Sea Loads On Floating Offshore Systems
Authors: Faltinsen O.M. 
Published: October 2008

High frequency and low frequency nonlinear wave loads on large volume offshore structures are discussed. The importance of ringing are pointed out. Mathematical models for slowdrift motions of floating production systems are discussed and weaknesses are pointed out. A main error source is due to damping. Hydrodynamic loads on risers are discussed.

INTRODUCTION

Environmental loads due to wind, waves and current cause mean and oscillatory motions of floating production systems, that are important in design and operation. The wave loads can be classified as wave frequency loads, low frequency loads and high frequency loads. By high and low frequency loads we mean relative to the wave frequencies of practical interest. There are mean loads included in the low frequency loads. The high frequency loads can cause springing and ringing of TLP's. The low frequency loads cause important slow drift oscillations and for a large volume structure with small waterplane area it causes also slowly varying heave, pitch and roll oscillations. By a large volume structure we mean that an incident wave length ? is sufficiently small relative to characteristic dimensions of the structure, so that the incident waves are modified by the presence of the structure. A rough estimate for a vertical circular cylinder, is that the cylinder is a large volume structure when ? /D &lt; ? (D = diameter). In the main text we will concentrate on high and low frequency wave loads and response and discuss leading edge research. The linear wave loads and responses are in general possible to determine accurately by model tests or numerical codes like 3-D diffraction programs for large-volume structures. One exception is rolling close to roll resonance of a ship or a barge, where viscous damping is important. Viscous effects cannot be totally neglected for an accurate determination of heave and pitch motions of ships with sharp corners like bilge keels (Beukelman1, Faltinsen2). Wind loads will not be dealt with in the main text. This does not mean that it is unimportant. Wind gusts can cause slowdrift motions, and wind loads represent an important mean load on a floating structure (Faltinsen3). The discussion in the main text on slowdrift damping has relevance for the damping of slow drift motions due to wind gusts. In the discussion of current loads we will emphasize the interaction between wave and current loads.

HIGH FREQUENCY MOTIONS

High frequency motions can be significant for a TLP and are often referred to as "ringing" and "springing". Springing is steady-state resonance oscillations in heave, pitch and roll. Typical resonance periods are between 2 and 4 s. In moderate sea conditions springing is excited by second order nonlinear excitation loads oscillating with frequencies 2?J, 2 ?k and ?J + ?k, where and ?J and ?k are frequency components described by a wave spectrum. The most important contribution to the vertical sum-frequency force on a TLP comes from the second-harmonic part of the second-order velocity potential (Kim & Yue4). Newman5 showed that one part of this potential is inversely proportional to the vertical distance from the mean free surface and will exist at large depth.