Background to the Use of Structural Clamps in the Strengthening of Two Gulf of Mexico Platforms
Authors: Lalani M. 
Published: October 2008

This paper presents the background to the design of eight stressed grouted clamps used to alleviate fatigue problems associated with T and X joints on two platforms in the Gulf of Mexico. Offshore design codes do not cater for the design and specification of clamp systems. The paper therefore addresses the design philosophy and approach adopted, together with the specifications and procedures developed during the design stage.

The clamp technology discussed in this paper represents the first major application of such systems in the Gulf of Mexico, and follows on from their extensive use for platforms in the North Sea. The need to strengthen or repair jacket structures has been increasing over recent years, and the clamp technology described is recognised as cost-effective, since it offers an alternative to the use of underwater welding techniques. The design of the clamp system is reported, covering both the static and fatigue aspects. Procedures related to sealing, grouting and bolt stressing are described with specific reference to the Gulf of Mexico environment.

Clamp systems are expected to find increasing favour in the strengthening and repair of existing installations in the Gulf of Mexico and elsewhere. The systems described in this paper have been designed and installed on two Gulf of Mexico platforms, and it is demonstrated that such systems are both technically efficient and cost-effective. Clamp systems are extremely versatile, and can be adapted to meet the strengthening/repair needs for all forms of damage, eg. fatigue cracking, corrosion, supply vessel collision, dropped objects, static overload or design omission.

INTRODUCTION

The repair of existing offshore installations is now an important part of offshore engineering. Design codes provide little or no guidance in this area. The need to repair or strengthen offshore installations is likely to increase as the number of installations increase and as the existing structures get older. In addition, this need becomes more likely as shipping movements in the oil and gas fields increase and as more becomes known about the environment in which installations operate.

There are anumber of causes of damage or defect which may require repair, and include:-dropped objectsinstallation damagevessel collisioncrack-inducing fatigue loadsdesign upgradecorrosionwelding and/or fabrication fault. This paper is concerned with the repair and strengthening of two Gulf of Mexico platforms, using clamping technology. The paper has been divided in two parts:-A - BackgroundB - Design philosophy, specifications, and procedures The design, fabrication and installation of the clamps were carried out in 1990. This paper concentrates on the design aspects.

PART A - BACKGROUND

Damage Appraisal

The two Gulf of Mexico platforms are referred to in this paper as Platforms 1 and 2. A part plan at El (-)120' is shown in Figure 1, and a part plan at El (-) 225' appears in Figure 2. Both platforms are similar in configuration, and the plan arrangements shown can be considered to be typical of both platforms.