Boat Impact Ultimate Capacity Analyses of Jacket Structures
Authors: Gidwani J.M. , Renault J.P. 
Published: October 2008

In the North Sea, in 45 meters of water, four different jacket structure concepts were analyzed to failure from boat impact loads. These concepts were (1) conventional jacket configuration 1, (2) grouted jacket configuration 1, (3) conventional jacket configuration 2, and (4) cast node jacket configuration 2. Each concept was analyzed to failure for two load conditions, one on jacket leg, and two, on the jacket brace. A total of eight load cases were considered. Nonlinear inelastic structural analysis methodology was established and used for the purpose. Results from this study can be used to evaluate the relative cost/performance statistics of different jacket configurations. The developed methodology can be effectively used for reassessment and maintenance of existing jacket structures.

INTRODUCTION

During their operating life, on site jacket structures may be subjected to boat impacts. It is useful to know the boat impact intensity a structure can withstand without considerable damage or disruption of normal operations. Furthermore, it is advantageous to know the maximum impact intensity that could cause platform failure.

The kinetic energy from boat impact is primarily absorbed as strain energy by the structure. Strain energy absorption criteria may be satisfied by the elastic response or by the inelastic straining of the structure and/or soil elements. The inelastic response also provides a measure of the overall platform ductility.

Four different jacket structure concepts were designed for a given location in the North Sea in 45 meters of water.

The purposes of this study were the following:statically increment the boat impact loads to platform failure. These failure loads would then be the maximum loads the structure could withstand;calculate the maximum strain energies the structures absorb for the maximum loads from step 1 above;provide the failure mechanisms and sequence of inelastic events;define the overall platform ductilities; and,evaluate the cost/performance of each of the concepts.

Each concept was analyzed for two load cases, one representing impact on the brace (load case 1) and the other reflecting impact on the leg (load case 2). A total of eight separate analyses were performed.

Four separate fully nonlinear structure-foundation models were created. Computer program INTRA-ISEC?(ref.l) was used. All analytical work was performed on in-house Sun Microsystem's SPARCstation? GXP series of workstations.

ANALYSES PROCEDURES

Configuration Description

Jacket number 1 is a conventional jacket with brace configurations shown in figure 1. The deck is supported on four legs. Piles are driven to a depth of 50 meters below mudline. Shims are used to connect the piles and legs at the mudline elevation of (-) 45 meters, and at elevations (-) 34, (-) 19, and (-) 6 meters, respectively. At elevation (+) 4.246 meters, the piles are directly connected to the leg.

Grouted jacket number 2 is similar to jacket number 1 except for two differences. One, the pile-leg sleeves are grouted, and two, all vertical braces and horizontals are grouted.

Layout of conventional jacket number 3 is shown in figure 2. Note the different brace configuration for this concept compared to jacket number 1. Shims are used to connect the piles and legs at the conductor bracing elevation of (-) 44 meters, and at elevations(-) 23.8, and (-) 7.9 meters, respectively.