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4 Ö. ÖZGÜÇ

operation loads. In the North Sea (see, e.g., ISO 2005a), the collision is described as one from a
vessel of 5,000 tons with a drifting speed of 2.0 m/s. The collision energy is 14 MJ for a broad-
side collision and 11 MJ for a head-on collision [3, 7].

In an analysis of a collision between a supply vessel and an FPSO in West Africa, Oh et al [10]
assumed that a collision could occur in three places: on the riser, the protector, and the deflector.
They selected to look into collisions that induce large deformations on the framed structure at a
colliding speed of 1 m/s. They estimated that the speed was a result of marine equipment
failure, or human error.

Lin et al [6] showed how FEM is used to simulate the collision process of two semi
submersibles. Stress and strain distributions, collision forces, and plastic energy absorption are
obtained. The motion lag of the struck submersible in the collision process is discussed and it is
found that it is sensitive to impact velocity which increases with the increasing velocity.

The objective of this analysis is to assess the collision strength of the riser protection frame
(RPF) and protection net (PN) supports of Ichthys Inpex CPF. The riser protection frame and
protection net supports are assessed for their collision strength when they are being collided
by a supply vessel. The colliding vessel from north-side is assumed to be of 18,000 tons and
from east/west side it is 7000 tons.

The finite element model is developed using MSC/PATRAN and analysis is performed in LS-
DYNA. The element size used in the FE model is one-longitudinal spacing away from the
concerned location. In the concerned location it is 100mm× 100mm. In the concerned location
only shell elements are used. 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.

The material assumed is EW420 which has a minimum yield strength of 380MPa and a tensile
strength of 530MPa. Initially the collision analysis is performed with an objective of
achieving zero plastic strain on the CPF column structure that is supporting the RPF and PN
supports, but later it is realized that practically it is not possible to achieve no plastic strain for
the given collision energy. Hence a plastic strain of 5% on the outer shell of the CPF column
structure is assumed to be acceptable. Since the final objective is to check any kind of leakage
into the CPF column due to the collision, it is important to control the plastic strain on the
outer shell of the column. Since the plastic strain of the internal members of the column, RPF
and PN structures, doesn't cause any leakage inside the column, it is proposed to have no
criterion for the plastic strain of these structural members [8, 9].

Thus with the above 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.

2. Project Description

In 1998, INPEX Exploration and Production acquired the petroleum exploration permit WA-
285-P, located in the Browse Basin, off the north-west coast of Western Australia and

GiDB|DERGi Sayı 3, 2015