Design and Analysis of a Flexible Jumper
The flexible jumper has found wide application as a connector between subsea structures. Flexible jumpers can be used to connect trees and pipelines (PLET) or manifolds. Its flexibility allows the connection to be made such that one side of the connection has sufficient movement. In the following sections, jumper in-place analyses and installation analyses are detailed for a GoM project.
Contents
Flexible Jumper In-Place Analysis
Allowable Jumper Loads for Jumper In-Place Analysis
The allowable jumper loads for a jumper in-place analysis are affected by the following issues:
- Maximum tension of flexible jumper. To ensure the integrity of supporting structures and the collet connectors of subsea structures, the horizontal tension should be limited to a maximum value (for example, 5 kips). The jumper loads and moments due to the maximum horizontal tension should be below the allowable limits.
- The minimum allowable bend radius should not below 1.25 MBR (minimum bend radius).
- The flexible jumper should experience a positive bottom tension at all times (i.e., no axial compressive load is permitted).
- The induced maximum jumper loads should ensure the integrity and stability of adjacent structures at all times.
Analysis Methodology
During installation, and throughout the design life of jumpers, loads will be transferred from the jumpers to the PLET collet connector and manifold structures. The global static analysis is performed to ensure the integrity of jumpers during installation and operating conditions, and to assess the induced load transfer from jumpers to the adjacent structures.
Static Jumper Analysis
All static analyses are 2D or 3D load cases, which account for all functional loads. In the analysis of the flexible jumper, the following external forces are considered:
- Uniformly distributed submerged weight of jumper and bend restrictor;
- Concentrated load due to gooseneck, end connector, running tool, etc.;
- Seabed foundation in the vertical plane, which is assumed to be a continuous elastic foundation;
- External hydrostatic pressure.
Free-End Catenary Analysis
The free-end catenary analysis is performed to define the gooseneck angle. The term free-end refers to the top end of the jumper, which is fixed at a predetermined elevation although it is free to rotate according to the natural catenary configuration of the jumper. The free-end catenary analysis is performed for the operating condition, which is the long running load case that the jumper will experience during its design life. For each jumper, the free-end catenary analysis assumes that the lower end of the jumper is resting on the seabed under a constant horizontal bottom tension. The predetermined elevation of the top end reflects the supporting structure height, with the nodal rotational degree of freedom unrestrained to allow the top end of the jumper to follow the natural catenary configuration of the free hanging string. For a range of bottom tension, the above analytical modeling allows study of jumper behavior to establish a range of touchdown locations and associated gooseneck angles. Based on this study, the gooseneck angle can be optimized to meet loading requirements. Based on the optimized gooseneck angle, the nominal bottom tension is defined for the operating condition. In-Place Jumper Analysis The in-place jumper analysis is performed to ensure that the gooseneck angle defined in the free-end catenary analysis provides a favorable solution for all other loading conditions. The in-place jumper analysis is also performed to provide a range of bottom tensions for which the integrity of the jumper and the supporting structure is maintained. Based on the axial soil resistance, it is concluded that jumper lengths, in most cases, are not long enough to fully or partially restrain the jumper in the longitudinal direction. Any variation in axial tension, therefore, is anticipated to freely travel from the first end to the second end of the jumper. For this reason, it is recommended that the entire jumper, including first- and second-end elevations, be modeled when performing in-place analysis.
As minimum the following three extreme cases are investigated:
- Near position;
- Far position;
- Maximum out-of-plane excursion.
The near position is defined as the loading direction that results in the touchdown point (TDP) moving toward the supporting structure. In this situation, bottom tension is at a minimum and the jumper bend radius at the touchdown location is approaching the MBR criteria limitation. The far position is defined as the loading direction that results in the TDP moving away from the supporting structure. For this load case, the minimum bend radius usually occurs at the jumper/end-fitting connection interface. The induced moment and top tension are high for this loading condition. The transverse loading direction results in the TDP moving in the out-of-plane direction to the jumper. The following criteria are used to establish jumper extreme positions:
- The minimum allowable bend radius should not below 1.25 MBR.
- The flexible jumper should experience a positive bottom tension at all times (i.e., no axial compressive load is permitted).
- The induced maximum jumper loads should ensure the integrity and stability of adjacent structures at all times.
- A pipe rigidity transition zone of 1.5 OD should be assumed after the jumper/end-fitting interface.
Flexible Jumper Installation
The finite element program Orcaflex is commonly used to simulate the flexible jumper installation. The software is developed by Orcina Ltd. The following installation steps should been analyzed in the installation analysis:
Step 1: Lowering of the jumper;
Step 2: Obtaining an installation configuration;
Step 3: Connecting to PLET;
Step 4: Connecting to another PLET or manifold.
The main parameters of the analysis and the items that should be investigated are:
- Wave height – constant throughout the analysis;
- Wave period – constant throughout the analysis;
- Wave type – Stokes fifth-order waves may be used in the analysis for conservatism;
- Current – assumed to be constant throughout the analysis;
- Vessel heading is maintained at 180 degrees or head seas, unless noted otherwise;
- Layback range (horizontal distance from overboard point to TDP);
- Curvature/bending radius;
- Tension in the jumper and deployment wire;
- Bending moment (BM) applied to bend restrictors (BRs).
Data required for the installation analysis include:
- Water depth;
- Current profile;
- Wave data;
- Seabed friction factors.
Design Criteria
The installation design criteria for a flexible jumper are listed below. The installation analysis is performed to ensure that these criteria could be safely and effectively achieved.
- MBR of flexible jumpers must not be exceeded.
- Tension at the PLETs and manifolds should not exceed a given value, for example, 5 kips.
- The bending moment (BM) applied to the BRs must remain within allowable limits.
- Maximum load on the lifting adapter should not be exceeded.
- Load capacity of the cranes should not be exceeded.
- Maximum load on the A&R wire should be limited to allowable value, for example, 45 tonne.
- There should be no compression in the jumpers.
Installation Steps
The installation steps for a flexible jumper for the project in the deep water of the GoM are given to show the procedure used for an installation analysis.
Step 1: Lowering of the Jumper
Analysis starts off with the jumper attached to crane hooks and hanging over the side of the vessel. This is the position from which the crane wires begin to pay out equally on both sides to begin lowering the jumper into the water.
Step 2: Obtaining an Installation Configuration
This step involves lowering the jumper by simultaneously paying out equal amounts of crane wire on both sides into; the crane hooks reach a certain
depth. After this depth is reached, only the crane wire attached to the first end is allowed to pay out further to lead into step 3.
Step 3: Connecting to First-End
This step starts when the wire attached to the first end is paid out to reach the depth at which the first end can be connected to the applicable PLET/
manifold. This is done while the other wire is held at the position where it was at the end of step 2. For some of the longer jumpers, the crane
separation on board the vessel may need to be increased to around 70 m. The vertical separation of the two connectors has to be increased so as to
assist in obtaining the vertical orientation of the connector being attached to the first end. Higher bending moments and curvatures are expected in the jumper due to this difference in elevation.
Step 4: Connecting to Second-End This step begins with the first end of the jumper being rigidly connected to the PLET/manifold. Thereafter, 10 m of the second-end crane wire is paid out (step 4, substep 1) to achieve touchdown and establish a layback. The subsequent steps involve paying out crane wire at the second end until the hub is within 10 m of its connection point and lowered further toward the second-end PLET/manifold in two steps (substeps 2 and 3), to be followed by the second end reaching the final connection position in the final step (substep 4).
References
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