FulI-Scale Load Test of Pile Gripper for Bullwinkle

ABSTRACT A full scale load test was required on a prototype pile gripper as part of its performance specifications. The test was conducted on a fully instrumented 84 inch diameter by 2.25 inch wall thickness, A572 - Grade 50 steel pile held in place by the gripper. Loading was increased in steps to a value of 7,50O kips. The test was successful and the prototype unit (one of four) was installed on the 1363 feet water depth Bullwinkle jacket. The four units were utilized during the platform installation, securing and stabilizing the jacket before grouting the "pin" piles and further pile driving. INTRODUCTION Pile grippers are constructed similar to grout packers. Each unit contains two inflatable inflatable actuation elements (nylon-reinforced rubber bladders) which force independent sets of flexural gripping members (steel fingers with machined teeth) against the surface of the pile. The entire unit is enclosed by an external steel containment cylinder. Grippers are usually mounted on %p of corner pile sleeves and serve as temporary structure to pile connections. Normally only used on an all skirt pilled jacket, their primary purposes are to provide some level of early resistance to storm loading and to prevent relative movement between the pile sleeves and piles during the time the permanent jacket to pile connection, consisting of grout, is setting and gaining its initial strength. A secondary purpose of the units comes in to action if a structure is set down or loaded such that it becomes out of tolerance with respect to levelness, Specific corners can be secured to a pile after adjustments are made and before installation of the other piles. As platforms were located in progressively deeper water, it became desirable to develop a fast method of attaching "pin" piles to help the temporary foundations (mudmats) in resisting loadings during installation. Attachment of piles penetrating only under their own weight would even be of great assistance. In the extreme, with as little few as one pile driven to adequate penetration at each corner, design events could be tolerated, reducing risk, In the design of the Bullwinkle platform, reduction of installation risk was a prime concern. A two-piece jacket was even considered (Ref. 1). The planned June installation could not be assured five years in advance, Early analyses indicated the jacket with no conductors installed, most legs still buoyant and no deck loading could safely survive major hurricane waves with only one pile driven to design penetration at each corner. The maximum pile load which could be resisted immediately after driving (50% of the design ultimate) was predicted as 7,600 kips compression or 5,000 hips tension. Shell had experience using grippers on two projects, platform Eureka a 700 feet water depth structure installed offshore California and the Boxer platform (Ref. 2) installed in the Gulf of Mexico in 750 feet of water. Eureka had four 1,000 ton units for 60 inch diameter piles.