For laying of offshore pipelines, the commonly known technique is referred to as the S-lay technique. In the S-lay technique, pipe sections are added to the previously launched pipeline in a horizontal position on a lay barge. The pipeline then curves over the stern of the barge, angles down toward the seabed, and curves back to horizontal and lays on the seabed. The profile of the pipeline from the lay barge to the seabed is in the form of a long “S”. Although S-lay technique has been widely utilized, the technique is limited to certain conditions, such as to limited water depth.
As an alternative, the J-lay technique has been developed. The J-lay technique is especially advantageous for deep water pipe laying. In the J-lay technique, pipe sections are added to the previously launched pipeline in an essentially vertical position. The pipeline runs down toward the seabed, curves back to horizontal, and lies on the seabed. The profile of the pipeline from the lay barge to the seabed is in the form of a long “J”. The definition of deep water, when referring to the use of J-lay technique, is a direct function of pipe diameter. This relationship is a result of the minimum water depth required for pipe of a certain diameter to achieve the proper flex from the vertical to the horizontal during the vertical laying operation.
Many different designs for J-lay technique apparatus are known in the current art. Many of the designs include a pivotable tower construction with a hoist system and a pipeline hoist cable suspended from the top of the tower. At the lower end of the tower, a static clamp is provided for holding the previously launched pipeline. The tower is provided with a mobile clamp that can engage the upper end of a new pipe section. The lower end of a new pipe section is connected to the previously launched pipeline, so that during a controlled lowering of the mobile clamp (after the static clamp has been released), the pipeline is lowered and a further new pipe section can be connected to the pipeline. Since the weight of the pipeline in deepwater situations can be enormous (several hundreds of tons), an extremely sturdy design of the tower is required.
Wilkens U.S. Pat. No. 5,464,307 discloses a J-lay technique system, wherein the tower extends downwardly from the deck of the vessel, essentially into the water. The static clamp in the Wilkens reference is provided at the lower end of the tower. The mobile clamp is movable up and down along a track formed in the tower and arranged above the static clamp. In another embodiment of Wilkens a stinger can extends downwardly from the static clamp. The static clamp in the Wilkens reference is mounted at deck level. The stinger extends into the water. The mobile clamp engages on the pipeline at a position below the static clamp and is guided along the stinger.
In many drilling techniques, vessels, particularly semi-submersible drilling vessels, are not used efficiently since drilling activities are not always needed while the vessel is under contract. Also other types of vessels are not used efficiently because those are designed for only one purpose. The Wilkens reference discloses an attempt to modify drilling vessels into pipe laying vessels. This was not successful.
Other references incorporated into this application include de Varax U.S. Pat. No. 6,361,250, and Seguin U.S. Pat. No. 6,352,388.
A need exists for an object to provide an improved pipelaying system for vertical or near vertical marine pipe laying, in particular in deepwater. A need exists for a (semi-submersible) drilling vessel that can be used effectively as pipe laying vessel for the vertical or near vertical laying of offshore pipelines. A need also exists for a floating production vessel that can be used effectively also as pipe laying vessel for the vertical or near vertical laying of offshore pipelines.
A need exists for a more efficient method for the vertical or near vertical laying of offshore pipelines.