ABSTRACT

A method for the calculation of design data for the transport of a jack-up platform on a barge is presented. The motions as well as the internal loads and the resulting deformations are considered.

With the aid of a computer program based on three-dimensional diffraction theory the hydrodynamic characteristics of the barge are determined. The effects of forward speed and non-linear roll damping are investigated. Also the loads in the jack-up and the dynamic hogging, sagging and torsion deformations of the barge are calculated.

The above mentioned aspects of the transport are reviewed for various wave conditions including directional seas. A procedure to determine design values based on long-term statistics is proposed.

INTRODUCTION

In the past decade the offshore exploration boom created a growing market for the transport of jack-up platforms. Generally the rig operator selects one out of two transport options: "wet" or "dry" tow. For the transportation over longer distances the second option is generally preferred because the sustained speed is higher and consequently the rig down-time and the risk of meeting bad weather during transport are reduced.

During the sea transport the jack-up construction has to endure the ever changing marine environment. Important aspects of the risks concern the motions of the barge affecting the inertia loads in the jack-up legs and the reaction forces at the barge/jack-up interface. The dynamic deformations of the barge in waves combined with the interface loads are a major design factor for the layout and strength of the seafastenings.

The main particulars of the case under consideration, a barge loaded with a hypothetical jack-up rig, are shown in Fig. 1 and Table 1. The seafastening layout is indicated.

THEORY
Motion Response

In the present approach motions are calculated based on the linearized equations of motion of the vessel) which are solved in the frequency domain. These equations can be written as follows:

  • (Mathematical equation available in full paper)

The hydrodynamic (motion induced) reaction forces and the (wave induced) excitation forces are calculated by means of a computer program based on three-dimensional linear potential theory for zero forward speed.

It is assumed that the flow field can be described by a velocity potential:

  • (Mathematical equation available in full paper)

This function is regarded as the sum of independent contributions of all modes of motion and of the incident and the diffracted wave field:

  • (Mathematical equation available in full paper)

A description of the numerical solution procedure for the above potentials was outlined by Van Oortmerssen [1].

Forward Speed

The correction for forward speed effects was described recently by Huijsmans [2]. The approach is based on the assumption that the velocity potential ? (x, t), describing the fluid motion for the case the vessel moves at a forward speed U, may be regarded as the sum of three independent contributions, viz.:

  • (Mathematical equation available in full paper)

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