University of Twente Student Theses


A standstill to get further : Leg sea-fastening system for frequently used jack-ups

Attema, S. (2008) A standstill to get further : Leg sea-fastening system for frequently used jack-ups.

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Abstract:Before sailing or towing of the platform, the legs of jack up platforms need to be sea fastened in order to prevent the platform and the legs from damage due to the movements of the leg originated from waves, wind and current. Also disturbing noises and vibrations from the legs hitting the platform need to be avoided because of the crew. In this thesis a new concept for leg sea fastening is discussed in order to meet the demands set by GustoMSC as far as possible. The thesis was conducted at GustoMSC in order to come up with a new leg sea fastening system as an alternative for the system used at Bard I platform with octagonal shaped plated legs. At Bard I sea fastening is done by pushing the leg to one side with hydraulic cylinders clamping the leg between two guides and the hydraulic cylinders. The purpose of this thesis is to come up with a solution to improve: Costs, safety, reliability, engagement time and costs and the necessary time for designing and calculation of the system. As a result of a force analysis, section 3, the decision was made to design the system only for field transit conditions, section 3.2. This decision leads to a cheaper design, but also a design which is only suitable for field transit and hence not usable for ocean transit conditions. Several alternative leg sea fastening systems had been designed. These have been compared to each other in section 4 with a multi criteria analysis on the demands earlier set. The chosen design from out the evaluation, is the double wedge variant. This system consists of two pairs of wedges, one pair placed at lower guide level and one pair at upper guide level. One wedge of each pair placed at the leg, the other at the jacking house. When retracting the leg the wedges run into each other by which the leg is forced sideways against the guides. The jacking house wedges all have a lubrication system for engagement to obtain an optimum wedge angle of about seven degrees for optimization of the pull out force and the force through the leg, as found in section 5.2. However, in order to create a tight fit the upper wedge at the jacking house needs to be adjustable mounted. The choice is made for a top spacer system, section 5.3. This system contains of six adjustable M20 bolts connected to an impact plate where the wedge runs into to create an adjustable connection. For disengagement the wedge can run into a console placed lower. Because of the guides the upper wedges cannot be placed on the stiffest parts of the leg, and should hence be placed on the leg hull, due to this the leg needs to be stiffened with the triangle shaped stiffening as described in section 5.4. This consists of two HE340A beams of 4.2 meters long and 2 extra stiffeners of the leg hull. These stiffeners are placed behind the wedge. For comparison the double wedge system was compared with the existing Bard I system. All concluded the features for improvement were partly met, as discussed in section 6. When costs will be referred to as weight the double wedge system might improve this (+). Both systems would not cause great safety hazards (+/-). Reliability might improve due to less moving parts, but overload should be prevented (+/-). Engagement time and costs are reduced because the system is passive (+). Calculation time however will increase because of the stiffening of the leg and calculation of the wedges would be more difficult than calculation of the jacks used on Bard I (-). The systems complexity of the sea fastening system would increase when choosing the double wedge system in exchange of faster and cheaper engagement.
Item Type:Essay (Bachelor)
GustoMSC, Schiedam, The Netherlands
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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