Feasibility of increasing the pedestal height on the J-class vessel by 10 meters

Feasibility of increasing the pedestal height on the J-class vessel by 10 meters

Lee, M.

Metrikine, A.V. (mentor)
De Oliveira Barbosa, J. (mentor)
Van der Heiden, K. (mentor)

Mechanical, Maritime and Materials Engineering

Offshore & Dredging Engineering

2016-05-19

In order for Jumbo to stay competitive in the offshore market, it's heavy lift vessels should be able to take on a wide range of lifting purposes. This can range from installation of transition pieces for wind turbines to installing modules of fpso's and platforms. One of the demands from the offshore market requires large lifting heights. It is proposed to increase the lifting height on existing mast cranes by increasing the pedestal by 10 meters. In this thesis it is tried to answer the question whether this is feasible. The pedestal should be increased such that it complies with geometric constraints, stability, vessel motions and structural integrity. This feasibility study is divided in two parts. In the first part it is investigated how the enhanced pedestal influences the vessel with regards to hydrostatic characteristics and vessel motions. This information is used to obtain the changed workability of the vessel. Stability characteristics were slightly worse compared to the original vessel layout. As a result of this the maximum offshore lifting capacity decreased from 648 ton to 580 ton at an outreach of 36.5m as measured from the centerline of the vessel. For three crane configurations the hydrostatic values were obtained and served as input for the vessel motion analysis. Since the Jumbo Javelin heavy lift vessel is equipped with a DP-2 system, only first order motions needed to be taken into account. The motions or Response Amplitude Operators (RAO's) express the ratio of the vessel motion amplitudes with the amplitudes of the incoming waves. In all crane configurations, the natural period shifted upwards. From all six degrees of freedom, the roll motion was the largest which is explained by the fact that roll has very little potential damping (energy transfer from ship motion to fluid motion). The final evaluation consisted of a workability study, expressed by the percentage of time that the vessel is able to operate in different sea states. Workability remained similar to the workability of the original vessel in cases where Jumbo performed operations before. A check for North sea circumstances led to an increased workability. This is due to the fact that the natural frequency of the enhanced vessel went further away from the most common sea states. In the second part of this study the pedestal is evaluated structurally. A pedestal geometry is chosen such that it resists normal bending stresses by using the least amount of material. The Fatigue limit stress was taken as the limiting value. In this way an optimal tapered pedestal shape was chosen with varying wall thicknesses ranging from 6.25cm till 4.85cm along the height of the pedestal. The obtained shape is further processed by studying the natural frequencies of the pedestal including the mast and jib. Ship motions should not interfere with natural frequencies of the crane structure due to the effects of resonance which could lead to disproportionate stresses. To study this a finite element model is created using MATLAB that can include the effects of pedestal taper, crane components and DOF's for different jib angles. Planar motions for a plane frame are considered. 6 DOF's for each element are considered and hence represent elements that take both bending and compression. Natural frequencies for two operating conditions and one stowed condition with tip mass are investigated. The lowest observed frequency has a value of 1.39 rad/s. While the lowest natural frequency of vessel motions considered in the workability study is 1.25 rad/s. This led to the conclusion that the improved pedestal geometry and stiffness are sufficient and hence workable.

offshore
heavy
lift
stability
motions
pedestal
crane
natural
frequencies
vessel
structural
workability
feasibility
jib
boom

http://resolver.tudelft.nl/uuid:2e95ed99-d6c4-45b7-8920-12341f6d9f98

2021-05-19

Student theses

master thesis

(c) 2016 Lee, M.