The condition of the pipe is closely monitored during the pipelay operation by a measurement system that provides state measurements from filtering of sensor data and from state estimators. The instrumentation varies with different vessels based on size, operational depth and installation method, and the following measurements are considered here:
1. Vessel position and velocity 2. Touchdown position 3. Axial tension 4. Departure angle
2.6. Measurements 33
5. Roller pressure 6. Distance to last roller 7. Free–span pipe length 8. Touchdown distance 9. Water depth
10. Environmental loads (current, wind, waves).
Vessel position and velocity
The vessel position may be obtained with high accuracy from a number of available position reference systems, including the global positioning system (GPS), hydroa-coustic position reference (HPR) and microwave position reference systems (Bray, 2003). Since pipelaying is done over long distances, using GPS with corrections (DGPS) is very common and provides an accuracy of up to 0.1 meters. GPS can also provides vessel velocity measurements, however, DP systems commonly applies an observer to compute the vessel velocity.
Touchdown position
A remotely operated vehicle (ROV) is used to hover over the touchdown point and provide a visual image. It is not easy to define the exact touchdown point since the seabed is uneven and penetrable, so the position of the touchdown point ptdis commonly assumed to coincide with the ROV position. Multiple ROVs may also be used to monitor other parts of the pipe in the water.
Axial tension
The purpose of applying tension to the pipeline through tension machines is to control the curvature of the sagbend and the moment at the stinger tip through supporting the submerged weight of the suspended part of the pipe. The ten-sion exerted on the tenten-sioners from the pipe depends on the pipe properties and configuration.
Departure angle
The departure angle of the pipe leaving the stinger is estimated from the contact force on the rollers and/or closed–circuit television (CCTV) equipment on the stinger. An equivalent term is lift–off angle.
Roller pressure
Rollers are spaced out along the stinger, supporting the overbend and reducing the strain. The rollers are equipped with pressure cells to measure the contact force from the pipe. Theses measurements are used to ensure that the pipe follows the
34 Offshore Pipelaying
stinger smoothly, and good practice indicates that the pipe should lift off from the stinger before the last roller. CCTV equipment may also be used for this purpose.
Distance to last roller
Frequently, the pipeline is required to lift off before the last roller on the stinger.
Hence, monitoring in this area is of importance, and laser measure or similar device can be used to measure this distance.
Free–span pipe length
Since each link joint is numbered sequentially, visual identification of the joints at the touchdown point and at the vessel means that the length of the suspended pipe L can be found.
Touchdown distance
The touchdown distance is the horizontal distance between ptmand ptd, also known as the lay–back distance.
Water depth
Measurements of the water depth are readily acquired from acoustic sensors located on the pipelay vessel. At the touchdown point, the ROV will measure its depth using pressure sensors.
Environmental loads
Wind will influence the vessel position, but the direction and speed of the wind is easily measured and compensated for by the DP system. Currents and second–
order wave loads can also be accounted for by the DP system. Loads that can not be avoided or compensated for are first–order wave effects.
Chapter 3
Pipelaying Mechanics
The structural analysis of an offshore pipeline under construction and installation deals with the computation of deformations, internal forces, and stresses as a result of external loads and the structural properties of the pipe. A short pipe section, like a single pipe joint appears to behave much like a rigid body, whereas a long pipe of several hundred meters is very elastic and behaves almost like a string.
Hence, the pipe string behavior is highly dependent on the water depth. For the structural analysis, it is seen to function as a continuous beam, a tension member, a compression member, a pressure pipe, an externally loaded conduit, and a suppression element. The static and dynamic loads due to the construction methods and the environment are numerous and varied (Small, 1970).
Structural analysis of pipelines experienced a significant increase in importance in the late 1960’s and 1970’s, when offshore oil development moved into deeper waters and more hostile environment. Simple approximations and rules of thumb used in the 1950’s and early 1960’s were no longer adequate, and more complex methods had to be designed. This progress brought with it new and more complex problems in structural evaluation and analysis.
Structural deformation of the pipe during construction depends on the method and equipment used for installation, the structural properties of the pipe and the environmental loads. In this chapter we consider the loads on the pipeline, and how these loads control the deformation of the pipe. The effects of the marine environ-ment, and the pipelay vessel are also considered. The pipelay mechanics presented in this chapter is the basis for the nonlinear dynamic pipe models developed in later chapters.