The Dry Transport of the Green Canyon Tension Leg Wellhead Platform by a Semisubmersible Heavy-Lift Ship

Abstract

A Tension Leg Platform poses unique challenges with respect to its dry transportability. The engineering that is required before such a transport can be effected is given step by step. The development of a new type of cribbing block is described. The predicted extreme design environmental conditions and resulting extreme motions are compared with the actual monitored weather conditions and motions. From the correlation, conclusions are drawn. INTRODUCTION On May 26, 1989, the first TLWP (Tension Leg Wellhead Platform) was delivered in the U.S. Gulf by a self-propelled semisubmersible heavy-lift ship. This ship departed Singapore only 41 days earlier after loading the TLWP by means of the float-on method, see figure 1. The dry transport of this platform broke new ground in various areas. The unique nature of the TLWP required various unorthodox solutions. Conventional design methods were revised and new materials were developed. The Green Canyon block 184 TLWP was built by a shipyard in Singapore, Republic of Singapore, which contracted with a HeavyLift Transporter for the dry transport of the TLWP to Pascagoula, Mississippi, where it was to be final outfitted before installation in the U.S. Gulf. Dry transport by a self-propelled heavylift vessel offers the highest degree of safety in combination with a minimum of environmental loads on the cargo. A wet tow was not a realistic option given the distance of over 12,000 miles and an anticipated tow speed of 2 - 3 knots. Transportation by tug/barge was considered, but the increased safety of the self-propelled option proved to be the most attractive. The TLWP is constructed from stiffened cylindrical shells. The truss deck is carried by four columns of 12.2 meters in diameter, 46.2 meters in height, spaced 42.67 meters apart (center to center). Pontoons, 7 meters in diameter, connect the four columns near the base. At the moment of load-out, the TLWP's displacement was 8,400 T with its center of gravity at 31.8 meters above its base. The draft measured 8.3 meters, leaving a freeboard on the pontoons of 1.2 meters. These pontoons were outfitted with bollards to facilitate the connection of tugboats and winch wires during on- and off-loading. Given the relatively small footprints of the columns, in combination with the fact that these would mostly overhang the carrier's 40 meter wide deck, four sponsors were required to increase the support area. Much attention was paid to the footprint loads in order to obtain the optimum support configuration.

Rolling response when navigating in sea environment with carriage of heavy-lifts

“…There are two commonly used inertial force calculation methods at this stage, namely:1) Based on the significant wave height and periodic joint distribution frequency of sea area waves, the wave once in 10 years is predicted as the design wave to carry out numerical simulation of ship motion and load response. Calculate the inertial acceleration of the structure through the simulation results to calculate its inertial force [1][4] ;2) According to the design requirements of sea area sailing time and exceeding probability, combined with the significant wave height and periodic joint distribution frequency of sea area waves, determine the response of the ship's numerical simulation design wave to the ship's navigation, and use the simulation results to calculate the inertial force [5][6] .…”

“…It is assumed that the sea state of the target sea area is a random stable sea state for 3 hours [4] , the sea state with a significant wave height greater than 4m is a storm sea state, the total time for the ship to sail through the sea area is T, and the number of stable storm sea states experienced by the ship is N. According to the design requirements of the exceeding probability P(E), the probability P(3h) that the ship safely navigates through a 3h stable storm sea state can be calculated as follows [5][6] :…”

Challenges of Heavy Load Logistics in Global Maritime Supply Chains

“…With these points in mind, the following table can be seen as a starting point to DfL and DfT in maritime transport for designers of heavy load products. It provides an overview of issues the designer has to take into consideration [15]: • Suitable loading equipment and skilled staff It has to be noted that this discussion is rather high level. In order to practically apply this or similar approaches in practice, a more detailed and thorough investigation, including physical design elements has to be considered.…”

The influence of a quay crane sea transportation on its further exploitation

“…Based on the limited data available, the ZHEN HUA 11 vessel was modelled based on the hull of a standard bulk carrier, converted into the crane carrier, with identical dimensions (similar to the real -full scale -conversion), see Van Hoorn (2005), Van Hoorn andDevoy (1990). For the computer model of the resulting hull (Fig.…”