Crane Vessel Research Articles

Basic Research on Automatic Installation of Underwater Flexible Structure Using Active Control

This paper examines application of active control technique to underwater installation as a new approach instead of the present method using crane vessels. In this paper, two kinds of active control problems are examined. The one problem is how to control a flexible structure on tracking the given trajectory or precise positioning on the operation of installation, not stimulating its elastic responses. The other is how to control a rigid body structure on tracking the given trajectory or precise positioning under the existence of unknown current.The structural flexibility imposes problems of suppressing elastic responses and securing stability of control system. In this paper, H∞ controller combined with LAC/HAC feedback controller is designed for the case that the structural flexiblity exists.When unknown current exists, the dynamical effect of the current must be compensated somehow. This paper presents learning trajectory tracking control when a structure is transported repeatedly along the objective trajectory.In order to confirm the control algorithm and verify the possibility of active control installation method, basin tests are executed using a neutrally buoyant flexible model with ultrasound ranging system and thrusters.

Parametric excitation of crane loads in moderate sea states

In order to investigate the parametric excitation of loads suspended from crane vessels in random seas a time domain numerical solution of the equations of motion is used. A computer program was developed to analyse in the time domain the equation of motion of a crane vessel and its suspended load. The system is described by the six rigid body degrees of freedom of the vessel, together with the three orthogonal displacements of the lift mass. Random force time histories were applied to the vessel based on the Pierson-Moskowitz spectral formulation. This time domain model is used here to demonstrate the influence of realistic sea states on the parametric excitation of crane loads.

Response Analysis and Operating Limits of Crane Ships

Results are presented of a recent research project with the objective of quantitatively predicting the dynamic response of crane ships during lifting operations offshore. For this purpose, a mathematical simulation model was developed. This model considers the elasticity and damping of the hoisting rope assembly and includes bi-angular swing of the hook load coupled with hull motions. The nonlinear, time-domain motion equations comprise effects of gravity, buoyancy, hydrodynamic reactions, viscous drag, wave excitation, and the mooring system. The mathematical model was verified by comparing calculated system response with corresponding measurements from scale-model tests. To derive transfer functions of system response, the dynamic motion equations were linearized about the system's equilibrium state. The dynamic behavior of a shear-leg crane barge and a semisubmersible crane vessel was investigated and operational limits were specified. Based on these limits, obtaining the crane ships' probability of successfully performing a heavy lift was demonstrated.

Concrete Structures for Development of Offshore Fields

Summary Concrete structures represent competitive solutions for developing many offshore fields. They are of particular interest when storage is required and as floating support structures. Furthermore, concrete reusable platforms may represent cost-saving solutions for many applications. To design optimum structures, designers must be realistic with respect to the structural capacity of concrete for offshore use. Introduction Experience in the North Sea has shown that concrete support structures are competitive with steel jacket structures in terms of initial investments and maintenance costs. This experience is considered relevant for other areas, particularly for countries that must import steel, for icy areas, for offshore areas without infrastructure, for places where oil storage facilities are required, and for areas where large crane vessels are not available to lift heavy steel structures or large topside modules. Furthermore, concrete floaters [tension-leg platforms (TLP's) and catenary anchored structures] are attractive as minimum-cost solutions for deepwater applications. This paper discusses the requirements and infrastructure needed to design and fabricate concrete structures. Particular attention is given to availability of cement, aggregate, reinforcement steel, and prestressing cables. This paper also discusses design competence and required computer facilities, as well as the availability of barges, a shipyard dock, or a special-purpose construction dock. We describe the fabrication, installation, and removal processes. Removal is becoming increasingly important in a world gradually becoming more aware of environmental stability. We give examples of efficient designs suitable for optimum development of small fields and fields requiring storage. These examples include traditional concrete structures constructed with form work or slip forming. Furthermore, novel concrete truss structures fabricated by joining precast tubes and nodes by prestressing cables and grout are discussed. We provide charts that give concrete volumes for a range of applications to enable operators to perform their own economic analyses.

Dynamic Aspects of Offshore Riser and Mooring Concepts

Summary. Recent research indicates that dynamic amplification of tension in lines and risers can be of the same order of magnitude as the so-called quasistatic values. Also, dynamic effects in the mooring system can affect low-frequency motions of the structure by increasing virtual stiffness and damping. This paper presents three-dimensional (3D) computational procedures that describe the motion, tension, and bending moment along a flexible pipe or mooring line. The paper also discusses correlations with model test data. Introduction The increasing number of mooring concepts for offshore structures and trends toward cheaper technology because of low oil prices put high demands on the design of the mooring arrangement. Important parameters are the large displacement of the structure, deep and hostile waters, and the need for year-round workability. The wide variety of mooring systems may be illustrated by the existence of shallow and deepwater single-point moorings with temporarily or permanently moored tankers. clump weight systems used for guyed towers, and wire moorings for semisubmersible crane vessels. The current design procedures include mainly a dynamic motion analysis of the moored object. This provides extreme positions of the structure. The mooring line tensions at these extreme positions can be found from the static-load excursion characteristics of the mooring system. In this quasistatic mooring analysis, all other phenomena that affect the maximum line load are taken into account in an overall safety factor. as required by certifying authorities. Typical values are three for operational and two for survival conditions. Experimental and theoretical research has shown that high-frequency oscillations (in the wave-frequency range) of the upper end of a mooring line can generate significant dynamic amplification of line loads. These dynamic effects depend onfrequency of oscillation,amplitude of oscillation,specific line mass.pretension, andhydromechanic line properties. From a systematic series of forced oscillation model tests, van Sluijs and Blok found that the ratios of maximum dynamic tension and maximum quasistatic tension strongly depend on the frequency of oscillation. This ratio is enhanced by increasing oscillation amplitude, increasing pre-tension, and reduction of line mass. Knowing the importance of dynamics for mooring systems, a similar behavior is expected for related "line-type" configurations such as flexible risers, pipe bundles, etc. The additional parameters concerning dynamic effects in these cases are the direct wave force and the bending stiffness. The traditional theoretical approach to solve the dynamic behavior of cable/riser systems is based on semi-analytical techniques. Geometrical nonlinearities are neglected to reduce the equations to differential equations that can be solved. Perturbation techniques are applied with success. but are restricted to certain areas. A more general approach to the problem is discretization techniques. The line is assumed to be composed of a limited number of discrete elements. These elements can have physical properties of their own. Thus, the formed system of partial differential equations describing the variables along the line can be replaced by equations of motion in an earth-bound system of coordinates. The most successful methods are widely known as the lumped mass method (LMM) and the finite-element method. This paper presents an LMM technique and the resultant algorithm (applied in the DYNFLX computer program). This approach was validated with results of an extensive study conducted by the Maritime Research Inst, Netherlands (MARIN) on behalf of the Netherlands Marine Technological Research (MATS) program (Fig. 1). Theory Basic Approach. The mathematical model for simulation of 3D behavior of flexible lines is an extension of the LMM technique used for mooring chains and wires. The spacewise discretization of the line is obtained by lumping the mass and all forces to a finite number of nodes. To derive the governing equations of motions for the jth lumped mass, Newton's law is written in global coordinates (Fig. 2): {[Mj] + [mj(t)]} ..........................(1) The added inertia matrix can be derived from the normal and tangential fluid inertia coefficients by directional transformations. [mj(t)]=nnj ...........................(2) where anj and ayj represent the normal and tangential added masses, respectively. anj=p(kIn-1), atj=p(KIt-1) and [ nj], [ tj]=directional transformation matrices. The nodal force vector ->F j contains the internal and external force componentssegment tension F sigma(t),shear forces caused by bending rigidity Fsno(t),fluid forces Ff (t),seafloor reactive forces Fr(t),buoyancy and weight Fw,buoy forces FB(t), andtether forces F sigma T(t). Because the tangential stiffness of the line (represented by its modulus of elasticity, EA) is an order of magnitude higher than the stiffness in normal direction, the tension is taken into account in the solution procedure. The tension vector on the jth node results from the tension and orientation of the adjacent line segments. ,............(3) where JPT P. 1609^