Control Objectives Based in Measurements Along the Riser 80

The second control objective principle is based on available measurements of the relative horizontal distance between the risers. In order to achieve near parallel risers, the horizontal distance between them should be constant and equal along the entire length of the risers. With only one measurement along the riser length, the distance between the risers ∆x_R12,m(z) should be equal to the distance at the top and bottom being the desired distance ∆x_d. Hence,

∆x_R12,m(z) = ∆x_d. (6.7)

The measurement should be placed where the risers are likely to be closest, which is determined by the current profile. Note that we in this control objective do not need to consider the elasticity of the riser material directly.

6.1.3 Discussion

For the first principle of control objectives, reliable and accurate measurements of payout and tension are available today. Furthermore, the measurements and the actuator (top tension force) are found at the same location, thus preserving passivity properties of the closed loop system more easily due to collocated con-trol. To calculate the total riser length as a function of time and tension, the initial riser length corresponding to the initial tension needs to be known.

If supervisory switched control is used, a system model is required and ad-ditional measurements of TLP motions, undisturbed current velocity profile and a good model for the hydrodynamic interaction are needed. These are used to calculate the smallest relative distance between the risers.

For the second principle of control objectives, the relative distance between the risers is measured directly. Hence, we do not need the additional measurements or a good process model for the linear controllers. The drawback of this method is that we are limited to a finite number of measurements, with predefined location along the riser. Also, we do not know if these are placed at the water depth where the risers are closest. As the measurements will be along the riser and the actuator is located at the top end, this set-up is not collocated. However, the system is much slower in the horizontal than the vertical direction, such that this may not be a problem after all. This may introduce some scattering in the vertical direction if the reference is not slow enough, but the horizontal direction will still be stable. We will in this work focus on the first principle of control objectives, as described in Section 6.1.1.

6.2 Riser Operational Conditions

The changes in the environmental conditions, in addition to the various riser types during operation and production, may require different control algorithms during the lifetime of a TLP/riser system. The purpose of the control system is to prevent collision for all environmental conditions, riser types and water depth, denoted as regimes or riser operational condition (ROC). A classification of the various regimes could help the design of appropriate controllers and smooth switching between them. The risers in an array may often have different phys-ical properties. The external diameter, wall thickness, material and density of the internal fluid could vary, depending on whether the riser is used for drilling, production, export or workover to mention some. Together these factors decide the dynamics of the riser. The applications and properties of the riser, the riser characteristics (RC), also affect the controller gains. A specification of the dif-ferent ROCs and the controllers based on these leads to a supervisory system where the time parameters and the necessary controller components are included according to the prevaling ROC. This concept of ROC is motivated by the work on vessel operational conditions (VOC) by Perez et al. (2006).

The aim of this section is to highlight the essential characteristics of the conditions that affect the dynamics of the riser system and use this information to decide which control action to perform.

6.2.1 Riser Characteristics

During drilling and production on offshore fields different riser types are used for different operations and purposes. Flexible risers or SCRs are not considered here. All risers referred to in this work are vertical steel risers, connected to a TLP. Some typical riser applications are:

• Drilling.

• Production.

• Workover/Maintenance.

• Export.

• Import.

• Injection.

The risers are specially made for each purpose, giving different properties which decides the risers physical behavior, both statically and dynamically. The most important parameters affecting the dynamics of a steel riser are:

• External diameter.

• Cross sectional area and wall thickness.

• Density of the riser contents.

• Riser length.

• Top tension.

• Elasticity.

Some of these parameters are closely related. The top tension level is dependent on the weight of the riser, i.e. the cross sectional area, the length of the riser and the density of the internal fluid.

The riser elongation is proportional to the modulus of elasticity times the cross sectional area. Hence, different cross sectional area will give different elongation according to the stiffness EA. The cross sectional parameters also contribute to a weight difference, increased or decreased by the difference in the density of the internal fluid between the risers. The effective weight decides the tension level along the riser, which together with the riser length is of major importance for the eigenperiods of the risers. For two otherwise equal risers, a difference in the variation of the contents affectes the total effective weight and the effective weight gradient. This could be seen statically as at which water depth the deflection is largest. A larger effective weight will have its maximum deflection at larger water depths than for otherwise equal risers.

The upper tension limit is decided by the yield stress for steel, whereas the lower limit is given by the effective weight plus a safety margin. Hence, for the otherwise equal risers with different contents, the upper tension limit is the same, but the lower limit is smaller for the riser with lighter contents.

Recall that the drag forces on the riser are proportional to the external di-ameter. For two risers with different external diameter, but equal weight per unit length, the riser with the largest diameter needs a larger top tension to compensate for the horizontal displacement due to the drag forces.

In extreme cases the longest riser eigenperiod can be close to the typical LF motions. For another riser case, the first eigenperiod could be close to the slowest WF motions. Hence, depending on the different dynamic properties and corre-sponding eigenperiods, resonance may occur at different frequencies. In addition it should be noted that the current field behind a riser and the shielding effect, is depending on the diameter of the upstream riser. The effects of the riser prop-erties are not further investigated here, but should be taken into consideration when deciding the control strategy and the controller gains.

Current

TLP

Dependening on environmental conditions Profile and velocity variation

Calm Strong

Stationary Dynamic Control

No Control

Riser Characteristics

Initial Conditions

Figure 6.2: Riser operational conditions and influence on the controller choice.