Ship Motions and Hull Deformations of a Non-Conventional Ship (the Pioneering Spirit) with Forward Speed in Waves

Ship Motions and Hull Deformations of a Non-Conventional Ship (the Pioneering Spirit) with Forward Speed in Waves

Morshuis, C.A.

Huijsmans, R.H.M. (mentor)

Mechanical, Maritime and Materials Engineering

Offshore & Dredging Engineering

Offshore Engineering

2016-12-20

The Pioneering Spirit is the world’s largest construction vessel. The twin-hulled vessel is 382 m long and 124 m wide. At the bow is a slot, 122 m long and 59 m wide, that enables the Pioneering Spirit to straddle a platform and remove or install entire topsides up to 48,000 t in a single lift using eight sets of horizontal lifting beams. Until now little is known about the vessels behaviour during platform transport at forward speed, which is part of the removal/installation operation. Vessel motions and internal loads are key elements in determining how much a platform must be reinforced during transport. The goal of this thesis therefore was to analyse those elements at forward speed in waves. For this thesis a time domain Rankine panel method (WASIM), has been chosen as the theoretical method to predict the vessel behaviour with forward speed. This method has been compared to an alternative, a greens function method (AQWA) that Allseas uses at the moment. The largest difference between the two boundary element methods is the different approach of the free-surface boundary condition. The Rankine panel method models the free surface and the body with singularities allowing forward speed terms to be included in the free surface boundary condition. The green functions method only models the body with singularities, and uses a conventional zero speed greens function by neglecting the forward speed terms in the free surface boundary condition. To validate the results of the two hydrodynamic solvers, motions from forward speed model tests have been used. Additionally to explain the vessel behaviour the added mass, damping and exciting forces have been analysed. Furthermore the internal loads have been investigated by calculating section loads. Those have subsequently been used in a beam model to estimate bow deformations. Due to forward speed, the Pioneering Spirit starts oscillating at a higher (encounter) frequency than the incoming wave frequency. This effect causes a large drop in added mass and damping of the vessel at certain frequencies. The wavelength with respect to the vessel dominating the excitation force however is unaffected by forward speed. This thesis shows that the change of balance in the equation of motion causes a significant change in vessel motions. The most significant is an increase in heave and pitch motions in bow wave directions at low frequencies. Peak heave and pitch accelerations are more than doubled in these situations, subsequently causing significant increases in bow deformations. Due to the unconventional shape of the Pioneering Spirit resonating wave modes are present in-between the bows at zero speed. This study shows that these resonating wave modes are translated away from the vessel with forward speed allowing no resonance to be build up. From this study it can also be confirmed that the greens function method (AQWA) does not correctly model the translation of waves in the free surface, causing wave resonance effects to be amplified instead of translating away. It can be concluded that the green functions method is an inaccurate solver for the Pioneering Spirit and ships with likewise geometry, regarding forward speed in waves. The time-domain Rankine panel method (WASIM) does a much better job, showing a good correlation with model test results in beam to bow wave directions. It may be concluded that a proper forward speed solver is highly recommended for the beam to bow wave situations that have been investigated in this thesis. A next step would be to investigate the effects stern incoming waves and different transport conditions.

Forward speed
Rankine panel
Non-conventional ship
greens function
wave resonance

http://resolver.tudelft.nl/uuid:87a519bc-e634-402a-b7b5-a4038ae10836

2021-12-20

Student theses

master thesis

(c) 2016 Morshuis, C.A.