SEACON II: An Instrumented Tri-Moor For Evaluating Cable Structure Design Methods

Kretschmer, T.R. (Civil Engineering Laboratory ) | Edgerton, G.A. (Civil Engineering Laboratory ) | Black, S.A. (Civil Engineering Laboratory ) | Albertsen, N.D. (Civil Engineering Laboratory )

OnePetro 

ABSTRACT

An instrumented, tri-moor cable structure was implanted successfully using a phased implant technique in 2,900 feet of water in the Santa Monica Basin off Southern California. The experimental structure was constructed to provide a platform for evaluating cable structure design techniques and recent developments in ocean engineering technology.

The analytical model used to design the structure is described and predicted structure displacements due to two assumed current profiles are presented. Guidelines used in the mechanical and electrical design of the structure are outlined, and the subsystems and major components are described. The techniques which resulted in the successful implant are presented.

Although sufficient data are not yet available to evaluate in detail the analytical model used to design the structure, data obtained do indicate gross agreement between theory and actual performance.

All mechanical and most electrical equipment survived the implant. Although some component failures have occurred, the design philosophy has been successful in avoiding any subsystem failures.

INTRODUCTION

During the past fifteen years numerous analytical models have been developed to analyze the steady-state behavior of moored cable systems. These models attempt to predict the tensions in the cables and the geometry of the moorings acted on by steady-state ocean currents. None of these models yield exact solutions because of the inexact assumptions regarding structural properties and hydrodynamic loading criteria and because of errors inherent in the computational techniques. 1 Because little experimental data exist to validate models, except those for designing very simple moors, precise validation data are needed to quantify the errors associated with the various techniques. 1 The SEACON II structure was designed and built primarily to satisfy this need for data on the steady-state response of a complex cable structure to ocean currents. An additional objective of the SEACON II project was to evaluate recent developments in ocean engineering technology while implanting and operating the cable structure.

The SEACON II structure (Figure 1) consists of a delta-shaped module tethered by three mooring legs (L1, L2, and L3) in 2,900 feet of water. Legs L1 and L2 are torque-balanced mechanical cables and L3 is a torque-balanced electromechanical (EM) cable. Each leg is 4,080 feet long. The delta module with 1,000-foot-long EM cable arms is positioned approximately 500 f be10w the surface and is buoyed at each apex by a 5½-foot-diameter spherical buoy (NBl, NB2, and NB3). The mechanical cable legs (Ll and L2) are anchored with experimental deep ocean explosive embedment anchors (AI and A2). The EM cable leg (L3) is anchored by a l2,500-pound clump anchor (A3) which contains a 10-watt radioisotope power generator (RPG). The anchors are positioned approximately 6600 feet apart. An EM wire rope crown line (CL) extends from the clump anchor to an 8-foot-diameter crown buoy (CB) 50 feet below the surface. Electronics and recording equipment is stored within a removable pressure canister in the crown buoy.