ABSTRACT

Global wave loads measured on the Tern platform during a very severe storm have been compared with predictions made on the basis of three different models for wave kinematics The first IS the "Newwave" theory, a broadbanded, probabilistic-based model for the extreme waves of a random seastate The second model involves complex time-domain simulation of random directional seas Both these theories are used with the Morison equation and realistic force coefficients to predict global forces The third wave model IS the Stokes flfth-order theory with artificial values for the force coefficients, as used In conventional design practice

Both the Newwave theory and the random directional simulationise add to predicted base shear forces that are comparable to the measured values over a wide range If non-hear effects In the ocean surface are accounted for, then the Newwave theory predicts the base shear generated by the largest wave of the storm with a very high degree of accuracy In contrast, calculations based on the Stokes kinematics of conventional design practice over predict the measured loads over the whole range studied

It appears that Newwave IS the most suitable available wave theory for routine use In the design and analysis of structures such as Tern

INTRODUCTION

The computation of wave forces on offshore, space-frame structures involves two steps The first is the determination of fluid kinematics around the structure associated with waves and currents The second stage IS the calculation of forces on the platform generated by the fluid motion Thus, realistic predictions of global loads require realistic, verified theories for the kinematics of the flow beneath the surface waves

In mid 1989 a structural monitoring system was put Into operation by Shell U K Exploration and Production on the Tern platform The platform IS located In the UK sector of the northern North Sea, as shown on Figure 1, and stands In a water depth of 167 m

The platform comprises a pilled steel space-frame sub-structure and a module support frame (MSF), which carries the topside facilities including drilling, process and accommodation packages The jacket was designed for an estimated 100-year wave of 30 5 m (see Figure 2)

Figure1 Platform location (Available in full paper)

Figure2 Jacket and MSF configuration and storm wave heights (Available in full paper)

The monitoring system on Tern (see Figure 3) consists of 68 strain gauges positioned near the base of the four corner legs, a wind sensor placed on top of the derrick, two wave-height sensors at different locations, and two water-particle velocity meters attached to the jacket at elevations of -13 m and -41 m The strain gauge readings are used to establish the axial load, bending moment (in two orthogonal directions) and shear force (In two orthogonal directions) for every instrumented brace and leg These member forces are then combined to give base shear and overturning moments for the entire structure Four accelerometers were also Installed to monitor the dynamic behaviour of the structure The specific objective of the system was to measure both the platform's global storm loading and environmental conditions at the same time

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