A feasibility study of the Versatruss system

Abstract

For the installation of a topside onto jacket, lift method with crane barge has been a common concept. However the increasing weight and offshore exposure hours has led designers to consider the float-over method as an alternative. Float-over installation exceeds the maximum capacity of crane vessels and allows platform topsides be installed as a single integrated package without heavy lift crane vessel. This allows topsides to be completed onshore eliminating the substantial costs associated with offshore hook-up and commissioning, reducing the offshore exposure hours according to ref. [25].

This thesis investigate the float-over technology Versatruss which uses two floating barges equipped with lifting booms to install a topside. Today it a trend in the offshore business to pursue low CAPEX. Therefor a cheap Standard Viking barge (day rates 40-50 000 NOK/per day), which is relative cheap compared SAIPEM S7000 and THIALF where the day rates is (6 000 000 NOK/per day). The barges are maneuvered around the legs of the jacket and the topside is lowered down to mate the topside onto the jacket. During the mating operation the topside and jacket experience impacts through the contact points.

The effect of impact is converted into a limiting acceleration and velocity, and the accelerations and velocities are evaluated with non-linear time domain analysis in SIMA. The purpose of this report is to find the limiting wave height for the Versatruss system based on the limiting acceleration and velocity. The structural limitations has not been investigated.

In order to do time domain analysis a conceptual model of the Standard Viking barge is made in GeniE and a hydrodynamic analysis is done to get the correct input (added mass, stiffness etc.) in SIMA. In addition hand calculations are done to verify the results from the hydrodynamic analysis. In SIMA the system is coupled in a Versatruss configuration and non-linear time domain analysis of the system is done with different wave heights, periods and directions.

In SIMA the model is coupled with fenders and wires to satisfy the assumption that the barges are free to move relative to each other. Since the Versatruss lifting system is allocated along the centreline of the barge individual roll motion of the barges does not influence the motion of the topside.

To assure that the results are correct a small time integration interval of 2,5e-3s is chosen. The most probable maximum (MPM) is taken from several analysis and the value with 10 % probability of exceedance (P90) is taken as the maximum value. This led to the limiting significant wave height (HS) and peak period (TP). In addition bootstrapping (ref. Section 3.5) is done to illustrate the confidence behind the numbers.