Design and Construction of the World's First Rock-Anchored Jacket

ABSTRACT A rock-anchored jacket as an economical support for offshore facilities on seabed rock and its designing, construction and quality control testing method are described to provide a guide to the design and construction of similar structures INTRODUCTION The rock anchored jacket method provides an economical support for various structures including intake facilities to be installed on seabed rock. The location of the jacket on seabed is dredged to a depth as required before it is installed. A hole of .a diameter and depth as specified will then be drilled in the seabed rock through the jacket leg. A tension member (bars or wire strands) will be inserted into the hole and cement-grouted to be anchored. When the cement group has been set, the tension member will be fixed at the top of the jacket under tension. Thus, the jacket will be fixed in the seabed rock in a pre-compressed condition. When horizontal forces such as wave force, current force, etc., act on the jacket, it will resist overturning, sliding, etc., by bearing capacity of the seabed rock, pretension of the rock anchor and tension being caused therein. For the intake structure of a thermal power plant constructed at Tocopilla, the Republic of Chile, South America, it was judged most advantageous to use the rock-anchored jacket method from the viewpoints of technique, cost and time. However, there have been no cases of its application to permanent offshore structures and, moreover, nothing has been clarified as to its designing, construction and completion test method as well as its evaluating method. The authors have confirmed the safety of the rock anchored jacket and completed it, as a structure, by performing 1) a basic test on the anchoring mechanism; 2) an assurance test of anchor force at site; and 3) a full-scale creep test, etc., in addition to a finite element analysis and a rheological model study. Special corrosion prevention measures have also been made for the tension members. OUTLINE AND BASIC PLAN OF PLANT INTAKE FACILITIES The intake is a deep water intake system by syphon. The intake-head was located 100 m off the coast line at a depth of 9 m. The site investigations revealed the following characteristics on the sea and seabed conditions:The location of the intake structure is in the midst of such a severe sea condition as being directly hit by high waves of the Pacific Ocean. They form the breaking waves at the coastal zone.The seabed consists of a sandy layer of 0.5 m with a layer of hard andesite underneath.The hard andesite has an unconfined compressive strength of more than 6 × 104 KPa