Replacement of a Damaged Diagonal Bracing in a Semisubmersible Drilling Rig
Authors: Schippers Johan , Boonstra Hotze , Koot Piet 
Published: October 2008

Due to the collision with a supply boat one of the bracings of the semi submersible drilling rig "West venture" was badly damaged calculation of residual strength indicated the necessity of repair 1.

The size of the damage was such that almost one third of the member needed to be removed and replaced by a new section. This paper describes the establishment of an adequate procedure to be followed during the repair. Main objective was to ensure that the diagonal after the repair would have the original system length. In that case no residual stresses or deformations would be generated in the structure after the repair.

The procedure to be used was developed by analyzing the intact damaged geometry and the cutter geometry. Since the member is in compression during normal loading Conditions measures were evaluated to obtain the longest stress level possible in the member before repair started.

Calculation of the effect of ballast rearrangement resulted in optimal loading conditions before and airing the repair.

During repair work strain gage measurements were made and adequate survey was required. This enabled the comparison of calculations with actual events during repair.

The various steps during repair are being illustrated.

Introduction

In this paper the repair of a side diagonal of the semi submersible drilling rig "West Venture" is being described. The diagonal was badly damaged due to the collision with a supply boat and almost one third of the member needed to be repaired.

Though the rig is a five column-floater design with bracings forming a rigid space frame with sufficient redundancy, figure 1, it was felt that attention had to be paid to the effect of removing part of the bracing

This resulted in a structural analysis of possible effects and complications that might be expected Also measures to be taken in case of difficulties during the repair were evaluated.

A detailed procedure for repair and survey of the repair was established based on the objectives that no excessive residual stresses or deformations of the original gentry were introduced in the structure and still be able to perform the job in as little time as possible.

Necessity Of Repair

The damage to the diagonal with 2 m. diameter was reported to be severe local denting although no axial deformation occurred. In this case the damage is sufficiently described with dent size, relative depth and location along the member.

The following data were established on the dent which is presented in figure 2 after removal.–dent size : length overall 5.0 m, width 1.2 a 1.5 m.–dent depth: approximately one fourth of diameter which results to 0.58 m.–location : centre of dent 14.0 m from lower member end.

Using a procedure developed by Taby, Moon band Rashed1 the necessity of repair was investigated.

Comparing the original intact capacity for axial compression 1000 with an estimated value for the dented member a reduction of 60 a 70% was determined which was regarded unacceptable.

Struciural Evaluation Of Repair

Before the actual work started structural analysis was performed of damaged, cutted and repaired geometries under the ballast and load conditions during repair.