PIPELINE SYSTEMS 

Though reelship pipelaying is not generally thought of as J-laying, it is in reality identical as far as the mechanical behaviour of the suspended span is concerned (see Figure 8.4, “Reel-lay configuration”. The reelship takes advantage of the low tension that J-lay allows: this is particularly useful because the reelship is dynamically positioned. The reelship method had been developed in parallel with the S-laybarge method. The original idea can be traced back to the PLUTO project which laid 3-inch products pipelines across the English Channel in 1944, from floating reels towed behind tugs, but the technology implemented in the reelship is much more sophisticated and controlled. The pipe is wound onto a vertical plane - horizontal axis reel with a 19.5m hub diameter and the ability to carry 2 x 2500 tonnes of pipe. In the actual reelship concept, the maximum diameter that can be handled is 18 inches.

Figure 8.4 - Reel-lay configuration

Pipe is paid out onto the reel from a ramp, with an aligner at its highest point which also serves as a level wind. The pipe leaves the reelship through the aligner, then through a straightener and tensioner (s) and down an adjustable steep ramp into the water. A pipe clamp is located at the foot of the ramp. This technique requires an onshore welding yard to prepare long strings (e.g. 1000m) which will be later joined together during the reeling process onto the lay vessel.

During the reeling process at quay-side and the subsequent unreeling offshore, the pipeline will experienced plastic Deformation and cumulative strain Deformation which are to be maintained within acceptable criterion.

The mechanics of reeled pipeline (moment versus curvature) is explained in the following figures:

This section will present the pipeline analysis and design for deep water application of Reel-lay method.

Buckling during pipelay is controlled by applied top tension. Increased tension at the top of the lay span reduces bending in the sag bend. Detailed assessment of bending during pipelay will normally involve the use of finite elements based software, which can model both dynamic and static effects. For first pass assessment of reel-lay installation, the pipe can be assumed to form a catenary to the seabed and basic catenary equations can be used for determining the static bending conditions in the pipe, as illustrated in the next section.

The pipe lay operations are divided into the following stages:

1. Initiation (initiation head or first end sled);

2. Normal pipe lay;

3. Midline installation (mid-line sled, Tee or Wye);

4. Laydown with (laydown head or second end sled);

5. Abandonment and recovery (A & R).

The following analyses should be carried out to ensure pipeline integrity during operations:

Operational parameters are driven by allowable pipe stress/strain, equipment limitations, catenary configuration, and weather conditions.

The basic catenary equations are useful to demonstrate how tension is used to control the bending in the sag bend and hence how to control buckling. It should be stated that catenary equations neglect bending stiffness and are only valid (if at all) in very deep water. The equations shown below are therefore for illustration only. The engineer should use time domain dynamic software e.g. Orcaflex, Flexcom, Deepline (or other approved specialised pipeline finite element programs) to find the configuration of the pipe under various loading such as bending, tension, etc.

Figure 8.5 - Schematic of the pipelay catenary

The lay tension also results in a residual tension (H) remaining in the laid pipeline, due to the reaction of the soil friction.

In relation to the diagram above:

d = Water depth (m);

α = Ramp angle with respect to horizontal (deg);

s = Pipe span length (m);

x = Distance to touchdown (m);

H = Horizontal force component (N);

T = Lay tension (N);

w = Submerged pipe weight (N/m);

c = Parameter of the catenary (the characteristic length) = H/w (m).

The variables of interested are:

T, α, w, d, s, x.

The submerged weight (w) and the water depth (d) are known and the lay tension (T) and ramp angle (α) can be altered in order to control the span length (s) and the length to touchdown (x).

These parameters in turn control the bending and axial stress.

Basic catenary equation:

For suspended pipeline:

Therefore:

Horizontal component of tension:

From these one can derive:

Pipe span length:

Touchdown length:

Looking at resultant stresses:

Bending stress:

Where:

Given:

Then:

One can now calculate bending stress.

Other stress components are derived as follows:

The axial force comprises residual tension and hydrostatic pressure end cap force;

Axial force in pipe (laid empty) :

Where Dc is the hydrodynamic diameter i.e. the outermost diameter over all coatings.

The axial stress is therefore:

Hoop stress is given:

The equivalent stress is given by:

Where is the total longitudinal stress given as:

The limitations of the installation vessel for the following parameters listed below should be obtained from the vessel latest data:

Normal lay operation are carried out under static empty conditions for a given water depth range. As the pipes are of same physical properties there is only one analysis for the normal lay stage that covers the maximum water depth range in the field. However, initiation, midline installation, and laydown are specifically analysed per structure and therefore pipe segment.

Checks should be performed against the local code if specified in the respective contract. A provision for a DAF (e.g. 1.1 to 1.3) should be considered to take into account dynamic effects.

Specific calculations such as fatigue analysis and curve route stability are implemented as required to ensure that pipe integrity is not jeopardized during operations. Due to the varied range of variables, it is required to implement iterative methods to ensure that all parameters are optimised and that the important limiting factors are identified. Basic methodology involves modelling the system with the dynamic software (e.g. Orcaflex). The first step is to investigate most probable environmental conditions for the area. Trial and error methods are used in the dynamic analysis to identify which weather conditions result in the pipe to reach 96% of yield. Weather limitations are checked for each of the pipelay stages and the results for the empty cases will become the maximum conditions for normal operations. This dynamic analysis is carried out for empty and flooded conditions. Flooded conditions will be used to check vessel equipment capabilities such as A&R system. Resulting loads are used to check localised stresses on the pipe due to reaction forces from pipe lay equipment and pipe interaction, for example when the pipe is being clamped at the HOM (hang off module). In some cases these reactions may be a limiting factor and impose a further decrease in maximum allowable operational conditions, as discussed in the next section.

Installation analyses, both static and dynamic, are performed primarily with the software Orcaflex (or equivalent). The program has detailed FEA capabilities addressing pipeline tensile, bending, shear, hoop stresses etc. However, the pipe needs to be assessed at the pipelay equipment interface.

The two pipelay equipment interfaces are the tensioners (reel lay only) and the HOM for both reel and J-lay. It is at these two interfaces that radial forces are imparted to the pipeline to resist the catenary tension and keep it from slipping. These radial forces exert hoop stresses in the pipe wall in addition to the tensile and bending stresses that are generated by the pipeline’s catenary.

The requirement is that the overall stress in the pipe wall is less than 96% SMYS. Equipment settings are calculated to ensure that localised stresses are maintained within allowable limits.

Vessel offsets may need to be optimised to reduce stresses at the pipe connection interface. The optimum vessel position is where the bending moment at the pipe top end is zero; this is where the departure angle (pipe angle at water level) should be close to the tower angle. Any vessel offset from this optimum position will increase the bending moment at the pipe top end and thus increase pipe stress at the pipe equipment interface.

This analysis is performed to confirm that the radii of the curves along the proposed routes are sufficiently large to ensure that the pipeline will not be dragged into a bend under the action of the axial and lateral components of the bottom tension in the pipeline during normal pipelay. The method of analysis adopted is based on “Hobbs formula” which considers the pipeline submerged weight, lateral coefficient of friction, axial bottom tension and curve radius.

Limiting Static Tension = (Pipeline Sub Weight x Curve Radius x μ)/ FOS

Where μ = coefficient of lateral friction.

The analysis is based on the minimum predicted lateral coefficient of friction. The calculation should incorporate a factor of safety of 1.5 against the limiting bottom tension.

Pipelines suffer a reduction in strength after they have been subjected to cyclic loading. This phenomenon is known as fatigue and is essentially a process of crack initiation and subsequent propagation and then failure. Therefore, when the pipe is clamped in the HOM, pipeline fatigue should be assessed.

A commonly used fatigue method of a material subject to repeated cyclic stresses of constant magnitude is known as the S-N curve in which N is the number of cycles of stress or stress range S which would cause failure.

Stress range is the difference between the maximum and minimum bending stresses at the point of interest as obtained by dynamic analysis. The stress range output from the dynamic analysis should be multiplied by the stress concentration factor (SCF), typical to the area of interest. The fatigue wave spectrum should be run for the dynamic analysis. The cumulative damage caused by different range of stresses may be assessed using Palmgren-Miner’s rule.