Methods of Strengthening Existing Offshore Structures
Authors: Tannahill C.A. 
Published: October 2008

This paper reviews methods of internal and external reinforcement to improve the structural integrity of existing offshore platforms in the Gulf of Mexico. It describes briefly the use of conventional reinforcement and grouting and gives 9 report on the onlyplatform to date reinforced with prestressing cables and high strength grout. External support by skirt piling and bracing is considered and compared to the use of supporting structures. The problems and analytical procedures for supporting a deficient four-pile platform with a new four-pile structure are also covered.

Introduction

Hurricane Hilda struck the Louisiana Coast in early October, 1964, causing damageto the petroleum industry estimated at ﹩125 million. It virtually ended use of the "25-year storm" design criteria by operators in the Gulf of Mexico. It also stirred a comprehensive appraisal of existing structures witha view to modifications which would enable them to withstand a future storm of like intensity. Much of the damage resulting from thisparticular storm was suffered by platformsbuilt with the deck too close to the water. In some instances, the danger of future loss was reduced by raising the deck elevation to enable the cellar deck to clear the crest of waves of this magnitude. Reduction of the area exposed to the extreme forces of the wave crest lessened the degree of overstress in many platforms. It did not, however, change a 25-year storm design into a 100-year storm design. The need for additional strength encouraged the development of adequate means for internal reinforcement of existing members or the provision of some means of external support.

Internal Reinforcement Of Existing Members

Internal reinforcement of existing members has the inherent advantage of achieving greater strength without increasing the area subjected to wave forces.- Early methods utilized the obvious materials available at time - conventional steel reinforcing rods and cement grout common to well cementing practices with a compressive strength of approximately 5,000 psi. Later developments introduced the use of prestressing cables or tendons and a nylon reinforced grout with a compressive strength of 15,000 psi.

Conventional Reinforcement and Grout The use of conventional reinforcing rods for additional pile strength required accessto the pile from an open end. For this. reason, it was economically feasible only in cases where the deck was to be removed in the process of raising to a higher elevation. Figure 1 illustrates the various steps taken in elevating the deck and strengthening a typical offshore platform.Existing well conductors were inter connected and tied to the cellar deck.A small areas of the cellar deck surrounding the wells was cut loose from the remaining deck.All deck columns were cut above the top of the pile and the deck removed.The internal mud plug was removed from the pile by jetting or drilling to the desired depth.A cage prepared from conventional reinforcing rods was lowered into each pile and covered with grout.Spool pieces were installed to provide the added deck elevation desired.The deck was replaced.