A Method for Rehabilitation of Offshore Platforms

Abstract

ABSTRACT Thousands of offshore structures have been erected in waters around the globe. Faced with the problems of fatigue, aging, and storm damage on these platforms, industry has for some time been seeking economical solutions for repairs. Sucha solution, employing prestressed concrete, is covered by this paper. INTRODUCTION The year 1947 ushered in a new era to the infant offshore petroleum industry, the use of the first prefabricated jacket template structure for supportingworking platforms. Just over two decades have elapsed and literally thousandsof these structures have been installed in the waters of the Gulf of Mexico, Persian Gulf, Gulf of Suez, Cook Inlet, North Sea, and Lake Maracaibo, to namea few. Paralleling this fantastic growth has also been the gradual decay of the earlygenerations of these structures. Old age and its accompanying deterioration, together with the wear and tear imposed by violent storms, has taken a greattoll. Realizing this, and faced with the rapid upward spiraling costs ofreplacement, the oil and gas industry has long sought an effective method forrepairing these units "in situ". Jointly developed by two service companies to the oil and gas industry, such aprocess is now available. This new system derives its effectiveness from theintrusion of prestressed concrete members into the legs and critical bracing ofthe jacket structure to be repaired. Each structure under consideration for repair by this process should beexamined by the owner from an economic viewpoint before a decision is made asto the extent of work to be performed. It is evident that the amount of repairnecessary to gain five more years of useful life from a platform wouldcertainly not be satisfactory for a unit requiring twenty additional years ofuse. PROBLEM DEFINITION AND SOLUTION In the late forties a prefabricated steel jacket template drilling structurewas erected in the offshore waters of the Gulf of Mexico changing for decadesto come the role of the infant offshore oil and gas industry. (Fig. 1) The development of these structures has not been without faulthowever. The worst natural enemy of steel is corrosion and ever since the firstplatform was installed, industry has fought long and hard to keep this elementfrom toppling their structures. Even with the aid of special corrosionpreventive paints and cathodic protection, the incessant rise and fall of thesea eventually triumphs and from then on corrosion of the metal cannot bereadily halted. On top of this, severe storms, usually of a magnitude far inexcess of design conditions have materially weakened or crippled manyplatforms. As the earlier structures have continued to be beaten and to age anderode away, so have design standards become more rigid. Criteria for wind andwave have increased to the point where many of the early generations ofplatforms not having been designed to withstand such forces would surely bydestroyed. Members of the oil and gas industry who own and operate offshore platforms arefaced daily with problems of replacement or repair of many older and damagedunits.