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FE-Analysis of an Integrated Plate Connection Between Jacket Structure and Skirt-Pile Sleeve - Presented during 4th Asia Pacific Structural Engineering & Construction Conference (ASPEC 2001)

Abstract

The finite element method (FEM) as a computational tool has been extensively used in the offshore industry. It has been applied both for global and local simulation to study the behavior of the offshore structures. Many finite element studies were done to study the behavior of complicated joints and connections in offshore structures. This paper discusses the behavior of the integrated plate assembly that connects the jacket structure to the skirt-pile sleeve, under operating and storm conditions, with three different wave directions, namely orthogonal and diagonal directions. The studies were based on a typical four-legged jacket structure. The studies were performed in two stages, global and local analyses.

Results obtained indicated that some components of the assembly, namely, yoke plate, external shear plate and stiffener plate were subjected to high stresses during storm conditions. The recommended solution of extending the external shear plate and making it thicker was also studied. Parametric studies were also performed in order to study the influence of the thickness variation of yoke plate, main shear plate and external shear plate, on the maximum stress.

Key words:  jacket structure, integrated plate assembly, FE analysis, Mises stress, yoke plate, shear plate.

Introduction

The finite element method (FEM) as a computational tool has been extensively used in the offshore industry. The jacket structure that supports the platform has several complicated joints. Since the early seventies, FEM has been widely utilized to study the tubular T-connection, the behavior of plate trunnions subjected to shear loads and many more. These studies are very important in advancing the use of FEM as a computational tool in oil to the pile sleeve, under operating and storm conditions with three different wave directions, namely, orthogonal and diagonal directions. and gas industry and at the same time, give us a glimpse of understanding of how the behavior of the joints or connections is influenced by various loading conditions, whether static or dynamic.

This paper presents the result of the study of the behavior of the integrated plate assembly that connects the jacket structure to the pile sleeve, under operating and storm conditions with three different wave directions, namely, orthogonal, and diagonal directions.

In order to examine the subject, the studies were based on a typical jacket structure that utilized skirt-pile sleeve connection. The studies, which utilized the ABAQUS [1] finite element software to evaluate the stresses, were performed on HP 9000, J 200 series hardware.

1.1 Model Platform

The model platform is a riser platform and located in a water depth of 74.37 meters. The platform's substructure has been designed to support facilities which include topsides, eight (8) risers, caisson and boatlandings which also serve as riser guards on both east and west faces of the jacket.

The jacket consists of a four legged structure with a vertical skirt pile at each corner leg. The frames forming the North and South faces are vertical while the East and West face frames are battered. The leg frames are fully x-braced. There are four plan levels which are all diamond braced. The base dimensions are 26m x 30.5m, and the top dimensions are 26m x 14m.

A skirt pile design was developed in preference to the conventional leg pile design to enable the pile diameter to be altered late in the detail design program without affecting the majority of drawings. This flexibility was required as the soil report was not to be available until late in the design program.

1.2 Environmental Loads

The analyses performed, ensures that the jacket and piles are capable of supporting the gas riser facilities in both normal operating and extreme storm conditions.

Environmental loads including wind, waves and current are as follows:

Operating Condition - 1 year return period

Wave height, 7.0 m
Associated period, 8.7 - 10.6 sec
1 minute mean wind speed 25 m/sec
Hourly mean surface current 0.6 m/sec
Hourly mean bottom current 0.4 m/sec


Extreme Storm Condition - 100 year return period

Wave height, 13.0 m
Associated period, 11.5 sec
1 minute mean wind speed 35 m/sec
Hourly mean surface current 0.8 m/sec
Hourly mean bottom current 0.5 m/sec


The wave directions chosen for the studies and the corresponding load : cases LC1,..,LC6 are shown in Fig.1.