Design of SYU Jackets for Wave Impact During Trans-Pacific Tow

ABSTRACT This paper presents methodology used to design the SYU jackets for wave immersion and slamming during the transpacific tow from Ulsan, Korea to the Santa Barbara Channel, California. This methodology has direct application to the design of deepwater structures for ocean transportation. Results given in this paper show the extent and severity of wave slamming and immersion predicted for the SYU jackets. Structural modifications developed to mitigate potential damage from immersion and slamming are also presented. Finally, a synopsis of the underlying assumptions associated with the methodology is presented to help readers obtain a better understanding of the results and their impact on jacket design. INTRODUCTION As the petroleum industry moves into deeper waters, numerous new technical challenges must be solved by design engineers. One of the challenges we faced for SYU was to design the deepwater jackets to withstand loads from wave slamming and immersion during the transpacific transportation. Immersion and slamming produce additional loads on jackets which are not traditionally included in jacket transportation analyses. Immersion can be an important consideration for deepwater structures like compliant towers and steel piled jackets. Wave immersion and slamming occur when jacket members, overhanging the barge, enter the water as a result of sea conditions and motions of the jacket and barge (Figure 1). Immersion refers to the submergence of members below the water surface. Entry into the water, which results in a "flat" impact with the water surface, is referred to as slamming. Slamming can be thought of as a subset of immersion. Our evaluation of immersion consisted of two main analyses:global immersion andlocal immersion, Immersion of overhanging portions of the structure usually involve multiple members. To evaluate the net effect on the structure, a global analysis was used. We used the local immersion analysis to evaluate the effects of immersion forces on individual members and to determine reaction forces on their supporting structure. Results from these analyses were used to identify areas on the SYU jackets that required strengthening to withstand immersion loads. METHODOLOGY Local Analysis The assessment of loads on individual members is referred to as the local analysis. Local immersion loads are calculated using a statistical approach based on linear frequency domain analyses, This approach is used to determine "design slam and immersion" events for each jacket member. Design events are associated with "expected" maximums. In the local analysis, member strength is evaluated on an individual basis. Loads from adjacent members are not included in the assessment of local member strength. The local analysis procedure is outlined below.Vessel motion transfer functions (RAOS) are generated using 3D diffraction theory methods. Displacement and velocity transfer functions for each member in the jacket are then developed from the vessel motion RAOS using rigid body kinematics. These frequency dependent transfer functions contain both amplitude and phase corresponding to each wave of unit amplitude and frequency ?.Relative displacement (?m(?)) and velocity (?m(?)) RAOs for each member are prepared.