methodology and
design of accident scenarios for CPF using more sophisticated tools such as the non-linear FEM which
predicts the structural responses during and after a collision more precisely. There are two different
designs for the RPF and PN supports and thus there are four structures i.e. RPF-01, RPF-02, PN-01 and
PN-02. The expected kinetic energy for the collision from the north side is 20MJ. The collision energy
from east/west side is 14MJ for side collision and 11MJ for bow/stern collision. With collision energy
and plastic strain criteria, the collision capacities of the/-RPF and PN supports are estimated along the
span of the structures for the given collision energies.

Keywords: Collision, Accident scenarios, Non-linear FEM, Riser Protection Frame (RPF), Protection
Net (PN)

1. Introduction

There are some limited reports on ship-to-platform collision and the consequential risks to the
damaged structures. Experts recognize that ship collisions are not likely to cause the push-over
failure of collided platforms that lose some individual structural members, especially in a benign
environment. However, minimum structures may see rapid deterioration of the overall
structural integrity if impact damages are left unrepaired [2].

Vessel collision during normal operations is one of the accidental loading possibilities. Such a
scenario for platforms in the Gulf of Mexico is possible when 1,000-ton supply vessel collides,
either head-on or broadside, with the platform at a speed of 0.5 m/s. The vessel is chosen to
represent typical OSVs in the U.S. Gulf of Mexico [4, 5]. This API RP [1] requires that the
platform survive the initial collision, and meet the post-impact criteria. During the described
collision, the offshore structure absorbs energy primarily from localized plastic deformation of
the tubular wall, elastic/plastic bending, and elongation of the member. In addition, if the
fendering device is fitted, then there is global platform deformation, in addition to ship
deformation and/or rotation. After collision, the damaged platform should retain sufficient
residual strength to withstand environmental storm loads for one-year in addition to normal

Sayı 3, 2015 GiDB|DERGi