Shape factors for five different configurations of a triangular jack-up leg have been determined through load measurement on scale models in a wind tunnel.
The drag, lift, and yaw-moment shape factors of the different configurations were determined in the search for an optimal arrangement of the three raw water pipes that minimize the current load on the combined jack-up leg - raw water pipe - structure.
In connection with a capability study for the DAN EARL, Lauritzen Offshore had a marine consultant company calculate the anticipated loads on the leg structure.
In this connection Lauritzen Offshore considered modifying the present raw water arrangement because the Department of Energy (DEN) had issued a Safety Notice No. 10/89 dated July 7, 1989 requiring the presence of an individual fire supply system. It was thus considered prudent to evaluate the overall loading situation in connection with such a modification.
The present raw water supply arrangement is arranged at the port leg, where four I-beams are attached to the leg structure, holding a set of three riser pipes with submerged pumps and electrical motors attached at the bottom of each pipe.
In fulfilling the DEN-requirement Lauritzen offshore was evaluating the possibility of maintaining two of the pumps at the port leg (close to where the connection manifold is located) and then locating one new pipe at the starboard leg. In order to minimize forces, a modification of the present arrangement was also considered which would present arrangement was also considered which would place one riser pipe at each chord of the port leg. place one riser pipe at each chord of the port leg. Irrespectively of what was considered, it was recognized that the influence such modification had on the overall load situation was not known.
From a literature study it was obvious that the selection of the drag shape factor was one of the most important factors in this respect. Through comparison it was found that there appeared to be several calculation models available (i.e. a random and a deterministic model), and that there were a variety of values for the drag shape factor which the marine consultant used and values that were used by the classification societies. In addition, most models only described the K- and X-brace design. The overall leg shape, including the rack / vertical ladder / jetting pipes and raw water supply lines seemed not to be described pipes and raw water supply lines seemed not to be described by standard shape factors due to the complexity of the members.
Since a slight difference in drag shape factors can result in a significant difference in the load, Lauritzen wanted to check not only the "pure" shape factor for a leg, but to carry out a set of tests which might show the most suitable location for a raw water arrangement. Further tests were conducted to investigate the contribution from the beams in an effort to determine whether the existing design could be improved.
In collaboration with the Danish Maritime Institute and our marine consultant a test programme was prepared providing shape factors for the various situations providing shape factors for the various situations encountered in connection with such modifications.
A model of the lower part of the jack-up leg previously used for hydraulic studies was available for the current load determination. The model was on the scale of 1:25 and a section of three bays with a total full-scale length of 14.6 metres was cut out of the existing model and used for the load measurement. A drawing of the bare model leg section is shown in Figure 1.
Shape factors were determined for the various configurations using the section model of the jack-up leg in a uniform air flow in a wind tunnel.