Design for Welding Offshore structures
Authors: Bennett R.W. 
Published: October 2008

This paper discusses twelve basic factors that are considered important to practical and economic Design for Welding of offshore structures. When all these factors are understood, coordinated and properly applied by structural design engineers, manufacturing engineers, as well as on-the-job supervision and craftsmen, weldments can be produced at a realistic cost, delivered on schedule, and have the properties to provide reliable service. Examples are given that illustrate the influence and limitations that metal properties, weld-joint designs, welding methods, and procedures have on the making of crack free welds with the dynamic and static properties required by properly drawn and sufficiently inclusive specifications.

INTRODUCTION

The rapidly increasing needs to develop the resources of the oceans has created a challenge to the engineering professions to provide the equipment necessary for men to work in a new and special environment. Many science and engineering disciplines must be integrated to design and build equipment that will function effectively, safely, and economically. The engineering challenge is particularly demanding for offshore equipment that must operate over the wide range of environmental conditions that are to be encountered in our oceans, seas, and lakes.

Engineering organizations involved in the design and operation of offshore structures and equipment have conducted extensive studies on the characteristics and magnitude of many large forces that must be resisted during severe storm conditions. They include the influence of waves and their frequency, as well as wind forces and ambient temperatures. Thus, unusually adverse potential environmental forces must be added to the expected operational loads and other stresses in the design and engineering of an effective, reliable, and economical structure. The organization responsible for engineering an offshore structure has the responsibility for establishing design specifications in sufficient detail to set forth structural features, to select' materials having the appropriate dynamic and static strength characteristics, and to define necessary quality-control measures. The effectiveness with which drawings, specifications, and control requirements are applied to the fabrication of the structure will directly influence its operating reliability and safety, as well as fabrication and maintenance costs.

In spite of the wealth of engineering information that has been developed and disseminated during the past twenty years, cracking and failures of structural metals and welded joints are frequently encountered during fabrication and service of all kinds of structures, whether they be offshore units, pressure vessels, ships, bridges, or piping, to name but a few. The detection and repair of cracks and other deficiencies during fabrication will determine the amount of profit or loss to the manufacturer. Late delivery resulting from excessive rework can, be a financial penalty to the owner and/or operator because of the consequent delay in start-up and use of the equipment. However, the potential losses in terms of life, money, time, salvage or replacement, and litigation that can develop from structures that fail in service are of a magnitude far beyond the losses encountered to correct deficiencies during the fabrication stage of the structure.