Fatigue Design Of Driven Piles For Deepwater Applications

Some codes of practice call for the consideration of the fatigue damage caused to piles during both installation and inplace operations. However, no codified methodology or criteria for fatigue design exist. Recent efforts to revise the foundation design for tendons in API RP2T made abundantly clear the need to establish both procedures and criteria. This paper addresses the load and criteria aspects involved in the fatigue damage evaluation of welded piles due to installation and operation, including piles with protruding attachments, such as padeyes for mooring lines or receptacles for tendon bottom terminations. A methodology to evaluate local stress histories due to driving is discussed, including pile response to driving, local and residual stresses, and cycle counting. A review of practices with respect to safety factors is presented and safety factors consistent with fracture-critical components are suggested. INTRODUCTION Driven piles are used to anchor floating structures to the seafloor via a mooring system. For floating production, storage, and offloading ships or deep draft caisson vessels, the mooring system consists of a combination of chain and cable lines, whereas for TLPs the system is a tendon made of welded and connected tubular members. Typically, each mooring line or tendon is anchored to a single pile. The means by which the mooring connects to the anchor pile is also different between lines and tendons, resulting in different loads applied to the pile. For the mooring line option, the anchor pile is primarily subjected to bending. Tendons, on the other hand, impart tensile axial loads on the pile. The distinctive characteristic of anchor piles for mooring lines is that the connection between the pile and the line is via an external padeye. Padeyes are needed due to the presence of a chain at the pile end. The preferred option is to make the padeye itself by a casting process, insert it in a window cut on the pile wall, and weld it to the pile wall using a two-sided groove weld. Alternatively, the padeye can be fabricated from plate and welded directly onto the pile wall. Because of the limitations on plate and weld sizes and the amount of welding that may be required, the fabricated padeye tends to be larger and intrinsically more fatigue susceptible than its cast counterpart. However, fabricated padeyes can afford more flexibility in fabrication schedule and, thus, offer potential cost savings. Anchoring tendons to piles requires more sophisticated mechanical connections, which may include a tendon receptacle at the top of the pile (Fig. 1-a). The receptacle often has internally protruding elements, such as shear lugs (Fig. 1-b), into which the tendon bottom termination latches. The attachments can be sufficiently large to warrant consideration of the effect of their mass inertia on the local stresses in the pile wall as the pile is driven. The fatigue life of the pile-mooring connection is dictated by the damage accumulated during installation, as the pile is driven into the seafloor and, during operation, as the mooring maintains station of the floating hull. Hence, the fatigue life of the connection may be controlled by either of those two conditions alone, or by a combination of both, depending on the application. Therefore, evaluation of the fatigue damage during driving needs to be addressed. One design example involving the fatigue damage at a girth weld in a pile anchoring a tendon is given in [Hunt et al]. The methodology is based on wave propagation and FE analyses. This paper discusses the methodology to arrive at the local fatigue load history during driving when attachments are present besides girth welds. Relevant fatigue criteria, including residual stresses and post-weld heat treatment and safety factors are also addressed. Although the connection of anchor piles to the mooring units can be significantly different in configuration and principle, the methodology to arrive at the damage inflicted to the piles during driving is the same. FATIGUE METHODOLOGY The fatigue methodology includes development of the acting loads due to driving, evaluation of the resistance of the pile to fatigue, and conversion of these loads to local stresses to enter the damage calculation. Evaluation of the in-place or Proceedings of The Thirteenth (2003) International Offshore and Polar Engineering Conference Honolulu, Hawaii, USA, May 25–30, 2003 Copyright © 2003 by The International Society of Offshore and Polar Engineers ISBN 1–880653-60–5 (Set); ISSN 1098–6189 (Set)